Trouble at WOT

[marc]

I was wondering how this thread is going on.

I also had problems with my carbs. A great advice is "never drop aftermarket parts inside your carbs", but since the price for OEM parts is a steal, everybody give them a chance.

I replaced the float needle and found that:

- Stock needle and K&L needle are slightly different. That difference is corrected through the valve seat. Hence, if you purchase a K&L float needle + seat kit, there would be no problem, but if you use a K&L needle with a stock seat you may come into trouble with an impropper fuel level in the bowl. A lazy mechanic may not notice it.

- Keyster needle size is same as stock, but the spring inside is very hard, compared to the stock one. Don't know if this may cause a difference.

So, check the fuel level.
Another check could be to turn the petcock to PRI, to avoid the well know fuel starvation issue.

[mr72]

I don't know about a well-known fuel starvation issue, but if you have fuel starvation that is masked by switching to PRI then the problem is most likely a vacuum leak or a faulty petcock, clogged or kinked fuel line, etc.

[md13]

Just an update - ran on prime and no difference. Thinking about draining the tank and checking the tank fuel filter... if not that then beginning to worry I'd need to start replacing rubber parts in the carb.


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[md13]

Update: drained tank and checked out fuel tank petcock - screen looked pretty good. Swapped in a brand new one and went on a test ride and the issue's still there. Low power at WOT.

[The Buddha]

I was suspecting you had a problem in the frame petcock not the tank petcock, specifically fuel getting into the vacuum side.
The way to test it would be to cap off the carb end and the petcock end of the vacuum line and run the thing on prime and see if it has the same problem.
Also you may have a tiny rip in the diaphragm - tiny tiny tiny rip will cause massive issues.
Cool.
Buddha.

[herennow]

My money's still on too big main jet or needle shimmed too high if its riding OK everywhere  except at WOT.

[marc]

Solved mine!

Sadly, I guess I will not give much help, simply because I replaced both carbs, and it started to work well.

My carbs probably had a leak somewhere I was not able to find.

The carbs I installed were purchased known as "nonworking", for parts only. I refurbished them: I replaced both jets with OEM Mikuni original sizes, replaced the needle with Keyster ones (they were damaged by previous owner), installed ALL NEW rubber rings, set up mixture to 3 turns out, needle clip in the middle and started from this setup.

I used the excellent post https://joshkarnes.blogspot.com/2017/09/fixing-common-gs500-carburetor-issues.html to adjunt the mixture screw (that was close to 3 turns out) and used the spark plug colour to set the needle to notch #2 starting from below.
Yay! GS500 workign again!
Will test to set the needle in notch #1, the one that keeps the needle more lifted.

[The Buddha]

Can I have your old carbs ?
My problem was worse on the 01-02 carb rack I had. And it got better with the 89-00 carb set.

I am pretty certain mine isn't in the carb per se. Your carbs may have a problem - but carb body leaks usually affect idle more than WOT.

I had a carb that was taking in air at the butterfly pivot hole that someone sent me to diagnose, and the bloody thing sounded raspy at idle, and showed all the symptoms of sucking air - wd40 test, smoke in the pipe etc etc. In fact spray wd40 a few times and it will seal up cos the Orings in there would swell just enough to seal up perfectly. But replacing boots etc obviously did nothing for the problem.

Anyway make sure your problem is truly gone too.

Cool.
Buddha.