Technical Timing issue, I'm stumped.

29 dodge fordor, 360-727, been on the road a few years, some time ago, changed carbs, hooked up distributor vacuum line and it ran like shit. 2 bbl carb, reversed lines to full vacuum at idle, worse yet, unhooked vacuum line, set timing, runs like a clock. This winter, a lifter let go, so, new cam, lifters, all stock specs, set timing, runs like a clock, hook up vacuum advance, runs terrible, stumbling, backfiring, but only if you crack the throttle hard. If you bring the rpm's up slowly, runs fine but has a poor idle. Advance or retard distributor, makes no difference.
Switch the line to manifold vacuum, runs like crap, ported vacuum, idles fine but won't run smoothly at anything above idle. Advance or retard distributor, no difference. Unhook vacuum line, runs like a clock at all times. Timing chain new, everything checked twice or three times.
1974 360, donor was a one ton motor home originally.

 

Bad distributor? Maybe vacuum advance and or breaker plate malfunction? It might seem to run okay to you because the mechanical advance portion (springs, etc.) are working as they should so it is getting some advance. Do you have a spare or donor distributor you can pop in to see if it sorts out the problem?

 

just a thought... maybe you have a bad vacuum canister membrane?

 

Put a timing light on it and tell us how much total mechanical advance is being made (vac. advance unit disconnected). Then how much total timing with vac. advance unit connected.

 

Advance unit works in reverse?

 

This gets my vote. When you hook up the advance, you just get a vacuum leak. It’s happened to me before.

 

I'm assuming, being a 74 engine, it has electronic ignition. Sounds like you have a problem inside the distributor. If it always works well with no vacuum to the advance unit I would suspect a worn out dist. advance plate or bad wiring to the dist. pick up unit. Reluctor air gap is important as well but it really sounds like something is going awry when the vacuum advance starts to move the dist. advance plate.

 

I can't tell you the exact timing numbers, not my car. I built most of the car and wired all of it, not a wiring issue. Engine has just under 60,000 miles on it, no appreciable play or anything wrong to the naked eye, distributor looks fine. HOWEVER, the fact that it may have a worn out or torn diagphram never occurred to the two of us and bears further investigation. Yes, it has electronic ignition. I'll get back to you guys.

 

I didn't mean a wiring issue external to the distributor, but rather a wiring problem or a ground problem for the wires internal to the distributor, as in the harness running from the distributor body inside to the pick up coil. It will move when the vacuum advance is working, and may be breaking a connection as it does so.

 

You may be right, we'll check. Thanks.