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Carb Troubleshooting Help. 1979 GS850

E

emlivings

Forum Apprentice
Recently cleaned the carbs out over the winter and bench synced and vacuum synced.
I am noticing some lurching at 3rd gear around 50mph and 5th gear around 75/80mph. When i accelerate in 3rd gear and 4th gear my bike backfires a bit.

I am still pretty new to this so I may be potentially omitting information. Anyone who might know the right questions to ask to pinpoint where to start first?
 
where are the mixture screws set at? General start point is two turns out from lightly seated. Also be sure you didnt accidentally pinch up a diaphragm when putting the tops back on.. I've done it.
 
The idle air screws are set about 1.75 turns. These are the VM26 slide carbs there was no diaphragm when i disassembled.
Would the idle screws affect cruising speeds?
 
Check the newbie mistakes thread linked in my signature for ideas
 
Did you clean the carbs per the tutorial?
Is the airbox on? Sealed?
 
Close the pilot screws to 7/8 out. they are way too open now. Side air screws at 1 1/2 to start. set pilots and lease them alone. adjust mixture with air screws only.
 
Another thing that can contribute to the issue described, is foam on the air filter frame and/or the foam on the airbox end caps. The foam deteriorates, then false air enters the system, causing lean running. The fix is to purchase high density weatherstrip foam from the hardware store, and make sure everything is sealed air tight.
 
I could look into the airbox seals. I know that the rubber seals are slightly torn a bit but i kinda just squish them in there and tighten it down.

I cleaned the carbs per the bike cliff archive guide.

One thing I forgot to check is the pilot fuel screw. I think i had it set at about 7/8 turn out from lightly seated when i bench synced the carbs. It is literally on the underside of the carb so i wanted to set it and forget it when i put everything back onto the bike before vacuum sync.

I have a few photos of the spark plugs too which is looking like a lean condition.
Still need to do a compression test to rule out tight valves.
 
The color of a spark plug doesn't tell us a whole lot. You need to do plug chops, where the bike is run for several minutes under the same conditions that are creating the running problems. Immediately pull over, allow to cool, then remove the plug. Then you will have some meaningful color to analyze.
 
Colortune is perfect on VM carbs. Just adjust air screws till the blue flames seen and done. Just looking at the plugs as they are shown id try 1/4 turns in on the air screws to richen it up. just remember this .. IN = ENRICHING THE FUEL MIX. OUT = LEANING THE FUEL MIX
 
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