Yet another carb issue.

I have a carb cough. When it started, it was one “miss” that always occurred during acceleration right at 50 MPH in 4th and right at 60 MPH in 5th. It has now progressed to happening in all gears, at 20 MPH in 1st, at 30 MPH in 2nd, etc. It does not backfire, it just misses. It does not happen consistently. In 5 miles, it may miss 10 or so times. The lights/electrics do NOT blink out when this happens. It does not happen when the motor is cold (having an Evo, I always warm up the motor until the rear rocker box is a bit warm). It increases in frequency not only with a warmed up motor, but also with increased air temperature.

I have a ’99 FLSTC, 80 inch Evo. Mods include an Andrews EV27 cam, a Thunderslide kit in the Keihin CV carb, Samson True Duals Fishtails, removal of the vacuum petcock (replacing it with non-vacuum) and a Dyna 2000 ignition module (set on curve 1 when the problem started, now on curve 2). All of these mods were in place and running fine prior to the cough development and nothing new was done just prior to the cough.

I sprayed carb cleaner on the intake manifold and could not detect a leak. I put a bigger idle jet in (48-->50) which made the problem worse. Putting the old one back in calmed it back down. Leaning or richening the fuel mixture has no effect. Plugs look fine.

It feels like an intake manifold leak.

Whaddya think?

 

A carb when cold will do things like that...Slightly lean in fuel mix. I've learned to deal with it or leave the choke out a bit more....Now if you running hot, in a Hot area... quite a different thing.

 

My stock carb used to cough. I replaced it with a S&S E and the cough went away.

 

How's the air filter?

 

Just cleaned it. Wasn't too dirty to begin with....

 

Doesn't miss a beat when cold, only when it warms up.

 

I feel your pain. Mine will do a hick-up only when colder & running just above idle, when going down the road. Since I'm used to it, I can give it a shot of throttle.

When did you last rebuild your carb? Your probably right about an intake leak. If you use a V.O.E.S hose look at that. Make sure that plug you used when you took off the vacuum petcock are still sealing good.

A bit of info may help us. How many miles on bike, miles since mods, how long in months/yrs since mods. Did you let it sit for awhile with E10 gas in the carb?

 

Have you checked your float level? With a warm motor does the problem go away if you pull the choke on? When did you last change your plugs? Evos can do some seriously weird **** when the plugs are getting on a bit, even though they look fine.

 

I'd suspect the beginnings of an intake leak if the problems comes on as it warms up , wouldn't hurt to check your exhaust pipes at the heads too .

 

I would do the basics first. I had plugs go south on my EVO about 2 months ago. Started coughing first and then finally just left me stranded. Luckily I was in my neighborhood.

I also had plug wires go bad on the twin cam I had. It had the same symptoms. The thing about bad wires is they almost never act up until after the bike is warmed up. I found out my wires were bad when I reached down for reserve and hit my wires. And they lit me up