98 c115 Only runs 3 min. at 4200rpm HELP

[dclachmann]

98 c115 2 stroke starts good, idles good, will run all day at 3000 rpms.
The problem if I increase to 4200 rpms it will run good for about 3 minutes
then fall off and shake , shutter, sputter, cough untill I briing it back to sloww speed. I can keep it at slow speed for about 2 min. then punch it up to 4200 rpms and it will run good another 3 min. and so on. If I run at 5000 rpms the same thing happens but the good run time is less than 3 min.
When the engine is sputtering the ball is full but not hard, squeezing the ball does not stop the engine from sputtering. Choking does not help.
I have replaced the ball and connectors, the on engine fuel pump,fuel filter
and have tried running with an externa fuel tank.
Would appreciate any comments or suggestions.Thanks

[dclachmann]

This was my first posting and so far disapointed with the lack of responce.

[robert graham]

Sounds like a fuel flow issue. Maybe check your fuel pump diaphragm for leaks, bad fuel line O-rings, fuel tank vent, restriction in fuel line, primer bulb check valves, sticking float needle valves, etc. Good luck and let us know what you find!

[99yam40]

you can always test fuel supply with a pressure/vacuum gauge for restrictions and proper pressure.
and see if there is air being sucked into the system with a piece of clear tubing installed temporarily.

check for spark with a inductive timing light at the problem RPM on all cylinders to see if it is loosing spark, check to make sure timing is going where it should.

If it has good compression and good spark on all cylinders all the time, no fuel restrictions or air being sucked in then I would be looking at trash in carbs or some restriction going into the carbs. Sounds like it is running low on fuel after 3 min