Very low (4") vacuum at idle
#1
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Very low (4") vacuum at idle
I thought my motor was running pretty well but I bought a vacuum gauge and realized I'm only pulling 4" at around 800 rpm. The cam is not that big its a flat tappet hydraulic
226/234
.458/.473
ADV 288/296
the initial timing is around 12 degrees
carb is a speed demon 750
it idles a little rough and rich and the vacuum advance isnt advancing the timing due to the very low vacuum. Distributor is an msd with vac advance.
Thanks in advance
226/234
.458/.473
ADV 288/296
the initial timing is around 12 degrees
carb is a speed demon 750
it idles a little rough and rich and the vacuum advance isnt advancing the timing due to the very low vacuum. Distributor is an msd with vac advance.
Thanks in advance
#2
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Re: Very low (4") vacuum at idle
You should be pulling well over 10" vacuum with that cam.
What adjustments have you played with to try to improve the situation? Where are your idle mixture screws set? Have you verified your indicated 12* advance at idle is accurate (TDC indicator actually is TDC)? Are the valves properly adjusted (none down too far and hanging a valve off it's seat slightly, no cam lobes going flat)? How do the plugs look?
Also, and not enough people use this powerful diagnostic tool, have you done a cranking compression test on all 8 cylinders?
What adjustments have you played with to try to improve the situation? Where are your idle mixture screws set? Have you verified your indicated 12* advance at idle is accurate (TDC indicator actually is TDC)? Are the valves properly adjusted (none down too far and hanging a valve off it's seat slightly, no cam lobes going flat)? How do the plugs look?
Also, and not enough people use this powerful diagnostic tool, have you done a cranking compression test on all 8 cylinders?
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Re: Very low (4") vacuum at idle
You should be pulling well over 10" vacuum with that cam.
What adjustments have you played with to try to improve the situation? Where are your idle mixture screws set? Have you verified your indicated 12* advance at idle is accurate (TDC indicator actually is TDC)? Are the valves properly adjusted (none down too far and hanging a valve off it's seat slightly, no cam lobes going flat)? How do the plugs look?
Also, and not enough people use this powerful diagnostic tool, have you done a cranking compression test on all 8 cylinders?
What adjustments have you played with to try to improve the situation? Where are your idle mixture screws set? Have you verified your indicated 12* advance at idle is accurate (TDC indicator actually is TDC)? Are the valves properly adjusted (none down too far and hanging a valve off it's seat slightly, no cam lobes going flat)? How do the plugs look?
Also, and not enough people use this powerful diagnostic tool, have you done a cranking compression test on all 8 cylinders?
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Re: Very low (4") vacuum at idle
Where are you hooking the guage up to? Which port specifically.
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Re: Very low (4") vacuum at idle
So I swapped the plugs because they were fouled. The vacuum is now around 11 inches after messing with the mixture and initial timing. Is 11" right for a cam like this one?
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#8
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Re: Very low (4") vacuum at idle
With only 12* of timing that sounds about right. If you hook up the vacuum advance to a full manifold vacuum source (so it's kicked in at idle as well as part throttle) you could pick up another 2-3" of vacuum.
#10
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Re: Very low (4") vacuum at idle
11" is fine. Power brakes get flaky below about 8 or 9. Just saying that there's a reasonable likelyhood that your combo might like vacuum advance attached to a full manifold vacuum source for drivability and mileage reasons, not just to raise idle vacuum.
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