Having had the carbs off for a clean up, I have fitted them back on and tried to balance them using my Carbtune guages. Problem is no matter what I do, the guages just won`t settle. I`ve had a job also trying to get the tickover smooth. I set the pilot air screws out at 1 & 3/4 turns each to begin with, tweaked about with them until I thought the tickover was smooth, but after a short time it drops off. With the balancer, setting a tickover about 1500rpm I get the guages lined up ok, but when I blip the throttle they are all out again. If the bikes left to tickover, the guages wander even more. The bike is a 1978 GS750, fitted with slide carbs. Where am I going wrong ?. I was`nt sure whether to adjust the idle using the air screws first, or to balance the carbs first then adjust that. Any help appreciated.
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GS750 Carb balancing problems
- Thread starter tomo
- Start date
Gidday Tomo
Those carbtunes are just the ticket.
Make sure that you have no airleaks anywhere in your inlet system.
When you cleaned your carbs did you remove the fuel mixure screws in front of the bowl to clean the passages. If you didn't this could be the cause of your being unable to get it to idle properly. If you did, make sure that they are adjusted correctly, they may be set too lean.
Once you've confirmed all your other tuning parameters are correct, hook up your tubes to each inlet. Make sure that you have made up the restricters correctly, and fit the restricter end of the hose closest to the engine. If you don't have a resticter on each hose the vacumm will fluctuate wildly with the pulses in the inlet tracts.
Warm your engine up and fit a big fan in front of your engine.
Clymer recommend setting idle at 1500 - 2000rpm. I do mine at 2000.
Don't be over pedantic about getting the cm/hg exact, within 2cm/hg will work fine. Obviously, if you can get them all perfect, then even better.
Just remember that you will have a lot of wear on carb and engine parts that will hamper your getting it perfect.
Did those compressions improve after your valve clearance check? If not that will be part of the problem.
Cheers.
Ian
Those carbtunes are just the ticket.
Make sure that you have no airleaks anywhere in your inlet system.
When you cleaned your carbs did you remove the fuel mixure screws in front of the bowl to clean the passages. If you didn't this could be the cause of your being unable to get it to idle properly. If you did, make sure that they are adjusted correctly, they may be set too lean.
Once you've confirmed all your other tuning parameters are correct, hook up your tubes to each inlet. Make sure that you have made up the restricters correctly, and fit the restricter end of the hose closest to the engine. If you don't have a resticter on each hose the vacumm will fluctuate wildly with the pulses in the inlet tracts.
Warm your engine up and fit a big fan in front of your engine.
Clymer recommend setting idle at 1500 - 2000rpm. I do mine at 2000.
Don't be over pedantic about getting the cm/hg exact, within 2cm/hg will work fine. Obviously, if you can get them all perfect, then even better.
Just remember that you will have a lot of wear on carb and engine parts that will hamper your getting it perfect.
Did those compressions improve after your valve clearance check? If not that will be part of the problem.
Cheers.
Ian
Thanks 49er. Must admit when I stripped the carbs I did`nt touch those screws at the front of the bowl, as my manual said not too cos they are factory adjusted. If I do take them out to clean, how do I set them up again as it does`nt give any guidance. Chances are a previous owner has messed with them anyway I guess as there is no sign of the yellow paint the manual says the factory applied after setting. Good idea about the fan too, I`ll get one. Yesterday even though it was colder than a witches tit, by the time I had fiddled about with the airscrews the engine really got hot, only realised when some oil on the head started smoking !. As for the compressions, I have`nt got around to checking yet. I was so chuffed to find the problem was just a dodgy connection that I just wanted to get the bike out on the road. Unfortunately my wife also wants me to get on with our loft conversion, new kitchen and bathroom, so its taken a back seat for now. I`ll check first chance I get and put a posting on the board.
Don't loose the plot
Don't loose the plot
Tomo, before you get committed to that domestic bliss, check those mixture screws. If they still have the paint covering them, remove it and count the turns that each one takes to lightly seat the needle. Record each one so you can reset them after cleaning the internals. If the figures vary wildly, set them all 1 turns off the seat. You will then need to do plug reads to adjust each one to its optimum setting.
Good luck.
Don't loose the plot
Tomo, before you get committed to that domestic bliss, check those mixture screws. If they still have the paint covering them, remove it and count the turns that each one takes to lightly seat the needle. Record each one so you can reset them after cleaning the internals. If the figures vary wildly, set them all 1 turns off the seat. You will then need to do plug reads to adjust each one to its optimum setting.
