# gx390 backfires, wont start



## HermitJohn

Ok, this is Honda gx390. Good compression, no didnt use gauge, but you sure can feel it when you pull the rope. The magneto coil went bad. I bought new one off ebay. Put it on and I get strong fat spark at sparkplug out of the engine held against a ground. Install plug, give shot starting fluid and it backfires, Does it a second time. Fine must have sheared flywheel key. So after huge effort at getting big nut off, get flywheel off and key looks like new. So is a valve hanging up? Take valve cover off and a cleaner valve train you couldnt ask for. And valve clearance ok and valves moving up and down smooth as you could want. No corrosion, carbon, broken spring, or any problems.

The only weird thing is that the original coil was thicker and had a double wire plugged into terminal on top. Plus the kill switch wire at bottom. The double wire goes to some little sensor on opposite side of flywheel from the coil. No idea what it is. Looking on web, dont see another coil like my OEM coil, just ones like the new one I put on that are thinner and just have the kill switch wire, not the other little sensor thingie wire. No mention anywhere of this other little sensor thingie whatever it is.

So whats up? This engine was running fine month ago. Then one day no spark which I trace down to bad coil. I know it was coil since I wired in an aftermarket ignition module to kill wire and still didnt get a spark. I've done this on several chainsaws and small engines where they bond the OEM module permanently to the coil. Saves buying a high dollar replacement coil/module combo.


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## HermitJohn

Played with it some more and same result. Just feels like ignition timing is off, but on a small engine unless flywheel key shears, really not any other way for that to happen. I suppose cam gear could have jumped a notch?? Grasping at straws here.

Have to dig out the compression tester and make sure of compression, but I've played with enough small engines to know they have adequate compression by how they feel when you crank them.


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## HermitJohn

One last chance before I drain fuel and mothball it. I once again today gave it shot ether and it backfired through carb with loud bang just like a car would when distributor is installed 180 degrees off. But that isnt possible on a one cylinder small engine with a magneto other than shearing flywheel key. As I say the flywheel key looks like new, it didnt shear the key.


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## agmantoo

I would associate this backfire with the intake valve. Either the valve stem is sticking or the valve spring broken. As for the starting issue. Does this engine have a low oil shutoff? If so, temporarily bypass it.


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## retire2$

I would replace the flywheel key. It may look OK but if it was slightly bent this could be the problem. Also double check the air gap between the coil and the flywheel. If there is rust on the flywheel (especially around the magnets) a little emory paper will remove the rust. Is the fuel fresh? Check the carb. Maybe it is flooding. 

Everyone has offered good suggestions.


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## HermitJohn

If you read my previous post, I was amazed to see the head and valve train absolutely new, and shiny and spotless looking. I assume previous owner must have put on new head and used it very little thereafter. With valve cover off, valves move easily and valve springs are very much intact. It amazingly looks like new, surprising since outside of head has oxidized to look like rest of engine which definitely doesnt look new. There is no sticky valve or broken valve spring.

Yes it has low oil shut off, but that simply grounds out the ignition when oil level gets low. This is getting a nice fat spark blue spark. The ignition is not disabled. There is no reason to suspect the low oil sensor.

The key is a half moon key. Its as straight as can be, there is no wear or indentation, if you've seen one of these there is no room for it to bend, there is no slop or wiggle, it would have to be partially or fully sheared to change timing and there is not even hint of an indentation.

Again this engine was starting easily and running normally. Then one day no spark. I replaced the coil which has ignition module molded in with it like most modern small engines. I then get fat blue spark but also the backfire, like the timing is 180 degree off.

As I said above this coil is what is sold for this application, but looks different than the original. The original is much taller and has another lead going to some sort of sensor or pickup mounted on the other side of the flywheel from the coil. (180 degrees!!!!) I could find no mention of this online and could find no replacement coil looking like the original. The coil I bought is what Honda sells as a replacement coil for all GX390. There is no mention in any parts lookup of the extra pickup sensor on opposite side of flywheel with lead going to coil. I've never seen such before either. 

If there was an easy and accurate way to do it, as an experiment I'd file new notch for flywheel key that is 180 degree from current one and mount flywheel using it. I just bet the engine would run. The backfire is just like when you have distributor installed on car 180 degree off. I dont have another one of these engines or I would compare flywheels. I am betting flywheel off engine using coil like one I bought has magnets in different position, probably 180 degree different.


