ok so that 12hp B&S engine...280000 series. finally got some time to work on it this past weekend and i am officially confused. my best guess is timing.

so if you guys remember, i had reground the valves, new HG on it, cleaned all the junk out of the inside. that fixed compression problems....then i thought it was overheating...tends to be the thought when it stalls, becomes hard to turn the engine (start couldn't do it) and you see small wisps of steam coming off it.

this time i fiddled with the carb, it did need some adjustment for sure. i intentionally tuned it a tad rich to account for any possible overheating caused by lack of fuel and sure enough i also had a clogging fuel filter which i replaced and put new fuel line in for the hell of it.

so it got much farther before stalling again. once again the starter could not turn the engine over. however, this time it didn't really seem that warm...but still hot. i poured a bit of gasoline down the carb after removing the air filter in an attempt to cool it down (as i had assumed that it was overheated again) and got it cool rather quickly. the strange part is that the starter STILL could not turn it over - and the engine was barely warm to the touch!

so it dawned on me...it can't turn the compression stroke for some reason...i pulled the spark plug, turned the engine over a bit without the plug to clear out any of the gas i had gotten in there, put the plug back in and it runs fine again! i can't make sense of it now. oh and theres definitely spark, i checked that.

any ideas? the exhaust is not clogged and it still doesn't run near as powerful as it did in the past. I'm stumped!

After it stalls, can you physically turn the motor with your hand with the plug still in? If so, then I'd want to make sure you don't have a starter problem. Poor electrical connections can cause severe enough voltage drop to cause starting problems(although I realize that wouldn't explain your ability to start it when cold, except the starter too can suffer from heat related problems).

Notwithstanding any potential electrical problem, have you ever had the case apart? I'm not sure without looking at the manual or IPL, but I wonder if your oil pumping mechanism is working properly. I don't think a 12HP uses an oil slinger, but I'm not positive. If you still think it's overheating, in spite of your most recent tests, it may be time to pull it apart and just verify the internals are all OK.

Let us know...

thanks for the ideas dawgpile. to answer your question no its very difficult to turn over by hand with the plug in. remember it starts just fine and runs for around 20-30 minutes before stalling. thats the point where it becomes difficult to turn - by hand as well. i'm assuming this is too high compression....i dont know how it could build up though. on relieving the pressure it runs fine again.

and yes there is an oil slinger in the ass end of it. havnt gone down to the basement yet, but i'm thinking i may have to come the fall.

It's possible that the compression release mechanism on the camshaft is not operating properly, this could cause excess compression.

When you did the valve work what did you set your valve clearance at?

i honestly can't remember what i set the valve clearance at as i did the work last fall/winter but i do remember that it was within spec that is specified in the B&S repair manual....exact number i dont remember.

so could you describe this pressure relief mechanisim to me? i didnt even know there was one. there seems to be a rectangular valve/vent over the cavity where you can access the valve springs/top of the tappets and vents into the carb....kinda like a PCV...could that cause these troubles if it was clogged?

i honestly can't remember what i set the valve clearance at as i did the work last fall/winter but i do remember that it was within spec that is specified in the B&S repair manual....exact number i dont remember.

so could you describe this pressure relief mechanisim to me? i didnt even know there was one. there seems to be a rectangular valve/vent over the cavity where you can access the valve springs/top of the tappets and vents into the carb....kinda like a PCV...could that cause these troubles if it was clogged?
The compression release mechanism is on the camshaft. It's a little flyweight with a spring attached to it that gives one of the valves a little lift on the compression stroke to release some of the compression for easier cranking. When the engine is running the flyweight pulls away and disengages the compression release.

There is also a possibility that there may be some damage on one of the crank journals causing the engine to bind up when it gets hot.

thanks again for the help, i'll have to tear the ass end of that engine down this fall. i seriously doubt overheating at this point, theres nothing left that i havnt checked to cause it. i'll check out that mechanisim on the camshaft you mentioned, have to dig out my diagrams...

so just out of curiosity, that valve i mentioned that covers the access to the valve springs and tappets....it looks like some sort of PCV to me...what happens if that cloggs?

You will get pressure in the crankcase, oil may leak out of the seals, the engine will usually smoke when run.