Is it carb or is it ignition?

[volks28]

83 850 GS
Any tips on diagnosing carb vs ignition issues?

I have a cyl #4 that the spark plug is sooty black. (all other plugs are light gray, but not white). I have a feeling that the #4 cyl is not always firing. I am wondering how I can troubleshoot if it comes from the carbs or from the ignition side? The bike starts and runs great, but had a slight hesitation at low RPMs. I already swapped plugs and caps and it follows the cyl.

[nert]

you could swap coils, complete with the plugs. that would eliminate ignition. You should do a compression check across all (4) cylinders to be sure they are mechanicly capable of good burn. #4 may be a little low on compression if its been firing poorly enough to wash down the cylinders. However once the fuel problem is corrected, #4 should return proper compression equal to the other three. You could have a too high fuel bowl level for the number 4 carb, or leaking needle and seat. You maybe able to detect that by removing the air cleaner assembly, setting the fuel cock to prime (engine off) and rocking the bike slightly side to side, and check for visible evidence of an overrunning #4 carburetor. OR could be baddly out of sync.

[Anonymous]

I have seen this in the past and almost always been the carb boots try spraying some wd40 at the front of the boot to see if you are sucking in air?? the rpms should rise if you are

[volks28]

No leaks, the boot rings are new. I think it would create a lean condition

I have seen this in the past and almost always been the carb boots try spraying some wd40 at the front of the boot to see if you are sucking in air?? the rpms should rise if you are

[flyingace]

I had an identical problem on my 83 850L. You can diagnose the problem by swapping the #1 and #4 plug wires. If your problem switches to cylinder #1 - it's electrical.

If not, it's the #4 carb. First, clean out your pilot mixture screw -this is on top in the front (head side) of the carb. Count the #turns in from the current setting so you can put it back just like it was. Spray carb cleaner into the hole and on the mixture screw. Blow out with compressed air if you have it.

If this doesn't work, the next step is to check your carb synch - the idle can be set higher (richer) on #4, but the engine isn't turning fast enough to use it, so extra fuel gets dumped into #4.

Then check for the integrity of the intake from airbox to cylinder head. Make sure there are no holes or cuts.

If that doesn't work, you'll need to dismantle #4 carb and check the float level and needle/seat to make sure you have the correct level of fuel in the bowl. Then take everything apart that comes apart and spary with carb cleaner and then compressed air.

Hope this helps,
Ace.

[volks28]

Thanks Ace, that's awesome!

I just did a sync, they all are within the same cm at 1k and 3500 rpm

Quick question, can I unscrew the mixture screw while everything is installed or do I have to take the carbs out?

Many thanks

[flyingace]

You probably need to take the tank off to really get to the pilot mixture screws. It's worth doing because of the need to count the number of turns to seat the screw ACURATELY before removing it.

One thing I left out was compression. I have low compression, but #4 has the most compression on my bike, so I ruled that out. Have you checked compression on #4 compared to the other cylinders?

For what it's worth, I really think dismantling the carb and cleaning it was what did the trick. It's just weird that it happened to your #4 carb too.

Ace.

[flyingace]

Volks,
I'm sorry to report my problem has returned. It was doing fine, then all of a sudden - poor idling and sooty #4 plug. It seems to happen when I'm doing stop and go driving, not so much at open throttle.

I hope you didn't spend to much time following my non-functional advice.


Did you have any luck resolving yours?

Thanks,
Ace.

[volks28]

Nope, however it has improved a bit. I changed caps (new), new spark plugs and adjusted the idle a bit.

My problem seems to be in the idle circuit, since the bogging occurs within the 1/8 throttle range. I think I will rebuild my carbs this winter.

For now, rich is better than too lean and willing to live with it.

[flyingace]

I resynched my carbs and it seemed to improve. It seems to happen when I drive around town 2-up. I need to keep the RPMs higher with a passenger on board.

I agree this is more of an annoyance than a problem. Let me know if you discover anything and I'll do the same.

Happy riding,
Ace

[duaneage]

Make sure that the float is not sticking open due to a dirty needle seat, this will cause fuel overflow into the carb, especially in traffic. Cleaning the needle seats in the carbs by spraying carb cleaner down the fuel line may do the trick.

Happened to me.

[flyingace]

Thanks Duane,
Both of us have been there and done that -but no T-shirt. I dismantled the carb and sprayed every orifice with carb cleaner and then compressed air. The needle and seat are operational and float levels OK.

RIght now my bike is running great, so further diagnoses is difficult. Sometimes I wake up at night wondering what is could be...

Ace.

[volks28]

Keep me posted ok? Sounds like I may have to do the same

RIght now my bike is running great, so further diagnoses is difficult. Sometimes I wake up at night wondering what is could be...

Ace.

[flyingace]

I'll do that. I'm convinced now it's a marginal situation with running rich in the idle or pilot circuit or both. Under normal conditions, everything is fine, but symptoms start when I lug the engine under a heavier than usual load. At this stage of the bikes life, better to just keep the revs up.

Good luck,
Ace.