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## OkieDavid

Interesting problem and seems like you've covered all the bases. I wonder if the oem sensor was a crankshaft position sensor that assisted in the firing by ensuring the coil only discharged on the compression cycle and without it functioning it is now firing every time the magneto charges resulting in firing on the intake stroke? Not sure how to disable or bypass it but I think that's where I would focus.


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## agmantoo

Hermit,
Under the flywheel of some of the more recent built small engines one will as you mention find more than one magnet. The flywheel is designed to not only fire the ignition but to also provide voltage for charging the battery. The voltage will be AC coming off the flywheel and then it will be fed to a rectifier. On some of these, the voltage will not be regulated and others will have a full setup of regulator and rectifier. If you ever want to have a charging system off this engine all you need to do is some research on the regulators off the Chinese ATV's. I bought and installed a $10 regulator/rectifier on a German diesel air cooled ditch packer saving nearly $400 compared to the OEM replacement. However, remember that on these engines, the engine (4 cycle) will fire once each revolution of the flywheel. Once on ignition and again on exhaust. Therefore filing another keyway should only give the same results.


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## HermitJohn

Looks like I guessed correct in last post. Top pic is my engine, just managed to go out before rain, remove everything to take pic showing position of magnet, keyway, and the pickup sensor opposite side of flywheel from coil. Second pic is one I stole off ebay showing new flywheel for this engine. Notice keyway is opposite side where it is on my flywheel. Apparently Honda originally had that separate trigger for who knows what reason and abandoned it. 

Since I cant get original type coil (probably high dollar if I could) if I want this engine to run will have to buy a new flywheel. One in pic is China clone for $60. Thats still lot more than I wanted to put into this engine, but short of cobbling up my own ignition system, about the only real option.


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## agmantoo

Hermit, I think it should still run at 180 degrees out as I stated before. The flywheel turns twice to one complete cycle of the engine.


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## HermitJohn

agmantoo said:


> Hermit, I think it should still run at 180 degrees out as I stated before. The flywheel turns twice to one complete cycle of the engine.


Then your car (if it still has a distributor) which is also 4cycle should run fine with distributor turned 180 degrees. I can tell you they dont, they backfire through the carb with a mighty bang. With magnet 180 degree off on flywheel, it fires on wrong cycles. And this flywheel only has ONE magnet. You can easily see the magnet in the pics.


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## agmantoo

Hermit, the camshaft, which drives the distributor, in a car does not turn at crankshaft speed. It turns at 1/2 crankshaft speed. If you want to try turning the flywheel 180 degrees I can tell you how without destroying anything. Put the flywheel exactly where you want it to be. Then drill into the joint where the flywheel and the crankshaft mate. Then drill and tap the hole and install a setscrew with 1/2 of the body of the screw in the crankshaft and 1/2 in the flywheel. You will be drilling parallel to the crankshaft. The magnet in your flywheel is different than I had envisioned it. I thought that it was similar to the ones seen in the newer engines.


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## Hip_Shot_Hanna

try putting your old coil along side the new coil and see if it is offset a little .


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## HermitJohn

Tapered shaft into flywheel, yes maybe doable but not a strong interface. I probably can remove key, jam something in there that would hold enough to start engine but wouldnt work for long under load. Decided to rain cats and dogs this morning here after I took the picture so not able to try it very soon. 

A four cycle engine fires at top of compression stroke and exhaust stroke. If it fires on intake stroke and power stroke then you have a problem. Having magneto 180 degrees off means you are firing on the intake and power strokes. This is exactly what happens in a traditional car engine if distributor is 180 degrees off. You get the backfire flame through carb cause ignition fired on intake stroke. There is fuel, but the intake valve is open.


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## HermitJohn

Hip_Shot_Hanna said:


> try putting your old coil along side the new coil and see if it is offset a little .


Old coil mounts to engine exactly same way. No offset. And even if you move timing slightly advanced or slightly retarded, engine will still run. A slight timing change doesnt cause major backfire through the carb. They even sell offset flywheel keys for small engines used in gokarts, to get some extra ignition advance and thus some extra power. Old coil just much taller with a two prong connection at the top that goes to that sensor on other side of flywheel along with a kill switch terminal down low. The new coil is short and just has kill switch terminal. I looked and looked and couldnt find a replacement coil like the old one and nobody mentions or stocks the pickup sensor you see opposite the coil in the picture. Mention it and you get blank stare or the internet equivalent. I am guessing my engine is one of very early ones in this series and Honda engineers were trying something that didnt prove to be beneficial.


