Coughs and sputters

My 75 Mk3 recently developed a problem with coughing and sputtering, especially at low throttle, but sometimes at mid throttle. She has become hard to start, but does start eventually. If I take it off the choke too early (5-7min) she dies. Used to idle really nicely too... The strange thing is that I have to stay on full choke for the first 10-15 minutes of riding, but then after a few minutes at 1/2 to 3/4 throttle she returns to normal... so it's as if I can 'ride through it'. As long as I keep the engine hot it's fine, but if it cools all the way down, the problem returns. I had the carb off and cleaned the pilot jet, sprayed the carb with carb cleaner, and she ran fine for a few days. Timing is good, but the plugs are a little darker than I like... but they're not bad or wet. The rods that the floats ride on have a tiny bit of discoloration near the top, but the floats seem to side on them fine. Any ideas?

If it idles ok but stumbles just as you open the throttle but improves as you open the throttle then sync the carbs, ie make sure the slides move off their stops together. Just have to do that myself with same symptoms which are now cured.

are you valves properly adjusted? if they have tightened up it makes it especially hard to warm up the engine. Choking it for 10-15 minutes when running is a good indication.

definitely check for throttle sync at idle and up top, that might just be compounding the problem. simple maintenance might be all your needing.

start with those and we can go from there

Classic symptoms of a clogged pilot jet. You say it ran fine after cleaning them once? I suspect you have some dirt in your fuel that's plugging one or both of the pilots.

Sorry, forgot to mention it has one Mikuni. Thanks for the info, I will check the pilot again and also if the tank has junk in it.
-paul

Well I pulled the carb off and recleaned it. I took a magnifier to the pilot jet and for the life of me, I could not see anything in there. SO I prodded each opening with a guitar string, squirted a little carb cleaner in there, blew it out with the compressor, and she's running fine again. Didn't get a chance to look into the gas tank, but I am assuming that is the source of the problem, right? One other thought, I put new fuel lines on a few months ago... not the clear plastic ones, but rubber auto fuel lines (black). Could this be creating problems? Thanks.
-paul

c.cito said:

Well I pulled the carb off and recleaned it. I took a magnifier to the pilot jet and for the life of me, I could not see anything in there. SO I prodded each opening with a guitar string, squirted a little carb cleaner in there, blew it out with the compressor, and she's running fine again. Didn't get a chance to look into the gas tank, but I am assuming that is the source of the problem, right? One other thought, I put new fuel lines on a few months ago... not the clear plastic ones, but rubber auto fuel lines (black). Could this be creating problems? Thanks.
-paul

Click to expand...

With inline filters?

Just a long shot, try new plugs.