Chaising the infamous hanging idle - Carb tops possibly?

Sci85

Sci85

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Ok, I've been chasing after a hanging high idle after a full rebuild '80 650 with BS34's. Carbs have been done, new boots, etc. I just replaced the shaft seals too. Spraying around boots and manifolds show no change. Shafts are good too now. However, I noticed that when I press down hard on the carb tops, the chrome part covering the slides, the idle dropped. I've not run into this before. Can air leak past the tops and lean out the idle circuit? I can't see any obvious issues with the slide diaphragms but didn't think air getting past the top would have an effect on the idle circuit.

Anyway, any ideas would be appreciated. Thanks!
 
Do the normal test for a leaky diaphragm, with carbs mounted and engine off. Then you can tell if you have a leak there and when it's present. Or could be when you're pushing on them you're seating them better in the holders. They're supposed to be pushed in pretty far.
 
Yeah the alignment tab is in the proper place. And I thought about the rubber boots idea xjwmx. But as an experiment, I pulled the carbs downward in the boots instead of pushing down on the tops and the idle did not drop. I also supported the bottom of the carb while pushing on the slide cover and it did indeed drop back down so it's definitely not the boot. These carbs were fine when I took them off the bike awhile back (this was a running bike that I had ridden). I will try the diaphragm test but from riding the bike, all feels fine except that dam high idle.

I was more curious if it is possible to have a leak at that location affect the idle circuit. Doesn't seem likely but what do I know. Just so odd...
 
^Without having a carb in front of me...I think an air leak there would have a path into the throat.
I once had an SR250 that had a hanging idle I couldn't figure out and one day I adjusted the valves and that cured it. I think the exhaust valve was slightly open during the intake stroke. Also, the other day somebody suggested to someone checking the flatness of carb tops by grinding it on a flat surface.
 
Hmm, I hadn't thought about valve's being an issue but i will check them along with the diaphragm test.
 
Ok guru's, I checked the valves and they are all in spec. I also performed the diaphragm test and that looks good too. I pulled the boots (new btw and seemed to check out ok) and cleaned up the gasket surfaces and put new gaskets on. And shoot me dead, it's still doing the same dang thing. :banghead:

So, I can't figure this one out for the life of me. Again, high idle unless I press down on the right side carb. Pushing down on the left carb has little to no effect. :wtf:

Any ideas short of setting it on fire and forgetting the whole thing? Maybe something really weird with the carb itself?
 
If it isn't the way it's fitting into the holder, then the pushing down is probably affecting the throttle plate or the shaft a hair. You did the seals, which means you had the throttle plates loose. Maybe some small misalignment or the body is worn out. Could be the new seals aren't sealing, too.
 
I will try that in the morning nj1639 but note that I did just replace the throttle shaft seals with OEM yamaha ones. I'll push on the shaft as well as spray them again in morn.

Here's where it gets even more weird...when i spray ether in the middle of the head gasket, between the cylinders, carb side, the idle drops. Again, makes no sense. I've never seen a head gasket leak cause a high idle condition. I did go and re-torque the head with no change but is there something back there that could be the culprit? Dang this is a head banger arghhhh.
 
You didn't mention syncing the carbs?
Even if you have I might try adjusting the sync screw an 1/8 turn one way then the other see if it makes a difference.
Did you snap the throttles shut a couple times before the final tightening of the butterflies?
Check pics here carefully, the two throttle return springs are close but different lengths.
I'm thinking throttle shaft is slightly binding.
Did you do a taper jet washer shim?
 
Yeah, I keep hoping it's something simple but so far, everything I've tried has eluded me. It ran fine before I tore it down so it's clearly something I did or didn't do. So, here's what I've done to date:

Fully cleaned the carbs and all passages
Replaced needle and seat
Floats set at 27mm
Replaced pilots (now using the 47.5 - have tried the 45)
Replaced Mains (137.5)
Replaced shaft seals (I followed the guide and did the snapping of the shafts before tightening) - had the high idle before i replaced the seals
New Carb boots
Replaced intake gaskets twice now
Carbs synced on bike with manometer
Checked intake and exh clearances
retorqued head

gggGary - not sure what a taper jet washer shim is...

Today, I will make a video and post it up. Maybe that will spur some thoughts.

Thanks guys.
 
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You mentioned the floats set at 27mm. You do still have the brass floats, right? Because if you've changed to the plastic ones, they have a different setting spec, 22mm. Setting them at 27mm would create a lean condition.
 
I do have brass. I fiddled with it again this morning and found that the right cylinder is misfiring. The rechecked the carb sync and they were good. But I did notice that if I turned the sync screw so they were out of sync, the high idle went away.

Also, if I spray at the head gasket, right in between the two cylinders at the back of the engine, the idle drops. So, I think I may have a bad head gasket that's causing the issue possibly.

Gonna recheck the right side valves once more and do a compression check. Will report the results...
 
How many miles on the carbs?
did you look at the face of the seal in the choke valve?
 
18K total miles on the motor before teardown. Carbs completely rebuilt so zero miles. All new gaskets, rings, valves lapped, new seals, etc.

I checked the compression on the motor cold without squirting oil in the cylinder I got 110 on the left and 100 on the right. The left cylinder runs good and I have a good strong exhaust note. The right cylinder is definitely weaker and it runs, but the exhaust is not as strong.

I've swapped plugs and wires with no difference.

The only thing that makes any difference is squirting the cylinder head gasket on the carb side. So, at this point I can only presume a cylinder head gasket issue. So I guess I'm gonna have to pull the head at this point and try replacing the gasket. Argh...
 
^Try re-torquing first if you didn't. If you're going to take the head off with compression like that you might as well throw in some rings, as well as valve stem seals. Cruzinimage on ebay for rings, Mama-yama for the stem seals. Unless the bad compression is due to valves...which you know how to check for with a dab of oil down the plug hole.
 
I just retorqued everything and added a few more foot lbs. no go. And i confirmed that spraying at the head gasket on both sides drops the idle down so I'm pretty certain at this point the gasket is crap.

Can the head be removed while the engine is in the bike? Also, recommendations on who makes the best head gasket? I think I used an Athena originally.
 

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