Compression test on H model engine

[Milt36]

Did a compression test today on my H model engine that was assembled about 10 years ago.  No new parts were used as the bore, piston and rings measured OK.  Result was 85 psi. which I think is pretty good, and power seems OK while riding.  However, I have never seen a number to compare with.  Has anyone checked their compression?  Just curious...

[pd]

Hi Milt ,

I've read that some of these motors can run on as little as 60 pounds compression .
My '57 M model has around 90 pounds , last I checked .
In more modern 2 strokes , I see upwards of 100#s . But even those need 90#s to run decent .

We have to consider the design of the piston and head Simplex uses . Much more surface than modern pistons and I think as a result , lower compression .
Just me speculating .

Pete .

[Milt36]

Thanks for the reply.  Since I hadn't started the bike in a couple of months, I sprayed a little oil in the cylinder and repeated the test.  This time around, it came up to 95 psi which seems to be a more reasonable value.

[Milt36]

For a comparison, I just checked the compression on my James ML 125cc, which has a fresh rebore, new piston and rings, and it registered 125 psi.  It starts with the kick starter with some difficulty if it sits more than a week.  The simplex is a push start model and it sometimes requires starter fluid it it has sat for a while.

[pd]

Oiled will bring the #s up a bit . Probably a bit more than what would be during regular use .

For a 75 year old machine , I'd say you're doing pretty good .

Pete .