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Not another Carb thread!!

  • Thread starter Thread starter cjanderson_90
  • Start date Start date
C

cjanderson_90

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Ok i request your guys assistance. Im trying to tun my bike to run better than it currently is. I have done full soak and rebuild of them, all new rubbers in the carbs, new intake boot orings, re sealed the air box. Doing everything I can to get this puppy running. Its a '79 GS750 with VM carbs. I set the bottom screw adjustment to factory of 3/4 turn out, then set my side screw to 1.5 turns, then did the "screw in it until the idle stops going up, then turn 1/4 back" After I have done that, the bike no longer idles(the idle can change from 1000 rpms to 2000 rpms, depending where its set, after I rev it), and I cant get it to go past 6k RPMs. It has the stock airbox, and a 4-1 of an unknown brand exhaust. I know the main jet went up one size from stock but i cant remember the exact jet size right now. Other than that, its stock. Any ideas? I just put a brand new petcock on it as well.
 
I am with Steve on adding another bit to the bottom pilot screws. And I would set all the mixture screws ( side ones ) at 2 out for beginning.

Is there an inline fuel filter installed in between the petcock and carbs?? If so its not needed and can actually not flow fast enough to keep up with the engines demands.

Have you done the valves and used a set of gauges to check the carb syncs???

What are the floats set at and how did you measure them???

How is the air filter? Clean or dirty??

And lastly, heres your service manual. http://www.mtsac.edu/~cliff/storage/gs/GS750_Service_Manual_early8valve.pdf
 
If the new petcock isnt stock and isnt vacuum actuated you MUST cap off the vac port on carb three to eliminate the massive vac leak its being open will create.
 
Yes the new petcock is vacuum operated and its on carb #3. No in line filter. I just put new intake boot orings in but did check all boots and they looked fine. Also sprayed some carb spray around them to see if I had a leak and nothing changed. Float heights were checked per the manual with a caliper and they are all in spec. I'll try the bottom screw out another 1/8 turn and set the side mixtures back at 2 turns out again. Air filter is clean. Valves were all set to spec. I have only bench syncs them but I doubt that would cause this big of an issues.
 
Sounds like its all done to spec so I cant see why its acting up other than needing more fuel and a good vac syncing.
 
Ill do some tuning of the bottom screw tonight. And ill double check my float heights although I only remember having to adjust one. Would needle position cause this many problems?
 
Oh my god. I just remembered that I never hooked up my crankcase breather back up to the airbox. Thats a HUGE air leak. Sorry fellas. I are dumb. Thats big enough to make this scenario right?
 
Hmm ok. I figured that was it since it was pulling in fresh air. I guess ill mess with the fuel mixtures a bit more.
 
All the hose does is suck blow by gasses from the crankcase and sends it back thru the carbs and it reburned. Just a motorcycles version of a PCV valve on a car.
 
I may have to make a float bowl height checker thing to make sure i actually set them correctly. This is very confusing
 
Ok but I do have a question. Would adding exhaust warrant a needle position change? Or just main jets? I really hope to get this thing going tonight as its been to nice to not be riding it!
 
A lot would depend on what exhaust you put on there. If it is rather free-flowing (loud), you will probably need to change the needle a bit. It will also require larger main jets than a quieter pipe.

The reason for the difference is directly related to how much air is flowing. By removing the restrictions of the airbox and or stock mufflers, you are allowing more air to flow, which needs more gas. This has a greater effect at, or near, wide-open throttle. Mid-throttle positions will put the restriction in the carbs, not at the intake or exhaust, so jetting changes (needle position) will be minimal. That is also why the pilot jets don't need to be changed, because there is simply no more air flowing at idle than there was before.

.
 
Its a fairly free flowing exhaust. I honestly have no idea what kind it is, it has a turn down tip and is black. Thats all I can see from it. I know the main jet was bumped up one size. I just dont see why it cant rev past 6k, and when it gets there it sounds ultra crappy and lean. But i havent had time to adjust the idle circuit yet. Ill do that, reset the air mixture screws and go from there.
 
OK ive set everything to what you guys suggested and we still have an issue. Still wont get past 3.5k revs. I have no idea where to start here....
 
Just got past 4.5k but then it makes a terrible bogging sound, almost like its too lean. So from like 1/4 to full throttle, basically nothing happens
 
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