Low Compression

[John_Aldrich]

I have a stock 1915 engine, rebuilt with 30 over pistons. Runs fine, starts easy.

Why would a newly rebuilt engine have low compression?

[JTT3]

When testing did you have the throttle wide open? If not that could be part of the problem.

[TXGOAT2]

How low is low?

I'd suspect non-seated rings. Engines run best after a few hundred miles.

If the rings are fitted properly, too little valve clearance could cause it, or mis-timed timing gears.

If you are certain the engine is assembled correctly and properly serviced, driving it on the road at moderate speeds and loads for a hundred miles or so might do it a world of good. Head retorquing is important.

[Jerry VanOoteghem]

I had an engine rebuilt several years ago. At first, it was a real dog. I wondered why I ever bothered with the rebuild. After driving it maybe 50 - 100 miles it really picked up on power and has been great since. I never checked compression, (and never do unless I have a real problem), but I'm willing to bet it went way up after the break-in period.

[Kevin Pharis]

Compression tests are always low when hand cranking. Take all the plugs out and give er a good spin. Need a few compression pulses to build up a real gage reading

[Norman Kling]

Compression will vary with altitude. As the air pressure decreases with altitude, the compression falls. Also poor fitting or worn rings will lower it also sticking or burnt valves.
Norm

[John_Aldrich]

When testing did you have the throttle wide open? If not that could be part of the problem.

No, I was alone at the time. I was getting 30-35 psi

A fellow T nut which has MANY more years in the hobby than I have cranked it and the first words out of his mouth were "you have no compression".

My Touring hand cranks MUCH harder than my 1915.

[JTT3]

The throttle needs to be open all the way especially if you’re hand cranking. Good advice about removing spark plugs, it will allow you to spin the engine over several times with relative ease. Others may have a better idea but on a non starter T, it is what it is. If you have a large drill you may be able to find some round stock long enough to put a crank ratchet on one end and the other in the drill to turn it over with the spark plugs out. Of course the OD OF THE STOCK HAS TO GO THROUGH THE HAND CRANK HOLE.

[Charlie B in N.J.]

30 to 35 is not no compression. First off as stated above you’ll probably have to put a few miles on it to seat everything but if you simply must know do a wet compression test. Do the dry test first. Pull the plugs, open the throttle fully and the choke open too. Get another set of hands to help. Do the dry test on all 4 holes and write the readings down. Shoot a few shirts of motor oil into the first hole and test again. If it goes up the rings haven’t seated yet. If it doesn’t the valves are usually the cause. You won’t be guessing what the problem is after doing both tests. You’ll know.

[Jerry VanOoteghem]

You say your car runs fine and starts easily. Are there any problems you're having that you have not mentioned? If not, this is why I don't check compression, or examine spark plugs, or use a motometer. As long as there is no problems, I leave that stuff alone. If something is not just right, it's a different story.

I'm fairly confident that your rebuilt engine will have better compression when it's all seated in.

[TXGOAT2]

I agree with Jerry. Be sure it has plenty of oil and water, and go drive it, avoiding hard pulls, lugging, or high speeds. I'd carry 3 gallons of water and a quart or two of oil. You probably won't need it, but it's good to have on hand. Keep a close check on water and oil. Watch or leaks, loose bolts or wires, or any other glitches. Expect some oil consumption. The engine may run a little on the warm side. Driving at varying speeds is good. Avoid extended periods of idling. If you have a lot of experience with this engine prior to the rebuild, don't be surprised if it behaves differently than it did before. Control settings and starting procedures may be a little different than before.

[Norman Kling]

Another thing to consider if you are hand cranking to check compression. Jack up one or both rear wheels and leave the parking brake in the neutral position. There is always a drag on the disk clutch so jacking the wheel will allow the rear axle to turn if it is dragging. Also remove all the spark plugs while testing. You should be able to make quick complete circles with the crank under those conditions.
Norm

[John_Aldrich]

I agree with Jerry. Be sure it has plenty of oil and water, and go drive it, avoiding hard pulls, lugging, or high speeds. I'd carry 3 gallons of water and a quart or two of oil. You probably won't need it, but it's good to have on hand. Keep a close check on water and oil. Watch or leaks, loose bolts or wires, or any other glitches. Expect some oil consumption. The engine may run a little on the warm side. Driving at varying speeds is good. Avoid extended periods of idling. If you have a lot of experience with this engine prior to the rebuild, don't be surprised if it behaves differently than it did before. Control settings and starting procedures may be a little different than before.

Nope... Not having any issues with it. Been running it for a couple of years like this and she just keeps chugging along. Compression has never changed (according to how hard it is to crank).

[TXGOAT2]

If it runs well and you have no issues, drive it and enjoy it.

Low compression is usually accompanied by excessive oil burning and leaking, low power, generally sluggish performance, plug fouling, poor fuel economy, and hard starting when cold.