Only 2 out of 4 bangin'

[Licensed to kill]

I've been working my model a trying the get it running better and it appears that I'm getting no bang in cylinders 3 and 4. I'm getting a very erratic vacuum reading at about 16 inches so thought perhaps a stuck valve. All valves are working freely, I have good spark, timing is right on, and with the "thumb" test, I have good compression (I will get real numbers when my adapter for my compression gauge arrives). Disable cylinders 3 or 4, no change, disable 1 or 2, change. The erratic vacuum reading would suggest that I'm losing cylinder seal someplace but I don't think it would be enough to prevent ignition??. Anything else I should check while waiting on my compression tester adaptor?.

[George Miller]

Originally Posted by Licensed to kill

I've been working my model a trying the get it running better and it appears that I'm getting no bang in cylinders 3 and 4. I'm getting a very erratic vacuum reading at about 16 inches so thought perhaps a stuck valve. All valves are working freely, I have good spark, timing is right on, and with the "thumb" test, I have good compression (I will get real numbers when my adapter for my compression gauge arrives). Disable cylinders 3 or 4, no change, disable 1 or 2, change. The erratic vacuum reading would suggest that I'm losing cylinder seal someplace but I don't think it would be enough to prevent ignition??. Anything else I should check while waiting on my compression tester adaptor?.

Intake manifold leak at the rear port.

[JDupuis]

Distributor body may have an internal short at #3 and #4.
They will spark, but they spark 3 +4 in unison. Causing no change when you manually short them out. Put an ohms tester on 3+4 to see if they are shorting.
Jeff

[Licensed to kill]

Originally Posted by George Miller

Intake manifold leak at the rear port.

We have a BINGO. Thanks George. Pulled the manifold and it's broken where it bolts to the block. I would have never thought to look there.

[Licensed to kill]

Originally Posted by JDupuis

Distributor body may have an internal short at #3 and #4.
They will spark, but they spark 3 +4 in unison. Causing no change when you manually short them out. Put an ohms tester on 3+4 to see if they are shorting.
Jeff

Thanks for the reply Jeff. It turned out to be a broken intake but I appreciate the reply and it's another piece of "knowledge" to tuck away for future reference.

[George Miller]

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Glad you found it.

[Tom Wesenberg]

Easy find and fix, but remember that a SLOW to CLOSE valve can show compression, but not close quickly enough to give compression on a running engine. This is where a vacuum gauge comes in handy.

If you ever have a sticking valve, the first thing I'd try is a can of SEA-FOAM in the oil, and another can in the gas. It sure did the trick for me with a sticking hydraulic lifter that rattled. Within 5 miles, the engine was running like new.

[George Miller]

Originally Posted by Licensed to kill

We have a BINGO. Thanks George. Pulled the manifold and it's broken where it bolts to the block. I would have never thought to look there.

It makes it easy when you give all the facts. Good job on your part.

[Licensed to kill]

Originally Posted by Tom Wesenberg

Easy find and fix, but remember that a SLOW to CLOSE valve can show compression, but not close quickly enough to give compression on a running engine. This is where a vacuum gauge comes in handy.

If you ever have a sticking valve, the first thing I'd try is a can of SEA-FOAM in the oil, and another can in the gas. It sure did the trick for me with a sticking hydraulic lifter that rattled. Within 5 miles, the engine was running like new.

I checked all of the valves with a ore scope through the sparkplug hole while turning it over with the crank and they all appeared to moved quite freely. I appreciate the heads up about seafoam. Wouldn't hurt to keep some on hand. Thanks

[100IH]

Most times that an intake is broken, it's because the heavy washers were on backwards. The dome of the washer must go on the outer side where the nut will contact it, when on wrong, the dome acts as a wedge to spread the ears of the casting. Renove the manifolds as a pair and check to see if there alignment rings. The exhaust manifold is prone to warp so intake and exhaust are no longer dead flat at the gasket surface. The bottom edge of the ports of both manifolds must also be in alignment "in a straight line" to match the block ports. Also, the exhaust will distort where it joins with the downpipe to the muffler, making a good seal difficult. All can be addressed by replacing both manifolds,gaskets and rings. Here is an opportunity to solve many common Model A problems in one hour. Now might be the time, fortunately the reproduction parts are finally excellent pieces if gotten from the major suppliers.

[Licensed to kill]

Originally Posted by 100IH

Most times that an intake is broken, it's because the heavy washers were on backwards.

Yup, I had them on the other way because that is how they came off. Should have looked it up rather than assuming that they were i the right way initially. So, I welded up the intake just as a test and am getting 22-23 inches of vacuum and, while not as steady as i would like, a LOT more steady than before. I have an exhaust leak now as the #4 exhaust wont seal but that is just because the welded intake is obviously not quite straight and is preventing the exhaust from sealing. No big deal as I will be relpacing the intake as ASAP anyway.

[100IH]

Worth a try but often each manifold will have it's own plane vertically. Surfacing can even them up against the block but your left with different heights than the ports on the block and issues mating with the muffler. without the rings , further warping will likely continue. Many ignition and carburation side tracks later, another manifold gasket will be needed. Use a straight edge to determine what your up against. A new pair will be perfectly matched.