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## HermitJohn

Getting it straight and all be hard to do with file, but guess I just use small file and cut out a new keyway 180 degree from one in the flywheel. Nothing to lose except some time. first try jamming flywheel on shaft in proper position without key and if engine starts even for bit, then I have my answer. Then try filing a new slot for the key. Probably need a new sharp file or it would take literally forever.


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## cfabe

I wonder how much a machine shop would charge to cut a new keyway for you? Might be worth a few calls.


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## HermitJohn

cfabe said:


> I wonder how much a machine shop would charge to cut a new keyway for you? Might be worth a few calls.


Dont get me started on machine shops (as opposed to automotive machine shops who generally are more reasonable). They have high minimums and just expensive pain in rear to deal with unless you have a friendly retired machinist neighbor with little shop out back of his house that likes to keep busy and putter. I actually found such a guy when I lived in Michigan, down here not so lucky.

Been hunting around on web and just found brand new gx390 flywheel (China made of course) for around $30 shipped. Even includes fan and recoil hub and new nut. Machinist wont talk to me for $30. 

I remember working for a guy once. We were adapting a Kubota backhoe attachment he'd bought somewhere and lot slop in couple pinned joints. He didnt have capability of drilling 1 inch holes so I said maybe a machine shop would do it, cant be much thinking of my childhood when Dad took some machine piece to local blacksmith and they drilled nice big hole for $1 ($1 per hole). Yea he knew of one close, guy worked in shop out back of his house. Didnt ask price, just left the two pieces of steel. When he picked them up, they charged him $100 and this was back in mid 90s. They didnt have to machine or measure anything, just punch couple holes in some steel he provided and then we would align them with pin on each side and weld. I think $100 was their minimum price for any job, they didnt want small jobs. Anymore I'd just have went and found some heavy wall tubing with inch hole, couple short pieces and burn hole to put them, then weld. Always a work around though not sometimes obvious.


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## agmantoo

Hermit, 
I am not being argumentive, but there is something you need to think through. Old one cylinder engines were fired from points driven by the cam shaft. The cam shaft rotated at 1/2 speed of the engine. Newer engines are fired from the flywheel at the speed of the engine rotation. Old engines when 180 degrees out of time did as you stated, backfire through the carb due to the 1/2 speed turning of the cam. On the engine you have now to rotate the fiywheel 180 degrees will do nothing but put it back where it is firing now IMO. Take time and sit down with a pencil and think this through. I do not want ot see you needlessly spend time and money and have the same results as now.


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## HermitJohn

Then why did Honda bother to change the keyway in relation to the magnet? It would be pointless according to you. On a magneto, the coil has to build a charge big enough for the module to fire. If it does it at wrong time, timing is off. If its 180 degrees out of place then timing is off 180 degrees and you are firing on wrong strokes. Now if there were two coils 180 degree from each other and one magnet on flywheel, then it would fire on every stroke. Dont think that would work well, as you just dont want sparkplug firing on intake stroke. Once there is fuel mix and spark together , something is going to burn. If intake valve is open, guess where that fire is going to travel?

Just been playing with it and thats HARD steel. Hardened screw couldnt get a grip. File could barely scratch it, let alone cut a slot. I could cut slot into the end of crank itself with a carbide cut off disk in dremel, but really hate doing that. Also hate idea of welding it. I bought an engine once that some fool did that and it for all purposes ended that engines life once magneto went bad as magneto coil was inside the flywheel on it. Welding this wouldnt do that but mean engine could never be taken apart again as this area where I'd weld is recessed and be hard to grind apart again.

Hmm dont think it would hurt strength of crank cutting another notch in it. gotta look again. Be nice not to buy anything even if it uses up few cut off wheels in dremel.


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## HermitJohn

woohoo, took 3 little cut off disks and about 10 minutes but gouged a hole for the half moon opposite side of crank from where it was. Not exactly machine shop job quality, but close enough for Arkansas... 

