A diagnostic challenge

[flyingace]

There are three of us with the same year/model (1983/GS850) with an identical problem. The bike runs fine for a while with a new plug, then the #4 spark plug gets black and sooty and the bike idles poorly and stutters coming off idle. The problem follows the #4 cylinder after swapping the #1 and #4 plug wire and plug. It happens to new plugs as well as old.

Between the three of us, we have gone thru the ignition system, cleaned&synched the carbs, checked the intake manifolds - just about everything we can think of. It is always the #4 cylinder on all three bikes.

Does anyone have any ideas what it can be? If not, heard of anything similar involving a particular year/model.

Thanks in advance,
Ace.

[LW_Icarus]

thats odd, maybe one of the grizzled old vets knows of something, but offhand Id say I cant think of any one unique thing with the #4 cylinder that would make it any more prone to foul than any other.

but it sure is an odd coincidence that 3 identical bikes have identical problems.

[flyingace]

LW,
Thanks. Something just occured to me about this. I wonder if it's possible the #4 carb's choke is not getting turned all the way off. Since it's at the end of the bank of carbs, if the mechanism that shuts the choke off was placed too far left (or right?)...

This would explain why cleaning the heck out of the carbs doesn't help.

I may try removing #4 from the choke circuit.

Ace.

[earlfor]

Re: A diagnostic challenge

Did the same person tune and synch all three bikes or give instructions to the other two?

Earl

There are three of us with the same year/model (1983/GS850) with an identical problem. The bike runs fine for a while with a new plug, then the #4 spark plug gets black and sooty and the bike idles poorly and stutters coming off idle. The problem follows the #4 cylinder after swapping the #1 and #4 plug wire and plug. It happens to new plugs as well as old.

Between the three of us, we have gone thru the ignition system, cleaned&synched the carbs, checked the intake manifolds - just about everything we can think of. It is always the #4 cylinder on all three bikes.

Does anyone have any ideas what it can be? If not, heard of anything similar involving a particular year/model.

Thanks in advance,
Ace.

[flyingace]

No. I started a thread about this problem a while back. Since then, 2 other users in different parts of the country have written to describe virtually the identical problem with their 83 850.

[earlfor]

Considering what you said you've already done, I would check compression, valve clearances, the seal on the rubber plug on the jet in the float bowl and the float height setting.

Earl

No. I started a thread about this problem a while back. Since then, 2 other users in different parts of the country have written to describe virtually the identical problem with their 83 850.

[flyingace]

Thanks Earl.
The compression on my bike is low - but #4 is the highest of them all (128). Also, I seem to recall the compression readings from the other users were better than mine. The float height is OK on mine, and I think the others checked that too. I don't remember a seal/rubber plug on the jet. I'll look again next time I'm in there.

I think my next step is to remove #4 from the choke linkage and see if the condition improves. If not, then look into swapping #3 and #4 carbs to eliminate or identify the #4 carb as the problem.

I think this is some kind of marginal factory defect that doesn't show itself until the bike is old and tired.

[earlfor]

On my bike, I cant swap the #3 carb with the #4 carb. There are fuel fittings on #3 on both sides. On #4 there is only a fuel fitting on the left side of the carb.

Earl

Thanks Earl.
The compression on my bike is low - but #4 is the highest of them all (128). Also, I seem to recall the compression readings from the other users were better than mine. The float height is OK on mine, and I think the others checked that too. I don't remember a seal/rubber plug on the jet. I'll look again next time I'm in there.

I think my next step is to remove #4 from the choke linkage and see if the condition improves. If not, then look into swapping #3 and #4 carbs to eliminate or identify the #4 carb as the problem.

I think this is some kind of marginal factory defect that doesn't show itself until the bike is old and tired.

[flyingace]

That's disappointing news, Earl. I'm sure mine is the same now that I think about it. I may try to jury-rig something temporary that will get fuel to all the carbs just to help diagnose this problem.

Are there other differences you can think of between #3 and #4 carb?

Thanks again,
Ace.

[earlfor]

ON bikes with CV carbs, the #3 carb is normally the control carb for a carb synch. #3t has no vacuum adjustment. #4 is adjustable.

Did you already synch your carbs using vacuum guages connected to all 4 carb simultaneously? A manual synch will not do and neither will trying to set vacuum levels with moving a single guage back and forth.

Earl

That's disappointing news, Earl. I'm sure mine is the same now that I think about it. I may try to jury-rig something temporary that will get fuel to all the carbs just to help diagnose this problem.

Are there other differences you can think of between #3 and #4 carb?

Thanks again,
Ace.

[flyingace]

Yes, I used a Motion Pro carb synch tool. It shows all 4 readings at once. All vacuum readings were within 1/4 MM/Hg of each other. The other users all synched their carbs too.

I'll just have to look into how difficult a chore it will be to swap carbs (if it can be done at all).

Thanks again Earl.

[earlfor]

its possible someone may have broken off a needle and drilled out a pilot jet, but it seems odd that it would happen to three people at a time.

Earl

Yes, I used a Motion Pro carb synch tool. It shows all 4 readings at once. All vacuum readings were within 1/4 MM/Hg of each other. The other users all synched their carbs too.

I'll just have to look into how difficult a chore it will be to swap carbs (if it can be done at all).

Thanks again Earl.

[NickNoo]

Hi all.
I am one of the trio that has had this problem of rich #4 cylinder. Flyingace and myself have been discussing it via email in the hope that we could jointly solve it.
Well, I have just today solved the problem on my bike and its now running perfectly from idle to redline. All plugs are nice and clean for the first time since I got the bike, so I am well pleased.

The problem was finally traced to a "loose" pilot jet.

After taking the bowls off the carbs and removing the black rubber plugs from each pilot jet passegeway, I measured the depth of each pilot jet from the lip of the passageway. #4 was about 2mm higher than the others - thats about one complete turn of the jet !

Maybe the thread on this carb was damaged when I first removed the pilot jet for cleaning ? Anyway, all it took was one turn with the screwdriver and the jet was properly seated.
Its hard to believe that this tiny adjustment cured the problem, but I guess it just shows how sensitive these pilot jets are.

[flyingace]

Nick,
Congrats on finding this out! Wouldn't it be weird if some guy in the Mikuni factory was just goofing off one day in 1983 and just didn't screw in the pilot jets of a bunch of carbs!

I'm wondering if we have the same carb types - you being from the UK and all. Are yours the 32mm Mikuni CV type? I just don't remember any rubber plugs covering any jets or passageways. But I was only inside the carb once, so I could have missed it.


Ace.

[NickNoo]

Ace,
Yes, they are the 32mm Mikuni CV carbs.
I think the loose pilot jet may have been my fault as I remember that it was difficult to remove when I first dismantled the carbs for cleaning.
I may have scored the thread and this would have caused the bad seating problem.
However, these carbs bodies are incredibly complex and I did wonder if there may have been a tiny fault in the casting that resulted in the pilot jet being a tight fit. Just a thought !
Anyway, it would be interesting to see whether your problem is the same as mine.

Nick