Confusing VM carb issue

[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.

[nert]

Nessism
Looking past the end result of "fixing" the hanging idle.
When turning out the mixture screws, what affect did it have on the engine idle speed?
Increase, decrease or stay the same.
EDIT
And was you idle staying elevated until you bliped it?
Did it slowly return to idle on its own?
what does blipping do to an elevated RPM to bring it back down?

[almarconi]

I still think that there is probably a vacuum leak somewhere. A leaky vacuum petcock diaphragm could be a possibility.

The PO also stated that a previous exhaust leak had been fixed, how was this repaired? Did the problem occur prior to the leak repair?

[dbarnes42]

First off, THANK YOU to everyone who has posted something in response to this issue. I do have good news and bad news to report.

In response to the exhaust leak, there was a small dime sized hole in the premuffler. The bike has a 80 exhaust on it, even though the bike is a 79. I fixed this problem by removing the premuffler and welding pipe into the area where the premuffler was. I basically turned the 80 exhaust into a 79. This fixed the problem of popping on decel.

The bike has a Dyna ignition, coils and wires with the mechanical advance. I will check the function of it tomorrow. I am using the stock plug, NGK B8ES, gapped at .25. I had read somewhere on this forum to gap it at .31, which I will do.

Now for the good news. Looking at the carbs today, I saw what could have been the problem. On the inside of the #1 carb is a bracket or clip that has a screw hole in the top and is "U" shaped on the bottom. The "U" part sildes over the rod that runs through the carbs that the slides bolt to. Well, this one was bent! And not just a little bit. I have a spare set of carbs so I pulled that one off and replaced it. I then turned to pilot fuel screws at 1/2 out and the pilot air screws at about 1 turn out.

The hanging idle is gone - mostly. Now it hangs up at about 1.5 to 2 K instead of the 3K it was doing.

But now the bad news. Bipping the throttle brings the idle back down, but now when the rpms drop, it falls below where I have it set and the bike wants to die. It did die twice on the way to work.

So, where do I go from here?

As a side note, I am positive I did the plug chops wrong and I only pulled #4 because it was easy to get to and I was running late for work.

[almarconi]

But now the bad news. Bipping the throttle brings the idle back down, but now when the rpms drop, it falls below where I have it set and the bike wants to die. It did die twice on the way to work.

Try opening the air screws a 1/8-1/4 turn and see if this improves the hanging idle. I think mixture wise you are pretty close. Make small adjustments and keep checking until the rpms rise quickly(when you turn the throttle) and then fall back to the baseline. Your baseline idle should be right around 1000 rpms or slightly less.

[Nessism]

Try opening the air screws a 1/8-1/4 turn and see if this improves the hanging idle. I think mixture wise you are pretty close. Make small adjustments and keep checking until the rpms rise quickly(when you turn the throttle) and then fall back to the baseline. Your baseline idle should be right around 1000 rpms or slightly less.

These are VM carbs, not CV. The pilot fuel screw needs to be opened up to allow extra fuel in.

[rustybronco]

I do have good news and bad news to report.

But now the bad news. Bipping the throttle brings the idle back down, but now when the rpms drop, it falls below where I have it set and the bike wants to die. It did die twice on the way to work.

So, where do I go from here?

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.

GS850 (77-79) the specs are for the VM's

idle r/min........... 950-1150
carb................... mik vm26ss
id no................... 45060
bore size ............ 26 (1.0)
float height ......... 23-25. (0.91" - 0.98")
fuel level ............ 3.0 - 5.0 (0.12" +/-0.20")
pilot air screw ..... 1.25 turns out
pilot screw........... 5/8 turns out
pilot air jet........... 1.2
pilot jet............... #15
cut away............. 1.5
jet needle........... 5DL36-2 (the -2 2nd notch)
needle jet .......... 0 - 6
pilot outlet........... 0.6
by pass ............. 0.8
main jet............. #102.5

My opinion? put the clips back to the 2nd notch and check the float levels by the wet method (a piece of clear tubing connected to the nipple on the bottom of the float bowls)

[Big T]

I think you need to do the last 3 posts

1. Fuel screw needs to be 7/8 to 1 turn out
2. If that doesn't work, check fuel level
3. Reset needles to stock

It's always some little thing, the problem is finding what that little thing is

[dbarnes42]

OK, I rode the bike to work and everything was ok, until I got to the first stop light off of the interstate. The rpm's hung up around 1800 to 2K and I barely blipped the throttle and they came down to idle. The bike did not want to die.

I had the pilot fuel screws about 1/2 turn out and the pilot air screws at 1 turn out.

Just out of curiosity and because I can't leave well enough alone, I adjusted the pilot fuel screws to 7/8 out before I left work. When I started the bike, I noticed that the little hickup it had before when the choke was on was gone. So, off I went.

Now the bad. The first stop light off of the interstate, the rpm's hung up around 3K again. Blipping the throttle made the bike die. I should point out that I live 42 miles from work and 30 miles of it is interstate. So the bike is nice and warm at this point.

I appreciate everyone's help with this. My plan is turn the fuel screws back in to 1/2 out (maybe a little more but not 7/8), put the needle back in the second notch, install new plugs and synch the carbs. One of these has to help.

Also, how to I check the float level using the wet method? I will try this, but I am a little lost on what to do other than put clear tubing on the float bowl drain with the bike at idle.

[almarconi]

Now the bad. The first stop light off of the interstate, the rpm's hung up around 3K again. Blipping the throttle made the bike die. I should point out that I live 42 miles from work and 30 miles of it is interstate. So the bike is nice and warm at this point.

The fact that the problem got worse as the bike warmed up means that you are too rich.

But now the bad news. Bipping the throttle brings the idle back down, but now when the rpms drop, it falls below where I have it set and the bike wants to die. It did die twice on the way to work.

This is also a classic symptom of a bike being rich at idle.

Leave the fuel screw at 5/8 turn out and reset the air screw back to 1.25 turns out. Measure your service fuel level to make sure this is within spec.
Clean or replace the plugs and try again. Also put the needles back into the second notch. I would also inspect your carbs to make sure nothing else is damaged.

[Nessism]

I'd try 3/4 for the fuel screw and 1.5 for the air screw. Running at 1/2 and 1 is defenitely different than most people use with a stock bike. One turn on the fuel screw is typically on the rich side but it works for some people, and the air screw should be tyically twice the number of turns as the fuel screw - to be adjusted for the highest idle speed although sometimes the engine doesn't respond to these minor tweeks with changes in idle speed.