stihl trimmer KM90R 4xmix brrr no power no speed

[r7000]

KM90R-Z
2006
4180-011-5310

always used stihl ultra oil, light duty at home use.
I have pulled the carb and cleaned, reinstalled, still basically starts right up especially when cold with choke,
but will only run for a little bit with the choke on and full trigger throttle, but rpm-wise is only maybe 1/4 speed and it's brrrrrrr no power and will only run for 5-10 seconds.

If I had to guess, being 4 mix, it sounds like the compression relief that opens the valve briefly when pull starting, is not disengaging once running, causing whichever valve to open on the compression stroke.

I've pulled the valve cover, the valve lash felt acceptable... i've done valves before from boat engines to honda gcv mowers so pretty sure not a valve lash problem.

Pulled the cam cover and pulled the cam wheel, no signs anywhere of where. I watched some youtube on the subject, anything mentioned on the topic for wear and problems I did not find. The plastic cam wheel lobe looks fine, as do the lifters that ride the cam and the pin bushing fits well into the cam cover.

i've ordered a replacement cam wheel.

Any insight that anyone can provide? I'm pretty sure it's not the carb, and I don't think it would be the spark plug or ignition coil.

question: in my haste I forget to observe before removing cam wheel... to reinstall cam wheel and have proper position/timing is it just the arrow on flywheel pointing to the ignition coil pickup, and then having cam installed with lobe straight up?

I can post pics later tonight if needed. thanks.

 

[Matt Ellerbee]

How’s the muffler screen look?

I’ve had saws and weed eaters at work not run worth a poop above idle. Blow torched screen, ran like a bandit.

Stop using Stihl oil.

 

[r7000]

stihl hp ultra oil in gray bottle no good? why?

what would be better ?

I'll check the muffler tonight, thanks.

 

[Matt Ellerbee]

stihl hp ultra oil in gray bottle no good? why?

what would be better ?

I'll check the muffler tonight, thanks.

Not to start an oil thread, but it has been known to gunk up. You can search here and see.

I personally use Red Armor, work still buys the orange and gray bottle Stihl.

Not so much the muffler, but the screen in the muffler is what clogs up.

 

[Matt Ellerbee]

Here’s a before and after of a MS 194 screen.

 

[r7000]

checked the muffler screen, was clear with zero crud.
waiting on new cam wheel to come in still before I put everything back together.
I got the valve cover off and cam cover off and cam wheel removed.
Still waiting on new cam wheel to come in.
Unless the existing cam wheel was sticking prior to me removing it, that seems fine with zero wear, as does everything else so I am perplexed as to what the cause of the problem is. Maybe spark plug, and I'd be surprised if it was the carb but those are the only other two things I can think of replacing if the new cam wheel doesn't work.
If I was smart, the first thing I should've tried was replacing the spark plug.