Backfire off Idle - 88 Twin Cam Carb'd

[DeanC]

Hello All!I've been having a problem with my 2001 deuce. I bought it several years back off a friend. He had bought it new and had the screaming eagle intake/carb put on... It's a Mikuni Carb. Anyway, it idles and runs great. The problem occurrs when it is at idle and then go to bring it up off idle. It will occasionally give a cough and may or may not stall. This only happens once or twice a day but is as embarassing as ____. The engine is at running temperature when it happens. I've had the idle mixture set at a few different places. It's currently running a little richer than what I had it before. Not rich enough to pop on decceleration, but almost. It happened before and after changing plugs. I put in an Accel coil after last year to try to help it idle. It can be jerky at low rpm's though I've just chalked that up to being carberated. The primary chain is in adjustment. I don't have a problem with working on it or even getting some performance parts, but looking for clues on what the best suspect is - is it carb, valves, timing... I know my way around engine basics, but this is my first time this deep on a HD. Hope someone has some tips from their experience. Thank you.

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[Marksman]

Can you see the accelerator pump working in the carb?

With the air cleaner off, looking in the carb, engine off, crack the throttle. Can you see a very healthy squirt of gas? You should.
Sounds like a fuel issue to me.

[krashed]

Is the air filter clean?

[DeanC]

I will do a visual check on the accelerator pump when I have time - good suggestion. I know that the linkage is all working. A couple of things that I should have mentioned - it usually happens just after returning to idle from decceleration. If it does cough, it will do it as soon as I move the throttle - it either coughs immediately or revs up fine. Air cleaner is good - it's a KN style - came with the SE intake kit, I believe. And I've cleaned it before.

[djl]

The post subject refers to backfire but you refer to coughing in the post. Is it backfiring (exhaust) or coughing (carburetor)? Assume it is couging through the carburetor from you description. Do you have the original CV carburetor? If so, install it and forget the Mikuni. The CV is a very adequate carburetor and much easier to tune. Not a Mikuni guy but that sort of coughing can usually be controlled with the A/F needle on the CV. Sounds like the problem is in the transition from idle to the slow jet; maybe some trash in the circuit? Could be that you just need to pull the bowl and blow some air through the jets and passages? Does the Mikuni have an enrichener circuit or a choke? The enrichener valve on the CV can leak and/or not seat properly and affect the idle and just off idle operation.

[big cahuna]

First thing I would do is pull the plugs and see how they look.How many miles on them? If they look good clean them up and over dose the gas with seafoam, then burn the entire tank of gas up, not just halfway and refuel. Then pour more seafoam in and ride. I think people forget that a carb will get gunked up over time and may need a internal cleaning now and then.,,

[DeanC]

The plugs come out a little black, but pretty good usually. I changed them (gapped) and it did not make a difference. It is a cough - just a single pfffffft (sound effect) coming up off of idle. Again, it only does it very infrequently. I have 18k miles now. Just rode an extended trip to Fla - put over a thousand on and it was consistent throughout the trip.

[1SG]

this is more than often a lean condition, with the motor warmed up and running spray wd 40 all around the manifold carb area if the idle changes you have an air leak.if there is no change re adjust your slow idle and when it is ideling good add a half turn out to add a tad more fuel, give this a try it has worked for me several times.