1977 OMC 235/Ford 351W Low Manifold Vacuum, Stalls when letting off throttle

Jrobbs3

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I've done a full rebuild on a 1977 OMC 235/Ford 351W. It fires up on second or third crank every time and idles very smoothly. I can throttle it up and hold it up at 2,000-3,000 RPM and it'll run there just fine. As soon as I let off the throttle, it'll stall (or nearly stall) if I don't keep on the throttle slightly. Not sure if this is related, but it's also pulling rather low intake manifold vacuum, 10-12 inHg. I should also note, it has very little running time on it since the rebuild, maybe 10 or 15 mins. Any thoughts are appreciated.
 

kenny nunez

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It could just a carburetor problem. Has it been rebuilt lately? Some time just the metering block gaskets are the problem from shrinking.
 

Jrobbs3

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It could just a carburetor problem. Has it been rebuilt lately? Some time just the metering block gaskets are the problem from shrinking.
It was rebuilt by a carburetor shop as part of the engine rebuild (maybe a month ago).
 

kenny nunez

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That is good. When every thing is right the idle mixture screws should be 1.5 turns out and the idle stop 1-1.5 turns. Are you sure the there are no vacuum leaks from under the carburetor? Are you sure the timing marks on the balancer’s “0” mark line up with the pointer with #1
If the cam is new was it broken in correctly With the special grease?
 

Jrobbs3

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That is good. When every thing is right the idle mixture screws should be 1.5 turns out and the idle stop 1-1.5 turns. Are you sure the there are no vacuum leaks from under the carburetor? Are you sure the timing marks on the balancer’s “0” mark line up with the pointer with #1
If the cam is new was it broken in correctly With the special grease?
Idle mixture screws are at 1.5 turns. I'll have to check idle stop screw, I just adjusted it to idle at 600-700 RPM.

I was thinking about leaks below the carburetor as well, but when I spray around with carb cleaner checking for leaks, I get no change in engine speed. Maybe I change my gaskets just to make sure. I could also replace the hose that goes from the carburetor riser to the PCV valve.

Timing marks on the balancer do line up at 0 when #1 is at TDC. I have set the timing to 10 degrees advanced in accordance with the shop manual.

I reused the old cam, but replaced the lifters. I coated all of the cam lobes and lifters in assembly lube prior to start up but no special grease. I've done no specific break in procedure since the rebuild, only idling and occasional throttling to check engine performance.

It might be worth noting, that I had my rocker arms adjusted WAY too tight initially and I tried to get the engine run in this condition repeatedly. I wonder if I caused damage to valvetrain components.
 

kenny nunez

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Go back over the valve adjustment. I only tighten the lock nut 3/4 of a turn after there is no slack and the push rod still spins. I doubt if there is any damage. There is a procedure to set the valves cold by starting @ tdc and rotating the crankshaft by hand and adjusting certain valves. If you have a Ford engine service manual it explains the method. Mark the rocker arms adjusted with chalk.
 
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