# Briggs and Stratton 18.5 twin cylinder not firing at all



## oldgaredneck (Jan 2, 2007)

HELP!B&S Model 42A707 was running great.
Now it turns over normally with good compression, but has NO spark on either cylinder - tested with inline spark tester (spark tester is good, checked on tiller, and indicates spark ok).
Has new plugs set at proper gap,
new solid state magneto set at proper air gap.
Flywheel magnet has strong pull on screwdriver at one inch, and is not rusty - key is not sheared. 
battery has full charge and solenoid checks good
New gas, fuel line and filter - gas getting to carb, and carb clean
Air filter is clean, oil has been changed and filled to correct capacity.
Engine is mounted on a 1996 Murray riding mower, all safety switches have been temporarily bypassed for testing - but I still get NO spark whatsoever on either cylinder!

Any ideas/suggestions? Could be possibly be the ignition switch which is a 6 tab?

I could definitely use some help on this, please!


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## oneokie (Aug 14, 2009)

Is the no spark condition the reason for the new solid state magneto?
Did the no spark condition start after installing the new magneto?


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## oldgaredneck (Jan 2, 2007)

oneokie said:


> Is the no spark condition the reason for the new solid state magneto?
> Did the no spark condition start after installing the new magneto?


No spark condition started before I installed the new magneto - this thing is driving me bonkers....


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## agmantoo (May 23, 2003)

I would like to see a schematic of the wiring for the machine. Simply bypassing the switches may not work. I do not know if the switches are all for making just continuity or if some of the switches function as normally open or if they function as normally closed in order for the engine to get fire to the plugs. For example, when the operator is sitting in the seat the switch may be open and the engine runs and when the operator is not in the seat does the magneto voltage get interrupted or does the hot wire from the magneto get shorted to ground? If the condition is the latter, then bypassing would kill the spark whereas in the former bypassing the switch would let the engine have spark. Therefore, bypassing the switch could work against you or for you and I think this could be the situation.

You may get something from reading this info that I copied
# Most electric start mowers have three safety devices. One is mounted under the seat, so that the machine can only be started with the driver seated. Another is usually on the clutch/brake linkage under the deck beneath the clutch pedal. The third is located either on the mower drive belt linkage under the frame on the right side (seated) of the mower, or in the electric mower clutch circuit if the mower is so equipped. Each of these has a plastic insulated plug with two wires going into the device, and a temporary jumper may be installed into them one at a time by unplugging each one and pushing a piece of copper wire under the terminal on the end attached to the wires. If you do not have any luck finding a defective safety device, the problem is narrowed down to the ignition switch itself.
# Lift the engine housing cover as you do to check the oil, and look immediately behind the ignition switch on the firewall of the mower. The switch component is on the back of the keyface, and will have a number of wires running to it, usually four. The primary voltage wire will be the largest, normally it is red, and often is found in the center. The other wires will be attached to terminals situated around the outside of the switch. Jumping from the primary wire to each terminal will result in different effects, one energizing the ignition circuit, one the starter motor, and on some mowers, another the lights or other accessories. If you can jump these terminals and the starter motor turns, you have probably located your trouble, and will need to change the switch.


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## oldgaredneck (Jan 2, 2007)

Thanks - I'll give your suggestions a shot.
No schematic for the wiring is available - it's a 1996 Murray, and I've looked everywhere online for any diagrams, etc.


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## oneokie (Aug 14, 2009)

agmantoo gives good suggestions on the safety switches. I will add this; if there is only one wire to the safety switch, disconnect it. If two wires, use the jumper wire. If you can access the magneto ground wire at the ignition switch block, disconnect it.

Use this search string for a google search >wiring diagrams for murray lawn mowers<. You will need the mower model number that is on the data plate attached to the mower. (not the engine model number)


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## retire2$ (Feb 12, 2003)

Here are a couple of other things to try. Disconnect the ground (kill wire) on the coil. See if you now have fire. Also, check the spark plug wire to see if it got nicked when putting the blower housing on. Probably not the latter since this was happening before you installed the new coil. Unless both wires got pinched.


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## Okie-Dokie (May 12, 2002)

Had the same problem with a 12-1/2 hp b&s. Turned out to be the ground wire where it is bolted to the frame was rusted up and not making a good connection.


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