Long time lurker/searcher, new member here. I cannot get my vacuum advance to work correctly, and it used to work just fine until the stator was replaced. Vehicle is a 1986 stock (I think) 460 F250 with a Holley carb.


Summary points:

1. New coil, ignition module, stator.
2. Retimed to 8* at 800rpm. Runs up fine with vacuum advance unhooked
3. Vacuum advance holds vacuum (no gauge, just by mouth) and smoothly moves through the range with varied pressure
4. When vacuum advance is hooked up (to ported vacuum) the engine will instantly die if throttle is applied. If backed all the way off (allen screw entirely CCW), it will let me get to 14-1600rpm before it starts to stumble, then misses and a large backfire through the exhaust (rarely carb).

How can I make my vac. advance work correctly? Are there any other adjustments the PO could have made that are casuing this? The only difference here is a new stator and I didn't really crank on that advance during the setup. Anything else to check?


Full background: Truck ran perfectly for 1.5 years, then suddenly started dying on the road with no spark after 15 miles or so. Replaced coil, ignition module, still the same issue. Thanks to searching this forum, I identified the stator as the issue- replaced with new stator (twice as the first was a dud) and it started but ran extremely roughly. I checked the timing and it was really advanced- enough that I could not turn the distributor far enough to retime as it would hit a nearby brace. I pulled the distributor out, set it back 1-2 teeth, and reinstalled. Timing was then set (with vacuum line unhooked and plugged and truck warmed up) at 8* at 800rpm. It would run great without the vacuum, but hen the vacuum was hooked up truck would die and not restart. Through searching here I found the PO had it hooked up to manifold vacuum (pressure at idle). Also, the PO has worked on the timing before as evidenced by marks/etches here and there.
I changed to ported vac and plugged the manifold vac. It starts and idles well (timing its still correct with vac unhooked/plugged). I took the vac off and found it was adjusted 1/2 turn from the endpoint (13.5 turns) CW. I have systematically gone through the entire adjustment range (1 turn increments) and found I can get to 1400-1500 before stumbling/stalling until I get to about 4 turns in and then it decreases to 11-13. It will wind up fine if you just gun it quickly. It is only having problems with easing on the throttle. I'm stumped.


I believe TDC is set correctly, my rotor does point to #1 when I have the crank TDC, and given how well it runs without vacuum I’m assuming I can’t be off 180*


All input welcome.
-SH

 

Wow! Such a good writeup, and from a newbie. Welcome to FTE!

As for your problem, I'm guessing that your harmonic balancer has slipped and what shows as TDC is actually not. If so, then your timing is badly advanced already and bringing the vacuum advance in is causing it to be too much.

Having said that, you have to have a LOT of timing to cause an engine to die. And if you had that much timing then it would probably kick back when starting. Does it?

 

My 74 390 did that when the vacuum advance pulled the breaker plate far enough that the points wouldn't contact anymore.

Pain in the neck trying to figure that one out.

 

Good point. (Pun intended.). And since changing the stator was part of the work done just prior, perhaps something so,liar is happening here. Like, maybe the stator and reluctor aren't properly aligned?

 

I would also check some of the wires inside the dist. You changed the stator which I am going to assume gave you new wires there, but check the ground wire. If there is a loose or broken wire it may give a intermittent connection when the plate moves.

 

Gary- before I retimed (upon putting in the new stator) it was kicking back REALLY HARD when I tried to turn it over. Where it is timed now it doesn't kick at all. I will pull #1 plug and see if I am TDC with the harmonic balancer when I get a chance next week.

If the reluctor is the paddled "gear" for the pickup that goes on the distributor, I believe I have it aligned just as before- the slot is on the roll pin ok. The stator only goes in 1 way. I do have 2 new wires that came attached to the stator and ground goes straight to the body of the distrib, but I will also check and make sure there aren't any slices or anything amiss there.
At first I was thinking I ruined my vac when I was pulling at the arm to get the stator out, but I wouldn't think this would result in the advance working "too well" and pulling too much advance. I can't think of anything else it would be doing in there besides advancing too much.

I am most confused as to how this could have run so well before with the old stator (until it got hot) and suddenly be so far off with the new one. Is it normal for these to be 'calibrated' really differently as to when they pulse?

Thanks for the input, I'll report back as soon as I get to follow up on these ideas.

 

I'm also not sure how he had it set up to idle ok off of manifold vacuum- it seems like with that much pressure and advance pulling on idle it would have to be set up oddly to make it work, but I see a lot of people support manifold over ported as well and make it work.

 

I'm not sure if this can be done.....but you didn't accidentally put the reluctor back on upside down?

The reason I ask is that I have seen some reluctors that didn't have the roll pin groove exactly in line with a tab; so if it was installed upside down, the slot could be offset in the opposite direction, affecting the timing significantly.

Regarding manifold vacuum vs ported......distributors using manifold vacuum do have a lot of advance at idle.

But because - a) the engine is under no load b) idle mixtures tend to be richer c) the cylinders aren't filling efficiently at idle...........pinging is not a problem.

As soon as any load is encountered, vacuum drops away upon opening the throttle; so the vacuum advance drops out of the equation.

 

I'm wondering about the reluctor too.

I've taken a couple '86 460 distributors apart and the reluctor wheels I've seen all have two grooves for the roll pin, meaning it is possible to install the reluctor 180* off and that could certainly explain your situation.