I have a 1919 engine that I bought over the summer for a power unit. After buying it I removed the head and wiped some heavy oil off of what appears to be relatively new aluminum pistons that are 40 thou oversize. The cylinders have no ridges at the top. I then reinstalled the head and did a compression check - 30 lbs of compression per cylinder.
Removed the head again and lapped the valves and got a good "ring" around each valve which means to me the valves are seating properly. I then sprayed WD-40 into each cylinder and the piston/rings held the WD-40 overnight. Pistons are tight in the cylinders. Reinstalled the head and still got 30 lbs of compression.
Removed the head AGAIN and installed a milled down low head, brand new copper gasket with copper gasket sealant - still 30 lbs of compression.
There are no manifolds on the engine and compression was checked using the starter to turn it over. I'm really not sure what else to look into. Suggestions would be appreciated.
Thanks in advance, Mark.
Problem with your compression tester?
I'd try it with oil and forget the WD-40.
Compression tester is good, have a new engine in my roadster and it reads 67-70 lbs per cylinder.
I put the WD-40 into the cylinders just to see if it would just disappear past the pistons, which it did not.
Worn cam? How far do your valves open? You may not be getting anything in the cylinder to compress if the valve opening event is too short in duration and the valve doesn't open very far.
I didn't measure the valve opening but I'd guess maybe a quarter inch or so...? Everything valve-related seemed ok after lapping.
Had this many a time on irrigation engines that had been sitting a long time, give it a run and then see what compression you have, usually an engine will come good.
Mark.. If the pistons are indeed "new" and the engine has a fresh bore/hone job, your compression will not be very high until you run the engine for a while and the rings seat in. That's usually about 500 miles or so. Try running it for a while.
I agree with James.
JJL sounds correct as far as the info given any way. Worn cam is possible too. I question a "re-build" that needs valve work. It will of course run on 30 lbs. comp. so that seems to me to be the next step. If needed as a power unit it'll probably work out OK in any case.
After you lapped the valves, did you check the clearance at the heel of the cam. When you remove material from the surface of the valve or valve seat, it decreases the clearance. If it is insufficient, the valve will not completely close.
More likely you need to put some oil in the cylinders. just a few drops so that the rings will seal in the grooves.