GS850 high idle, one more time....

[Guest]

Hi Guys. I know this topic has been beaten to death, and the answer to my question is probably buried somewhere in the archives, but starting a new thread guarantees that I will ask exactly the right question.

First, this is what my bike is doing (BTW, it's an '81 GS850G with 17k miles): when I start the bike cold, I pull the choke about 1/2 way and hit the start button. It starts very easily and idles at about 1200 RPM. As it warms up (2-3 minutes), engine speed increases to nearly 3000 RPM, even with the choke fully disengaged. Nothing I do will make it settle back down.

This is what has been done to the bike and carbs relating to this issue:
1) new OEM throttle and choke cables. Each has been carefully adjusted to have a slight amount of slack when relaxed.
2) new OEM carb boots and large O-rings. I have double-checked the mating surfaces of the o-rings and the head; both are spotless. The sync port screws are snug - as are the screws that hold the boots to the head (new stainless steel, with anti-seize on the threads)
3) I have an OEM air box with a new OEM filter installed - lightly coated with oil (though there is still a small gap between the top of the air filter and the ceiling of the air box). The four rubber boots between the air box and carbs aren't new, but they are still nice and pliable, and correctly oriented in rotation. The rubber gaskets on each end of the air box aren't perfect, but they look like they're probably good enough to make a pretty good seal.
4) all eight rubber boots are fully seated on their mating surfaces, with the clamping bands properly aligned and snug.
5) the carbs have been rebuilt - twice - the 2nd time by one of the gurus on this forum who clearly knows what he's doing. I've tried setting the mixture adjustment screws to 2.5, then 3 full turns from seated. I couldn't detect any difference. After I made the adjustments, I rode the bike around the block a couple of times to see if anything might change; it didn't.

ARGH!!!! It's a good thing I wasn't working in an upstairs shop with windows, and I'm not nearly strong enough to throw a 600 # bike very far, 'cause if BOTH had been the case, that's what I'd have done!

The only thing that may be slightly suspect is my vacuum line to the fuel petcock. It is slightly stiff, but still in good enough shape to provide the vacuum required to actuate the diaphragm inside the petcock. Also... the #3 carb is from a different bike, and the butterfly disc is stamped "135", whereas the three other (original) ones are stamped "130".

Does anyone have any idea what's going on?

[hillsy]

The only thing that may be slightly suspect is my vacuum line to the fuel petcock. It is slightly stiff, but still in good enough shape to provide the vacuum required to actuate the diaphragm inside the petcock. Also... the #3 carb is from a different bike, and the butterfly disc is stamped "135", whereas the three other (original) ones are stamped "130".

Does anyone have any idea what's going on?

The two things above throw out red flags. I have no idea what the difference is between the butterflies, but if they are different that might be the issue.

First of all, replace your vacuum hose. It's cheap to do - take your hose to an auto store and buy a length of the same diameter. That way you rule that out.

If there's still no improvement, have you balanced the carbs? If so, and the problem is still there I'd be thinking the odd butterfly / carb might be the issue.

[Nessism]

The odd butterfly won't cause this issue. There is a vacuum leak somewhere. The air filter must fit the airbox. On my old 1000S removing the air filter caused a high idle.

[Guest]

Ok. So I've been planning to fill that gap between the air filter and the air box; this would seem to be the perfect time to do that.

And if that doesn't fix the problem, do you know of a good way to find a vacuum leak? I've heard others on this forum say that using WD-40 isn't sufficient.

[Burque73]

What about propane. Like a torch just don't light it. Wave around various locations till it gets drawn in.

Smoke or vape thing if you've got that on hand. Slowly gets sucked in. Just a couple of thoughts.

[hillsy]

You can use brake cleaner or starter fluid - spray it around the intake manifolds and listen for increases in RPM.

You will want to balance the carbs as well. At least bench sync them.

BTW - when you said carb 3 is from "a different bike" did you mean the same model?

[Guest]

You can use brake cleaner or starter fluid - spray it around the intake manifolds and listen for increases in RPM.

You will want to balance the carbs as well. At least bench sync them.

BTW - when you said carb 3 is from "a different bike" did you mean the same model?

The carbs were bench synced by the forum member who rebuilt them, and then he did a vacuum sync, and then installed them on his own 850 and got them perfect before he shipped them back to me. That's what's so frustrating about this.

