Some help required with JETTING a 82 750

[david suzuki]

Hi, I've got a 1982 gs(x) 750 e, with an unknown 4 into 1 header on and a stock airbox. The bike really struggles at high rpm so i decided to fiddle with the jetting. I tried downsizing from 117.5 mains to 115 mains, and it just made it worse, so obviously ive got to richen things up a little. Ive seen a lot of threads on this site with people running everything from 120's to 130's and messing around with needle heights and so on. Im just wondering what a good starting point would be for this sought of set up??? Im thinking of starting with 125's now. SOund good???? whadya reckon?????

[ToddStyles]

I've got an '81 GS750E with pods and a stock exhaust. I'm running a 125 main jet and it seems to be working out okay. I did have to raise the needle a bit to get the midrange working, though.
Todd.

[brveagle]

How's that air filter look? I'd try upping your mains and then replacing your stock filter with a k&n drop in... Opening things up, up top.

[david suzuki]

hey, i tried going up to 125's and it seemed a little better, but still coughing and stumbling when i open up the throttle. So, i know that 117.5's dont work and i know that 125's dont work, maybe i should have a go at 120's???? or would you suggest raising the needle??? bearing in mind that the engine plays up only from 3rd gear onward under load, and with open throttle.

[ToddStyles]

The idle circuit controls everything from idle to about 1/4 throttle position. The needle position and needle jet controls everything from 1/4 to nearly WOT throttle position. The main jet controls everything at WOT. This is a very general statement as the circuits do overlap. These functions relate more to the throttle position than the actual RPM of the engine. You need to determine at what throttle position you start experiencing the problem so you can isolate the particular circuit.

If the initial throttle movement is okay and then it stumbles once you crack open the throttle a little more, then you can adjust the needle position for the midrange. If everything is okay until you hit WOT, then you can start looking at the main jet.

First, you need to make sure that the idle circuit is working properly and that the fuel mixture screws are adjusted correctly. If the idle circuit is too lean, it could be stumbling on the transition to the needle. Adjust the fuel mixtures screws by backing them out until you reach the peak RPM. If that ends up being 4 or more turns out from the bottom, then you need to swap out the idle air jet (smaller) to reduce the amount of air you are letting into the idle circuit. Reading the plugs is a good way of determining if you are running rich or lean on the idle circuit. Also, if you rev up the engine from idle and release the throttle, the engine should idle back down. If it hangs for a bit before idling down, that is an indication of a lean mixture. If the RPMs drop down below the normal idle speed and then recovers, that typically means it is too rich.

Once you are sure the idle circuit is working okay, then you can play with the needle settings.

If everything is fine until around 7K RPM's and WOT, then you can look at the main jet. I would image that the 125 main jet would be fine. The smaller 117 might work if you can adjust the needle.

It is imperative that carbs are clean, too. If you haven't synced the carbs, you can experience similar situations too.
Todd

[nabrams]

Hold on there a second!
Are you sure it's not ignition related?
My bike had issues identical to yours over 7000 RPM. I fiddled with carbs forever, till one day the coil that had been gradually failing for weeks finally failed altogether and the bike would not fire those two cylinders.
That's when I realized it was weak spark all along. Coil performance degrades at higher RPM (see Wikipedia), so that a bike that runs just fine at low RPM with a border-line coil, will lose power at high RPM.

Bad spark-plug wires and plug caps can result in the same symptom.
A slowly failing igniter can do the same.

If you've got "known-good" coils to test with, then you should swap. If that still does not help, try new wires and caps. If still no luck, then try a "known-good" igniter.

You'll find plenty of posts where one (or more) of the above were the culprit.

[t3rmin]

Have you seen this site?

http://www.factorypro.com/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html

Excellent carb tuning guide. It'll walk you through the whole process systematically but easily.