Engine starts and runs. Really mostly happy to have it working again, though winning a friendly argument with a respected and knowledgeable regular here kinda nice too. I wasnt absolutely positively 100% sure, but my argument above made sense to me. Also couldnt believe Honda changed keyway direction just for the heck of it. Rarely are engineering changes made to existing design without good reason. There just wasnt any other answer as to why it backfired like it did.


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## HermitJohn

One does wonder why I seem to get all the oddball stuff thats just enough different to cause these kind of problems.


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## agmantoo

Hermit
I am happy that you have the engine running. I will not question success but I do not know any more than I did. I do not understand your explanation of the 180 degree out of position and I am unsure if you understand my inputs of the firing once per revolution off the current flywheel and the different of the older point system where the engine fires once per 2 revolutions. There is obviously something with the engine you have that I do not understand and possibly never will. I make mistakes and can live with them. It is never my intent to mislead anyone but it is difficult to sometimes "see" what is happening from this side of the keyboard. If I mislead you I apologize.


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## HermitJohn

agmantoo said:


> Hermit
> I am happy that you have the engine running. I will not question success but I do not know any more than I did. I do not understand your explanation of the 180 degree out of position and I am unsure if you understand my inputs of the firing once per revolution off the current flywheel and the different of the older point system where the engine fires once per 2 revolutions. There is obviously something with the engine you have that I do not understand and possibly never will. I make mistakes and can live with them. It is never my intent to mislead anyone but it is difficult to sometimes "see" what is happening from this side of the keyboard. If I mislead you I apologize.


Nothing to apologize for. On a four stroke engine, (intake, compression, power, exhaust), the engine makes two complete revolutions to accomplish this cycle. It fires once on each 360 degree revolution of crankshaft by whatever method so twice in that 4 stroke cycle. It has to fire on compression stroke for engine to run and the firing on exhaust stroke is meaningless since there is no fuel left to burn. If ignition is 180 degrees off, then it will still fire twice but on the intake and power strokes. Firing on the intake stroke with intake valve open gives the flaming backfire through the carburetor. Thats what my Honda was doing. No matter what ignition system is used, you cant fire on the intake stroke so no 4 stroke engine fires on every stroke.

Also this has a magneto, engine has to turn to build up a charge in coil which then either points or ignition module fires. There is no constant charge ready all time like on a battery system. Thus even electronic module cant trigger coil to fire if there is no charge built up by magneto at the proper time. Well it could trigger, but without a built up charge, nothing happens.


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## Hip_Shot_Hanna

I am with agmantoo here 
there are 360 degrees in one revolution ,one revolution is only half of a 4 stroke cycle 
so the spark fires at Top dead Center ( 0) degrees , with one magnet in the flywheel it fires again at the top of the exhaust stroke , one complete turn of the crankshaft ,360 degrees (power ,, Exhaust ) then it starts its induction stroke , ( induction compression ) followed by the compresion stroke , 360 degrees and the piston is back at TDC ready for the spark . 720 degrees for a complete cycle or 2 complete turns of the crank . 
spiting back thru the intake unles the timing is way out and as the flywheel is keyed thats unlikley i would say that with the engine standing you have a inlet valve sticking .

there has to be TWO magnets in the flywheel to fire at 180 degrees


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## HermitJohn

Hip_Shot_Hanna said:


> I am with agmantoo here
> there are 360 degrees in one revolution ,one revolution is only half of a 4 stroke cycle
> so the spark fires at Top dead Center ( 0) degrees , with one magnet in the flywheel it fires again at the top of the exhaust stroke , one complete turn of the crankshaft ,360 degrees (power ,, Exhaust ) then it starts its induction stroke , ( induction compression ) followed by the compresion stroke , 360 degrees and the piston is back at TDC ready for the spark . 720 degrees for a complete cycle or 2 complete turns of the crank .
> spiting back thru the intake unles the timing is way out and as the flywheel is keyed thats unlikley i would say that with the engine standing you have a inlet valve sticking .
> 
> there has to be TWO magnets in the flywheel to fire at 180 degrees


Either you are calling me a liar or you didnt read this thread completely and talking out your rear orifice? Thats the only conclusion I can come to. Cause neither you nor Agmantoo has explained why it now runs when I made new keyway on opposite side of crank and remounted flywheel using that keyway. If it doesnt matter why did HONDA change the flywheel keyway, HMMM?? Did you look at the pictures or do you think I photoshopped them. 

