Need Stronger Vacuum Advance Module

Cobra 29 · #1 (**permalink**) 04-07-2002, 07:51 PM

I have a 351W SVO crate motor with the FMS ignition system made by MSD. The distributor is a Duraspark with a vacuum advance module. The vacuum advance is not working. I thought it was because of a broken advance module. Wanting the ability to limit (adjust) the vacuum advance I ordered and installed the adjustable Crane Cam vacuum advance module. It too did not advance the timing. The vaccum port on the carb is working per my vacuum gauge. The signal is timed for part trottle (no vacuum at idle). Inspecting the top plate in the distributor, I found that it will rotate clockwise toward the vacuum advance module but it takes a lot of force. After manually rotating (clockwise to advance) the top plate it will rotate counter clockwise back into the original position. It seems to have strong springs. I believe these springs are too strong for either vacuum module. What could be wrong with the distributor? I know it is not a MSD! Thanks in advance !!!

Jeff Frigo · #2 (**permalink**) 04-07-2002, 08:41 PM

Cobra 29:

There is really no reason to run vacuum advance in a high power to weight ratio car. Just disconnet it and be happy.

SCOBRAC · #3 (**permalink**) 04-07-2002, 08:55 PM

It may be disconnected now.. Mechanically blocked.. Be happy.. But be sure the mechanical advance is doing its job, and had a respectable curve dialed in for your Cobra... Most find a steep curve works pretty well without detonation. I get 38 degrees by 2300 rpm with my 428. I think your 351 can use more.. maybe 40?? but verify that with a small block guy before you do it... start at 10-12 degrees initial and you should have no problem. Good luck.

Jack21 · #4 (**permalink**) 04-08-2002, 05:11 PM

Have similar setup. Try hooking vacuum line to manifold vacuum port on carb for testing. Timed vacuum port isn't giving enough vacuum to work vacuum advance until engine is under load at part throttle (cruise). The return spring in the vacuum advance is not too strong to work properly. (Keep your fingers out of the way when connecting vacuum line, and plunger pulls in!) Is your distributor plate binding? It should rotate freely with vac unit out of the dist.

Gave up on the Duraspark, and got the MSD dist w/ vacuum. It too doesn't come with an adjustable vacuum, you have to buy one from Crane or Accel. And...the MSD dist takes a '57 - '73 Delco (Chevy) vacuum unit. Nice unit though. Basically a HD, ball bearing, Delco dist on a Ford stalk. Roller cam - steel gear. Flat cam - iron gear.

The 351W tends to like 12 - 14 deg initial, 20 deg centrifugal @ 2500 RPM, but only 8 - 12 deg vacuum. Any more vacuum and it will develop a miss at light load highway cruise. (Which is why the OEM non-adjustable MSD vacuum was replaced.)