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carb question

  • Thread starter Thread starter sissonk
  • Start date Start date
S

sissonk

Guest
I have a 81 GS850 been struggling with getting it to run past 2500 rpm , but have noticed that I can not get it to idle until I get the mixture screws out 3 1/2 to 4 1/2 turns and then using a color tune plug the further turning does not make any difference to the adjustment . some cylinders run reasonably smooth but 2 miss fire regularly .If I turn them in it just miss fires and backfires more . I had the exhaust valves and guides changed and did a little surface grinding in the inlet tracts as well , but only the surface . Before I did the head it idled perfectly . This is a US import bike into Ireland .
This is my question, in the Haynes manual , section for the carb says for the US model the pilot jet is 40 , pilot air jet is 190 , no value for the mixture screw. For the UK bike the pilot jet is 42.5 , pilot air jet is 200 and 3 1/2 turns is given for the mixture screw. Also the main and needle jets are different.
Anyone know why these are different? And could my grinding of the inlet port have made the engine so that it needs larger pilot jets to get the screws into lower area so the the adjustment makes a difference ?
 
The different jets were because the U.S. EPA (Environmental Protection Agency) was mandating leaner running for pollution control. The rest of the world had the better-running setup.

Depending on just how much you took out of the head, it should not affect mixture at low speeds. Actually, what you are describing sould a LOT like you are trying to run the bike without an airbox on it. :shock: You either need an airbox or individual pods installed to make it run right enough for jetting checks. A quick option just to see if it runs If you have NO airbox installed), is to fold a shop rag in half, drape it over the carb inlets. Clamp it around the throats of carbs 1 & 4 with tape, hose clamp or wire tie. This smooths the airflow enough for it to run for a quick check, but do not rely on it for jetting adjustments.


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Thanks Steve,
Yes I have the air box on and the vac line plugged. Do you think if I change the pilot circuit to the UK values that it would run better and get the adjustment down to say 2 1/2 turns ?
 
With your screws out that far, going to the UK-spec jets would help, but maybe not enough. Going from a 40 to 42.5 is only a half size, might want to go to a 45. The 200 air jet might be OK, but I am not really the expert on fine-tuning the carbs.

Hope Keith Krause tunes in to this thread and sheds some light. 8-[


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Thanks again Steve , I think I will try to change to the 42.5 jet . Can't think of anything else yet .
 
This may be obvious to you, but have you checked compression since the head has been re-installed? Low compression (most commonly due to insufficient valve clearance or poor valve sealing) will kill any chance of the bike idling properly, no matter how much you mess around with the carbs.
 
nabrams said:
This may be obvious to you, but have you checked compression since the head has been re-installed? Low compression (most commonly due to insufficient valve clearance or poor valve sealing) will kill any chance of the bike idling properly, no matter how much you mess around with the carbs.
Click to expand...
Ditto! Did your new valves seat. Probably not carb related. Do a compression check.
 
Yah dun that , compression on all cylinders is 160 -180 psi . And checked valve clearances 2 and 4 cylinder were out of spec , changed the shims and engine did run abit better, I will change the pilot jets over this weekend and see what that does .
thanks for your help.
 
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