I've been tuning my ignition and carbs. Set my timing and idle mixture with the vacuum gauge but can only get the vacuum to get up to around 15" at idle then it drops to 13" in gear.

What could be causing this? Or is this normal?

58 Ford Custom 300 - San Pedro, CA (So-Cal)

Is the engine mostly stock or modified, with a bigger cam and other stuff?


Lawrenceville, GA

Not sure about the cam but I do have the ford 2x4 intake with 2 edelbrock 500cfm carbs. I also have the accel points eliminator in the dizzy.

58 Ford Custom 300 - San Pedro, CA (So-Cal)

Might be about the best you can get if your motor is worn.. Condition of your motor? Cam? Or with a twin carb setup Perhaps there is a loss of velocity causing low vacuum? Ted may tell us if that happens..

Is your reading steady, double check for vacuum leaks..



"Came too close to dying to stop living now!"

I was wondering if maybe I have some burnt valves or maybe the timing chain jumped as the vacuum is not that steady. I currently don't have any exhaust hooked up only the exhaust manifolds. I'm afraid of pulling the heads

58 Ford Custom 300 - San Pedro, CA (So-Cal)

How's your valve clearance? If they're too tight they can cause low/erratic vacuum readings by not closing completely.

I get about 14" at idle (~500 rpm), and less with the FOM in Drive. Mummert Y265S cam, .040 over 292, "B" manifold/holley 1848-1 (465CFM), '113 heads. Runs great, but has enough valve overlap that the vacuum is a little jumpy at idle, and increasing valve lash won't take that away. No vacuum leaks, runs very strong, but doesn't have much vacuum assist for the brakes!

I've learned to live with it, and just enjoy the car!

"Came too close to dying to stop living now!"

I adjusted the valve lash before taking the vacuum reading. It jumps around by about 2-3" of vacuum.

58 Ford Custom 300 - San Pedro, CA (So-Cal)

I'd recommend a compression test if the erratic vacuum bugs you. Much less intrusive than pulling heads.