I swear talk about EGO, you guys have it in SPADES!!!! You just cant admit you are wrong when you have a picture right in front of you and that there is something you can be wrong about.


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## Hip_Shot_Hanna

well after 40 years of working on engines from 50 cc 2 strokes to 50 liter V 16 gen sets i have never seen a engine need a keyway recut to get the timing in place , I would love to take that engine apart and see why a recut keyway made it work .


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## cfabe

HermitJohn, I don't understand why these other guys are having problems with your engine problem & solution. It's pretty clear to me. 

On earlier GX390 engines, honda used a magneto with a remote trigger sensor and a flywheel that matched that. At some point they changed over to a magneto that did not need the remote sensor, probably as a cost savings, and changed the clocking of the keyway on the flywheel. You can only find replacement parts of the new style, so you had to modify the keyway clocking to match the new style flywheel. 

With the mismatched parts it was firing at 180 *crankshaft* degrees off. So it was firing when the bottom of the intake stroke when the cylinder was full of fuel but the intake valve still open, making it backfire through the carb.


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## HermitJohn

Hip_Shot_Hanna said:


> well after 40 years of working on engines from 50 cc 2 strokes to 50 liter V 16 gen sets i have never seen a engine need a keyway recut to get the timing in place , I would love to take that engine apart and see why a recut keyway made it work .


Then I guess you havent seen everything yet. And apparently you cant believe your own eyes with the pics I posted. YOU CAN SEE THE KEYWAY IN THE NEW HONDA GX390 FLYWHEEL IS 180 DEGREES FROM WHERE THE KEYWAY IS IN THE OLD FLYWHEEL IN MY ENGINE. Sorry if you cant believe something is possible without previous experience in your own life, but hey at least I can adapt. I've never seen anything like this before either. But look AT THE EFFIN' PICTURES I POSTED, you can see the keyway is 180 degrees different. 

Trying to tell me I have sticky valves AFTER I have said again and again this is a CLEAN NEW LOOKING HEAD and valves move fine is telling me I am lieing or that I am stupid and dont know the difference. And after I told you I cut a new keyway THAT SOLVED THE PROBLEM.

Dont want to believe me, then pick any four stroke engine you have setting around, cut new keyway so flywheel is 180 degrees from normal, THEN explain to me however you want why it fires on the intake stroke and backfires through the carburetor. Or wouldnt you even believe your own eyes?


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## HermitJohn

cfabe said:


> HermitJohn, I don't understand why these other guys are having problems with your engine problem & solution. It's pretty clear to me.
> 
> On earlier GX390 engines, honda used a magneto with a remote trigger sensor and a flywheel that matched that. At some point they changed over to a magneto that did not need the remote sensor, probably as a cost savings, and changed the clocking of the keyway on the flywheel. You can only find replacement parts of the new style, so you had to modify the keyway clocking to match the new style flywheel.
> 
> With the mismatched parts it was firing at 180 *crankshaft* degrees off. So it was firing when the bottom of the intake stroke when the cylinder was full of fuel but the intake valve still open, making it backfire through the carb.


YES!!!!! at least there is somebody here that gets it. It was beginning to feel like I was in some HT episode of the Twilight Zone with usually very knowledgeable people insisting up is down and red is blue.


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## Hip_Shot_Hanna

there can be Only 2 reasons for 180 degree spark timing out , mis matched parts or a broken crank , and yup the key is cut 180 out ,


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## HermitJohn

Hip_Shot_Hanna said:


> there can be Only 2 reasons for 180 degree spark timing out , mis matched parts or a broken crank , and yup the key is cut 180 out ,


Yes mismatched parts AS HAS BEEN STATED OVER AND OVER AGAIN BY ME, though you and Agmantoo dont seem to thoroughly read my long boring posts and just jump to conclusions based on your own experiences. As has been stated over and over, Honda briefly used a remote trigger on their GX390 magneto (SEE MY FIRST PICTURE, the sensor is still there though there is no place to plug it into the new magneto coil) and had flywheel clocked to match. It must have only been done this way for a very brief time as NOBODY seems to have ever seen this before, thus no warning about flywheel clocking, and magneto parts for this oddball system ARE NOT AVAILABLE or mentioned anywhere though I dont have a Honda factory service manual for this engine.

