40 V8 compression change all cylinders same time

Russ/40 said:
I'm thinking the valve lash is off. Bring each valve to max lift and rotate another full turn. Then adjust the lash.
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It ran good for 2,000 miles with the valve lash the same as it is now. Would that not eliminate the valve lash as the cause?
 
L78CHEVELLE said:
It ran good for 2,000 miles with the valve lash the same as it is now. Would that not eliminate the valve lash as the cause?
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Not if the degradation is due to burnt valves. Run them too tight and they will burn. Not having an engine open currently to check your adjustment technique, frankly I don't know if it is valid. I do know that is not going to put you directly on the heel of the cam. You mention your engine was built. When assembled, who adjusted the valves initially? Was it you with the method you have described?
 
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A gradual degradation of performance as a result of compression loss, to me anyway, screams burnt valves.
 
Russ/40 said:
Not if the degradation is due to burnt valves. Run them too tight and they will burn. Not having an engine open currently to check your adjustment technique, frankly I don't know if it is valid. I do know that is not going to put you directly on the heel of the cam. You mention your engine was built. When assembled, who adjusted the valves initially? Was it you with the method you have described?
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The valves were adjusted by the machine shop. The valves would not all change at the same exact time resulting in the exact same drop from 100 to 65 in each cylinder of the engine. Only change I have made on the valve lash is to increase the exhaust valve lash from 12 to 14. We used the exhaust valve lash setting Ford gave in 1940 and then changed it to the new listing of 14 Ford came out with in 1947.
 
Russ/40 said:
A gradual degradation of performance as a result of compression loss, to me anyway, screams burnt valves.
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Burnt valves could be the answer, but would all the cylinders react the exact same way going from 100 to 65 at the same exact time? I have only used straight gasoline in the car since the rebuild.
 
Russ/40 said:
A gradual degradation of performance as a result of compression loss, to me anyway, screams burnt valves.
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I have the intake off now. This week if nothing new comes up I plan on taking one of the heads off and checking the valves to see if any show signs of damage. I have the tools needed to take the valves out.
 
before you take anything apart, do a cylinder leak test to see where the compression is going. if you dont have the fancy tool, i dont, you just weld an air fitting to an old spark plug, screw it in, get that cylinder to TDC and put low pressure, like 20lbs to it and listen. air out the carb=intake valve. air out the exhaust pipe=exhaust valve, and air out the oil fill/breather cap=rings. if you get all the same valves, i would suspect cam timing. this one is a puzzler!
 
L78CHEVELLE said:
Burnt valves could be the answer, but would all the cylinders react the exact same way going from 100 to 65 at the same exact time? I have only used straight gasoline in the car since the rebuild.
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It would be very weird or highly coincidentally that every cylinder went from 100 to 65 in a yr. Might need to take the intake off.
 
cas3 said:
before you take anything apart, do a cylinder leak test to see where the compression is going. if you dont have the fancy tool, i dont, you just weld an air fitting to an old spark plug, screw it in, get that cylinder to TDC and put low pressure, like 20lbs to it and listen. air out the carb=intake valve. air out the exhaust pipe=exhaust valve, and air out the oil fill/breather cap=rings. if you get all the same valves, i would suspect cam timing. this one is a puzzler!
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I am going to try and buy a piece that will allow me to put air into the cylinders to check for leaks before I take the heads off this week. If it was not for the help on the Ford Barn I would have to push the car to the back of the garage and wait until I came up with a new approach to the problem. THANKS TO EVERYONE.
 
Craig CT said:
Hi L78CHEVELLE; If it's a faulty compression gauge that will take you back to carburetor and or ignition, Craig.
Hi L78CHEVELLE; I'd make sure it's not a faulty compression gauge before I took anything else apart. Good luck, Craig.
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Post your procedure for compression testing when you try new compression gauge etc Something might be different which might give us a clue to why compression drop across all cylinders. Mike
 
Mike in AZ said:
Post your procedure for compression testing when you try new compression gauge etc Something might be different which might give us a clue to why compression drop across all cylinders. Mike
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I used another compression gauge and got the same readings on each cylinder. Will try the leak down with air on each cylinder as soon as I can get the tool. I will post the results.
 
I think everyone should be aware that the O/P said the engine started running poorly and when he checked the compression was down uniformly. I think that points to something more serious than bad testing technique or a bad compression gauge. The leakdown test sounds like the next best test.
 
i will mention this also, TDC on compression stroke, and if you are not right on top dead, some times the air will cause the crank to turn so keep your hands-fingers-trouble light cords-or cat away from the fan. good luck let us know what you find
 
Just curious about compression testing procedure. If throttle is not full open, will compression numbers be lower than at full open? Thanks. Mike

Never mind. OP is testing with intake off. CRAFT. Sigh. M
 
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