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Need carb tuning help

  • Thread starter Thread starter tfholly
  • Start date Start date
T

tfholly

Guest
Hello,
I just rebuilt the carbs on my '82 GS850. It was a one-week long saga due to stripping out the air screw on one carb. I had to find a donor carb to replace the damaged one. It is all back together and the bike started easily. I experimented with the mixture screws. Best seems to be about 3 to 3.5 turns out. It wouldn't idle at 2.5 and was missing at 4.0.
The bike now idles smoothly and runs well with low throttle. Still runs OK at higher throttle but seems starved for gas at higher throttles, and lacking in oomph at full throttle. When at full throttle and giving it full choke, it surges forward.
I used the following rebuild kit:
K&L Kit 18-2591
One Kit fixes one carb,
Kit includes:
includes float gasket; Y99 slide needle and clip, 2.0 float needle and seat; air screw and spring; #180 main jet, #190 air jet. #115 air jet and #40 slow jet; misc packing and gaskets and "O" rings as applicable

Are my jets the right size? Or can the fuel flow just not keep up? Could it be the petcock? Any insights would be appreciated.

Tom
 
Do the sliders move up and down smoothly with no hangups?
Have you done a sync? Did you check the float levels?
Did you follow the cleaning instructions on this site? In particular
did you remove and clean the needle jet and all the little holes.
Have you checked the boots? Boot o-rings? You need a good
seal at each stage of the intake. Can't let any air get in.
 
I completely disassembled the carbs and put in all new jets and o-rings, etc. I made sure the passages and holes were clear. The intake O-rings were recently replaced and I replaced the clamps and rear-boots to the airbox. I think there aren't any leaks.
I used my mercury sticks and did a synchronization successfully. It runs smoothly, just start to have troulbe at higher RPMs.
 
I had a restriction at an inline fuel filter on my bike. It would run great at part throttle but would starve at WOT for any length of time. It's easy to figure out if you have a petcock issue: can you apply WOT smoothly for a few seconds? what happens when you hold it open for longer than a few seconds? If its the petcock, there should be enough fuel in the bowls for a quick opening and closing of the throttle, but if you hold it open for too long it will starve and eventually stall. Once it stalls, there won't be any fuel in the bowls to start up again until you re-prime it. I found that this is more likely to happen when I'm on reserve (or close to it).

What size main jets did you take off the carbs to replace with? It sounds like you replaced the slider-needles and springs. Was there a problem with the original parts? Maybe you can try putting the originals back (at least you can do this without removing the carbs).
 
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Did you check the air hoses for the carb vents? Try adn remove them. Are we DARN sure the floats are set correctly?
 
To tell the truth, I didn't check the float height. I didn't realize they were adjustable. Reading some posts here, it seems like you have to bend the metal part that pushes on the needle. I just put back all the floats as originally. The needle valve assembly looked slightly different from the original. It had a little filter at the bottom. It all fit well and popped right in, though.
I didn't try WOT for very long. The revs do go up high, just not as fast as they might. Not developing maximum acceleration. I will try replacing the petcock diaphragm.
 
Ok, here's an update. I took it all apart and carefully set the float levels. The running improved (seemed smoother, overall), but still fuel starved, and improved with choke.
I tried to put the petcock to prime. There was a big improvement. Almost ran great. Put it back to normal and it was fuel starved again. I ordered a diaphragm kit for the petcock. Should this fix it? Definitely seems like a fuel supply, not a carb problem now.
Thanks for the suggestions so far.
 
petcock will do it. so will a plugged gas tank cap vent. try it on prime with one gallon of gas and loosen the gas cap. if it picks up a bit clean the vent in the cap. another test is to have a gallon of gas in the tank, swish it around a lot, wait a few seconds and then slowly open the gas cap. If a great deal of pressure comes out (watch your eyes) then the vent may be blocked. You should hear the vent hissing after the shake test, relieving the pressure.
 
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