Hello there fellow FT enthusiasts, I have a problem with my 300 i6 in my 1984 F-150 and am really stumped.

Let me preface this by saying that my truck has been running a duraspark II ignition system with a non-feedback (no computer) carb for several years with no issues, ran very smooth. Nothing on the truck has been changed or adjusted since then. Last month on a road trip, the truck suddenly died on the highway, sputtering and popping as it lost power. Now it won't start at all and it will just make loud backfires through the carburetor with smoke.

I suspected timing had gotten messed up by sheared timing gears so I pulled the front cover off the engine to check timing but sure enough it was dead on. The distributor rotor was at no. 1 when the two marks on the timing gears were aligned.

Next I did a compression test and several cylinders were low. I suspected that worn, non sealing intake valves were reducing compression and allowing the combustion to go back up through the intake when the cylinder fired. I pulled the head and had it rebuilt. The machinist confirmed several intake valves were worn and not seating.

I got it back together this weekend and to my disappointment the problem of the engine not running and backfiring through the carburetor while cranking persists. I now am thinking it could be that somehow the ignition system is firing on the intake stroke, causing these backfires.

From what I've read about the distributor on the duraspark II system, it tells the ignition module when to signal the coil based on the pickup position of the distributor.

Could a faulty pickup coil on the distributor be causing cylinders to fire at the wrong time like on the intake stroke?

Could this also be a broken distributor gear pin moving timing all over the place?

 

I don't bet but I would put my money on the DSII box.


Also a bad cam shaft can cause a back fire up the carb. If an exh lobe was bad the exh valve cannot open to let the gases out so when the intake opens it goes out the intake track.
Pull the valve cover and when cranking the motor over check to see if all the rockers move the same amount.
Dave ----

 

I checked the movement of the valves with a mechanic friend before I pulled the head and we both saw them actuating fully across the whole rocker assembly. Which led us to believe a valve was burnt or not seating properly was causing the issue instead.

So can these DSII boxes actually cause spark plugs to fire at the wrong time?
From what I've read they tend to just die when they get warm and then just die completely. I will call my local parts stores and see if they test them.

 

The DS2 box can die intermittently. Mine did that several times so I took it to Napa for testing. They ran three tests and it came up bad one out of the three times so if you get it tested make sure multiple tests are run.

 

I had the DSII module tested 5 times today and it passed. The rotor spins with the engine cranking and can't spin freely so the pin breaking and letting the timing go I think is ruled out.

I checked the spark at with my light up spark tester and only the no 1 cylinder made it light up dimly at first and then nothing as the engine cranked, nothing lit up with the other cylinders.

I replaced the coil and DSII box and neither helped.

One thing I noticed is the tachometer isn't moving while the engine is cranking. It jumped a few times but that's it. I also noticed that backfires only if I have the choke open and the keep the throttle wide open.

Is the only thing left that could cause this issue the pick up inside the distributor?

It seems like I have a no spark situation and the coil and ignition module are ruled out.

 

I'll throw another possibility out there - Maybe you're chasing down the wrong rabbit hole.

In my experience,"intermittent" spark usually leads to backfire out the exhaust.
Backfire out through the intake is usually caused by a way too lean mixture.

Maybe your spark tester gizmo isn't very reliable. Maybe you DO have spark to all cylinders. An actual inductive-pickup timing light would show if you have spark at each cylinder, by clipping it over each plug wire one by one.

If you weren't getting spark to each cylinder, very soon each plug tip would get wet with gas with all the cranking you probably have been doing. Pull a few out and see. Dry them off if they are. If they aren't wet, then that increases the probability that a fuel delivery issue is the problem.

With choke and throttle wide open, pour in a bit of gas down the carb's barrel. Then close the choke, or leave it open only a tiny bit. Depress gas pedal just a little at most. Turn key. Does it then fire a few cylinders or maybe run for an instant without intake backfires? If so, then it's definitely a fuel delivery problem.

