The Ford Barn - View Single Post - No. 7 Cyl never gives evidence of a power stroke on '46 Flathead

rotorwrench · 04-01-2017, 02:15 PM

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I only do differential compression tests if I find a problem like this. Some folks call them a leak down test. The tester has two gauges and a regulator with a calibrated orifice in the pressure manifold in between the two gauges. I put just enough air into the cylinder so that the engine can still be rotated though. By watching the gauge and feeling the pressure on the crank, a person can find top dead center on compression. Once it's at TDC, I then increase pressure in the cylinder to 80 PSI. If the other gauge reads between 65 and 79 PSI then there may be no problem there. You can listen at the carb or exhaust to check for valve leakage and you can check at the breather for excessive ring leakage. If it checks out to be 70 to 79 PSI then I would pull the intake a check for obstructions.

We have mud daubers here in Texas and they can build a nest overnight that would plug an intake manifold or even an intake port. A blocked exhaust port can happen too but it depends on what type of manifolds it has or whether someone put a port divider in wrong or something of that nature. If #7 gets blocked, it would likely be blocking #6 too with the siamese ports in there.

04-01-2017, 02:15 PM	#26
rotorwrench Senior Member Join Date: Jun 2010 Location: San Antonio, Texas Posts: 18,018	Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead Sponsored Links (Register now to hide all advertisements) I only do differential compression tests if I find a problem like this. Some folks call them a leak down test. The tester has two gauges and a regulator with a calibrated orifice in the pressure manifold in between the two gauges. I put just enough air into the cylinder so that the engine can still be rotated though. By watching the gauge and feeling the pressure on the crank, a person can find top dead center on compression. Once it's at TDC, I then increase pressure in the cylinder to 80 PSI. If the other gauge reads between 65 and 79 PSI then there may be no problem there. You can listen at the carb or exhaust to check for valve leakage and you can check at the breather for excessive ring leakage. If it checks out to be 70 to 79 PSI then I would pull the intake a check for obstructions. We have mud daubers here in Texas and they can build a nest overnight that would plug an intake manifold or even an intake port. A blocked exhaust port can happen too but it depends on what type of manifolds it has or whether someone put a port divider in wrong or something of that nature. If #7 gets blocked, it would likely be blocking #6 too with the siamese ports in there.