Motorcycle Repair: Tuning, rubber diaphragms, float bowls

Question
Hello, I know your expertise isn't suzuki, but my question shouldnt really be brand specific. I have a gs550-4cyl, it is a little cold natured, (has to idle a few minutes before it will take gas) but the real problem is that during acceleration it runs great, but when held at a steady throttle it kind of surges, like its running out of gas. The float bowls are set perfectly, did the test with the tube coming from the bowls upward to check, I'm sure it is getting fuel, I have replaced the petcock, and even tried another tank without the vacuum port. when it is does surge all you have to do is roll the throttle a bit and it accelerates well, but then you constantly have to run too fast then let off, and repeat.

Answer
Travis, I'll do my best here, with a few comments.

You mentioned using a different fuel tank, but did you use the same gas cap? The tank has to vent somehow and often gas caps that have plugged/partially blocked vent systems will starve for fuel at a certain throttle setting.

Other areas to check would be the whole carburetor rack and the interconnected fuel feed passages between them. Dirt/debris can pile up behind the float valves, restricting fuel to one or more cylinders. If the carbs have never been overhauled, it may be a good time to consider that option.

These carbs, I see, are CV with rubber diaphragms (depending upon the year). If there are any pinholes or tears in the rubber, the carburetor won't function properly and the other cylinders have to try to compensate for the lack of power in the affected cylinder. I see they used some radically different carburetors on these bikes, but you didn't give the year of yours, so I don't know what else to offer about them. If your bike is older with standard slide carbs, then that doesn't apply.

There have been cases where carb venting is an issue, either with pinched off vent hoses, blocked vent passages in the float bowls and when long hoses have been used off the overflow tube fittings, the air blowing across the ends of the hoses causes a vacuum drop, which can affect float bowl pressure levels.

I does sound like fuel flow issues are affecting your bike. Sometimes it is fuel flow restrictions and sometimes it is a venting issue that is holding the fuel flow back.

That's about all I can offer for your question, at this time.

Bill Silver