Mild 454/4-speed,performer rpm, 750 holley vac. secondaries. I just installed new rpm and a new fuel pump. I set timing to about 16 where it seems to run good. I have trouble with back fire under load and it sputters when accel. in nuetral. I checked vaccum and it says 6, But living in denver w/ adjustment for altitude Im at 11. I have sprayed manifold for leaks but don't seem to find anything wrong with that leaking. Accel. pump is distributing a good shot that I can tell. What can I check for next???

Vacume leaks?

sounds like a/f ratio 6 inches of vaccume is not good at idle.... sea leval that is

<BLOCKQUOTE>quote:</font><HR>Originally posted by Metalmechanic:
sounds like a/f ratio 6 inches of vaccume is not good at idle.... sea leval that is<HR></BLOCKQUOTE>

Vacuum was actually 11 with correction factor on passenger side top vac. port. But when I checked the front lower vac. port that is capped I got about 20 which is good. I set timing at about 13 now and even took my holley 750 off and put a temporary 750 edelbrock on and it still back fired a little through carb. I have put new plugs new cap/rotor also. I checked tdc and I'm right on with that. Any more ideas would be very welcome.

It sounds to me that you probably need to check your timing components. If you backfire through the carb, you have combustion that is escaping past an intake valve,... backwards into the intake manifold. I cant think of any other way you can get this without an intake valve being open at the "wrong" time. Just my .02, good luck!

Could you possibly have some fouled plugs that wont fire? If so, the cylinders would be loading up with raw fuel, and then getting back into the intake manifold. ...just brainstorming here.