Good luck.
M
mark
Guest
Obvious maybe, but make sure that you plug your vacuum line to the petcock and a good fuel supply going to the carbs.
Are those metering screws connected with higher speeds than idling, or do they affect both ?. I`ve never understood what they are there for really as if I remember my carb theory right the pilot air screws adjust mixture from idle to 1/4 throttle, the slide cut out 1/4 to half and the slide needle 1/2 to full. Do I set these up first before adjusting the pilot air screws and balancing, or do I balance the carbs first ?. I hate carbs !.
The fuel metering/mixture screws supply the fuel demand from idle to 1/3 throttle. If you set them all at 1 turn off the seat, you can then adjust the air screws until you get the highest idle. 1/3 to 3/4 throttle is governed by the needle hight. 3/4 to full throttle is on the main jet size.
All optimum settings should be confirmed by plug chop readings at the appropriate rpms. You can then change the mixtures to achieve your highest rpm within the recommended settings on the air screws. If you're happy with the mid and top range, just concentrate on the pilot circuit (idle to 1/3 throttle). If the whole range doesn't feel right, you should work backwards, ie full throttle plug checks, mid range plug checks and finally pilot circuit checks. If the idle to 1/3 throttle feels soggy when the air screws are tuned as recommended 1 &1/4 to 2 turns off the seat, you may need to lower the float level. If it feels hesitant at the correct settings, you should raise the floats to richen it up. An incorrrect float level has the most impact on the pilot circuit. We are assuming that your carb jetting is as recommended for a standard motor, ie carbs, airbox and exhaust. If you or the PO have changed any of these specs then the jetting may have to be corrected to match the existing setup.
Once you have all these parameters correct its time to balance the carbs.
Carbs do get challenging, don't they?
All optimum settings should be confirmed by plug chop readings at the appropriate rpms. You can then change the mixtures to achieve your highest rpm within the recommended settings on the air screws. If you're happy with the mid and top range, just concentrate on the pilot circuit (idle to 1/3 throttle). If the whole range doesn't feel right, you should work backwards, ie full throttle plug checks, mid range plug checks and finally pilot circuit checks. If the idle to 1/3 throttle feels soggy when the air screws are tuned as recommended 1 &1/4 to 2 turns off the seat, you may need to lower the float level. If it feels hesitant at the correct settings, you should raise the floats to richen it up. An incorrrect float level has the most impact on the pilot circuit. We are assuming that your carb jetting is as recommended for a standard motor, ie carbs, airbox and exhaust. If you or the PO have changed any of these specs then the jetting may have to be corrected to match the existing setup.
Once you have all these parameters correct its time to balance the carbs.
Carbs do get challenging, don't they?
Last edited:
P
ptm
Guest
This may have a few ideas to help you through the process. I'd like to have one that covers more details about initial setup and fine tuning, but this has some suggestions that might be helpful...
http://www.thegsresources.com/files/vm_carb_rebuild.pdf
Here are a few other links that have some good info about the VM carbs and carb theory in general. Some interesting reading.
http://www.motocross.com/motoprof/mo...1/carb101.html
http://www.dansmc.com/carbs.htm
http://www.dansmc.com/carbs2.htm
http://oldmanhonda.com/MC/Rcarbs.html
http://www.mikuni.com/pdf/vmmanual.pdf
http://www.mikuni.com/fs-tuning_guide.html
http://www.mikuni.com/fs-performance_guide.html
http://www.sudco.com/ see the diagrams section
There are also many, many threads in the forum that cover carb rebuild and tuning challenges.
Good luck and have fun!
http://www.thegsresources.com/files/vm_carb_rebuild.pdf
Here are a few other links that have some good info about the VM carbs and carb theory in general. Some interesting reading.
http://www.motocross.com/motoprof/mo...1/carb101.html
http://www.dansmc.com/carbs.htm
http://www.dansmc.com/carbs2.htm
http://oldmanhonda.com/MC/Rcarbs.html
http://www.mikuni.com/pdf/vmmanual.pdf
http://www.mikuni.com/fs-tuning_guide.html
http://www.mikuni.com/fs-performance_guide.html
http://www.sudco.com/ see the diagrams section
There are also many, many threads in the forum that cover carb rebuild and tuning challenges.
Good luck and have fun!
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