Strange carb issues: Riding home the bike started to blubber a bit when going up hills at high throttle. Gradually got worse so now anything above about 1/3 throttle it hardly runs it blubbers rich so badly. Startup and idle are perfect. Pulled the air filter, pulled the gas cap, same thing.

Here is the thing: Reach over and turn off the fuel and right away it runs like normal (till it runs the bowl dry).

So I would have thought the float is stuck down or the needle isn't sealing. But it can't be because the Carb does not overflow or leak gas anywhere anytime. No fuel in the engine oil. Needle and float must be good?

Stripped the carb down and figured the diaphragm had a hole or the carb was full of junk. Nope all looks good. Slide and diaphragm move up and down freely. I must have missed something, but where?

Bowl vent hose is clear.

Thank you Mikuni experts.

Air-filter didn't oil-up, gum-up somehow?

Main jet and "carrier" were checked for being loose?

Choke cable (somehow) turning choke full on?

It's not likely, but the internal overflow tube and/or it's plastic draining tubing could be clogged, and then the floats and float seal could be compromised, but no leakage shows.

Alexd

The clincher is it runs perfectly as soon as either the petcock is off or the hose is pulled off the petcock.
I tried it with the air filter and the gas cap off. Same thing.
The overflow tube and drain line are clear, can blow through them while hooked up and verified by plugging each with my finger that they are all clear.

The "choke" enrichment circuit is a knob on the carb not a cable to the handlebars on this old model.

This is not my first carb rodeo and this one is strange.

I'll pull the bowl off again tomorrow and make sure the jet isn't loose but I wouldn't mind some more ideas while I'm in there again.

Thanks

Well, the "clincher" is that it will begin to run more and more lean until all the fuel in the float-bowl is exhausted.

But, you know that.

The question is WHY it's running rich in the first place.

Perhaps a tiny leak in the floats, which takes time to fill up and then cause increasing richness. Later, when the float-bowl is empty, the leaking float drains dry again.

The fix is to probably replace the float, rather than to attempt a repair...assuming you can find a leaking spot or crack.

While you're in there, check your float-heights for being within specs. This probably won't be part of the problem, but it's a good idea to do it anyway.

Alexd

Cracked float or sticky needle/seat are all I can think of

I ride along bucking at half throttle and shut off the fuel. By the time my hand is back on the grip it's running perfect. not better, perfect. Ride for 100 feet turn fuel back to on or res and it immediately goes back to bucking at anything more than 1/3 throttle.
Below 1/3 throttle, startup, idle etc is perfectly normal.
half sunk floats don't idle in my experience, that's normally the first symptom.
When I took the carb apart I checked the float carefully for cracks and for if they had any fuel in them.

I will check the float level with the clear hose method and when I have it apart next I'll stick it in a bowl of gas and watch it

Did you actually unscrew the float needle valve to inspect things? It sounds like perhaps gunk build-up causing erratic float valve action.

Jet needle got a burr on it that's causing it to stick at a particular throttle opening (vacuum level)? Or it might have a nick or worn spot that's causing it to pull in way too much fuel?

1/3rd throttle should be about where its transitioning from pilot to needle.

So it goes against my grain to change 2 things at once because then you never know what fixed it....

Checked gas level with tube method, gas was 1/4" high. Took the carb completely apart again just like last time. Float measured about 17mm which should have made gas level lower not higher ... see below as to why.
Float is not cracked, floats evenly in a bowl of gas, main and jet needles looks great. Boiled it in Pinesol and water then into the ultrasonic cleaner.
When assembling I noticed the float needle slipped back and forth when tipping the carb. In carbs that are made loose I'm used to seeing a narrow spot in the bowl to keep the pin from slipping out of the ears. The pin that this carb had in it was short enough to slip out of one ear before hitting the bowl.
Now I'm not sure if it was supposed to be a longer pin or tighter ears but I made a longer pin out of a 3/32" welding rod.

I believe the pin would slip to one side and give me erratic level and fuel pressure on the needle would make it very rich.
And that is why turning fuel on and off changed thing immediately.


Put it back together and all is back to running great.

Good to know that you solved the problem!

Alexd