Backfire

[Stevepshipley]

Hi, one of my engines is playing up a bit. Starts and runs well but occasionally backfires. I can't pin it down to anything specific though. I changed the cdi, and also for other reasons swapped a 26mm jetex for a 26mm delorto some time ago, but I'm still getting the backfire. It's an early (2005 ish) mugello 186, not done a huge mileage and recently replaced rings.
Any ideas?
Cheers steve

[Feersum Injun]

LT coil failing on the stator? Easy enough to check....

[Stevepshipley]

Hi FI,
How do you check the LT?
Cheers

[Feersum Injun]

You’ll need a electronic multi-meter to check resistance. Pull stator cables out of the cdi. Attach one meter lead to the white wire, this I the earth. Then attach other to the red, this will measure the pick-up, it should be above 95. Then attach to the green, this is the lt coil. This should read over 400 I think, can’t remember exact figure... it’s in the spanner’s manual if you have that....?

[Stevepshipley]

Thanks, got a meter and manual. I'll see what's what.

[Fast n Furious]

Steve.....In my experience it is unlikely that you will measure anything out of the ordinary here.
As the engine warms up and becomes subjected to vibration the finite break in the hair thickness wire that makes up the LT coil begins to mess with the ignition. The break usually occurs around the earthing connection of the coil. This tension + vibration causes it to fracture.
Just fit a new coil.
I dont connect the earthing wire directly to the laminates earthing point. I prefer to make an extra hole in the coils plastic former and insert a small ferrule. The coil earth is terminated here and then a small piece of multi-strand flexible wire connects from this ferrule to the earthing point on the stator's laminates. Solder up, job done. Never had a failure.

You might actually be able to see the fault with a timing gun as it sometimes shows the timing to be all over the place. Just depends on how bad it is.

[Stevepshipley]

Bang on F&F,
I can't see anything wrong with the meter, I've got a spare stator so I'll try that and see what happens.
I'll check with a strobe before i swap it just to see if anything is odd with the timing.
Many thanks
cheers steve

[CHRIS in MARGATE]

Got 2 stators here that check out 100% on my meter. Neither work !
The resistance check will definitely tell you that you have an electrical continuity fault if the readings are not correct but it won't tell you it's faulty when the readings are correct !