I agree with this. I think since the air is being heated as it is compressed the actual pressure produced for a given compression ratio (assuming the air in the cylinder is at atmospheric pressure to begin with), could possibly be a little higher, but yeah there is no way you could hit 150 psi. That pressure is what modern car engines with compression ratios of 10:1 make.
Gauge pressure for isothermal compression should be about 14.7 * (compression ratio - 1). Minus whatever is lost from valves and rings. At cranking speeds, a compression gauge will read close to isothermal compression. 150 psi would indicate >11:1 compression ratio. Which is a damn lot for a small engine, even with perfect rings.
I've seen inaccurate readings before but they always had read low due to leakage at the valve...also this formula must not apply to car engines since they typically get around 150 psi with 8.5:1 CR, and about 130 psi on my 8.0:1 V8? I believe the camshaft also affects the "dynamic" CR? I'm not trying to argue with it, just want to understand since I've seen a ton of compression tests on automotive engines and didn't understand why small engines had so much less (and also aircraft engines it seems). I'll see if my friend can lend me his tester again and I'll redo the test and if I get the same I'll throw it on my Oldsmobile...if the previous 130 becomes like 170 clearly the gauge is bad.
1980s V8 engines in good health make it with 8.5:1 CR. I have less experience with newer engines but likely the increase in compression ratio is also accompanied by higher lift cam profile which probably offsets the higher compression giving a similar dynamic compression ratio. It's been a long time since I read up on this so hopefully I'm explaining it right.
Your last sentence says you bought a new carb. Did you do the starting fluid test like I said or no?
I haven't had a chance yet. It needs to sit overnight and I leave in the dark for work in the morning. For the price of the carb I figured I should get it ordered ASAP so I might have a chance to put it on before it's so cold that it wouldn't be worth doing. (not going to adjust it for cold weather usage). I will do that test next chance I get but I've got a ton of jobs to do around the house before winter unfortunately.
The closest I had to starting fluid or gasoline was a bottle of mass air flow cleaner (labelled "very flammable" so I used that tonight as I needed the tractor to move some sod with my trailer...it still took several tries but definitely fired off quicker than by itself. Not sure if it would have been different with actual gas or starting fluid. As usual it ran great after I got it going.
Finally an update.... broke the choke cable installing the new carb and the new one is different than what was on there..... couldn't figure out why it was still not wanting to cold start when I manually operated the choke and throttle....tried it again today in near freezing temperatures and it became immediately apparent. Oil and smoke puffing out from the head gasket when I crank it. It must have been sealing up with heat from combustion until the temperatures got colder.
