Stihl BR600 lack of high RPM.

[Silverado 2500]

Hello all, first time here. Can usually figure out most problems myself , but this 4 mix is a new animal to me and has me stumped. Having a problem with not being able to maintain or achieve full throttle. When first started, it will tend to operate just fine for up to the first minute. Then will not be able to gain full throttle. I'd say it gets about just past 3/4 throttle. Here's what I've done so far. Rebuilt carb with a Zama kit, than replaced the carb with a genuine Zama carb. Checked emulsion tube for cracks and for a tight fit. Checked fuel filter( looked pretty clean) but replaced anyway along with fuel tank vent. Adjusted valves. Removed muffler to check exhaust port and spark arrestor.Have heard about faulty ignition coils on these, but would like to be able to confirm somehow before spending more money unnecessarily. It just feels to me to be a fuel issue. But again,this is a new animal to me. Thanks for any info that may help.

 

[hedge hog]

Double check your valves sounds like there I little tight
Only seen it once but when the motor gets hot and expansion takes up the tight clearance and won’t let the valves seat


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