I know this topic has already been discussed at nauseum but,

After cleaning my carburetors, polishing the throats so the clunk test is so fast it's like a magnet is pulling them down, the jets are open, the enrichment circuit is clean, adjusted the float levels and vac synced the carbs on the bike.

It idles great, revs great, but comes down too slowly (eventually it comes down to idle). I did the propane test thinking I might have a vacuum leak but there was none. The throttle butterflies come back immediately when I release the throttle. I had a spare rack of carbs so I cleaned those and put them on. Same issue. What have I overlooked? I haven't checked the valves but compression was really good. Could tight valves cause this issue? and wouldn't tight valves cause compression loss?

Thanks!

 

Did you check every thing hot? I mean good and hot?

Reason I ask is last year I had a rock steady idle, even synch gauge etc would idle in my driveway all day long. Ride it 30 miles on the free way @65 or whatever, pull off and stop at the light on the ramp it would only come down to 2,500.

I had to back off the idle adjust screw to close the blades a bit and that fixed it. Barely a 1/2 turn on the screw, no mix adjustments.

 

I've never ridden it more than a mile -- it's really annoying to shift when it doesn't come down when you pull the clutch in. But eventually (at least most of the time) it will casually come back down to idle.

 

Try disconnecting the throttle cable and work the linkage with a pair of pliers.

Maybe your cable is binding.

 

The Main reason for High rpm's after a Carb Cleaning is usually AIR, Mixtures and Sync.

Did you do a Closed Throttles Bench Sync?
Manual Sync of Throttles with a THIN Feeler Gauge.
Strips of 3X5 Card.

Manifolds getting Air in.
Clamps loose
Carbs Not fully seated.

Mechanical Issues
Linkage help open by clamps.
Connected to wrong Cable.
Linkage binding because Carbs were removed from rack.

And, ... Cable Pull stuck under head and not letting linkage close.

How high is it over-revving?

 

Re: I know this topic has already been discussed at nauseum

It doesn't really over rev by itself. I give it some throttle and then it takes it time back down. So if I pull the throttle it will immediately go up to whatever -- say 5000 RPM, but then takes it time back down once released. I don't see any mechanical issues, the butterflies snap right back, cables are clear. Maybe I didn't do a good job with the vac sync or something -- is a sync problem indicated? I also haven't adjusted the fuel/air mixture screws -- is this an indication that some cylinder(s) is/are too rich?

 

Re: I know this topic has already been discussed at nauseum

Hanging idle with carbs that have been synchronized indicates a lean condition. Usually it's an air leak somewhere.

Did you check for a leak at the petcock and in its vacuum line?

 

Hmmm. Well, my petcock doesn't have a vacuum line, they guy who had it before must have replaced it with an after market. I have 'hats' on all the intake manifolds though. I have wondered if changing the petcock to the stock one would make any difference. Maybe I should check the air filter to see if that is sealing up properly.....

 

I would still suspect the shaft seals, if they haven't been replaced. I had the same problem. Idle would stay high after I warmed the engine. It would pass the propane test when the engine was fairly cool. I replaced the shaft seals and all was good.

 

Ok DK, That is something I haven't done. I'll check that out.

Thanks everyone!

 

HOW did you check the float levels? Clear tube, with fuel? Dry setting's only a start...

And yes, one cylinder being way off on mixture could hang the others too; vac sync is still also a possibility.

Is this a YICS motor and if so did you use a YICS tool for the sync?

Vacuum leak also a good possibility.

 

5000 rpm's ?

I'm thinking you didn't do the Sync right and turned the Throttles too far open.

See if the Idle Rod is to blame.
I doubt Shaft Seals would be enough Air to sustain 5-G's.

Make sure you didn't cut a Manifold or two reinstalling the Intake Clamps.
Those clamp leaks are sneaky.

5000 rpm's means you are getting Main Jet Fuel to run it that fast.
The Throttles are opened.
You can;t get to 5000 rpms without the gas.

Unhook the Throttle Cable.
Back-out the Idle Rod
Put a screwdriver on the Linkaage and "Help" it to close.
If its stuck ... you have to remove the carbs and true the rack so it doesn't bind.

 

So Rick, I twisted the throttle to reach 5000 rpm. I was just saying that when I then shut it off, it floats down slowly to whatever I set the idle screw to. It doesn't all of sudden go to 5000 rpm by itself. Does that make sense?

 

Fitz,

to answer your questions, I did do a wet float level check with clear tubes, not a YICS. 1981 650. I'll try another sync and look for more leaks.

Thanks for your help everyone. This is a great forum!

 

see if the problem is present when the petcock is on PRI.

i have seen it a few times now with these silly vacuum petcocks, the ON position doesnt supply enough fuel to maintain the bowl levels, changine the mixture as you ride. PRI is wide open fuel, and keeps the carbs full.

just something easy and free to try.

 

Cool, just trying to "pick off" possibilities.

Here's one from left field, but it happened to me: Make sure all of your little rubber caps on the intake manifolds are still rubber and not black stone. They can LOOK fine but be hard as rocks, and "dance around" with vibration, creating a phantom vacuum gremlin that will drive you crazy.

Make sure your petcock vacuum line is really vacuum line. Possibly a PO has replaced the vacuum line with something that collapses under vacuum.

Then there's always throttle shaft seals. (Sorry.)

 

If they never have been done, the Diaphragm Piston Bores should be refinished.
The Bores get a coating of Oxidation that causes the Piston to slow-down, chatter or just not move.

The easiest way to clean them is with ScotchBrite Medium Gray Pad.
Scrub them all out.
If you have a Dremel and Polish 'em up nice, ... that's even better!

SCOTCHBRITE SCRUB:



POLISHED BORE:

 

my guess would be gummed up or binding linkage somewhere. since the bike DOES idle. just takes it sweet time getting back there.