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.

 

[nert]

seems like we have established nothing!
First, an elevated idle does not mean a "lean" mixture. It can mean an air leak or vacuum leak. (which may result in a mixture being lean) the reason the RPM increases with an air leak is because its much like opening the throttle more, letting in more air. Lean mixture by itself (not an air leak) will cause RPM to drop and leaner yet, misfire.

When the RPM is hung, back off the idle speed screw and see if that has an effect. If the nothing happens, more than likely you have a mechanical cable, throttle, linkage problem. Then try pushing the throttle linkage at the carb to less throttle to see see if that confirms a mechanical hangup.

Not familiar with this bike, but does the Dyna ignition have a mechanical advance or electronic? I have experienced Lucas mechanical advance units on my britIron hang up, advance the timing, and therefore increase the idle. Alittle blip of the throttle would help reset the advance to rest position.

 

[almarconi]

First, an elevated idle does not mean a "lean" mixture.

I have to disagree. When the idle is rev'd passed the base line (idle) and does not return to the base idle quickly but hangs and returns slowly to the base idle, this is a sign it is lean at idle. On the bikes I've tuned, opening the mixture screws (adding fuel) cured the problem.

Is the bike lean on the mains ?? Who knows until the PO does a proper plug chop.

The point of going to a forum is to get other peoples opinions. Since none of us are there in the garage with the bike in question we can only make suggestions as to what may be the problem. You say " seems like we have established nothing !" That is not true, by following the suggestions the OP can at least rule out that he does not have an air leak.

 

[Nessism]

I'd vacuum sync at about 2000 rpm or so (use a fan in front of the engine) and if the idle is still hanging, open the fuel screw another 1/8th turn and see if this helps.

The hanging idle does mean too lean, either due to the mixture itself or from an air leak of some sort. Spraying stuff on the carbs and boots is an unreliable method to find air leaks so keep this in mind although I think it's unlikely this is the problem based on what's been done already.

One last thing to check is the mechanical spark advance. If you twist the rotor with your hand it should snap back smoothly once you release it. Worth a check if everything else seems to be checking out.

Good luck.

 

[nert]

His idles is not coming back on its own. Elevated idle means air leak, not lean mixture. If you take ANY engine thats adjusted properly all systems working correctly and lean the mixture the, rpm will fall. Induce an air leak, by pulling a vacuum leak or other means and rpm will rise. I didn't make this up. Its Fact

Think about how you adjust an idle fuel mixture screw. You adjust rich till RPM begins to fall. You then adjust lean until RPM begins to fall. Then some where, (in small increments) between those (2) settings is the proper fuel mixture. The proper mixture is that which gives the best smoothest highest RPM (neither rich or lean). It is at that point, where if you adjust richer, RPM will drop, adjust leaner, RPM will drop.

Unfortunately, the perception of vacuum leaks has clouded what a lean mixture is. Lean means, not enough fuel to properly support combustion. And when there is not enough fuel, RPM fall, reduce fuel even more, and you get misfire.

Making a mixture rich will cause RPM to drop as you stated. It may be masking a different problem, not the solution or cause of the problem. If making a mixture richer, causes RPM to fall, then you are either already adjusted correctly or, already too rich.

I am not suggesting that an air leak is the problem here., OR am I denying that it could be the problem. A faulty mechanical spark advance
(if it has one) is certainly a reasonable cause as well. And with
(4) carburetors newly assembled, a bunch of different issues could be causing this problem, including mechanical, linkage, misassemble, parts, etc, etc.

We need to know compression in all 4 cylinders is in spec, that all spark plugs, wires and coils are operating correctly. We need to know, for sure the throttle linkage is returning fully to the idle speed set screw when this problem occurs. We need to know if the (4) throttle slides are retuning from the open position. We need to know for sure all (4) fuel bowl levels are reasonably the same, and the bench sync is accurate. NOT, just that the twist grip and cable feel free and there appears to be slack in the cable. These are far different threads of information leading to far different paths while trying to evaluate and discover the accurate problem solving solution..

We need to understand the difference between lean mixture, vacuum leaks, rich mixture, and air intrusion, etc, to make sense of troubleshooting procedures, and symptoms, and to analyze the results of diagnostic testing.

 

[Nessism]

When the engine is leaking air into the intake track it LEANS the mixture at idle and causes the hanging idle. You can also get the hanging idle if the mixture screws are in too far because again, the mixture is too LEAN. My old 550T did this; hanging idle due to the pilot screws being in too far. It wasn't until the mixture screws were opened to about 3 turns before the hanging idle went away, and the bike ran best at 3.5 turns.