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EFV-8 Club Forum / General Ford Discussion / Flat head v8 with a miss.

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Discussion Topic: Flat head v8 with a miss.

gunner

07-10-2018 @ 9:34 AM

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40 lbs of compression, Tried 3 new plugs, and one new wire. Still missis on the back cylinder on passenger side. Any ideas.

gunner

kirkstad

07-10-2018 @ 10:06 AM

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If you have only 40 lbs of compression in that cylinder you could have a bad valve or worn or broken compression rings.Try squirting oil in that cylinder if the compression reading increases than it is probably rings,if not, possibly a bad valve.You should check compression with a fully charged battery all plugs removed and make sure the choke and throttle are fully opened.Be sure you have the ignition off as not to get zapped by a plug wire.

gunner

07-10-2018 @ 3:29 PM

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I did squirt oil in it but no increase in compression. I did not have all the plugs out and throttle open. I will try that. Thanks.

gunner

shogun1940

07-10-2018 @ 4:19 PM

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The next test is a cylinder leak down test. With the piston on tdc, the firing stroke. Put shop air in the spark plug hole and see where it comes out. Carb is intake valve, exhaust is the exhaust valve bubbles in the radiator a crack or gasket and out the breather is the rings. GOODLUCK

4dFordSC

07-13-2018 @ 8:09 PM

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Also make sure the ferrule is firmly seated on the end of the no. 4 plug wire, and that the plug wire itself is firmly seated in the distributor cap. I chased an elusive miss for several days before narrowing it down to the plug wire ferrule.

gunner

07-20-2018 @ 5:32 AM

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I did as you said and opened the throttle. My lowest was 40 lbs ( and that cyl. fires good) The high was 55 lbs. The one that don't fire is 45 lbs. I bought a pressure leak down tester a few years ago and never used it until now. Not sure if I read it correctly but no bubbles in the radiator. I set the air pressure at various levels. 7 to 50 lbs. All cylinders read about the same with no air leaking anywhere when both valves are shut. I can distinctly hear air in carb when intake is open and exhaust when exhaust valve is open. Maybe a little in the oil dip stick but not sure ?

gunner

pauls39coupe

07-20-2018 @ 7:18 AM

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40-55 psi is extremely low compression. Are you using a conventional compression tester to get that reading or the leak down tester?
The leak down unit will read a percentage of the air you put in. If your air compressor is only putting out 50 psi then 45lbs would only be a 10% drop. Up the incoming pressure to 100PSI at 10% you should see 90 PSI. You would need to have the tested cylinder at TDC with both valves closed.
A conventional compression tester just reads the pressure produced by that cylinder. Which would normally be above 90 PSI .
Let us know what method you are using.

alanwoodieman

07-20-2018 @ 7:32 AM

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did you run a vacuum test? a steady 20 lbs would be ideal, sticking valves make the reading jump around, a sudden drop would indicate a serious valve problem. has the engine been sitting for a long time? try a little MMO in the crankcase oil/gas and down the carb to help remove the sticking ring/valve problems

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