Rough Idle on 312, Intake Gasket failure.

[wearymicrobe]

Quick question I have a 312 y Block that has ~1500 miles on a rebuild. Only problem was it was rebuilt built in ~1983.

I have been going through the motor and it runs just fine with a rebuilt carb and a F100 distributor with the proper vacuum advance.

Only issue is at idles at ~600rpm the motor shakes a good bit more then I remember. Engine mounts are fresh and anything above idle it smooths out immediately. Plug are the proper color and the car does not hesitate off idle.

Manifold vacuum is very very low though, ~11in/mg, it jumps right up to where it is supposed to though at anything above ~800rpm. Compression is clean as well.

Sounds like a intake gasket right? Anything else that I should do while I have the manifold off the motor. Its a major pain to remove and I do not want to do it twice. Carb gaskets are already new and of good quality.

Also is there a preferred manufacturer for the 312. I have had a few gaskets in the past be to think to get to seat properly.

Anything else to look for.

[scicala]

Hello,

I'm not sure if you have a 2 bbl. or 4 bbl. carb, but if you can't increase the manifold vacuum and make it idle smooth by using the two idle mixture screws, then you may have dirt or a restriction in the idle circuit of the carb, especially if the carb was left with fuel in it and not run for years. I don't think you have any leaky intake valves if you say the engine runs well everywhere except idle and it had good compression. It's possible you have a vacuum leak like the intake gasket or somewhere else. Should be easy to check for that before you remove the intake. Sal Cicala

[Motorhead6]

An easy test for that vacuum leak Mr. Cicala alluded to would be to spray some starting fluid around the intake while the engine is idling. If there is a leak, the engine will immediately smooth out while spraying near the problem area. Be careful not to spay that stuff near anything real hot as it is very explosive.

Just went through this with my 312. Turned out to be the power valve in a Holley. A while back I had the same issue and resolution with a tea pot. You didn't mention which carburetor you have. I also had a rich mixture at idle up to about 1500 RPM and float bowl leak down while sitting for 15 minutes or so and you didn't mention any of these other issues.

In talking to a local carburetor rebuilder, he didn't believe the power valve was bad, but they fixed my problems and I can't argue with results. They were not "blown", but were apparently leaking thanks to the ethanol in gas these days. My guess is that trash found it's way onto the power valve internal seat. For some reason I have been finding stuff that looks like silty calcium sediment in my float bowls even though I use 2 fine filters in the fuel line; one at the tank and another between the pump and carburetor and add ethanol fuel treatment at every fill up. Also, new tanks and fuel lines; one of the tanks is plastic.

Maybe someone has a resolution to this problem other than to avoid use of non-alcohol gas. Maybe a better additive.

[Ole Don]

The vacuum you have sounds like my engine. Do you have a clue what cam is in the motor? My 312 has Isky, and runs 11 inches on a good day, but still powers the wipers. My experiments with cams and valve lash lean towards more lash, - more vacuum. Are your rockers set to .018 or more? Dwell correct? Points clean?

[wearymicrobe]

Originally Posted by Ole Don

The vacuum you have sounds like my engine. Do you have a clue what cam is in the motor? My 312 has Isky, and runs 11 inches on a good day, but still powers the wipers. My experiments with cams and valve lash lean towards more lash, - more vacuum. Are your rockers set to .018 or more? Dwell correct? Points clean?

It is supposed to be a 1/4 Isky. I have run it on a few different carbs that I know are good. With a tri power setup i get 10in/mg with a single 97 I get a bit more say 13.5in/mg.

Same rough idle though.

New points and dwell was set to book.

With the gauge on manifold vacuum I get the highest signal set to ~4 degrees above factory timimg mark but that I have seen in the past. Still only ~1in/mg above.

Only thing I can think is that my tach is reading a 100-150 rpm lower then I would expect. As soon as I bring the idle up everything feels and looks good.

Can you even get a Y block to idle at ~450-500 rpm.

[Herman Munster]

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Yes, my 292 does idle at about 450 RPMs.