What causes "dieseling"

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Princess Valiant

Princess Valiant

A.K.A. Rainy Day Auto
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So I have a 74 dodge D100

Slant six, 4 speed

I recently rebuilt the carb, 2 barrel. Adjusted the carb per instructions

Then I tuned it to 800 rpm per the emissions sticker, set the timing and everything was running perfect.

Then I turned it off and it "dieseled" pretty good and then went off. I restarted it after that and it started on the first shot as usual.

What did I miss

Thanks

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Assuming compression is stock? Timing is the likely culprit. And most the time it's too advanced. Many like it advanced for starting and low rpm stability.

How's it sounding near WOT? Wouldn't surprise me of a slight "Popping?" Another sign of too advanced.
 
I see you have the timing light out so can you tell us where you set initial timing? Could also be crappy ethanol gas.
 
IF you don't have to pass emissions, throw the book numbers in the trash. Even with emissions testing, set it where it runs best, then set it to the trash numbers when you have to test.

Not enough initial timing which requires opening the throttle plates too much to maintain idle.
 
crackedback said:
IF you don't have to pass emissions, throw the book numbers in the trash. Even with emissions testing, set it where it runs best, then set it to the trash numbers when you have to test.

Not enough initial timing which requires opening the throttle plates too much to maintain idle.
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Yup. Likely what it could be.
 
ok, so timing issue.

I did set it to the factory numbers per the sticker and instructions.

We do not have emmisions testing here and this truck sure isn't a regularly driven street truck (it's a captive yard truck) so I can set it to whatever and nobody cares.

I will forget the sticker and set it to run good ....Thanks so much
 
Keep your total timing number from going out of bounds. The distributor may have 30*+ of mechanical timing in it if the book says time it at TDC. Nothing should be timed at TDC... BARF

You may have to bust into the distributor to limit mechanical. I'd bump it to 8-10, reset carb, see if it shuts off OK. Then see where total lands.
 
Ignition timing and or bad gas is my guess.
Both are easy to check.

Also, pull out the spark plugs, check them and clean them if needed.

Short story,
I had a nasty knock coming from my old Kohler K series in my tractor. The issue turned out to be bad gas that I had in the can which was causing the worst diesel/spark knock I have ever heard.
 
crackedback said:
Keep your total timing number from going out of bounds. The distributor may have 30*+ of mechanical timing in it if the book says time it at TDC. Nothing should be timed at TDC... BARF

You may have to bust into the distributor to limit mechanical. I'd bump it to 8-10, reset carb, see if it shuts off OK. Then see where total lands.
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Barf. So 80s. lol
 
I have never had a diesel issue but it seems that hot spots in the combustion chamber (carbon buildup, too hot of a spark plug, sharp edges on the valves or pistons and the throttle blades being open too far at idle (due to several causes) would provide the things needed to keep an engine running with the ignition shut off.

I'm not sure how advance world have any effect once the key is turned off, but again I have never had that issue.
 
Dana67Dart said:
I have never had a diesel issue but it seems that hot spots in the combustion chamber (carbon buildup, too hot of a spark plug, sharp edges on the valves or pistons and the throttle blades being open too far at idle (due to several causes) would provide the things needed to keep an engine running with the ignition shut off.

I'm not sure how advance world have any effect once the key is turned off, but again I have never had that issue.
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Thats why most late 70s and early 80s vehicles had electric solenoid dashpots on the carbs to allow the throttle blades to close all the way on shut down.
 
I vote carbon build up and hot spots.
Does she burn any noticeable mounts of oil?
 
I would as said play with timing and run it maybe at least 5 advanced from spec. Be sure to set the idle when warmed, and if the engine is running "hot" that can aggravate the problem. I've often suspected that an engine that uses a little oil might be worse for "run on"
 
My stone stock 170 liked 12 degrees initial best. I tried 15, but it ran rough so I backed it off to 12 and it ran great. Just some FYI.
 
Princess Valiant said:
Yes, because I replaced it with new line.

And new air filter.

Good thinking though
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Is the vacuum advance working? Not that it would cause your issue. Remember this though. As the timing chain stretches over time, the camshaft timing becomes more retarded. So more ignition timing may be needed to compensate for that.
 
TT5.9mag said:
@Princess Valiant shortbed? Fleet side? Wanna sell it if both of the answers to the above questions are yes.
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It started life as a step side but either the factory or someone put the tool boxes on, which is the main reason I bought it because I wanted a tool truck because I am too lazy to lug heavy tools everywhere I go.

I'm not sure what a fleet side is but it is a shorty.

At this time I am not selling, because I want to play with it.

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Dieseling is usually caused by a combination of things:
- high idle speed. Rotating assy has more momentum at higher rpm & tries 'to keep going'. Lower the idle speed. See no reason for 800 rpm, 4 speed, & stock engine.
- retarded timing. Timing being 'late' helps the engine to run on; it also causes hotter engine temps which causes hot spots. Try advancing the timing a few degrees.
- hot spots, hot carbon particles in the combustion chamber, usually with high mileage engines. Engine at fast idle, try pouring water slowly down the carb bores to steam-clean the chambers.
 
Check you vacuum caps on the carb those super six carbs have a bunch of ports that have full manifold vacuum, stock slants like 4-6 degrees, set it there see if the idle goes up, if it does then lower the curb idle. Should solve your dieseling. If not spray around your carb with a spray bottle with water and check for leaks
 
Princess Valiant said:
It started life as a step side but either the factory or someone put the tool boxes on, which is the main reason I bought it because I wanted a tool truck because I am too lazy to lug heavy tools everywhere I go.

I'm not sure what a fleet side is but it is a shorty.

At this time I am not selling, because I want to play with it.

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View attachment 1716279597
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That's kinda cool! Better have good locks on that tool body!
 
Princess Valiant said:
It started life as a step side but either the factory or someone put the tool boxes on, which is the main reason I bought it because I wanted a tool truck because I am too lazy to lug heavy tools everywhere I go.

I'm not sure what a fleet side is but it is a shorty.

At this time I am not selling, because I want to play with it.

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Click to expand...
That’s awesome. Very cool work truck. A fleet side is just a straight regular truck bed.
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