Tuning my 1978

[pjgoeman]

Yes, the air cut valve has a new diaphragm and new O rings. I also disconnected the vacuum line going to the air cut valve with no effect. All of my popping/misfire is coming from the #1 cylinder. The other three run OK.

[pjgoeman]

Thanks for all the help and suggestions that were offered in helping me figure out what was going on with my bike. I took a few days away from working on this although I did accomplish a couple of other things on the bike that I had been putting off.

While staring at the bike idling in the driveway my thoughts were that I felt like I had definitely eliminated both the ignition system and any mechanical issues but the #1 carburetor might still be a problem. I didn’t find any issues with it when I partially disassembled it but it’s not like you can easily swap the carbs side to side to trouble shoot them. I had tried to run it rich by partially closing the choke but that had no effect. The #1 plug had the same visual appearance as the other three so there wasn’t any glaring mixture problem.

So while it was idling I decided to try cracking the throttle slightly more open just on #1. I figured it would mess up my carb synch but so what. I opened the #1 throttle screw about one quarter turn and the thing smoothed right out, no more miss at idle. I rode it around the block a few times and the exhaust popping was almost gone. I went back home and hooked up my TwinMax to #1 and #3 and somehow they were still in synch. I’ve ridden it two or three more times and it’s continued to run well. It’s sort of weird, all the checking, testing, carbs in and out, etc. that I went through and a slight change like this made all the difference.

At this point I’m just happy that it’s running much better and I can move forward on finishing it so I can get it licensed and insured and put some legal miles on it.

[Don R]

Randakks said bowl gaskets may cause an internal leak, other than trying another set of carbs I'm stumped. With the leakdown fitting on, was there any noise from either the exhaust or intake port?

[Don R]

The shop I go to keeps a set of 83 carbs around for checking problem bikes. I ran them to Sturgis and back a couple years ago. Contrary to what I've read, they worked great.

[redglbx]

Peter, have you checked the spark plug caps ? Not just for firing but pull them off the wire and use a VOM to check the resistance across the cap, should be less than 5k ohms. While you’re at it trim a small amount off the end of the wire and spread the wires before putting the cap back on. I also add a very small ring of rtv around the outside of the wire to seal the wire to cap joint as well as glue it in place.

Over the years I’ve had little tuning issues that I blamed on carbs/points/and everything else and ultimately found it to be a bad spark plug cap.

Also if you have the carbs off, make sure you can spray carb cleaner through the idle jet and have it come out of the idle mixture screw hole, be careful to not lose the little spring, washer & o’ring on the end of the mixture screw.

Does your carbs have stock jetting ?

[pjgoeman]

Thanks for the continued ideas on things to check.

With my home made leakdown test I could not hear any air out the exhaust or intake, only the the air leaking at the quick disconnect coupling. When timing the pressure loss when connected to the cylinder it was equal to the pressure loss when connected to a nail gun with the same quick disconnect m and f connectors so no apparent leakage in the cylinder.

I isolated the misfire to the number one cylinder by separately grounding each spark plug. I then swapped the number 1 and 2 spark plugs/leads/caps and the misfire stayed at number one instead of moving to number two so no apparent problem with the ignition components.

I had carefully rebuilt the carbs prior to startup using Randakk’s kit and video. All jets and carb parts are original, all passages blown out with brake cleaner and compressed air, all O rings and gaskets replaced.

I learned long ago to never say never about any possible cause to a problem. Right now the bike runs pretty good but I have less than 100 miles on it since start up. I’ve got a couple things to finish up and then I’ll get it registered and insured, put some miles on it, and really see what I have.