Experts needed! Very sticky situation - carburation

[GG]

Hello all!

It's been awhile since I've posted - my 1966 Superhawk has been serving me just great. BUT - little by little one cylinder has been running richer and richer, until now between 3000 and 4000 rpm's the motor "4 cycles" (gurgles and bucks). Under and over those rpm's it runs fine, but the right side spark plug is always very black.

Compression between both cylinders perfect (110 lbs at 5,000 ft elevation). Valves adjusted. Timing spot on and even between the two cylinders. Carbs cleaned. Jetted down a bit (120's main) and left side white / brown spark plug and right side black.

Carb throttle plungers "even" as close as you can get by eye. I've fussed trying to raise or lower the bad side with no difference.

I'm stuck! Any advice is very welcome!!!

Frustrated in Costa Rica

GG

[Michael Stoic]

Hello GG,

The symptoms you've described point to either fuel restriction or ignition degradation on the affected side.

I think this one will come down to systematic testing of each component. I'd start with electrical. Easy enough to swap lt/rt on some of the components involved. Even before that, clean all electrical connections, test resistance, and make sure they have good ground.

It's often said that all carb problems are electrical - and vice versa...

--
Mike Stoic

[GG]

Thanks Michael - that gives me something to go on. I'll start there. The strange thing is that just over 4000 rpm's it clears up and runs fine all the way up. You'd think that under great stress, if it were electrical, it would fail even more. No fuel restrictions - I've even changed jets, etc with no improvement on the rich side. The other side responds - leaner (whiter spark plug). One side seems to be dumping fuel in. The carb needles are set the same (right now to their lowest point). The bike idles fine, and runs good over 4000. From 2 to 4 thousand is "4 cycles" terribly on the right cylinder. Grrrr.

I'll check out the electrical in the meantime!! Thanks for the response.

BG

[Michael Stoic]

BG, With the last detail - fine at higher RPMs - fuel supply is now more of a suspect, IMO.

Still, try to lt-rt swap whatever you can (plugs, leads, coils, carbs), but only one item at a time, to see whether the issue travels side to side. It might.

If your bike had been laid up for a while, you could have a varnish restriction in a carb passage. I've had very good results with Stabil which does dissolve build up if given enough time.

It never hurts to re-establish the factory baseline (jets, needle positions, etc.) as the first step in further diagnosis.

--
Mike Stoic