idles all over the place

[fuzzyruttin]

Sounds like a leak, right? I'm stumped.

On first startup it's fine. Coming to a stop, the idle hangs. Blipping the throttle once or twice brings it back down.

RPMs change with idle screw adjustment, however, it's constantly searching for its happy idle speed. It is not in a tunable state.

The carb has been off countless times now, and has been thoroughly cleaned. I removed the manifold and checked the gaskets. Button everything back up and no change in behavior. The throttle cable is not binding. Throwing starter fluid at it doesn't seem to make any difference.

26mm Keihin carb, GY6 150.
For reference, the old 24mm knock-off carb ran best with 114 main, 41 slow. Came stock with 102/35.
Used Keihin carb came with 82/35s. Currently has 110/38. I've gone as high as 125/41 in various combinations. The idle is so bad right now, the only degradation in performance with swapping out the jets is it is clearly rich with a 120+ main. Basically, there is bigger problem here than mere jet size.

I cut a juice jug to make an air filter shroud to help prevent it from sucking in starter fluid while messing around. There are some holes on the right side that I started poking around on in the video, the one on top I think releases pressure on the diaphragm? Anyway, I rode around a few blocks and came back to make to record this. When I walked around to the left side to spray starter fluid, I think it may have sucked in a tiny bit, as the idle dropped just a tad. At the very end of the video, you can hear the RPMs deciding to pick up the pace again, without even touching the bike. Any help is much appreciated, as I am at wits end.

[Happypancake!]

Once had a hairline crack in the intake manifold which made the RPMs hunt like that

[tortoise2]

Mar 9, 2019 12:29:44 GMT -5 fuzzyruttin said:

There are some holes on the right side that I started poking around on in the video, the one on top I think releases pressure on the diaphragm? 

Top hole is carb bowl vent port . . blocking inflow air restricts outflow fuel to low-speed and main jets.

Why was air-cut-valve removed?

[fuzzyruttin]

This carb doesn't have air cut-off valve. I confirmed this because that was my first question. Since it's part of the mold, the form is still there. There are no thru-holes, but yeah, it looks like something is missing otherwise.

[fuzzyruttin]

Mar 9, 2019 14:44:58 GMT -5 Happypancake! said:

Once had a hairline crack in the intake manifold which made the RPMs hunt like that

I would expect the RPMs to go down after drenching it with starter fluid, tho. Did you do the starter fluid test on it?

[ryan_ott]

An unlit propane torch works great for tracking down air leaks. A lot less messy also.

[fuzzyruttin]

For kicks I put the 35s "original" slow jet back in, because the pick-up end is about 5mm shorter than the hoca jets. It shouldn't matter, but wanted to check off that box. No difference. It's really surprising that there is not an obvious difference between a 35s and a 41.

As I was fiddling with the idle screw with the 35s installed, I noticed the exhaust was not clean. So I checked the oil level, and it was quite a bit over. Wondering if I filled it up when the engine was off, don't recall now. After draining it to a reasonable level, ~3/4 full on the dipstick, I rode some WOT. It didn't get quite the top speed that it usually does. Possibly bump the 114 main down a notch, which is 110 in my kit. Regardless, came back and the exhaust is still smokey. So what now, you think my rings are blown? I did not notice anything last year.

As far as the air leak, I don't feel any progress has been made. Most likely I'll end up buying a new manifold, insulator, and gaskets (and possibly find that's a futile waste of money). Optionally, I could try getting the old carb to work with the throttle cable mod that was needed for the new carb, and see how it idles (and if still smokey).

[fuzzyruttin]

Guy at work suggested that the carb might be leaking gas into the cylinder head. This makes a lot of sense because I'm pretty sure I could not have overfilled the oil by that far (1/2 qt?) I'm visiting folks in FL right now, otherwise I would be on that. After doing a proper bowl height adjustment, the carb is starved of fuel after running WOT for a minute or two. I went back in and deliberately allowed more fuel to flow, and that solved the 2-minute bog, so it must be getting TOO much now? Dunno.

[fuzzyruttin]

Been a long time posting here, but I'm still battling the idle gremlin. I had pretty much given up cuz it's good enough to ride. Nonetheless it's eating at me, idles rough and pop-pop-bangs when I need to cruise with traffic at 25-30 mph. Took the carb off again tonight, removed the idle screw, and gave it a few good shots of carb cleaner and forced air. After putting the carb back on the bike, wow whadaya know, the idle screw had to be backed off about 3 or 4 turns! Alright, let's move up the slow from a 30 to a 32. Nope, idle screw had to be screwed back down tight again. So figured the 30 was probably the better choice. Well, damn, after putting the 30 back in, it's back behaving the way it was. Idle screw all the way in, running rich. This is getting old. So, what, take it apart again and shoot the idle screw port with more carb cleaner?? Slightly frustrating.

[pinkscoot]

Just a crazy question, is the o-ring still on the idle screw?

[fuzzyruttin]

Jun 19, 2019 19:55:53 GMT -5 pinkscoot said:

Just a crazy question, is the o-ring still on the idle screw?

Yup

[SMALL CC TEK]

That is bad looks like a tall manifold is that factory or aftermarket ? I think your buddy hit it on the head look at the angle of your carb the way it sits your fuel level is way high dripping most likely . Find a stock spacer and just try that ..