engine vacuum problems

[hmmech]

i have a rebuilt 53 ford flathead engine ,ever sense i 1st started it ,it starts right up but rough idle and vacuum gauge jumps all over from 5-12"and increase rpm from 8-15"with little pops out pass. side of exh., i did a compression and cyl. leak down test looks ok, the vacuum chart shows could be leaking head gasket, put new head gasket in and the same thing, i do have new alum heads which look ok, could they be warped and causing this problem, also checked my 2x2 intake manifold and carbs with no leaks, not sure where to look now. any help out there ?

[big job]

sounds like tight valve lash from here....

[cadillac512]

First I'd make sure there aren't two plug wires switched. Then I'd idle the engine and short each plug wire in succession to see if each drops rpm slightly as it's shorted. If you can isolate the affected cyls it will be easier to diagnose.



Terry

[hmmech]

readjusted already

[tubman]

Sounds like a vacuum leak to me.

[Kurt in NJ]

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Sticking valves perhaps--- try some oil--- memo--

[JSeery]

Any kind of head gasket or warped head issues would have shown up on the leak-down test. I'm with tubman, sounds like a vacuum leak somewhere.

[1931 flamingo]

Vacuum wipers??
Paul in CT

[Drbrown]

I'm with others here re vacuum leak. As a possible point of reference, best my '47 with 30,000 m will do at running temp is a steady 19.

[corvette8n]

Try spraying brake parts cleaner around base of manifold,carb,anyplace where you could think of where it would leak. The rpm will change when you get close. Wiper hose is another possibility, mine was all cracked, I found it when I tried to lube the wiper motor with a syringe and I had a puddle of MMO under the dash.

[hmmech]

my 53 has no vacuum wipers so i will try spraying around the intake and check for leaks

[JSeery]

Originally Posted by hmmech

readjusted already

How did you go about adjusting the lifter clearance?

[hmmech]

I went by the book to adj the lifters,had holes drilled in the housing to hold the lifter in place while adjusting and they were tight locked in place

[JSeery]

Which book, LOL?

[hmmech]

How to Rebuild and modify ford flathead v8 engines. maybe i should have read it twice.

[Tinker]

If valves adjusted correctly. I'd guess its the valves sticking with tight new guides. mmo down the carb till it smokes out and then drive it. Repeat.

[Tinker]

I would think a vacuum leak in the intake would just create a consistent low vacuum reading through out. Not a erratic vacuum reading, that would show a individual cylinder or valve issue.

[hmmech]

i will try mmo tomorrow, i put a little in the oil and gas tank ,but didn't do the carb

[Talkwrench]

Just going to ask what cam, so we can judge roughly what vacuum it should be.

[JSeery]

Originally Posted by hmmech

How to Rebuild and modify ford flathead v8 engines. maybe i should have read it twice.

If you happen to have the intake off again for something I would check a couple of valves again for clearance being sure the lifter is on the cam heel for each one you check. But the issue is more likely a vacuum leak somewhere. Also, if the cam was not degreed when it was installed there is always the possibility of the cam being off or the timing gear not being marked correctly.