351M
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Senior User
Joined: Apr 2006
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From: Phoenix
351M
Have a 351M I rebuilt. Just barely runs, can not get much vacuum. Indication is the valve timing. Can I put a wrong timing chain and gear set on or maybe a wrong cam. Any ideas. Have good cranking compression and Ignition. Timing is set correctly.
Laughing Gas
Joined: Nov 2003
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From: southeren Oh
Yes to all your questions. What do you mean timing is correct? To factory specs?
Is this a stock rebuild?
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From: Phoenix
Stock rebuild, both ignition timing and alignment of marks on the timing gear set are correct. I know that other cams for that engine will interchange but could I have a cam for a different engine series in this?
Elder User
Joined: Apr 2009
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From: San Diego
If you got an excessively agressive cam for the components that you have, that could be what is causing your low vacuum. The vacuum is created by the cylinders drawing in air for combustion and you have to have the right combination of lift/duration/overlap combined with flow characteristics of your intake and heads.
That very reason is why some racers and performance monsters switch to a hydroboost assisted brake system, to conserve vacuum (or because they don't have enough vac).
That very reason is why some racers and performance monsters switch to a hydroboost assisted brake system, to conserve vacuum (or because they don't have enough vac).
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351m
how much cranking compression do you have? I had a 400 with 7/8 top rings cracked that always gave fits to start, but when it would, ran just fine. Had I known that damage was there, I would have overhauled it 2 years earlier. With low vacuum, your carb won't have enough signal to function much past the idle and transfer circuits. I would check out the intake/carb for leaks, and all vacuum ports are functioning/capped and accounted for. additionally, like 6fifty pointed out too, that too large of a cam will cause lower than stock vacuum. could u share any specifics?
)
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From: Phoenix
The vacuum leak is a possible. Have not removed the intake yet. My first thought was the timing gear alignment. I have never goofed on that in hundreds of rebuilds but there is always that first time. It check out OK. I had the machine work done at a machine shop/parts house so I got everything as a kit. I thought maybe it was possible the cam and or the timing gear set was for a different engine and causing my problem. I have not put a degree wheel on it yet to check valve timing. Kids have lost the wheel and I have to buy another. All of the parts are stock replacement. The engine would idle if I advanced the ignition timing and all cylinders check out pretty even, RPM drop test. Have opened up the exaust by removing the exaust back to the headers. I got pis'd at this and have not touched it in a year. During the year the shop I bought the parts from went out of business and I can't find the receipt. I am just having to much fun. Thanks for responding
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bruce351
335 Series- 5.8/351M, 6.6/400, 351 Cleveland
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Aug 28, 2003 10:09 AM
