[Moparman1972]

I ran into a problem with my 318 that has me scratching my head. The engine starts up and idles fine, but at half throttle, stumbles and misfires. Its very noticeable on the road, enough to buck the car. At full throttle it doesnt buck, but its like the engine is unbalanced, and it shakes quite a bit.

I switched distributors a week ago, to a new vaccum advance with new plug wires, and it was running fine, but I took it on a 3 hour drive to Massachusetts and on the highway on the way back it started doing this.

The coil is brand new, and I've checked that the plug wires are snug several times. Any ideas as to what is causing this?

[Eddie]

At part throttle the vacum is higher than at full throttle. What amount of vacum advance did that new dizzy have in it? *(The amount of vac. advance is stamped on the arm of the vac. adv canister X2 so if it has a 12 on it it means it's capable of 24 degrees of vacum advance in addition to the mechanical advance).The new M.P. dizzys have as much as 24 degrees V.A. in them I think. This may be too much for your engine. I also think the newer dizzys have an adjustment screw inside the nipple. 3/32 allen wrench comes to mind. Try adjusting that a little bit and see if that helps. If I remember right clockwise increases the amount of advance. So you might want to go counterclock wise to slow the vacum advance rate. I would try a full turn CCW and drive it.

[Moparman1972]

Thanks Eddie. I'm at work so I'll take a look at that when I get out.

So your line of thinking is that the advance is too much at part throttle with max vaccum? Wouldn't it ping at that point, not just run rougher?

I put the vaccum advance distributor on to begin with because I switched to a mechanical advance dual point, and it wasnt advancing quick enough. I had it advanced something like 30 degrees static to get it to perform.

The car used to smoke the tires off after we first built it, but after switching to the mechanical advance distrib, it was such a dog down low I couldnt break it loose. I thought that the mechanical distributor was the problem, but there were years between when we first built it and thrashed it to when we took it out on the road again with the mechanical advance, so maybe it was something else.

[Eddie]

I had my 360/380 Magnum crate engine with the M.P. dizzy and it did the same thing. I installed the new dizzy, rev the engine to 3000, it showed 34 degrees. I buttoned it back up, took it out on the road and it ran fine until I let off the gas or gave it light throttle. At that point it ran horribly, bucking and kicking like a bronco! I took out most of the vacum advance but it still had a tad of too much advance in it. I plugged the advance and it ran like a champ! With too much vac advance it will pull the rotor to the next terminal and when it fires it's actually misfiring due to too much advance. It's weird though, it never did 'Ping" or 'rattle' as some do.

[Moparman1972]

Thanks for the advice, I backed it way off on the timing yesterday and its still doing it, so I guess I have to go more. I am scratching my head at this one. With the mechanical distributor I was able to advance it enough to ping, and back it off and it ran fine, no rough running problems like I'm having now. Even if it is catching the next cylinder and misfiring, why wouldnt it do that with the mechanical advance distributor too?

Another problem I found when going through everything was the points. I bought a rebuilt distributor from Advanced Auto Parts, and I was checking the condenser when I noticed that the points were moving! The rivet that holds one pad on was installed incorrectly and was loose, allowing the pad to move about .050, so my gapping meant nothing. So, probably something to look for if any of you have a rebuilt distributor from them.

[dave-r]

Points?

Horrible horrible things. I hate them.

[Moparman1972]

So I let Eddie know, but for everyone else, Eddie was right, and the engine smoothed right out when I backed the timing off enough. Thanks again Eddie!