GS750 re-jetting

[TJBergli]

I have some trouble with the jetting on my 77 GS 750 . I've pod filters on and just about standard exhaust, but even with 120 main-jet (standard is 102,5) the engine seems to suffer from starvation (at least when the engine is kold). I think the idle-jet is standard. Anyone out there who can tell me if I have to go for bigger main jets? Or look for some other faults?

TJ from Norway

[Rudeman]

The engine usually requires a richer mixture for decent drivability until it warms up. Did you check/adjust the floats? Make sure your carbs are venting also.

Assuming that the mixture screws are set to factory spec, I'd turn them out about 1/2 to 1 full turn. I would go up 2 sizes on the pilot jets as well. I ran mine witha header on 120 mains andit ran perfect so I don't think you'll need to go up in main jet size.

[Dave8338]

In addition: Make sure you have plenty of fuel flow. Verify the petcock, screen, and inline filter (if you have one).

Does it bog out while under full throttle?

Heavy load at partial throttle to 3/4 twisted?

You may be fine on the mains but need a bump to the pilots instead.
Plug chops will confirm where things are falling flat.

[TJBergli]

Thanks!

The floats should be ok. The mixture screws are set to 1,25 turns out (those one side of the carbs) The other ones are set 0,75 turn out.

[Rudeman]

Thanks!

The floats should be ok.

Make sure. CHeck for vacum leaks also. Do you know if the manifold boot O-rings have been replaced?

[Coady]

Assuming that the mixture screws are set to factory spec, I'd turn them out about 1/2 to 1 full turn. I would go up 2 sizes on the pilot jets as well. I ran mine witha header on 120 mains andit ran perfect so I don't think you'll need to go up in main jet size.

Definitely try this with the mixture screws first - 1/2 turn out on each to start.

[TJBergli]

I've been away for a few days, but I'll go ahead with the mixture screws. And yes, manifold boot O-rings have been replaced. Whar about the needle, should I try to lift up one step?

[BassCliff]

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[KEITH KRAUSE]

Thanks!

The floats should be ok. The mixture screws are set to 1,25 turns out (those one side of the carbs) The other ones are set 0,75 turn out.

The side air screws are tuned using the highest rpm method, not a "pre-set" position. Also, a stock exhaust and pods are a poor flow match and you can expect problems. Your description of "just about stock" for the exhaust makes me wonder too. You also say the engine seems to suffer from fuel starvation at least when cold but you can't expect good performance on the main circuit (3/4 to full throttle) when the motor isn't even warmed up yet.
If the bike has been tuned correctly first...valve clearances, ignition timing, carbs/filters serviced, etc....
Try 1 position richer for the jet needles than the factory position.
Based on past experience of my self and others here, a 117.5 to 122.5 main jet figures to work with pods only.
You can try richening the pilot fuel screws (underneath) up to approx' 3 turns out to see if that can assist the stock 15 pilot jet well. If not, a 17.5 pilot jet is as large as you'll need. Be sure you buy the correct length of pilot jet if you do. Re-set the pilot fuel screws to something near factory (1/2 to 3/4 turn) if you go to the 17.5 jet. Fine tuning will always be needed.
REMOVE the two floatbowl vent lines and leave the vents open. Running the stock vent lines with pods does result in varying degrees of fuel starvation.
Bench synch the carbs. Whenever you disturb the jet needles the VM carbs must be re-synched. This must be done carefully.
Set the side air screws to 1 3/4 turns out for now.
Be sure the fuel line is routed correctly and there's no kinking. Check for good fuel flow when on prime.
After complete warm up, place the bike on the centerstand. Set idle to 1,000 rpm's using the idle knob adjuster.
Now adjust the side air screws using the highest rpm method.
Now synch the carbs with a vacuum tool.
Test the pilot, jet needle, and main circuits, at minimum, 1/3 to 1/2 throttle, and full throttle respectively.
Do what the plugs/performance say to do.

[TJBergli]

Thank you all for your help!

The history behind this bike is that 9 years ago, I bought a 77 GS750. And after 2-3 years i bought a donor-bike. I sold some parts (which i thought i would not need) from the donor-bike, among them the seat, fenders, speedo and air filter! 2 years ago i sold the original bike, and left in the garage was an engine, 2 wheels, an exhaust (that looks and sounds like the orginale Suzuki but is not), and a petrol tank. So, last winter I started putting these parts together, and supplied the project with other parts that are leftovers from restoring a 1993 Ural with sidecar and a 1957 Ural M-72. So now I've got this hybrid, bobber-look-a-like bike (may be i will post some pictures later on)

If I had known that i would need the airfilter, i wouldn't have sold it.

It's getting colder here in Norway, so i don't know how much further i will proceed with the project this year, but my goal is to get it running properly.

It starts easy (withe choke on), and idles ok after some minute with out choke. But when I twist the trottle, it say "booop". Driving at higher revs is quite ok when the engine is warm, accelerating is still a probleme.

Let me add that I have not yet balanced the carbs.