 

I would start with some basic voltage reading in the Ignition Primary circuit.

With the meter negative lead left on the battery post negative, check these voltage points, with the IGN system all connected.

Key ON,
coil positive
coil negative

Also check voltage across your battery post, just to see what the max supply voltage is.

Get back to me with the voltage numbers for the step.

You are using a stock DS2 round coil????

Jim

 

If you have changed out the DS II and coil, this would be my next guess .Visually checking the distributor cap and rotor for burned or corroded contact carbon tracks or moisture. Your on the right track cause the distributor causes problems as stated. If you have a no spark (Why), spark present then move to the fuel carb issues .Let us know what you find .TR

 

I really don't think the carburetor is suspect because it was new along with the ignition components roughly 2 years ago when I did the DSII conversion. That and the truck rand perfect with the tune that I set up for 2 years before it died abruptly back in October, the accel ump squirts gas into the throat . I will be replacing the pick up coil tomorrow and post results, It doesn't even try to pop off with starting fluid and the spark tester has worked fine for other issues so I'm confident that it has been narrowed down to an ignition issue.

 

I've had one of my vehicles with the DSII die going down the road, three times, the way you described yours.

The first time was the rotor.
The second was the pick up coil.
The third was a sheared distributor gear roll pin.

Since you ruled out the roll pin on the dist. gear. Have you checked for spark at the coil wire ( distributor cap end )?

I would also pull the rotor and check the underside where it goes over the distributor shaft for any burn marks. I've had them were the spark burnt thru the rotor and would ground out to the shaft.

If that's not the issue then I would suspect the pick up coil inside the distributor.

 

Solid Diagnostic Advice! Check those voltages!

I would suggest getting Ohm's to the distributor pickup wires and those to ground. There should be continuity between the wires, and no continuity to ground (Distributor Housing).

 

So I had an interesting development. I replaced the pickup coil today along with doing a re curve of the distributor. I put the dizzy back in to the same position as I pulled it out of. Hooked everything up and still no spark seen on my tester, either from the coil wire to the center of the dizzy cap or when the tester was connected from a plug terminal on the cap to a spark plug. After some time to regroup and research troubleshooting the DSII system I got the idea from a write up to install everything in the ignition system as normal but leave the dizzy uninstalled from the block and just ground the body of it to the block using a jumper cable. I also connected my spark tester between the coil and the center of the dizzy cap. I spun the distributor shaft by hand and confirmed the no spark condition again as the tester did not light up.

I then quickly thrust the shaft and lo and behold I got spark between the coil and distributor as shown on the spark tester!

So what does this mean? A bad grounding when the distributor is bolted down to the block somehow? Or did me spinning it quickly "shock" the system to life and it will suddenly work tomorrow when I put it back together? It would seem the everything functions fine when the distributor is not on the block but instead grounded using a jumper wire.

 

Try another ground wire from the distributor housing to the block.

 

I cleaned up the contact surface of the distributor to block and reinstalled the distributor. I then hooked up one side of my multi meter to the + battery terminal and the other other lead to my distributor body and got right around 12.1 volts.

So my distributor when bolted to the block does in fact have good ground.

I then put my spark tester between the coil High Voltage port and the coil terminal on the center of the distributor. I got a short spark when the key was turned to the on position, nothing when the engine was cranking, another another short spark when I stopped cranking and the engine came to rest.

So it seems when distributor is spun rapidly by hand the ignition system puts out good spark but when the engine turns it at cranking speed i get nothing. Since the old pick up coil and the new pickup coil gave the same results the only thing I can thing is the armature inside the distributor is suspect. From what I have read elsewhere when you only get spark by rapidly spinning the distributor by hand and nothing by having the engine cranking the armature is the cause.

What do you guys think?

 

I don't believe this necessarily has anything to do with your problem but 12.1 is real low for battery voltage. 12.8 is 100% for maintenance free batts.