Backfiring 81

[radke]

Hi, I've been working on an newly acquired 1981 SH for the past week and this forum has been an incredibly helpful resource.

I'm getting backfires out my left exhaust. She'll start up just fine but when I open the throttle it starts backfiring, going back to idle it'll backfire some more before generally settling. So far I've set my cam chain, valve clearances, cleaned and synced my carbs. No change. I put in new spark plugs and swapped out the cables and boots. No change.

I switched the secondary spark plug wires around at the coil and the backfiring jumped from the left exhaust to the right. Pulled the coil and tested it with my meter and everything checked out ok (I think). Think I have a bad coil? Something different? Thanks!

 

[radke]

Just wanted to update this. I double checked the coils and they are within spec, could they still be faulty even if the primary and secondary test ok? Should I suspect the TCI?

Thanks again in advance, any help is greatly appreciated!

 

[DogBunny]

If you swapped the plugs, boots and wires and nothing changed, and if you then swapped which cylinder the plug wires went to and the backfire changed sides, then it has to be the coil. It isn't the TCI -- the TCI fires both plugs at the same time (it is a "wasted spark" system). If the TCI is able to fire one cylinder, then it is able to fire both cylinders.

EDIT: I guess you couldn't swap the plug wires because they are molded onto the coil, you can only swap which cylinder they go to. In any case, it's your coil, or the molded-on wires. You can try trimming a little off of the wire that doesn't fire to expose fresh conductor for the boot to grab on to. If that doesn't work, you need a new coil.

 

[radke]

Thanks DogBunny,

The coils that are on the bike look newish, like its been replaced at some point. The wires aren't molded on so I was able to swap them where they screw into the coil to make the backfiring switch sides.

New coils ordered from mikes, I'll let you know how I make out. Thanks again!

 

[DogBunny]

Cool, I just hope you didn't order the $37.50 coil from MikesXS. It is complete junk, there has been a lot of chatter about it on this Forum. Here's the good coil:
http://www.mikesxs.net/product/17-6903.html

 

[radke]

Ha, yep, I did so I just canceled that order and ordered the better one. Thanks!

 

[Revanoff2]

If you were able to swap only the wire and the backfiring changed with the wire, then it's the wire. Not the Coil.....

Take a multimeter and set it to Ohms. Start at the lowest setting and check the resistance of each wire. I believe you will find that one is much higher than the other. The one with higher resistance will be the bad wire.

~Revanoff~

 

[XSLeo]

I agree with Revanoff2, if the backfiring moved by swapping the wires then it's probably the wires.
Pick up some new wires, ones with a copper conductor, no suppressor wire.
Leo

 

[5twins]

..... or plug cap, they can go bad too, especially originals. Something about being 30 years old or something .....

 

[radke]

Yep, it was a bad wire, discovered it when I retested everything after I replaced the coil. Even though it was new and I tried clipping the ends shorter it was still bad. Guess it's good to have the new better coil regardless. Thanks again for all the help!