Not knowing this, I bought the magneto coil that was sold as a replacement for ALL GX340 and GX390 engines and was stumped why my engine was backfiring UNTIL I compared my flywheel to the replacement GX390 flywheels. The clocking of flywheel has to match the magneto used. DUH!!!


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## agmantoo

Hermit,
You said you had an engine that had been running and that it failed. You said that you replaced the magneto. At no time did you state that you had elected to rewire anything other than you had an extra wire on the replacement magneto. If you have no more ability to replicate what you removed and to understand what YOU changed creating the problem do not expect those of us that only have what poorly communicated information that is provided to try to assist. I told you we cannot see your problem and certainly we were attempting to assist depending on the information provided to come to a conclusion. Now you want to come back an waive a banner and to down those that offered help. Do not expect me to ever offer to help you again. End of discussion. Have a good day.


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## HermitJohn

agmantoo said:


> Hermit,
> You said you had an engine that had been running and that it failed. You said that you replaced the magneto. At no time did you state that you had elected to rewire anything other than you had an extra wire on the replacement magneto. If you have no more ability to replicate what you removed and to understand what YOU changed creating the problem do not expect those of us that only have what poorly communicated information that is provided to try to assist. I told you we cannot see your problem and certainly we were attempting to assist depending on the information provided to come to a conclusion. Now you want to come back an waive a banner and to down those that offered help. Do not expect me to ever offer to help you again. End of discussion. Have a good day.


Hello, I stated I had replaced the magneto with a new one that claimed to be a replacement for ALL gx340 and gx390 Honda engines. I SAID that it only had the kill wire terminal, not the terminal at the top like the old coil. I SAID there was some sort of sensor pickup on opposite side of flywheel which YOU SAID was to charge a battery. So much for your vast knowledge! I POSTED PICS which apparently YOU and the other fellow couldnt be bothered to look at. Not my problem if you either have poor eyesight, poor reading comprehension, or poor attention span. If you notice, CFABE read my posts, looked at my pictures and comprehended at once what I was saying. So much for my poor communication abilities. Its your comprehension abilities that are apparently lacking and now you are apparently experiencing the 'sour grapes' phenomena trying to cover your behind.

I DIDNT REWIRE anything, just connected the kill wire and left the sensor wire unhooked since there was no terminal on new coil for it. No mention of the old system anywhere on web that I could find, maybe a footnote in the official Honda shop manual for these engines, but I dont have that so wasnt even sure what the extra pickup sensor was. I had to assume when Honda sells a magneto coil for ALL GX390/GX340, that any changes were internal and the external sensor was no longer needed. No warning about any flywheel change out if one went to the new "improved" coil.

You and Hipshot offered no help but to tell me I'm an idiot or a liar or that I couldnt possibly have observed what I observed and that I couldnt possibly have gotten the engine running by reclocking the flywheel. Then you tried to tell me a four stroke engine fires on every stroke and that clocking the flywheel would have no effect. This AFTER I said reclocking flywheel solved my problem.

If you cant be bothered to actually look at pics I post nor read my posts thoroughly then I suggest when you see any thread started by me that you IGNORE IT SINCE YOUR ADVICE IS NO LONGER RESPECTED BY ME and any response you make to any of my threads will be ignored and not read. I will ignore any thread started by you or Hipshot. And you have a delightful day too.


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## ledsel

HermitJohn. I just had that exact problem. Bought a new coil and just back fired and spit. Read your post and took of the flywheel but instead I just took a file and made a new keyway. very easy maybe 10 minutes it is just cast iron so very soft to file. put it back on and first pull runs like before. I think all that senser does is cut the spark every second rotation. Now it sparks every rotation like a two cycle which doesn't matter. I think they tried that maybe to stop the unnecessary sparking. ?? It doesn't matter because the second spark is just before the fresh fuel comes in. Thank you very much for your post it saved me lots of thinking on my own..


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## HermitJohn

I am glad this old thread helped you. I know back at the time, I found ZERO mention of this remote magneto trigger on older Honda engines.

