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Good point although 48 should have the windshield wiper port at the front of the manifold -- but maybe it has a later manifold or engine installed. Did the 288/327 manifolds ever have the raw gas drain port on the bottom of the manifold near 7 and 8? If so I know leaking or broken check valves caused problems on some 356 engines with that setup.

Tim: I believe my windshield wipers are plumbed to the auxiliary vacuum pump on top of the fuel pump, but I will verify this tomorrow. Still worth a try to block that hose to see if there is any improvement in the vacuum.

Howard: I was not aware that there might be an opening somewhere beneath the intake manifold. I'm very interested to pursue that possibility.

Ernie: I'm getting a strong spark to both the 7 & 8 plugs, but I will double check the distributor cap to see it there are any abnormalities.

Thank you all for your suggestions.

Quote:

The aux vacuum pump is in series between the wiper motor (and vacuum antenna if you have that option) and the intake manifold. In normal use the manifold provides all the vacuum and because it is stronger than pump vacuum the valves inside the pump are kept open and pump just acts as a pass thru. In cases of engine load or wide open throttle when manifold vacuum is minimal or non existent the valves in the pump start operating and the pump takes over and provides vacuum to the wiper motor at a reduced volume to keep the wipers moving at a slowed speed.

On a 48-50 there should be port at the top front of the manifold and via a fitting and curved tube enters the fuel pump vacuum section at the front. Another tube exits the rear side and follows the block to the firewall area where via tubing and hoses vacuum reaches the wiper motor. If the vacuum antenna is present, a tee in the wiper hose near the motor feeds the valve for the antenna.

I'm making some headway hopefully toward a solution

I've verified that wires from the distributor to the plugs are indeed in the correct firing order.

I have blocked the vacuum line to the wipers with no improvement to the low idle vacuum, so there is no leak there.

I have also blocked the vacuum line for the carburetor to the vacuum advance. No improvement there either.

I have yet to find an additional plug hole anywhere beneath the intake manifold, but I just purchased an inspection camera that may allow me to investigate the bottom of the intake manifold, and in addition allow me to examine the back portion of the intake manifold from the inside where it meets the 7 and 8 cylinders. Wish me luck.

Any additional theories are most welcome

It's probably the intake manifold gasket. To test spray throttle body cleaner, Gum Out, or anything containing toluene around the manifold. If the motor changes the manifold is leaking. Some starter fluid on a rag can achieve the same result.

After that you can do a running compression test. Put the tester in a cylinder and run the motor. Make sure to ground out the plug wire to avoid coil damage. This test will check for broken rings, sticking rings, and sticking valves.

Tim, thanks for your thoughts. Dave Czirr suggested a while back that I spray oil along the manifold to block connection in an attempt to perhaps seal the connection and improve performance. This approach did not produce discernible improvement, but your suggestion to introduce a flammable substance to that area might give a more pronounced effect, assuming there is a gasket leak. A leak on the rear intake gasket would certainly explain the lack of contribution from those last two cylinders.

What should I expect to see when doing a running compression check on cylinders 7 and 8, and how should I interpret the readings?

Mystery solved, thanks to Dave, Tim and 50 2382 who all suggested the spraying of something onto the gasket area of the intake manifold to block connection. When I sprayed the Gumout on that connection the idle smoothed right out. So this leak is evidently the culprit for both my low vacuum readings and the failure of 7&8 to fire. Glory Hallelujah.

Now for some guidance on what to do to replace the gaskets. Can I remove just the intake manifold, will I have to wrestle with the exhaust manifold as well. There appear to be eight nuts that must be removed to release the manifolds plus a lot of auxiliary connections. Can I replace just the intake gaskets?

Thanks to all for your help. I'm almost home free, but not quite yet.

Unfortunately you'll need to remove them as a pair and, in fact, it's better not to separate them at all unless there is some really good reason for going so. Actually it's easier to get them off together as a pair anyway. Be sure to check for flatness or have them milled. As for gaskets, you can get only those you need from Olson's Gaskets in Oregon.

PackarDon: thanks for the advice. Are the two manifold actually connected to one another? If I take them to a machine shop to be milled, can I assume that the block is straight and true? I'm assuming I may need a lot of PB Blaster to free up some of the exhaust connectors. I ordered a three piece gasket set. Are there any sealers required to be applied during installation and is there a tightening sequence or torque specification to be mindful of?

The exhaust and intake manifolds are bolted together from the bottom under the carburetor where the choke's heat exchanger is. There is, I believe, also a gasket between the two halves but no need to get one if you're not taking them apart.

In addition to the three-piece gasket set, you'll need the doughnut gasket at the exhaust connection. No, no sealer in any of these areas. As for tightening sequence, you would have to refer to the service manual but a typical rule of thumb is to tighten center out.