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

 

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?

 

I tried changing the timing a little and the idle mixture, none of which changed the reading. TDC is accurate as verified by my engine builder last year. I'll pull a plug and check them out. Would fouled plugs cause the low vacuum? The plugs were new in September. A cranking compression test will reveal improperly adjusted valves right?

 

Possibly and Yes.

 

Where are you hooking the guage up to? Which port specifically.

 

The port behind the timed one on the baseplate.

 

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?

 

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.

 

The vacuum port does bump up the timing at idle. Is 11" really too low? It seems to run ok and brakes work fine.

 

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.