HELP !!! I have a 1984 F-350 Duly with a 460/7.5L
I had rebuilt this motor 5 years ago. Never had a problem! NOW when the truck gets up to temp it would shut off and would not start until it cooled down. So it ended up being the dizzy. So put a new one in. Now the thing is backfiring though carb. I have done EVERYTHING ! The only thing I have not tried is a fuel reg. It is driving me crazy because I had Two fuel pumps running with the Old dizzy never skipped a beat. I have taken #1 plug out turn over on compression stock top dead center and put dizzy back in like 4 or 5 time.

 

First thing to do is check the firing order. The firing order is 15426378 and the distributor rotates counter-clockwise as the motor runs.

The cylinders are numbered as follows:

Firewall

4 - 8
3 - 7
2 - 6
1 - 5
-Fan-
Radiator

Once you know the firing order is correct, crank it and put a timing light on it. 8* or 10* will be right on. If you know the timing and the firing order is right, you'll want to crank the motor up and pull each spark plug wire individually and see if you can narrow it down to a specific cylinder. If you pull it and no change, that's a dead cylinder. If you pull it and gets worse, that's a good cylinder and you should put the plug wire back. Check all of the cylinders this way.

If you find a specific one that's bad, change that spark plug and spark plug wire for new, good replacements.

If this doesn't solve the issue, I'd be looking at the distributor. Many parts that come from parts stores these days are known to be defective out of the box, including ignition modules, solenoids, coils, etc.

Also, as a final tidbit, when the motor runs cold but shuts off when it gets hot, and won't run for a while until it cools down, then it's the ignition module. Duraspark II ignition systems are notorious for this on these old vehicles, due to leaking capacitors inside of the module. These ignition modules are sensitive to heat, and should be replaced when it starts to give warning signs. Eventually it'll fail and it won't start the motor at all. The pickup coils in the distributors are known to go out sometimes, but 9x out of 10, it's the module.

If the motor cranks, runs, and revs up (even with a backfire), then it's getting fuel, air, spark, and compression. Since you changed the distributor, the only things it could be are ignition related components.

Also, if you're running a Holley 4bbl carburetor, you may have to replace the power valve if the backfire was bad enough. A key sign the power valve is bad is if the carburetor is dumping fuel into the motor with the motor off. You have to take off the air cleaner and look down the carburetor to see this, though.

Good luck and let us know what you find.

 

I changed the title to something describing the problem in an attempt to attract people who might be able to help.

How much time elapsed between:

- Engine Rebuild and Distributor Replacement
- Distributor Replacement and Backfiring Through Carb



IOW did the backfiring begin immediately after replacing the distributor?

 

Thank you for your time to Help ! But Everything is NEW ! I have gone over everything more time than I care !

 

So, are you saying this is a newly-rebuilt engine with a brand-new distributor that's backfiring through the carb?

Or, do you really mean that you replaced the distributor and now it won't start and it backfires?

I'm just trying to better understand the situation. I can understand your being anxious and under pressure to get this working by Monday so maybe you just need to call it a night and relax for a bit.

If the only thing that changed is the distributor and now it's behaving like this, then the problem is in the distributor or the installation of it and not in some unrelated component you never touched to change it.

If you have checked everything, then I'd say you've missed something or are looking at something incorrectly, e.g. you're looking at the oppositte side of the timing pointe you should be looking at, or your harmonic balancer has slipped and the way you determined #1 is at TDC compression stroke is invalid, or something like that.

 

Here's another thought... the replacement distributor you got is the wrong one for that engine. Happened to me once when I got a Cardone reman dizzy for my Tempo... the box was right (had the right number) but the part inside of it was the wrong one and my engine refused to run well with it.

Put your old distributor back in, can you get the engine to run as before with it installed?

Did you get a reman distributor with a TFI module installed on it? Ford put out basically two variations of those TFI modules - one gray, the other black - and they're not interchangeable. No idea about the colors of aftermarket modules.

Or, do you have a separate DS2 ignition box, probably on the driver's side wheel cover?

 

Yes I got A reman distributor. This is now the second reman distributor I have put in. I don't have the old one. I gave it to them for a core. If I had the old one I darn shore would put it back in. And DS2 ignition box, yes Everything Is New 750 EDELBROCK
ELECTRIC CHOKE CARBURETORS , EDELBROCK PERFORMER RPM™ MANIFOLDS (1500 to 6500 rpm)

 

460 of this vintage won't have a TFI, Chris.
Edelbrock carb doesn't have the powervalve to blow out.
But they do require lower fuel pressure than the Holley that came on this truck.
5-6 psi rather than the 7-8 of the Holley.

I would suspect that the markings on the harmonic damper have slipped.
Chris is right, determine TDC mechanically (like with a straw in the spark plug hole) and compare to the timing marks on the damper.
10* BTDC with the vacuum disconnected and plugged is about right for stock cam and heads.

 

I didn't notice in you rpost if you said "when" the backfire occurs. But recognize that backfiring through the carb when throttle is opened also/can be from too lean a mixture. Check for vacuum leaks. Spray some carb cleaner around base of carb and manifold gasket and any idle speed change indicates a leak.

 

I would definately check for a slipped balancer, one qick clue, if it looks like the outer ring has moved back away from the pulleys (maybe even far enough to hit the external counterweight) then it is bad. Remember, you are dealing with a roughly 30 year old part that was made by bonding rubber to steel.

CountryBumkin is correct, a lean mixture (bad or weak accelerator pump) will cause a backfire when you open the throttle suddenly. Timing needs to be set with the vacuum advance disconnected.

 

Did the OP ever find out what the issue was with this truck? Just curious to know what was causing the backfire.

 

Check the vacuum advance canister, apply vacuum and check the connecting rod and plate for movement. Also check the centrifigal advance with a timing light. I had a problem with mine backfiring, drove me crazy throwing parts at it with no change, turned out to be insufficient fuel delivery caused by a partially plugged 3 way fuel/vapor separator,$12 part cured it.