Craftsman 2-cycle trimmer varying RPM at full throttle

[bkenobi]

I had an issue with the carb purge system on my ~10yo line trimmer power unit (check valve must have been partially blocked). I swapped out for a similar Walbro WT carb from my spares box and the motor spun up great and ran without incident for 15 minutes of testing/clearing leaves. I noticed that after 5-10 minutes the RPM started to vary slightly. I wouldn't call it surging, just slowly up and down a bit. It didn't cause me any issues with what I was doing, but I'm curious if anyone has any thoughts on what might cause this? I can do more research but not even sure what to search for to understand the potential problem.

I'm getting to the point in the year where I won't need it. I just have to get the leaves moved in over the next few weeks and then I can do any needed maintenance. Now would be a good time to test things and order parts so I can fix after it's not needed and before spring.

 

[Fuddyduddy1952]

Should be a high speed adjustment screw. I bought an assortment kit that fits all the different crazy screws on carbs.

 

[bkenobi]

I've adjusted many carbs on line trimmers including this carb when it was on the last trimmer. I was actually surprised it worked great without touching the adjustments. I don't think this is something that can be adjusted by the high/low/idle screws though.

 

[CalG]

Really? Play with the high speed screw. back it out just a scooch until you KNOW the engine is running fat (Won't really rev up and sing) Then tweek it in , just a bit.

Wavering at is a sure sign of lean condition.

 

[bkenobi]

Huh, ok. Ill give it a try. Doesnt hurt to give it a go. Figured if it was stable for several minutes it could be something else. But then, they are pretty simple so not sure what it could be. Thanks.

 

[mrmikey]

Any chance the exhaust screen is carboned up a bit?

 

[bkenobi]

When i started having problems starting it after the first tank that was one thing i looked at. Worth checking again though.