Cam Master needed to solve mystery

[c9zx]

Recent build, 408C, 9.5:1 CR, blue thunder intake, 750 Holley HP, iron Ford CC 4V heads minor porting good valve job, 305/197 cfm @ .600. cam is solid flat tappet. Dynoed 529 HP@ 6400rpm, 496 TQ @ 4900rpm. No vacuum leaks. Cam specs: 272/282 adv. dur., 238/248 dur. @ .050, 146/155 dur. @ .200, 110 LCA, installed @ 106. The mystery is best vacuum in neutral @ 1250 RPM is 10.5 in./hg. In gear, 1050 RPM, 8.5 in./hg. Best vacuum occurs with 16 degrees initial, no vacuum increase at 20 degrees initial, total without vacuum advance is 34 degrees. Best vacuum/idle quality is with idle volume screws set at 1 3/8 turn out, about .028 of all 4 transfer slot uncovered. I was expecting 13-15 in./hg. I have wasted days trying to find a problem or an explanation for the low vacuum. I'm officially stumped. Anyone have any thoughts on the subject? All input appreciated.  Thanks, Chuck

[machoneman]

Power brake booster? Clamp off the vacuum hose, carb-to-booster, with a vise-grips and see if vacuum improves. Or, per an old post from Rich F and yours truly....you may have a leaking intake gasket causing a vacuum leak. It could be leaking externally (pulling in air from the atmosphere) or it could be leaking internally (pulling in air from the crankcase).  To test for external leak, use some propane around the intake, carb base etc. and see if the idle changes.  Yes, I know you said no leak but...there is a 2nd and more accurate test.

To test for an internal leak, block off all vacuum sources such as brake booster, pcv, etc. and block all vents such as valve cover breathers, oil cap etc. Crankcase needs to be fully sealed from the outside atmosphere for this test. Pull the oil dipstick and attach a vacuum gauge to the dipstick tube. If you get a vacuum reading with the engine running, you have an internal vacuum leak and most likely it is a sucked in intake gasket. Could also be a cracked intake, cracked intake port runner, etc.  Most likely an intake gasket though. Using the cork end gaskets on the intake can cause a leak if the cork gaskets are too big for the opening they are supposed to seal. The cork gaskets can actually hold the intake up enough to prevent a good seal at the intake runners. Resurfaced heads/intakes can negate the use of the cork end gaskets and a sealer like "Right Stuff" or similar could be used.

One other possibility too. See Boomyal's post from our little (but mighty!) SBF Net 54 Forum and the use of a Ford OEM carb spacer that doesn't match the intake's carb pad size.

http://www.network54.com/Forum/119417/thread/1229625787/FYI-Ford+Carb+spacers+%26amp%3B+narrow+pad+intakes