Backfire

[jerryjerry05]

1988 125 Force
​Rebuilt powerhead.
Compression good and spark on all cyl.
Trigger readings 40OHMS on all 4 cyl.
​5-6 volt on 3 cyl and 25 volts on the last.
​Packs are all good. Coils are good.
​Wiring is ok.
​Regulator is good.

It backfires when running.
​But when you undo the top plug it stops backfiring???
Unhooked the tach.
​Been driving me crazy!!!!

​Gonna redo the stator readings and post them. soon.

CDI suggested a double fire but I couldn't find another one firing?

​Started as a simple change 2 pistons.
​Then: no spark, bad stator,
no fire #4 , bad coil,
bad connections everywhere.(all been replaced)
​ CDI suggested replacing the bottom pack, now the top cyl. when hooked up makes it backfire?
Before I changed it it did the same on cyl.4

 

[pnwboat]

Might try hooking up a timing light to the top cylinder to see if the spark is firing when it should. Might even try that with each cylinder to see if you're getting a good steady spark on each cylinder.

Trigger voltage should only be around 5V on all 4. The trigger itself is only capable of generating around 5V. If you're seeing more than that on a particular set of trigger leads, I would think that something is not right. Might try checking the voltage with the trigger lead disconnected? If the voltage drops to around 5V, then I would suspect a bad CD Module, or something is inducing voltage into that trigger wire? I'm sure you already have, but double check the wiring again paying particular attention to the trigger wires..

Just another thing to put in the back of your mind. If the isolation diode in a CD Module goes bad, the Module can still produce spark, however, it can interfere with the operation of the other CD Module (which is good). You can test this by disconnecting the White kill circuit wires that go from the CD Modules to the terminal block and see if this helps. To stop the motor just choke the motor to stall it out, or touch the White wires to ground. Whichever is easier for you to do.

 

[jerryjerry05]

I tested the trigger unhooked, that's when I found the 25v DVA
​I replaced both packs and coils.NO
I did the white wire disconnect.NO!
​I checked the firing on each cyl.NO
​Been over this thing so much I'm seeing it in my sleep!!!

CDI suggested the blocking diode in the pack. Had already checked it.
OI.COM suggested the firing on each cyl.

​CDI finnaly said the trigger "might" be the trouble.
A replacement should be here Monday.


I've never really had trouble with a trigger before .
Either the connectors break under the shrink or it just doesn't produce signal??

I can usually figure out what's wrong?? But not on this one??

​Thanks for the help.

 

[60sboater]

Could it be a some type of ground or insulation problem?

 

[pnwboat]

Sure is a strange one, but the fact that you're seeing 25V on one of the trigger seems to be unusual.

Haven't seen any trigger failures on the Prestolite ignition systems other than what you mention about the broken wires or other physical damage to the trigger assy. The only other items that is associated with the ignition system is the flywheel magnets. The large strip magnet for the stator voltage and the smaller one around the center hole for the 5V ignition trigger signal to the CD Modules.

 

[jerryjerry05]

Gonna change the flywheel (I got all kinds of spare stuff)
​I just reconditioned 2 and they're just waiting to be used.

 

[pnwboat]

Just remember one other thing. Disconnect the rectifier and see if that helps.

 

[jerryjerry05]

Thanks, did that and even changed to a different one.
​Trigger is supposed to be here Monday and ?????????

I even changed the flywheel??? Just in case.