I know this forum isn’t the right place for a question about a small engine but I’m getting desperate. I have a vintage 1976 Briggs and Stratton 8hp vertical shaft engine in my Wheel Horse lawn tractor. It has a loud intermittent knock that only happens after the engine has warmed up. It is not the rod going out, (trust me, I’ve torn it down to the crankcase twice in the last week). It has decent compression, like 85 psi. The valves and piston are not hitting the spark plug. I’ve isolated the noise using a mechanics stethoscope, otherwise known as a long screwdriver, to the front of the engine, under the head. But once again, I don’t think anything is hitting. I’ve had the head off probably four or five times in the past week😤. I just don’t understand why this engine won’t run right. Granted, I had to bore the cylinder out just to get it running, but as I said, decent compression. So that can’t be the issue. Could it be out of adjustment valves? That wouldn’t make sense though because it only happens when the motor is warm. I am at the end of my rope, any ideas? Any help I can get would be greatly appreciated. Also, the engine doesn’t run very good in general, but I think that’s the crappy carburetor that I don’t think is replaceable, just thought it was worth mentioning.
85 psi is high compression for a Briggs. Run 93 octane in it or E85. Ex-Brother in Law used to race Go Karts. He was clocked doing 65 “testing” it on the street. He would raise his compression and get bad knock on regular gas.
Just FYI, most Briggs motors only run 60-65 psi in compression.
Thanks, I’ll give that a try. I guess it’s still got really good compression for 45 year old lawnmower engine that was locked up last week lol.
For a 45 year old engine it’s actually high. Lol. You may have an exhaust valve sticking or not opening completely. The newer ones run a lot higher compression but the ratio is different, able to run on regular gas.
The ignition timing may be too far advanced, causing the ping. Is it adjustable? I always use Clear Gas in these motors, 93 octane and no alcohol.
I’ve never tried 93 octane in a lawnmower before, but I’ll give it a try. This thing is driving me nuts lol.
I just realized something else. When the engine is warm, even with the spark plug is unhooked, It still knocks when cranking until it cools down. But only when the spark plug is in causing compression. If I take the plug out the noise stops, but it’s not hitting the plug. And the timing is to factory spec as far as I know. I don’t think it is adjustable. Also, the valves fully open as far as I can tell. The engine is throwing me for a loop. It can just be puttering along smooth as can be, and be knocking intermittently at the same time. It also lets out a random puff of smoke on occasion, but once again, probably that awful carburetor.