Weird carb issue't

[Jasper66]

GS750B, stock airbox and exhaust. I cleaned the carbs well last year and the bike has been running like a champ, until a little while ago.

After 10 min of running great I get popping and lurching with anything over 1/8 throttle. After 15-20 min I have to give it almost half throttle to run and I'm barely making it home. Next day same thing; starts up and runs great then I get the aforementioned problems.

It sounds like a fuel starvation issue, but I can't find any faults with the system. If I suck on the vacuum line from the petcock gas flows freely, and I can easily move air through the carbs fuel and two vent hoses. If floats or jetting were off it wouldn't run great for 10 min, and it ran great for two years how it is now... A few people told me to check the tank/cap vents but I'm convinced my model doesn't have either, and loosening the cap mid-ride does nothing. Choke does nothing. Plugs look normal. I'm getting 13.5v at idle 14 at higher RPM. New stator/RR, Coils with relay mod, points.

I'm running out of options, unless it's something electrical getting hot and malfunctioning exactly at 10 min, but I think the float bowls draining is more believable.

Any ideas?

[rphillips]

Did you reach down and switch to PRI., That should tell if petcock is problem or not. No offense, but remove the gas cap, and put it back on, just loosening it may not relieve vacuum, if that is the problem.... Oh, 1st post... Welcome to GSR. and we here love pictures... of bike.

[Jasper66]

I turned it on prime while riding for about a min with no results.

[rphillips]

1 min. "should" be enough, though much longer wouldn't hurt anything. Just be sure it's back to "ON" before turning it off and leaving it for very long.

[Jasper66]

So, if I leave it on prime, and nothing changes, then what?

[rphillips]

If it doesn't change while running on PRI, you're positive the petcock is not the problem. Don't leave it on PRI when parked, could possibly leak gas through the carbs and into the crankcase.

[Jasper66]

Yea, the petcock, carb vents and fuel line aren't the problem. They all flow beautifully.

[Nessism]

Are the valve clearances all in spec?

[Jasper66]

yea, valves done maybe 500 miles ago. it was running perfectly all year round, and never pinged or overheated,

[gustovh]

Take your gas cap apart and clean it..all of it, there is a round flat plastic plate that covers passages , it is right in the base of the cap, under all the stuff you remove, be careful it is plastic. If that isn’t confusing enough, I could put it another way. GL

V

[Suzukian]

You cleaned the carbs last year, how did you store it? If you didn't put fuel stabilizer in it,you may want to drop the float bowls, drain them first, and see if there is anything accumulated in there. You could try running some Sea Foam to see if it cleans it up first. I might add a bit of Marvel Mystery Oil too. Do that after you empty the float bowls ( which is a good time to see how the gas is flowing into them). Add the additives before you drop the float bowls, they may clean up the carbs.

[unfocused]

Everyone has added so let me add... check your fuel filter. Especially if you have a small one. They plug fast. I know, I just plugged up 2 of them from a dirty fuel tank purchased online. I went and picked up a 20 micron inline filter from a local parts house. IF this is the case, I would removed the petcock and check it as well. I've switched to an adapter plate and a 3/8 NPT petcock. Last but not least, replace your fuel lines. I switched to a silicon fuel hose.

Finally - use ethonal free fuel as fuel stabilizers don't really work for long term storage, or don't work at all. Something I refuse to use. I'll get synthetic fuel for storage and some of my small engines ONLY get synthetic fuels. Chain saw starts everytime, all the time...!