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78 gs750 carb issue

  • Thread starter Thread starter mac65
  • Start date Start date
M

mac65

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Hello all just found your forum was looking for a little help with my gs750 1978 vintage it has an 850big bore top end had the head checked out at local machine shop new valve seals installed carbs very clean and new o-rings installed but cant find anything onair needle? On bottom of carb my manuel says factory set dont jack with it but that is not how Iplay the main problem is crappy bottom end once you get going it runs out pretty good it has k&n filter in factory airbox factory exaust 110 lbs compression across the board +or -1-2lbs valves set to spec new points plugs and wires gapped and timed per clymers good hot battery starts and idles nice andpulls strong above about 3000rpm but crappy upto that also has the green dyna coils installed good blue spark on all four check onsparkboard would jump almost a1/4 inch 1 2&4 plugs are tanning nicely but three always seems alittle wet thats all Ican think of right now so close yet so far away! Anything worth doing ,is worth doing twice?This thing is making me crazy please help.Thanks in advance for any info
 
Number 3 may be why it's running bad at lower rpm's. If stock, the fuel petcock may have a failing diaphragm leaking fuel into the cylinder via the vacuum line.
Turn the petcock lever to PRIME, test, and see if cylinder 3 remains wet. To be sure of no fuel entry through the vacuum line, temporarily plug the vacuum line or remove it and cap the carb vacuum inlet.
If the petcock is good, (and the carbs are truly clean) then be sure the carbs side air screws are adjusted using the "highest rpm method". Initially set side air screws to approx' 1 1/2 turns out, warm up fully, set idle to factory recommended idle (no higher), slowly turn a screw in either direction until you hear the rpm's max out, stop, then return the idle to factory recommendation by using the idle adjuster knob. Repeat to next carb until all are done.
The above assumes the pilot fuel screws (underneath) are set reasonably close already. Somewhere between 1 and 1 1/2 turns out from lightly seated for most bikes.
The carbs must also be bench synched well (for starting purposes), then the air screw adjustments, and then synced with a vacuum tool. A vacuum synch is basic maintenance that must be done.
 
Definately check the petcock, The compression is a little low? maybe due to the 850 top end ? Was it rejetted for the pods ??? If the motor is sound ? I would think about maybe surfacing the head about .030 thousands to bump the compression and degreeing the cams would help take advantage of the 850 kit
 
SqDancerLynn1 said:
Definately check the petcock, The compression is a little low? maybe due to the 850 top end ? Was it rejetted for the pods ??? If the motor is sound ? I would think about maybe surfacing the head about .030 thousands to bump the compression and degreeing the cams would help take advantage of the 850 kit
Click to expand...
Lynn..you seem versed in the 750/850 mod, so I will ask you (sorry for the hijack) I am about to do the same to the cafe motor, as Mike Riddle just purchased a 79 850 parts bike. I bought the motor off of him for this very reason...

When you say get the head decked minus .030 is that inches or mms? Also, besides new rings and a hone job, supposedly this is a drop on mod..any advice otherwise? you think 106/107 on the stock cams would punch it up a lil??
 
Thanks

Thanks

:clap:Thanks guys problem solved main thing i found was #3 choke cocked a little not letting it shut off all the way therefor wet plug and no bottom end, now dialed in and tanning all 4 plugs nicely ,I was thinking that compression might come up some with a few miles on? Saved from the crusher back on the road po just gave up a little to soon Thanks again Mac:D:D
 
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