Confusing VM carb issue

[dbarnes42]

Here is what I have - 79 GS850, all stock except Dyna ignition and coils. Carbs rebuilt, new o rings in carbs, new o rings on intake boots, new intake and airbox boots, airbox sealed and valves adjusted. Carbs are all stock except for clip on needle, which is in 3rd notch from the top and previous exhaust leak fixed.

Pilot fuel screws are 5/8 out and pilot air screws are 1 1/4 out. Uni air filter, freshly cleaned and oil. When I oiled it, I misted it once.

The bike startes easily with the choke. When started, the rpm's climb to about 3K and I start to close the choke. After about a minute or so, the choke is fully off. The bike accelerates fine and has plenty of power.

My issue is this - when the bike is nice and warm and I stop, the rpm's hang up around 2-3K and won't come down unless I blip the throttle once or twice. After I blip it, it idles fine, around 1100 rpm. This happens every time I stop, but only when the bike is really warmed up, say after 30 minutes or so on the interstate. It doesn't seem to happen after the bike is first started and the choke is turned off, only after it is really warm. Any one have a problem like this before?

[almarconi]

I would check for an intake leak to rule out this possibility. Usually a hanging idle is a sign that the bike is lean.

When the idle hangs is the engine getting unusually hot?

You could try opening your fuel screw an additional 1/8 turn and see if this helps.

[Nessism]

I would check for an intake leak to rule out this possibility. Usually a hanging idle is a sign that the bike is lean.

When the idle hangs is the engine getting unusually hot?

You could try opening your fuel screw an additional 1/8 turn and see if this helps.

Good advice here.

[Jagir]

Does it have the original vacuum petcock or has it been replaced with a manual one?

If it is the latter, check for an air intake leak.
Ask me how I know

[tkent02]

Could certainly be an air leak, could also be the throttle cables adjusted a little too tight, or kinked, worn out, unlubed or routed poorly so that the throttle is not snapping shut as it should, might be staying just an RCH from closing. Maybe the grease inside the throttle twist grip has turned to goo, or maybe the linkage on the carb rack is not quite straight, not moving freely.
When you blip it maybe it snaps shut a little better than when you just gradually roll off the throttle to slow down.

[dbarnes42]

The petcock was replaced with a new one last winter, which is vacuum. I totally avoided getting a rebuild kit. As far as the engine getting hotter when the idle hangs, it does not seem to.
For the intake leak, the o rings and boots were replaced last winter also. Maybe one of the o rings has gone bad already or even a boot? What is the best way to check for an air leak?
The throttle cable has a little play in it, maybe 1/8" or so. I will check the lube on it though as it was replaced last winter also. I will also check the grease on the throttle tube. I had planned on turning the fuel screws out in the morning and I will keep everyone posted. Thanks for all of the help.

[almarconi]

What is the best way to check for an air leak?

Spray the boots with some carb cleaner with the engine running. If you have a leak the engine rpms will increase.

[SqDancerLynn1]

You should do a carb sync too!!

[Big T]

An air leak can come from around the carbs also. Check your clamps and make sure they're tight

Also, check your spark plugs and see what color they are

Got access to a Colortune?

[tkent02]

The throttle cable has a little play in it, maybe 1/8" or so. I will check the lube on it though as it was replaced last winter also. I will also check the grease on the throttle tube.

Just check if it is closing all the way when you gently close the throttle, the idle speed lever should be resting on the stop screw. There should be almost no friction in the entire throttle mechanism.

[dbarnes42]

I will check all of these things today. Big T, no I do not have access to a Colortune. The synch gages I have are just that, the gages. Nothing fancy.

With regards to the carb synch, should I do this at idle or at a certain RPM?

[almarconi]

With regards to the carb synch, should I do this at idle or at a certain RPM?

Consult your service manual but usually the rpms are in the 1500-1800 range. It helps to have a fan blowing on the engine while you sync the carbs.

[spyder2120]

ditto on previous replies. i sprayed silicon lube on the linkage at the carb rack. i would definately spray some carb cleaner around to check for vacuum leaks. after i dipped mine and replaced all the o-rings last spring i found a leak. good luck. its the little things that get you.

[dbarnes42]

Ok, today I pulled the throttle housing apart and lubed it. I lubed the carb rack and made sure the throttle cable wasn't to tight. I started the bike and let it warm up good.

I noticed if I bipped the throttle, the problem of the rpm's hanging up wasn't there. It only seemed to happen if I took the rpm's above 3K and then relased the throttle. It didn't matter if I closed it quickly or slowly, the problem was still there.

I then sparyed carb cleaned EVERYWHERE and I don't have any leaks. I rode the bike to work and everytime I stopped, I had this problem. Once I got to work, I pulled the clutch in and hit the kill switch. I was in 2nd gear and about 20 mph when I did this. I then pulled the #4 plug and it was black, like it is running rich. The sysmptoms show lean but the plugs show rich.

I was wondering if the position of the clip on the needle has anything to do with this problem. I currently have the clip in the 3 notch from the top. Stock position is 2nd from the top.

Anyone have any thoughts or suggestions?

[almarconi]

Once I got to work, I pulled the clutch in and hit the kill switch. I was in 2nd gear and about 20 mph when I did this. I then pulled the #4 plug and it was black, like it is running rich. The sysmptoms show lean but the plugs show rich.

I'm not sure you are doing a plug chop correctly since the plug chop is related to throttle position. For example to test the main jet you would hold full throttle for a mile or so and then pull in clutch and kill engine. You also need to clean your plugs before doing the plug chops. Why did you look at only plug #4?

I would return the needle to the stock position. Did you measure your fuel level? Incorrect fuel level (height of gas in the bowls) will effect all needle positions so this needs to be set correctly. Hook a clear piece of tubing to your carb drain fitting with the engine at idle and measure the height. Your service manual should have the proper specs.