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carbs or ignition, not sure

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My 80 gs1100 has run flawlessly for years until this past week. I have a miss in the engine from idle to about 3500 rpm. The cylinder seems to kick in when you bring the revs up past that. If you hold it steady on the road in 3rd gear at about 3000 rpm, it spits and pops and backfires. If I accelerate it seems ok. I thought it was a carb issue so I completely disassembled them and ran them through my ultrasonic.....no difference.
I thought maybe a plug was dead so I replaced all 4. Nope. I also checked the plug wires and all wires coming from the coil, nothing.
I'm baffled. Any ideas?
 
did you check to see if it was a plug cap they have resistors in them and fail from time to time
 
I've had weak coils do this. Coils have been causing my biggest problems lately. I think i'm down to tuning the idle mixtures now. Could also be your idle/low speed fuel mixtures aren't set right where your engine needs them. Where are the idle mix screws set at?
 
Plug caps are dyna one piece in good condition. Coils are Bandit that I modded to accept the new wires a few years ago. Idle or pilot screws are set to highest idle method
 
If you read my original post you will recall the bike suddenly started missing and running rough.

Ok, here's the latest. I rebuilt the carbs again and it still won't run on all 4. It runs really rough and pops back through a couple of the carbs. But, if I put my finger over the #3 and #4 pilot air jet screw holes, the idle picks right up and smooths out and seems to be running on all 4.
What is this telling me?
 
I was hoping to get some suggestions on where to look for my issue. It appears to be definitely fuel/carb related.
 
It seems to me that it's electrical. Can you swap coils to see if the problem moves with the coil.
How about putting a timing light on the problem cylinder.
Are the valve clearances within specifications?
Broken valve spring?

It's easier to ignite a rich fuel mixture with a weak spark than a lean one.
 
If putting your fingers over the mixture screws causes a change, I would be ordering an O-ring kit or check to see if you left them out.
 
rustybronco said:
It seems to me that it's electrical. Can you swap coils to see if the problem moves with the coil.
How about putting a timing light on the problem cylinder.
Are the valve clearances within specifications?
Broken valve spring?

It's easier to ignite a rich fuel mixture with a weak spark than a lean one.
Click to expand...

I've been running these 36mm 1150 carbs for several years now. I've ultrasonically cleaned them twice in the last week. All orings are new. Just for laughs I put my old carbs back on and the problem disappeared. That's why it's carb related.
 
Do your old carbs run richer than the ones you are currently running?
 
rustybronco said:
Do your old carbs run richer than the ones you are currently running?
Click to expand...

I'm not sure, since the old carbs aren't set up for this motor. All I know is going back to the old carbs made the engine run better. It seems down on power but I think that can be attributed to the increase the 36 mm carbs gave me over the old 34's.
 
You're right, it's either carb (#4) or ignition. Now to remove the coil from the equation. It's too bad you don't have access to an old automotive 'scope that shows the firing voltages.

Did you pull the plug(s) to inspect when the bike was misfiring?

The backfiring make's me think it's electrical.
 
Last edited:
rustybronco said:
You're right, it's either carb (#4) or ignition. Now to remove the coil from the equation. It's too bad you don't have access to an old automotive 'scope that shows the firing voltages.

Did you pull the plug(s) to inspect when the bike was misfiring?

The backfiring make's me think it's electrical.
Click to expand...

I'll definitely consider electrical. Testing the coils will be next then.
 
OldVet66 said:
If putting your fingers over the mixture screws causes a change, I would be ordering an O-ring kit or check to see if you left them out.
Click to expand...

+1 with checking the mixture screw O-rings. You should see NO change in engine running characteristics by simply putting your finger over the fuel enrichment screw towers. Your pulling in air from there and that means that the sealing O-ring/Washer and spring are not setup correctly. The order from bottom to top should be O-ring first then washer and then spring. By swapping out the carbs and having it run better you've verified that it's a fuel issue as well.
 
Solved, I think. Using my multimeter I get no reading from the 2-3 coil. Those are the cylinders that have the cooler headers than 3-4.

I am using Bandit coils but I'll use originals if they aren't available. Anyone have a spare coil laying around that they're willing to ship to Canada?
 
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