But NO, not any 4cycle engine fires every stroke. As I mentioned it simply cant be allowed to fire on intake stroke without causing lot problems. Firing on intake stroke will cause LOUD BACKFIRE. The fuel is being burned before it is compressed and will exhaust through the intake valve and the carburetor. Also by doing this will try to force the engine backward rotation, not good at all. The external sensor was what triggered the coil on old setup. The new magneto was self contained, but mounted same position as old magneto, thus trigger was 180 degree off.

This engine by way finally threw a rod. I saved it cause it had a nearly new head, not to mention a new magneto. I imagine most of these older Hondas with remote sensor are either scrap or have been converted by now. I have never seen the original type magneto available anywhere. Maybe a well stocked small engine shop that has been in buisiness for a very long time.....


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## ledsel

Thanks for the reply. Wouldn't it fire every rotation? It pretty much has to as the mag is on the flywheel not the cam.


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## HermitJohn

But a stroke isnt a full 360 degree rotation. Say engine starts with piston at top of cylinder. The intake stroke is piston going from top of cylinder to bottom with intake valve open and exhaust valve closed. The compression stroke is then piston going from bottom of cylinder to the top with both valves closed. The power stroke is then piston going from top to bottom with both valves closed. And exhaust stroke is piston going bottom to top with exhaust valve open and intake valve closed.

So a "stroke" on four stroke engine is half a revolution, 180 degrees. So yes it does fire once per revolution, but not every stroke. It should fire towards end of the compression stroke, just before the power stroke, and again on the exhaust stroke. Sure you have heard of timing a car engine "before top dead center". Well thats what they are talking about. When piston is at top of cylinder at end of the compression stroke. Most engines you set ignition to fire few degrees before top dead center.

Here is animation of a four stroke engine: http://www.animatedengines.com/otto.html if that helps you see how it works.


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## ledsel

Yes I know that but what I am saying is that it fires every revolution. where with that other censer it would short out every second revolution.


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## HermitJohn

On the old system, the magneto coil was triggered when magnet on flywheel went under the remote sensor and ONLY then. On modern system, the magneto coil is triggered when magnet on flywheel passes under coil itself and there is no remote coil, nor any need for one. It didnt fire twice per revolution with the old system. The remote sensor didnt cancel anything. The current system is like every other small engine does it. That remote sensor thing was just oddball and I have absolutely no idea why Honda did it that way originally. Maybe they were avoiding paying for some patent or something. It would have no engineering advantage and just adds complexity with more parts.


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## wy_white_wolf

Just wondering and didn't read all the posts. But if your 180 out couldn't you also set the cam 180 out to correct it rather than replace the flywheel? They should cancel each other out.

WWW


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## HermitJohn

This is a very old thread ledsel found. I solved my problem long ago by gouging a hole for the moon key 180 degrees on crankshaft from original hole. Ledsel filed a new keyway in his flywheel 180 degrees from original. I dont know how long it took him, but it was relatively short and painless for me as I used carbide cutoff disk in a dremel. Taking apart the engine to reposition camshaft around seems lot more involved but without thinking lot on it, should work. And anymore if you are really adverse to work, brand new Chinese flywheel for a clone engine probably not horrible price.


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## Auspete

HermitJohn said:


> This is a very old thread ledsel found. I solved my problem long ago by gouging a hole for the moon key 180 degrees on crankshaft from original hole. Ledsel filed a new keyway in his flywheel 180 degrees from original. I dont know how long it took him, but it was relatively short and painless for me as I used carbide cutoff disk in a dremel. Taking apart the engine to reposition camshaft around seems lot more involved but without thinking lot on it, should work. And anymore if you are really adverse to work, brand new Chinese flywheel for a clone engine probably not horrible price.


Thanks for your info. I was given this generator a while ago by my brother in law. It was in the shed on his new farm, so unknown problems. Finally spent the day on it prepping it, great compression like yours, went to start it and bang, dog headed for the hills. Checked the valve timing and clearance, pulled the fan shroud off and everything looked ok apart from incorrect fan installation. Even checked the key on the crank like you. When something breaks it’s usually fairly easy to figure out, but when someone else “fixes” it, it can be a lot harder. I figured the timing was out but that looked impossible without the original parts in place. So thank you for your post or it would have been a mystery how the timing could have been out with a fixed coil and magnet. Marvel of the internet, 400k from the city in the bush and connected to the world. Thanks!


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## Danaus29

Old threads are good for something, occasionally.


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