I have a 1982 F-150 with a I6 300.

The motor has dieseled after shutoff since I've owned it, even with re-gapping spark plugs, and consistent Seafoam as well as other attempts to clear out carbon deposits.

The idle speed is about 600.

I wonder if I have a vacuum leak I am unaware of? I sprayed WD40 around the carb/intake seal as well as all other vacuum nipples to check for air leaks and I found nothing.

I don't fully understand how the Ignition system adjusts the timing, but is there a possibility that could be a factor? Is it worth replacing?

I will also say I think something is very wrong in general as even with a swapped in 5 Speed manual I am getting about 11-12mpg. (I suspect the same issue is causing both of these facts)

Any and all help is appreciated!!

 

How are you measuring the RPM?
Also "about" is not good it is or is not at 600 RPM
What is the timing set to?
What brand and model number plugs are you running?

There are only 2 things that I know of that can cause run on.
Idle speed set to high and something lighting off the fuel mix like hot carbon or spark plugs.
Dave ----

 

I am measuring the RPM with the tachometer (I installed a junkyard gauge cluster and ran a wire from the coil)
It is 600 RPM when warm.
Just checked my timing, and I am at 3 degrees ATDC while idling (Could be my issue?)
I am running Autolite 45s gapped to 0.43 (I have not replaced since I bought the truck last summer)

 

Your initial timing should be 10 deg BTDC at idle......correcting that alone will noticeably improve mpg & power.
Being as retarded as yours is currently causes more heat to be absorbed into the head because the burn is still occurring as the exhaust valve opens; that extra heat will encourage run-on.


There are three aspects to the timing advance assuming you have a vacuum advance canister on your distributor....

1. The first is your initial timing ( the 10 deg BTDC which stays constant when you clamp the distributor down).

2. The second is the mechanical advance which constantly changes while you drive; this is done by springs & centrifugal weights spinning inside the distributor & is solely rpm based (more advance as rpms increase). The amount of mechanical advance varies according to the distributor & engine, but commonly can be another 20+ deg.

3. Thirdly, the engine's manifold vacuum varies according to the load on the engine (high vacuum under light loads & low under heavy loads). The vacuum hose going to the canister causes a diaphragm to pull or release a linkage advancing & retarding the timing as load changes.
At high rpm & low load, vacuum is high & total advance increases towards its maximum, this gives a good fuel saving & more light-throttle response.

All three are independent of each other but work together for power & economy.

You probably know, but it's important to pull the hose from the vacuum canister (plugging it so there's no vacuum leak) when you are checking/adjusting the initial & mechanical timing......otherwise your timing light readings can be false.

In factory form, most of these trucks used a ported vacuum source for the advance canister. So there shouldn't be vacuum to the canister until the throttle plate opens slightly because the nipple the hose connects to is above the throttle plate. The topic of ported vs non-ported timing is a can of worms that's as endless as 'the best oil to use'.
If I were you, I would stay with the factory set up until you have ironed out other bugs.

You can see if your mechanical advance is operating by shining your timing light on the balancer marks while you work the revs up & down a bit.......the timing will increase as engine revs do. If you don't see this happening something is stuck inside the distributor.
You probably won't see the timing begin to advance until you are above 800 rpm or so because of the springs resisting the centrifugal weights inside the distributor at low rpm......this is normal.

If you have a hand vacuum pump, you can check that the vacuum canister is working by applying vacuum to it with the distributor cap off......you'll see the base plate rotate slightly & hold until you release the vacuum. If that isn't happening the canister diaphragm is probably bad, which would affect your timing & create a slight vacuum leak as well.

A previous owner of one of my trucks had the distributor springs changed for 'performance' but the springs that were used were too light & not tensioned properly either......as a result it advanced waay too early, pinging & self destructing.
To compensate they retarded the initial timing back to 3-4 deg ATDC like yours, so it had no power & poor economy too. It can take a while to get to the bottom of these historic issues sometimes.

PS John Denver fan?

 

Ken is right
Follow what he posted and go from there but I bet the issue will be gone.

When you adjust the timing to 10* BTDC the idle will go up you will have to adjust the idle back down to the 600 RPM.

Oh I think the timing where you have it as Ken said it making to mich heat and keeping the end of the spark plug hot enough, like a glow plug, to fire any fuel in the motor.
Unlike a EFI motor where the fuel & spark is turned off, on a carb motor it is spark only that is turned off and with fuel still in the motor anything that can light it off, carbon or hot tip spark plugs, will keep it running.

Let us know how you make out
Dave ----

 

If it truly is 3 ATDC(I am surprised it runs there) then he has the carb butterflies open a lot at idle to keep it running. That is letting a lot of air into the engine, which contributes to dieseling.

 

How did you go here?

 

Update!

I timed it and tuned the carb for the correct timing and it's like an entirely different truck! No longer diesels and the economy and performance are much better.

You guys have been a big help!