Re jetting an '83 GS750ES

[email2r2]

Hi guys,

I completely restored an old '83 GS750ES last winter an the only mod I made was the installation of a supertrapp exhaust. Everything else is stock.
I found out after a couple of rides that I now have a flat spot in revs between 4500 and 6000 rpm. Since the bike has been sitting in a garage for almost 20 years, I've dismantled all the carbs and cleaned every jets and small holes to be sure there wasn't any debris in them.

Since it haven't changed anything, I'm in the process of retuning the needle jet since I thing that it's the problem for that kind of range.

I first tried to lift the needle 1 groove but that doen't change anything.

It originally had a #120 jet and a D10 needle.

I have a set of #124 jet sitting somewhere. Do you think that only changing them would be enough? When do I have to think of changing the needle too.

Any suggestion?

I'm kind of confused here because I've put 4 into 1 exhaust systems on other motorcycles in the past and it never changed throttle response.

Thanks

[Billy Ricks]

The 120s should be about right, I think I used to run 116s with my DJ kit. The range you're having trouble with would be with the needle and a little overlap on the main. Start with changing the mains and see what happens. Do a plug chop at that throttle setting then pull the plugs and see what they look like. If you don't know what a plug chop is it means running at different rpm ranges and shutting the ignition off without changing the throttle setting. Then pull over and check your plug color. That will help tell you where you're at on jetting. Fuel level in the bowls has alot to do with the mixture at those rpm. Make certain the floats are set right.

[email2r2]

Thanks Billy for the quick reply.

Sorry if I was mixed up a little earlier. When I talked about the needle jet, it was in fact the main that was #120. Since they're one in front of the other, it confused me.

If I'm following you right, you're saying that #120 should be OK but also that I should change the main jet. Isn't it what putting a #124 would be.

Also do you think that the exhaust system alone would be enough to change the response that much?

Thanks

[Billy Ricks]

Just depends on how free flowing the exhaust is. Do you know if it has a K&N replacement filter in the airbox? It would be a pleated gauze type filter.

The only reason I said to try changing the mains is because you can probably do it without pulling the carbs. I don't remember if the drains on the bottoms of my stock carbs were large enough to get the mains out through them or not. The mains will have some effect on the midrange though where you are having trouble. Usually you want to get the mains right then work from there. If I were you I would go ahead and do a plug read first so you know what direction to go. Try a high speed plug chop where you are up in the rpm range close to wide open. That will tell you if the mains are about right. Then drop the throttle to about half throttle and check the plugs again to see how you are on the needle jet. After that you can adjust the pilots for the highest, smoothest idle you can get out of them. And do make sure the floats are set right.

[email2r2]

Thanks again Billy,

The air filter element is stock Suzuki genuine part.

I'll change the main and see if it will resolve or just move the problem and will keep you posted.