Precision ?

Had a nearly new Weed-Whacker with similar symptoms. It would start and kinda run. Hit the throttle and it would die out. Also die out if you let it idle for several seconds...

Mud wasps had built a nest in the exhaust pipe/muffler almost completely blocking it off. I didn't notice in your trouble shooting anything about the exhaust...

No , you didn't ... a bit late now since I've fitted the drive coupler to a larger crankshaft but I will check that . If I can figure it out I'll keep this motor for a replacement for the tiller . The new motor was originally intended for the tiller , but it recovered from it's problem and has been running fine .

It's a long shot, I know you do good work👍️

I felt like an idiot when I found that mud nest clogging the exhaust after having the carb all apart among other things🙄️

One other odd thing I've seen is a partially sheared half-moon key on the crank shaft to flywheel. May happen after a particularly violent stall-out type mishap or the flywheel bolt has loosened a tad. It threw the timing off just enough that it would still run but poorly.

My dad once spent a year fighting with one of the two 4 cylinder Chevy engines in a Searay sport fisher. He finally broke down the exhaust system and found a wide thin wedge of rust about 75% obstructing the exhaust.

I never think of checking the exhaust either. 19 out of 20 times when I have small engine troubles its air box or carburetor. Usually plugged. Sometimes float stuck. The 20th time its usually down to being completely worn out.

Rarely is a spark plug bad, but I have had bad coils or coils that were to far from the flywheel.

I haven't had that happen, but I follow some of the hotrod go cart guys on YouTube like Cars and Camera, and I have seen them show that problem. Of course you expect failures on those small engines every other trip to the track when you are chasing tenths of a second on the 1/8 mile at RPMs in the multiples of what those engines were designed for.

I have to replace the engine on my log splitter* ... fortunately I happen to have a suitable engine on hand for the replacement .

--------------------------------- I've mentioned similar problems and solutions I had, the leaky carb float, self-destructing Lovejoy coupler, incorrectly machined replacement pump and warped motor mounting plate.

I added a tee and pressure gauge to my splitter to confirm that the pressure release opened at 2500PSI, as the control valve had been rebuilt, plus a spin-on filter in the oil return line to trap the wear debris that had required the complete seal replacement. The gauge shows that the pump switches to lower flow around 600PSI, simply by recirculating the output of the wider gear. 26 heating seasons later it's still OK.

A previous car had the clogged exhaust problem, it would idle but not take much load, so I crept down the breakdown lane to an exit. A corroded baffle in the manifold/resonator had collapsed over the outlet.

IIRC a Lovejoy coupler shaft should be bored centered in the OD and perpendicular to the jaw faces so you can use a feeler gauge between the motor and load shaft couplers to adjust them parallel. Slide the coupler(s) back until the outer jaw faces almost meet. If they seem poorly made you could true the OD and ends. The spiders won't survive much misalignment.

The 5/16"-24 bolts holding the pump to the FRP nylon bracket and engine tended to loosen until I drilled the heads for safety wire. The industry standard hole pattern and thread is the same on the HF 6.5HP 212 Predator engine I bought for $99.99. jsw

Of course you expect failures on those small engines every other trip to the track when you are chasing tenths of a second on the 1/8 mile at RPMs in the multiples of what those engines were designed for.

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Here's an example of a 212 Predator set up for racing:

The limit on the HF 212 on my sawmill is blade tension which is only about

I have a Saito FA-80 (stamped on the block) displayed on a bookshelf, it cost me $30 and satisfied my quest for a model aero engine with visible pushrods. It has compression, the throttle and choke were stuck.