Shovelhead ignition problem - missing under load

[jboomer]

hello,

Desperate to keep my buddies 81 FLHT out of the dealership to pay for troubleshooting time, we've decided to figure this one out on our own. "Betty" is an 81 shovel with a S&S Super B(i'm pretty sure, don't believe it has an accelerator pump,) and a Crane Hi-4 ignition Dual fire ignition unit. It was rebuilt 4 years and 12,000 miles ago when it was purchased. Not sure about cams or internals. The compression tests ran earlier in the month came back good and it just had a new rotor installed (spun the old one.) After getting it back form the local wrench it ran excellent for 50 miles. We stopped, got some lunch and the following issue started an hour later.

The issue is this, when under any type of load the bike starts missing and backfiring, and makes no power. It will idle fine and sometimes revs in neutral fine. Other times when reving without load, the bike will miss at about 2500 RPM. But, under load she fails completely with horrible backfiring and lots of unburnt fuel smell. We were worried the Crane Hi-4 was dying so we ran the prescribed test of idling the bike and heating up the ignition module with a blow dryer. She never died. Actually, when the issue first started the bike would always start back up, but under load she would start missing and failing. We have checked for bad gas, clogged fuel filter without any success.

I am wondering if a worn out coil could cause these types of problems? I have wrenched on cars a little, but never on a shovel, so any help in diagnosis would be great.

Thanks,
Boomer

[fauskar]

Begin by back tracking what your local wrench touched. Usually the problem and soultion are related. Did he/she touch the ignition, carb... Is there anything in the repair path that got touched. You get the picture.

As for the carb, go to sscycle.com to begin. Find the installation manual for the super B carb. Good data on troubleshooting and setup of the carb.

I have a super B on my shovel. Yes, there's no accelerator pump or choke. Only an enhrichment valve/lever for priming before start. Make sure that's not engaged. Assuming you don't have a manifold leak, I would drop the bowl and check to make sure that intermediate jet is tight and clean. The intermediate operates approxiatemately in the region between 1200rpm and 3400rpm. By dropping the bowl you can also check for any sediment that may have gotten into the fuel system. Total time 15 minutes.

Check the ignition timing path for tightness.

Most electronics (i.e, coils) fail completely. If the coil sparks and measures around 3-4 ohms, you're probably okay.

[jboomer]

thanks for the reply fausker. I will drop the bowl tonight. We are still waiting for a call back from the local wrench. He did install a new rotor, but that's all I know.

Saturday we checked all ignition components for solid connection and tightness except for the coil. I will get to that tonight. Yesterday I started the bike up, ran it for about 3 minutes and then road around the block. Definately not enough time to heat the engine upto operating temp. There were no signs of the missing for my brief 4 minute ride.

I will check the carb tonight, and start looking for a potential manifold leak. Is there a good way to check for that when the engine is warm. I need to get the bike reproducing the issue, then troubleshoot.

Thanks again,
Boomer

[fauskar]

A manifold leak would cause you to run lean because of to much air. The plugs are a good indication. Haven't heard of a intermittent leak at RPM but anything is possible. I have no good procedure for checking the engine while hot.

I'm not familiar with the 81's electrics/ignition but I would check the ignition's advance mechanism to see if evrything looks okay. Ignition advance is a rpm related function. Ihave a old points system with mechanical advance. Maybe someone else out can help you with this.

[miles to go]

Check to make sure both cylinders are firing. Check each exhaust pipe to see if the exhaust is warm. If it's cold then the cylinder is not firing. Could be plug, plug wire, coil, ignition. Good luck and let us know what you find.

[jboomer]

Fellas,

Last night we tore out the old coil and installed a new one, mainly because we didn't know how long the existing one had been in there (2? 5? 10? years.) With the new coil installed we checked all the wire connections, grounds and for scraped housings. Let the bike warm up to operating temp and roped down the freeway, no issues. Bike runs great. Upon close examination, we found a half circle crack on the older coil that worked it's way from the side toward the center of the body. If that was the main cause, my guess is it would heat up and seperate, causing loss of spark. We tore into the coil more and found rust. Not sure how significant this is, but for now we're riding.

Thanks for all your help. If something else develops, I will post. I know how frustrating it is to research for a specific issue.

Boomer