MAP sensor

[hombre912]

I have a 2000 Fat Boy (carburetor) that runs well in warm weather but coughs and bucks a little in cooler temps.  I suspect a manifold leak or perhaps a problem with the MAP sensor itself.  Can anyone shed any light on this?  Thanks in advance.

[smoserx1]

I seriously doubt it is either one as I am still riding a 99 FLHT that behaved the same way when new (for years before I fixed it).  The easiest fix is probably shimming the stock needle with one or two small brass washers so the needle sits a little higher in the needle jet and lets in more fuel at idle and especially the off idle "transition" where this issue often occurs.  Or you could try an N65C needle that lots of folks used to fix the problem.

https://cv-performance.com/cvp-n65-needle

Either way you will lose a few MPG, but IMO it is worth it.  Next time the weather is cold try riding with the enrichner out a little way and see if the coughing and bucking stops.  If it does, that carburetor needs tuning and the above modifications are good starting points.

[Hossamania]

It could be something as simple the carb needing a good cleaning. The jets can get a small buildup of varnish (and mold with ethynol fuel) that can change the mixture slightly. Has it always done this? Has the idle mixture screw been adjusted? Any other mods to the carb and air cleaner, exhaust?
You can check for manifold leaks by spraying some carb cleaner around manifold seals and listening for a change in engine idle. Don't get confused by spray being pulled into the carb itself and giving a false reading.

[Hossamania]

One other thing, you say it does this when cold. Are you running with the enrichiner on for a while when cold, or are you turning it off right away, or is it going in on its own too early? A little choke is needed until the motor comes up to temperature.

[Hossamania]

A quick side story, because that's what I do. In Sturgis one year, a guy could not get his bike to start and stay running. After listening to him torture his starter for a couple minutes, I asked him if he had the choke on. He said no, it will flood the motor, especially in this lean air. I told him to give it a try, don't touch the throttle, just pull it out and start it. It fired right off, went to high idle just fine. I said "You're welcome!", and walked away.
I'm not trying to move this in a different direction of anecdotes, just relaying a story that always pops into my head when dealing with cold start/run issues.

[motorhogman]

My 01 FLHT will do that once in a while in cold weather until warmed up good.  Now that I'm an OLD fair weather rider who seldom rides unless the temps are at least 65 degrees and the sun is shining bright it seldom ever happens, Again though it is only likely to happen during the 1st few miles of a ride maybe once or twice.

[motorhogman]

One other thing, you say it does this when cold. Are you running with the enrichiner on for a while when cold, or are you turning it off right away, or is it going in on its own too early? A little choke is needed until the motor comes up to temperature.

True, I always start a ride from a cold start with the fuel enrichener out part way and push it in as the rpms come up,  That is when the damn thing is adjusted properly and doesn't slide itself in.
Thinking about replacing it with the after market unit that mounts on the right side,  Somebody on here posted about that unit a while ago.