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Low manifold vacuum

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War Eagle
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Posts: 14
Joined: Tue Nov 26, 2013 4:19 pm
Location: Pike Road, Alabama

Low manifold vacuum

Post #1 by War Eagle » Wed Feb 26, 2014 6:15 am

I knew my 300 wasn't running/idling quite right after the rebuild, so I hooked a vacuum gauge up to my intake manifold port and was only getting 5-7 in HG. So I check the compression and #1 and #3 were at zero!?!? I readjust/backoff the rocker arm nuts on those 2 intake valves and get the following readings:
1 - 150
2 - 160
3 - 160
4 - 150
5 - 155
6 - 160

So the compression problem is fixed, but I am still getting 5-7 in hg from the manifold port. I then try the carb vac port and get the same reading.
I retighten all intake bolts, same problem. Do the stock intake manifolds ever warp enough to cause a problem?
Does this sound more like late valve timing or a vacuum leak? When the cam was installed the dots lined up and the Comp Cam(252) is supposed to 4 degrees advanced. If I go back in and advance the cam gear one tooth, would this help the problem? Thanks
1972 Ford F100 styleside, 300+ cubes

1986F150six
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Posts: 307
Joined: Wed Oct 24, 2012 3:11 pm
Location: Northwest Alabama

Re: Low manifold vacuum

Post #2 by 1986F150six » Wed Feb 26, 2014 8:57 am

Have you tried adjusting the timing while reading the vacuum? Does the vacuum reading vary as you rotate the distributor?

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Dr Jay
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Posts: 202
Joined: Sat Oct 05, 2013 6:12 pm
Location: North Alabama

Re: Low manifold vacuum

Post #3 by Dr Jay » Wed Feb 26, 2014 11:09 am

I'm excited to hear what the real authorities say about this.
My understanding was that the stock cam had 4 degrees
of retard built into the cam (not the gears). So
the 252 is 4 ahead of stock. I installed my 252 straight up. My real
Thought is that you have not found an egr, PVC, power brake
or other sizable leak to cause that vacuum reading.
I agree with the post concerning timing. If you are a tooth off on that distributor
vacuum can drop and idle speed adjustment could take you
off the idle circuit. (My timing light 0 adjustment knob gets bumped
off causing crazy mistakes)
Blessings,
Dr Jay

1978 F100 Shortie, Lowered front and rear, 300 .030 over, 300 carb head, Clifford, Holley 390
268 Comp Cam, Cloyes, Fuel Inj. exhaust manifolds, MSD 6A, TOD, 9"

Baron Von Ottomatic
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Posts: 197
Joined: Sat Jul 24, 2010 11:56 pm

Re: Low manifold vacuum

Post #4 by Baron Von Ottomatic » Wed Feb 26, 2014 4:39 pm

The stock cam is ground 6° retarded and the Comp cam is ground 4° advanced - a total of 10° advance over the stock cam.

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War Eagle
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Joined: Tue Nov 26, 2013 4:19 pm
Location: Pike Road, Alabama

Re: Low manifold vacuum

Post #5 by War Eagle » Wed Feb 26, 2014 9:30 pm

DR Jay - did you mean a tooth off of the distributor or cam gear?

1986F150six - The vacuum did not change when adjusting the timing.

But then I put some tape on the vacuum lines at the carb and manifold and the vac needle jumped to 20 in hg! I guess they were not snug enough. No power brakes so no leak there. But I still could not get the motor to run down around 600 rpm, it died on me. Now that the vac gauge shows 20ish, I can work on the carb mixture and idle speed a bit.

Getting closer !!!
1972 Ford F100 styleside, 300+ cubes

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Dr Jay
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Posts: 202
Joined: Sat Oct 05, 2013 6:12 pm
Location: North Alabama

Re: Low manifold vacuum

Post #6 by Dr Jay » Thu Feb 27, 2014 7:44 pm

War Eagle,
I have installed a distributor with the first cylinder on top dead center; but the rotor not pointing at the number one terminal in the cap. I was one tooth off on that cam gear. It is a helical gear and as the dizzy seated downward the rotor button rotated to a position off the first cylinder spark plug terminal. A simple thing but I could not get timed right until I reinstalled the distributor 1 tooth back (so that in its fully seated position the rotor pointed to a place I could spin the distributor housing to line up with that first terminal). So I was referring to the distributor gear there. Nice work. I believe you have already solved this vacuum problem.
With Baron's info, you can see how advancing that cam timing could have a dramatic effect. (referring to the cam/crank gear set here)
My last thought is about that 1 tooth adjustment in cam timing. I believe that moving that gear set 1 tooth off in any direction is to large of a change. I have never tried it, but contributors on this site have convinced me with some good math that the adjustment is to coarse. No Bueno.
Blessings,
Dr Jay

1978 F100 Shortie, Lowered front and rear, 300 .030 over, 300 carb head, Clifford, Holley 390
268 Comp Cam, Cloyes, Fuel Inj. exhaust manifolds, MSD 6A, TOD, 9"

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