40 V8 compression change all cylinders same time

[L78CHEVELLE]

I have a original 1940 85HP engine that has been rebuilt professionally about 3 years ago. The engine ran great for the first year and half for about 2,000 miles. Compression on each cylinder was 100 to 105. Now the engine runs poorly, misses, hard to start when hot, and is not reliable. I checked the compression today and every cylinder is 60 to 65 pounds. Valve adjustments are good. I am confused and discouraged. Can any one give me some options?

[Fordestes]

Check for a plugged exhaust, bad timing gear. cam gear may have lost a tooth, some gears were made of a phenolic material .

[WABOOM]

Watching this thread out of curiosity.

[Tinker]

Engines need to be run. Hot Hard to start is a coil indicator, for starts.

[Tinker]

not sure how relative upper engine lucubration is. L heads rely on it. Lead gas reason.

[flatheadmurre]

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Was it sitting for a long time before this happened ??
Since it was rebuilt and i assume no taper in cylinders...rings may just be stuck.
You favourite penetrating oil down the cylinders...then run it up to working temp...see if it improves.
Or long periods of idling...having given you ring to wall sealing issues.
Over rich carburation also has this effect.

[Ol' Ron]

Was the throttles wide open during tests. Screw driver works well.

[Kurt in NJ]

Power valve?? running very rich, gasoline washing cylinder walls reducing compression??, does the oil smell like gasoline??

[joe 1950]

don’t no what grade fuel your using but i’m sure the rebuilder would have put the hardened seats in ,don’t use any fuel that has corn in it ,these old girls fuel systems just don’t like it something to try ,add the proper amount of marvel mystery oil to the gas and oil and drive it ,these old flatheads always had problems with valves sticking just enough to lower compression witch will cause hard starting it may take a while but it’s easier than throwing a bunch of parts at it and still have the same problem ,i’ve used mmo in my model a and my 50 cpe with great results keep us informed

[deuce_roadster]

How often have you retorqued the heads? If aluminum heads, I have to do this at least 4 or 5 times before they stop moving. Alum heads torque stone cold, cast iron torque hot.

[Flathead Fever]

Like Ron said, You need to hold the throttle wide open when doing a compression test. Otherwise the cylinder will not fill with enough air to get an accurate test. You should have a battery charger on the battery so each cylinder is checked with a full charge on the battery. You should pull all the plugs so the engine can crank faster. That's officially how its done. I've done it without doing any of that stuff and have gotten good readings but since your readings are bad you probably should perform the test correctly.

If this was a quality rebuild, its unlikely all the cylinders have poor compression after only 2000 miles. I try a different compression gauge just to verify yours is accurate. You can have a faulty compression gauge and the problem could be elsewhere.

The first thing would be to give each cylinder a squirt oil and see if the compression comes up. The oil gives the rings a temporary seal. If the rings are bad the compression will come right up to specs with a squirt of oil. If you got the engine hot the rings could have lost their tension.

If one cylinder was low you would look for a valve problem, worm cam lobe, broken ring, blown head gasket on that one cylinder. But when all cylinders are low you look for something common to all of the cylinders. If it had a timing chain, it could be stretched. But with gears its unlikely it jumped a tooth. It would probably strip a tooth off and then not run at all. If you had timing marks on the crankshaft pulley you could check it to see if the timing had changed do to the gears not being aligned properly. Always make timing marks and a pointer on your flatheads when you have the heads off!

A plugged exhaust can be checked with a vacuum gauge. I've mostly seen it happen with melted catalytic converters. It happens a lot on those cars. But mufflers can have baffles break loose and then cover the outlet hole. Usually you will hear a rattling muffler. It acts like a governor and will only let you go a few mph but it doesn't run bad it just will only go so fast and it cause the engine to run hot. I suppose a rodent could build a nest inside the pipe or you could run over something and collapse the pipe. You could try separating the muffler from the inlet pipe and see if it makes any difference. You could also do a compression test with the muffler disconnected. I think thins is unlikely your problem, it's pretty rare, at least here in SoCal. Here is a video showing you home to check it with a vacuum gauge. They are neat tools, everybody working on cars should have one.

https://www.ericthecarguy.com/faq/so...t-restrictions

You can also take a rubber tipped air nozzle and with the piston at TDC blow air into the cylinder and see where it comes out. If it comes out the intake the intake valve is burned. Out the exhaust, the exhaust valve is burned. Out the radiator you have a crack or bad head gasket. Out the oil fill the rings are worn. Some air will normally get by the rings. The only way to accurately test them is with a special tool that measures the percentage of leakage. Without that tool about all you get check is for a major failure like a hole in a piston.

