Low Compression

[Clownfish]

The photos below show the current status of my restoration. The engine was bored to .040 oversize and new valves, pistons, rings etc. were installed by a local machine shop about 3 months ago. While installing the governor and magneto I have noticed that there is very little compression when turned over using the hand crank. It is very difficult to feel any compression with your finger when trying to set the # 1 cylinder at TDC. I have not yet primed the oil pump but I did add a small amount of oil through the spark plug hole. The valves have been set to .015. Is it normal not to have compression at this stage and will it go up when I get the oil pump primed and use the electric starter? If I have a problem I want to know it before I get everything reassembled.

IMG_1304.JPG (401.25 KiB) Viewed 554 times

IMG_1301.JPG (436.59 KiB) Viewed 554 times

[Eugene]

You should have compression at this point. The problem is that when cranking the engine over by hand the compression will be lower than if you were using the starter.

Put a tablespoon of engine oil on top of each piston. Rotate engine over by hand 3 or 4 times. Then see if you have compression, use compression tester.

On a fresh overhaul you should see around 120 psi when cranking the engine over with the starter.

Prime the oil pump - first thing. Priming the oil pump will not help the compression issue.

[Matt Kirsch]

Compression will probably be lower than 120, possibly much lower, because the rings have not seated yet. That will happen with some run time, and especially some work.

[Hengy]

Every time that I have had my engine apart, I have had to pull start it to build enough compression to make it run. Once I did that the first time, POOF...started on the first pull of the starter every time after that...

[Clownfish]

I put more oil in each cylinder and using the hand crank the compression is better but not good. Nos 1 & 4 are about 30-35 psi No 2 is 60 and No 3 is 100. I primed the oil pump and it seems to be pumping good. Later today after my battery is charged I'll try using the starter for about 30 Sec and see if the compression improves. If nothing changes I guess I'll just wait until it is ready to crank and if it won't start I'll try pulling it to get it started. Thanks for the info that this situation isn't unusual. At least I won't worry about it at this time.

Glenn