Ms 290 engine failure after replacement

[big_d]

Hi, I am new to the site, although I have been lurking for some time, The advice I have seen here seems to be top notch so I thought I would ask a question. If it is already in the archive I apologize, although I could not seem to find a specific answer. It concerns a Stihl MS290 that I recently installed an aftermarket engine into, a complete unit, already assembled, supposedly by a Stihl trained technician, as I wanted to get back up and running. I killed my previous engine I believe, by running it leaned out by either a vacuum leak or maybe weak gas, anyways, my fault, live and learn. This new (Chinese)engine went in without a hitch, used Motoseal to join the halves, fueled it with Motomix, all adjustments set up per Stihl owners manual, started up no problem, let it idle 3 or 4 minutes adjusted low speed and then the chain and brought it up to about three quarter throttle and then a metallic pop, and saw died. It will not restart and when you pull the starter rope then it is hard to pull as if something is binding. What did I do wrong or was it defective right out of the box? I admit I not a pro mechanic but most small engine repairs I am capable of doing. I have the original 290 engine, would I have been better off just buying the piston and rings? Cylinder above the intake and exhaust I believe was good enough to rebuild, I wanted to rebuild at a later date a keep it as a spare. Any anwers or advice will be appreciated. Thanks.

 

[buzz sawyer]

Have you pulled the muffler yet to check the P&C? What is motomix? Why not 89 gas mixed 50:1?

 

[big_d]

Motomix is Stihl's own canned fuel, no alcohol, plus the oil, premixed 93 0ctane. its like Trufuel. I wanted to cover all the bases on this new startup, no I have not pulled muffler yet, will do it tomorrow, I work nights, will do it after I get up tomorrow.

 

[big_d]

Update: Here's what I found on further inspection, took off muffler and looked thru exhaust port, everything looked good, same with intake, all looked good, put it all back together, decided to check flywheel and key was sheared. Could have cracked it when old engine was in place, I think that is likely, as the new engine had not been really revved yet. Have a new one coming Friday, then we'll see.

 

[buzz sawyer]

Good to hear.
The key is only there to position the flywheel. The taper is what locks it into place. It should run fine without it. Make sure the tapers are clean and free of any oil.

 

[big_d]

" Make sure the tapers are clean and free of any oil."
And there's my sign. Duh.
As soon as I read that I knew what I forgot...
Other threads on here confirm it.
Thanks for that advice!