1979 GS550 Jet experience

[Anonymous]

Hi everyone,
I have 1979 GS550 E. It's all stock except for Dyna ignition and K&N air filter. I've always felt that it ran rather lean but since adding the K&N it seemed to become more boggy. So I set about changing the main jet size. The stock jets are .80 so I tried .90. Seemed to help a little but not very much. I upped to 1.025 and the bike pulls good to about 4000rpm then big flat spot and "burbling" at wide open throttle. Eventually at around 7000 rpm it pulls again. My guess is that the 1.025 is too rich. Any thoughts on this? Has anyone found a decent jet size for the type of setup I have? Thanks
Jeff

[Anonymous]

I'm not familar with your bike specifically, but in general, the main jet only controls fuel above half throttle or so. Since your bike is pulling well at higher RPM, it could mean that your main size is OK. The burble in the middle could mean that you need to raise your needles one notch (if you can make that adjusment).

The first test you ought to do is to install new plugs, then take the bike to a straight section of road with little traffic and open her up. Then, hit the kill switch and coast to a stop. Pull out the plugs, and look at them, If they are sooty and black, you are too rich and need to drop back down at least one jet size. If they look tan or gray, your mains are OK.

On my 1979 GS750L, the Mikuni carburators have a mechanical slide that lifts the needle out of the needle jet in response to throttle position. This controls mid range response. On my carburetor, the needle has five grooves in its top end. Position of the needle is adjusted by placing an "E" clip into one of the grooves. Moving to a lower groove raises the needle and enrinches the mixture. If you can make this adjustment, you may need to do it to lose the burble in the mid range.

My carburetors also have air screws on the sides, near the air inlets. These control mixture throughout the range, but have the biggest impact at the low end.

I hope this helps a little.

[Anonymous]

I agree, kind of. If the flat spot is in the middle, it's normally a jet needle issue. Lower needle to lean, raise needle to richen. The part I question is the air screw/adjust air throughout range. I thought that this was an adjustment for the idle circuit only. It's my understanding that anything above idle is controlled by the cutaway on the slide(air) and needle/ emulsion tube (fuel) until 7/8 throttle. From 7/8 to WFO is venturi (air) and main jet (fuel). Anybody else want to chime in here and straighten us out? Bob

[Anonymous]

If you're using a K&N filter in the stock airbox it should have very little, if any, effect on carb jetting. But if you're running separate K&N's on the carbs then you may have to richen the pilot, needle and main jets.
Axel