Slant 6 confusion

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Thomas Shirley

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I have a 79 d100 with the slant 6 Holley 1920 1bbl. I was going home the other night and the truck was running great all that day, then it started spitting and sputtering until it left me stranded. So the next day I took a look and my carb and fuel. Everything was clean and in spec, spark was also good. But when I turn it over it only backfires out the exhaust. I checked valve clearance about 4 months ago and everything was in spec and I’ve been driving the truck about everyday since then. I went back and checked for any wires that could have been touched on the ignition system and everything was good. Good spark on all plugs. So I’m believing that the engine has jumped timing or lost compression. Because I have great fuel and spark but it’s acting like it’s firing at the wrong time. Is there anything else I could check? It’s still sitting on the side of the road at my grandfathers house so simple things I could check first before I have to tow it to my shop. Thanks guys.
 
Not sure if this will help, but this sounds like what happened with an old lawn mower I had. That problem was that the ignition switch/control module went bad and it gave the motor bad signals causing it to be unsure if it had spark or not causing it to backfire and not run. Also, the valve clearance could of gone off. While highly unlikely, it is possible.

What I meant to say was the motor got unstable and out of time spark.
 
shittyslant6 said:
Not sure if this will help, but this sounds like what happened with an old lawn mower I had. That problem was that the ignition switch/control module went bad and it gave the motor bad signals causing it to be unsure if it had spark or not causing it to backfire and not run. Also, the valve clearance could of gone off. While highly unlikely, it is possible.

What I meant to say was the motor got unstable and out of time spark.
Click to expand...

There is a module on the firewall right behind the valve cover and about 4 months ago when I got done checking the valves. I forgot to put the wire back on the firewall and when I started the truck it ran for like 1 minute until that wire got burned but I repaired it and everything was good. Also the same module has like glue from the factory that’s been melted for a long time coming out behind it.
 
Probably something????? hard to tell because you said you fixed it but it could very well still be broken. maybe half broken and deteriorating slowly over the course of the 4 months until it finally stopped working.
 
check the plastic dist gear it may have stripped or the roll pin may have moved
 
Firing out the exhaust could just be flooded.

I would pull the plugs, disable the ignition system, and crank it until the gas stops flying out of the cylinders .
Then put a couple a shots of oil in each cylinder, let it sit for 20 minutes, while you dry and regap the plugs.
Then put the plugs back in, and fire it up. If it happens again; fix the carb.

Caution; if gas does come flying out, remember that gas likes to burn, especially when it is mixed with air! Don't set your truck on fire; disable the ignition.
If your muffler is full of gas, anything could happen.


Other causes not yet mentioned for "after-firing", are valve timing, rotor phasing/ distributor out of time, and carbon tracks/or just plain moisture in the cap. One thing for sure is that it does in fact have spark!
 
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yup check taht distributor cap and rotor..... could be a simple fix. Havent had to mess with a bad cap in a while (though everything I own is old enough to have a real distributor)
reminds me.... I have caps rotors wires and plugs for both my Dakotas sitting here, both are a bit overdue (but still running OK) I gotta throw a tune up on both while they are still running good so I DON'T have problems..... one 3.9 and 1, 318. both Magnum engines.
 
Have you ever had the gas tank out? If not, it probably has a bunch of sediment buildup and needs to be cleaned out. Once that's done, blow the fuel lines out good, put a new filter on and a carburetor kit. Start at the source (the tank), or you'll forever be chasing it. That's always the very first thing I do when I get an old vehicle.
 
Use the dist to check if [a] chain has jumped a tooth or dist gear is damaged.
Rotate the crank to the initial timing for that engine. I am assuming it has elec ign. Remove the dist cap; the reluctor tip should just be on the point of passing the centre of the p/up coil.
 
Put a timing light on it and while the engine is cranking see if the initial timing is where it should be.
It could be as simple as the distributor hold down bolt got loose and the distributor housing rotated.
 
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