Question about 1977 Suzuki GS 750

2 Answers

High idle on start up with choke off.

When i first start this 1978 gs750 the rpm hang at around 3,000, with the choke off. after 30 seconds or so the rpms slowly come down to 1200. the bike seems to be running really rich at idle (smells bad and burns your eyes to be around), but if i try lean out the air screws the idle will just hang. head is rebuilt, second set of carbs rebuilt, new rings, petcock rebuilt, new points and condensors, nothing has helped this problem. past idle the bike runs strong, doesn'tsmoke and doesn't seem to be running rich higher in the rpms. please help me i'm starting to loose my hair!

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  • 6proudfeet Jun 15, 2009

    thanks for your response. i appreciate your sugestions. i have checked and double checked the intake boots, air box, and carbs for any air leaks. the boots are in good shape, no cracks, clamps are tight, carbs are inserted in boots good. i also just lapped the valves, they were in good shape before and are in better condition after i lapped them. i installed new valve guide seals and of course a new head gasket when i reinstalled the head, along with new piston rings. the problem was there before and after the top end work. i also installed another set of carbs and rebuilt them with new rebuild kits from k&l. problem was there before and after carb swap. carbs are set at 3.5 turns on fuel screw and i can't set the air screws(screw towards back of carbs} out any furter than 1/4 turn or the idle will hang even after engine warms up. with the settings that i have the carbs at now the idle doesn't hang after the bike runs for 30 seconds or so, but the fumes this bike puts off are overwelming at idle. i also synced the carbs after rebuild and reinstalling. if i'm missing something then i'm missing something, but would there be any other possibilities. i've rebuilt 100's of carbs and rebuilt as many motors and i seem to be just chasing my tail on this one.

  • dragonkustoms Jun 09, 2012

    I'm having the same problem with my 79 GS750,

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This sounds like an air leak is making the fuel/air mixture too lean. Any air leaks between the carb and the head will lean the mixture. Old, hard and cracked rubber manifold parts are the usual culprits. Old gaskets and "O" ring seals can leak air also. A bad head gasket seal happens at times also, as does an exhaust valve not fully seating. Air leaks are the idle speed problem.

A different problem is likely present in the carbs and that is what is causing the rich fuel smell. The carbs must be balanced. Example > If three of four carbs are perfectly balanced and one carb is not, then the 3 are "driving" the 4th carb. This will cause gas to spit into the air cleaner from the "bad" carb and can cause a backfire or two. Special vacuum gauges are needed to balance the carbs. Also be aware the carbs must be scycronizsed as well. The slides on all four carbs must lift at exactly the same time when the throttle is cracked.

Go to the site below where you can see a parts diagram for your specific bike. You will select the actual brand, year, model, etc., once you go to the site. Part numbers and prices are also shown. You can order parts from this site. In the event no price is shown on a particular part, the part is not in stock. www.babbittsonline.com/pages/parts/viewbybrandand/parts.aspx

I don't get paid for my answers but I do take pride in them, and I am only trying to help. So if you will, > > A “very helpful” rating for these answers? Thanks!

Posted on Jun 14, 2009

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There are 3 things you can do, the 1st is to get some 29mm smoothbore micuni's, or ajust tour idle screw thats located in between the 2nd and 3rd carb. idle rmp's down thur left, idle rpm up turn right.
and i had the same problem i solved it by re-jetting the main jets

Posted on Aug 14, 2009

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