I originally posted this under the big block section but things are moving slow over there so I thought I'd give it a try here.

I have an 86 F250 with a 460 big block. Ive only had it a month. It's running a Holley 80457-8 600cfm with electric choke on an Edlebrock Performer 460 intake. The motor runs strong up to 2000 RPM. At 2000 RPM it begins to backfire out the exhaust and shake violently. Doesnt matter what gear im in. The timing is set to 10*. The PO had the distributor off by one tooth but that's been fixed.

I'm barely a shade tree mechanic so this is moving beyond my knowledge quickly. I've checked the distributor cap and rotor. They look good. Seems like the coil was recently replaced by the PO.

Am I in left field for thinking its an ignition problem? Could it be the carb? I don't know when the secondaries are supposed to kick in on the 460 but 2000 RPM seemed early.

 

We have very similar setups, and I have owned my truck since new.

I would say check the distributor for a sloppy shaft and check the ohms reading of the pickup inside.
If you grab the rotor and try to wiggle it sideways in any direction, does it move at all?
The cam gear is always pushing in one direction, and the bushings get worn out.

If you unplug the distributor it should read 450-950 Ohms across the orange and purple wires.
Also check the black wire in that plug for continuity to ground, while manually advancing the backing plate in the distributor.

The ground wire can fatigue inside the insulation at the lug, and if the ground goes away so does your spark.
The centrifugal advance moves the wire, and it disconnects.

If the ignition dies for an instant and then comes back on it will ignite the unburnt fuel that traveled through your engine and into the exhaust.

Is the coil HOT (to the touch) when this happens?
There must be a reason it was changed, unless someone was just throwing parts at it.

These are some things to check first.
The DSII system is pretty simple, and easy to troubleshoot.

 

I had several problems very similar and they were all ignition. Either bad ign module or wiring. upgraded to hei style dist and no problems since. Money well spent

 

If it will rev past 2000rpm in the driveway, I would lean toward it being a fuel problem. If it still has trouble in the driveway(no load on the engine) then it must be ignition or something else.

 

Check the plug wires. Are they in good shape? Have they been changed in a while?

The symptoms you describe sounds like bad plug wires especially if under load when

it happens or possibly even when revving it up sitting still.

 

The motor has the problem whether in the driveway or under load. I'm going out to work on it now.

The plug wires look new. I'm going to start at the motor and work back to the ignition module tonight. I'll check the spark on each plug then look at the dizzy shaft and so on.

Should I advance the timing at idle beyond the 10* first or wait until the other items are ruled out?

I'll keep you posted...

 

If you can't find it, I have a suspicion. Take the vacuum line off the dist, plug it off, and then try it. How does it react to that?

 

So here's the update

All spark plugs checked out

Removing the vacuum advance helped the engine smooth out at idle but I didn't have anything useful to plug the holes so no different at 2K rpm. I'll have something for that tomorrow.

The coil checked out as well

I checked the timing when the engine was stumbling and the timing was steady. I was worried that it wouldn't hold time. I ended up advancing the timing around 12-15*.

Why would the engine run better without the vacuum advance hooked up?

I still think its the ignition system. I don't know what in the carb would be causing the prob at exactly 2K rpm in every gear.

 

Some of the stock vacuum advance systems advance the timing alot. Once you d-commission a lot of the smog stuff, it can be too much.

 

So by removing the vacuum advance I'm not removing the vacuum source that opens the secondaries?

Sorry if that's a dumb question.

 

The source of vacuum for the secondaries is internal to the carburetor.
There is a port with a check-ball leading to the diaphragm housing.

By disconnecting and plugging the hose to the distributor you are disabling the vacuum advance.... but NOT the centrifugal advance I mentioned in post #2.

 

Vacuum advance affects and controls the timing. That's the line that goes to the distributor. It has nothing to do with the carb and the fuel delivery.

3 things affect the timing;

1. The initial setting where you tighten the dist down.

2. The weights and springs that control the timing depending on what rpm the engine is turning.

3. The vacuum advance, which controls the timing according to engine load.

 

It's finally running right! I started tonight by switching out the ignition module. That appeared to be original to the truck. I fired it up and it revved over 2K so I took it for a drive to see what happened under load. After 10 minutes of running, the stumbling came back. I made it home, popped the hood and started looking things over again. The coil was incredibly hot, which didn't seem right. I went out and bought a new one. Bingo! I drove it around the same loop an no problems. She'll rev up pretty easy.

Thanks for all the help.

 

There is a reason the coil was new, and you've had to change it again.
Check to see how much voltage is going to the coil while the truck is running.
It should be ~7V. (it will show ~12V with the key on, engine off)

I suspect someone has removed or bypassed the heavy red resistor wire near the ignition switch.

 

BTW, save that 'spare' original ignition module.
Just toss it under the seat.
Those original Motorcraft units seem much more robust than the Chinese units available today.

Some day (when, not IF), your truck will let out a single backfire, and die.
Open the hood and and connect the two plugs of the spare DSII module.
Drive home secure in the knowledge you had a simple solution to your problem, and avoided a tow.
Bolt it down to the inner fender when you get to your destination.