Topic: High idle help

[Hutch]

I got my bike started, 1982 cb650sc, but its idling at 4200 rpm.

I couldn't find anything in the manual or online related to idle adjustment for CV carbs.  Here are the changes made since removing the carbs.

complete carb cleaning and rebuild
replaced vacuum-assisted petcock with a regular petcock capped vacuum hole
bench synced
backed out master throttle control/throttle stop

It fires right up when cold with full choke.

[Hutch]

Update:

Checked the slack in the throttle cables, that's not the problem.

Also sprayed carb cleaner around the carb boots with no change in rpm, so ruling out an air leak at the boots.

[CBJoe]

Idle adjust backed out?  Maybe this is what you called the throttle control stop?

Super high idle can also be caused by air leak at intake boots between carbs and head.




Sent from my iPhone using Tapatalk

[Hutch]

Idle adjust backed out?  Maybe this is what you called the throttle control stop?

Super high idle can also be caused by air leak at intake boots between carbs and head.

I checked the parts diagram, can't find the Idle adjust screw.  Where is it typically located?  I backed out the throttle stop, which is the black knob on the bottom side of the carbs. 

Also sprayed carb cleaner around the carb boots while it was running, with no change in rpm.  Any other way to diagnose if it might be a leak at the boots?

[calj737]

It may well be that your slide heights are too high and not returning to the carb “floor”. What work did you do to the carbs? Have you bench synched them or vacuum synched them yet?

[Hutch]

Complete carb cleaning and rebuild, and bench synced.  I have not vacuum sync'd, its my understanding it should run OK until I can vacuum sync.  I ordered new carb boots, which I suspect is the problem.  I'll check to ensure the slide is working properly when I remove the carbs.  I also found a video on bench testing slide lift with a bit of compressed air.

[Hutch]

I removed the carbs while waiting for the new boots to arrive, found that the butterfly plates are all gaped differently when closed as you can see from the pics.  #4 is completely closed, #1 & #3 have about a 1/32" gap while #2 has over a 1/16" gap.

Also, the throttle mechanism sticks after I snap it back, but I can't find anything binding.

[calj737]

Those are the choke flaps. Looks like you need to pull the crabs and give them a FULL cleaning.

If the throttle doesn’t snap back, either your cable is improperly adjusted, incorrectly routed, or perhaps the throttle tube/grip is binding against the switch housing. There should be a credit card gap between the grip and housing.

[Hutch]

As an update, I made the following changes which by and large resolved the issues.

It seems that the high idle was due to a vacuum leak which was fixed with new carb boots.  Check that off the list.

I corrected the problems with the butterflies by re-aligning the carbs.  Problem fixed.

I had it running, but soon after it would not start.  I found some very small debris when draining the tank which clogged slow speed jet, pilot screw opening, and accelerator pump check valve.  Aside from rinsing the tank several times with gas, I did a thorough carb cleaning with chem-tool carb cleaner spray, e-string, and compressed air.  It seamed to do the trick.  I set the idle mixture screws at 1.5 turns out which I'm not sure is the right starting point for KEIHIN  VB44c carbs.  The mixture screws are on the air side. 

Here's where I'm at today...

Starts cold with full choke, runs with no choke after 30 seconds or so.  Idles at 900rpm, although somewhat labored, occasionally stalls with no throttle.  There's a stutter or lag with initial throttle, but then it picks right up.  All cylinders seem to be equally hot when checking the exhaust pipes.

I'll put a couple of miles riding in then check the plugs, right now its been ridden less than a mile.  It seems to be running a bit rich, other than adjusting the mixture screws and a proper sync, I'm at a loss. 

[odioanonimo]

Is everything stock? Airbox, exhaust, etc?

And my experience is that the only way to be certain where the rich condition originates is to do some plug chops.

That being said if you've ridden less than a mile I assume it wasn't at WOT. 

What makes you think rich?  Is it the smell, or behavior, or does it start easy no choke etc.

The biggest lesson I've learned i think is that everything effects everything else.  Do an idle drop mixture adjustment.  Make sure your timing is dead on, adjust valve clearance, vacuum sync, plug chop is probably how I would proceed.

[Hutch]

Is everything stock? Airbox, exhaust, etc?

And my experience is that the only way to be certain where the rich condition originates is to do some plug chops.

That being said if you've ridden less than a mile I assume it wasn't at WOT. 

What makes you think rich?  Is it the smell, or behavior, or does it start easy no choke etc.

The biggest lesson I've learned i think is that everything effects everything else.  Do an idle drop mixture adjustment.  Make sure your timing is dead on, adjust valve clearance, vacuum sync, plug chop is probably how I would proceed.

Everything is almost stock, I kept the plenum to maintain calm air, but eliminated the air cleaner case replaced by one large pod about the same size as the stock filter.  I also replaced the vacuum-assisted petcock and capped the vacuum line at carb #2.

No riding at WOT yet, it's currently 38 degrees plus I didn't want to stray too far until I was confident I wouldn't get stranded.  I guessed it may be running rich due to smell and a bit of rough or labored idle.  If the plugs a sooty, that would be one more clue.  I can't seem to find any definitive information on mixture screw setting, I've read everything from 1.5 to 2.5 turns to start.  I haven't checked timing or valve clearance.

[calj737]

You should definitely adjust your idle once engine is warm to above 1100 and leave it there. 900 is TOO LOW.

That air filter change will make your bike run more lean. Once you get a chance, some IDLE plug chops with fresh plugs would be a sensible starting point for final fine tuning of your carbs.

[Hutch]

Thanks for all the great advice.  Don't have fresh plugs ATM, so based on smell, labored idle, and pretty much starting w/o choke, I turned the idle mixture screws out a half turn from 1 1/2 turns to 2.  This seems to be right, and makes me feel that 2 1/8 or 2 1/2 turns out is the proper factory starting point.  I'm assuming my filter set up is causing it to run a bit leaner than the stock filter box. I also ratcheted up the warm idle to 1100 rpm.  Everything is good so far.  The bike fires right up cold, and is idling well.  No popping or backfire.  Fingers crossed for a day or two above freezing without sleet or cold rain so I can run it hard through all gears.  I'll do both an idle and WOT plug chop, and build a manometer for syncing the carbs. 

Next up, figuring out how to wire aftermarket Emgo right and left switches and new throttle.  At that point, I'll be ready for body work, paint and shaping the seat.

[odioanonimo]

if youve come this far those switches will be no problem at all.

[kerryb]

Watching this one carefully because I'll be going through the same procedures fairly soon (spring).
My question involves the valve clearances.  Assuming only normal wear which would make the gaps larger (not dangerous to motor, just a loss of power), how important is checking valve clearance to solve the mixture question?

learn something new every day...

[calj737]

Watching this one carefully because I'll be going through the same procedures fairly soon (spring).
My question involves the valve clearances.  Assuming only normal wear which would make the gaps larger (not dangerous to motor, just a loss of power), how important is checking valve clearance to solve the mixture question?

learn something new every day...

Pretty much required. Set valves. Then timing. Then deal with carbs. Vacuum needs to be right, timing must be on for proper combustion, and THAT determines fuel requirements.

[kerryb]

Good to know, thanks