Originally Posted by st3gamefarm
70 is good using a differential pressure guage. 80 psi is about the max that a person can keep the crank from turning. so holding 70/80 is pretty good. It's a leakdown test, you can listen where the air is escaping, and determine what's wrong.
In this particular case it's a 360 cubic inch aircraft engine. Has 550 hours since rebuild. (should have gone over 2000 hours)
Found the trouble to have been caused by a combo of two things. #1 leaded fuel, and #2 valve guides on the tight side, but still within allowance.
Lead buildup on the valve stem, caused stickin' valve, thus causing burned valve. (note the discolored stuff on the intake valve That's lead)
That's ok because he can get a complete cylinder assembly New from the engine manufacturer, including new piston, and rings, for about $1100 +/- a few bucks.
Now just exactly how izzit that lead is supposed to prevent valve troubles?
A low compression, air cooled, valve sticking airplane motor and your surprised by a burnt valve?
Lead in fuel was found to prevent seat erosion. But, it also contributes to sticking valves by both gumming up the lower part of the guide and stem as well as by contaminating the oil which gets into the upper guide. Add that to typical hotter valve temps of an air-cooled engine vis-a-vis a liquid cooled with a little tight guide clearance and you've got a recipe for a sticking valve. Then add to that an airplane motor, like a boat motor, works a lot harder than the typical car motor. Take off power, everything its got till you’re a up a ways, climb power, almost everything its got till you get to altitude which might be a while, cruise power what 60-70 percent of everything's its got till you throttle back for descent. If you operated the typical car engine that way it would be junk and a couple months.
Is this low lead av gas or something else? And I guess with such a low compression motor why lead at all?