As for what model the #3 carb came off of, I got it from yet another forum member, and I think he said it was from a GS1000 - though I'm not certain of that. Since it's the same diameter of the other three, no one can figure out what those numbers refer to.

[hillsy]

OK - have you ever had this bike running right? Is this a new problem per se or has this been happening since the start?

Assuming the carbs are OK, have you checked the ignition timing / distance from the rotor to the pick-ups?

[Guest]

OK - have you ever had this bike running right? Is this a new problem per se or has this been happening since the start?

Assuming the carbs are OK, have you checked the ignition timing / distance from the rotor to the pick-ups?

Hi Hillsy. Thanks for the suggestions. No, the bike has never run "right". It has been close, but never right. When I bought it last December, it had a gaping hole in one of the intake boots (so of course it wouldn't run at all), so I replaced all four of them with new OEM items. I really don't suspect the ignition elements because once I get the bike off of idle, it pulls pretty strongly. Plus, the bike DID idle normally before this last rebuild, but had other carb-related issues - which is why I had them rebuilt a second time. So the high idle issue is new, after this most recent rebuild. I'm going to seal my air cleaner element, and if that doesn't fix the problem, I'll start checking for vacuum leaks. There IS an answer to this - as frustrating as it may be.

[2BRacing]

Have you tried backing out the idle speed adjuster just a bit?

[Guest]

Have you tried backing out the idle speed adjuster just a bit?

The idle speed adjuster has no affect on the idle at this point - which would seem to point to a vacuum leak somewhere.

[londonboards]

Here's something about the butterflies; the number stamped on them refers to the cut of the sealing surface. The butterfly valves are sort of egg shaped and they need to completely close a perfectly round hole. They do this by being fully closed at about 3 degrees off horizontal. The edge of the valve is champhered in order to effect the seal.







Now if the butterfly was changed and placed into a carb body in which the champher did not match the internal bore of the carburettor, then that butterfly would not close properly. That could be one issue. I would test this by removing the plug on cylinder 3 and seeing if you get the same high idle. Obviously on 3 cylinders it will not idle as well but I'm sure you would be able to tell if it had the same urge to spin higher.

Then the other thing to check is that the carb no.3 that you say was changed over has the same internal diameter at the engine end and that it is jetted the same as the other carbs and that the float height is correct.

If all that is good, remove the vacuum hose completely, turn gas to Prime, put your thumb over the vacuum hole and try that. It night still be that the vacuum hose or indeed the petcock itself, has problems.

[Guest]

So now we know! But I'm not clear on what you mean by "cut of the sealing surface". Does the number refer to the angle at which the chamfer is cut? And knowing this, would you expect a valve that is stamped "135" to be fully compatible with three others that are stamped "130"?

I did notice when I had the idle adjustment screw backed out far enough to be disengaged from the adjuster on the #3 carb, that the valves kind of got "sticky" when they were fully closed. Could the one "135" valve cause this?

[2BRacing]

I would think that the whole carb was replaced, instead of just a butterfly being swopped into an existing carb body. Therefore the "135" butterfly would be matched to that carb, and not cause the high idle problem.

The part that I find interesting, is the fact that you say the set of carbs was synced on another bike. I know that the particular forum member who rebuilt the carbs would have synced them perfectly on his bike, but that is not to say the situation is still the same on your bike. Most importantly, airflow through the carbs is affected by valve clearances, and therefore will not be the same on your bike compared to his.

I would suggest that you first check the valve clearances on your bike and then vacuum sync the carbs on your bike.

[Guest]

I would think that the whole carb was replaced, instead of just a butterfly being swopped into an existing carb body. Therefore the "135" butterfly would be matched to that carb, and not cause the high idle problem.

The part that I find interesting, is the fact that you say the set of carbs was synced on another bike. I know that the particular forum member who rebuilt the carbs would have synced them perfectly on his bike, but that is not to say the situation is still the same on your bike. Most importantly, airflow through the carbs is affected by valve clearances, and therefore will not be the same on your bike compared to his.

I would suggest that you first check the valve clearances on your bike and then vacuum sync the carbs on your bike.

Yes, the entire carburetor was installed, since the issue was the broken float tower in the original carb.

And yes, I am aware that the settings of the valves has a large influence on how much air the engine draws through the carbs. I have only checked four of the valves on my bike, and since the smallest gauge on my set was .002", I was only able to confirm that the four valves I checked were not too loose. Can there be that large of a difference between HIS bike and MY bike that it creates this problem?