[L78CHEVELLE]

Did the first compression check with the engine complete together just like I pulled it in the garage with all the spark plugs out. Got 60 to 65 on each cylinder. Took the intake off and checked the compression again on each cylinder. Same results of 60 to 65 on each cylinder. I am going to try another gauge tomorrow. When I had the engine rebuilt it had only 26,000 original miles and had the sleeves in each cylinder. The machine shop took the sleeves out and put in Ford 8BA standard pistons telling me that was the normal thing to do. Like I said it ran good for 2,000 miles after that.

[Craig CT]

Originally Posted by L78CHEVELLE

I have a original 1940 85HP engine that has been rebuilt professionally about 3 years ago. The engine ran great for the first year and half for about 2,000 miles. Compression on each cylinder was 100 to 105. Now the engine runs poorly, misses, hard to start when hot, and is not reliable. I checked the compression today and every cylinder is 60 to 65 pounds. Valve adjustments are good. I am confused and discouraged. Can any one give me some options?

Hi L78CHEVELLE; It could be your cam timing. Good luck, Craig.

[Kruzn40]

Do a leak-down test on each cylinder with compressed air. That will give you an idea of the path that your cylinder pressure is using to make its escape.

[BUBBAS IGNITION]

Cam , cam gear issue is the only issue that would lower compression on all cylinders. I would look for a messed up worn cam gear.......

[Craig CT]

Originally Posted by BUBBAS IGNITION

Cam , cam gear issue is the only issue that would lower compression on all cylinders. I would look for a messed up worn cam gear.......

Hi Bubba; If it's a press on it probably just moved a little, Craig.

[L78CHEVELLE]

Tried another compression gauge and put some oil in the cylinders with the same results of 60 to 65 in each cylinder. Checked the exhaust today to see if anything was plugged. Everyone may laugh at my method for check the exhaust system. I put a shop vacuum hose in the tail pipe and made it good and tight. The shop vacuum pulled good air from the front of the engine with the intake off and me turning the engine over with the starter. Next step is to take out the radiator, fan, crank pulley, distributor, front engine cover and see what I find. I can find no one local to help me with this deal and have become very discouraged. Thanks to all on the Ford Barn for giving me ideas of how to correct this problem. New rebuilt engine with a compression problem has taken the wind out of my sails. Will post what I find when I take off the front engine cover if I can figure out what I am looking at. Should there be a dot on the cam gear and a dot on the crank that line up together if correctly installed?

[Fordestes]

Sounds like you are headed in the right direction, the cam gear and crank gear have a mark on each and should match together , is it possible the builder re used the old cam gear ,as you stated it was a low mileage engine to start with , this has happened ,when a part looked good sometimes it gets re installed, someone not knowing the habits of a fiber gear may have assumed it would be ok to re use?

[BUBBAS IGNITION]

Originally Posted by L78CHEVELLE

Tried another compression gauge and put some oil in the cylinders with the same results of 60 to 65 in each cylinder. Checked the exhaust today to see if anything was plugged. Everyone may laugh at my method for check the exhaust system. I put a shop vacuum hose in the tail pipe and made it good and tight. The shop vacuum pulled good air from the front of the engine with the intake off and me turning the engine over with the starter. Next step is to take out the radiator, fan, crank pulley, distributor, front engine cover and see what I find. I can find no one local to help me with this deal and have become very discouraged. Thanks to all on the Ford Barn for giving me ideas of how to correct this problem. New rebuilt engine with a compression problem has taken the wind out of my sails. Will post what I find when I take off the front engine cover if I can figure out what I am looking at. Should there be a dot on the cam gear and a dot on the crank that line up together if correctly installed?

Wouldnt worry so much in regards to the dots as i would TDC versus actual valve position....

[STEVE O]

Just wondering, being you have the intake off and put it on top dead center (not sure how to be sure it is exactly on TDC) exhaust stroke you should be able rock the crankshaft one way then the other and see the intake valve move one way and the exhaust the other way. If not something has slipped. If this is true on these flatheads of this era as later v8s that would check cam timing without pulling the timing cover????