Why Can't I stop My 351w From Dieseling (Engine Runon)

[jikelly]

I figured the engine was 'fresh', I figured it wouldn't have more than fluffy carbon from, say, idling rich.

Things I left out about hot spots, any kind of a raised edge in the combustion chamber can result in a hot spot. Way back when there were tails of detonation traced back to casting flash.

Worn valves will also lead to dieseling, knife edging or insuficent seat can lead to a glowing plug. At 6,000 miles that shouldn't be the problem.


There is an obvious answer, Your running too hot of a plug. Drop down a heat range or two. If you drop 2 ranges and the problem goes away you have nailed the problem, then you only have to find the right heat range in the right plug manufacturer. I'm sitting here laughing at my losing track of the number of sets of plugs I had laying around.

(plugs are like shoes, size 11B from one manufacturer is a 10EE from another, and just cause they look the same doesn't make them the same).




Idle or main jet?

Concern yourself only with the idle circuit.




Seems to me the most interesting things happen in #s 4 (and 8).

Thing is soon as the engine is ran up the fluffy carbon from idle should be gone (except for maybe the deepest recesses of the insulator).

If I were you my next move would be to set the timing where it's "suppose to be" and install a colder plug and see what happens.

Thanks for the suggestions. Um, colder plug???

I'm stupid and have never understood this hot cold plug thing. Hence I've never experimented with that.

 

[67coupe]

Is it truly dieseling?

Do you have an electric fan?

I have seen electric fans on older cars back feed the ignition system with current and provide enough spark to keep the engine running.

 

[jikelly]

Is it truly dieseling?

Do you have an electric fan?

I have seen electric fans on older cars back feed the ignition system with current and provide enough spark to keep the engine running.

There may be feedback causing this issue, but no I don't have an electric fan. I reckon I could isolate the ignition box and coil with diodes.

 

[dennis112]

I saw that you tried to retarded the timing, but that is the wrong way. Retarding the timing will only make it worse. As I said in post #6, advance the timing a few degrees (4-6) and see what happens--that WILL help but be sure to drop the idle down to specs after doing it. Just don't advance it so much that it pings or the starter bogs when the car is hot.

SERIOUSLY--it costs you nothing to try.

 

[Wart]

Huh?

I thought that was the preferred method.

Thanks for the suggestions. Um, colder plug???

I'm stupid and have never understood this hot cold plug thing. Hence I've never experimented with that.

From The Secret Life of Spark Plugs {LINK}

Different lengths of insulator nose result in varied characteristics and different heat ranges. Heat is dissipated from the insulator nose through the center electrode and the inner seal ring to the spark-plug shell. If the insulator nose is long, the heat-transfer point formed by the seal ring is further away from the hottest point on the insulator nose than with a short one. So it follows that spark plugs with a long insulator nose can absorb more heat than plugs with a short insulator nose.

Quoted material is on page 4 , the section starts on page 3.

 

[jikelly]

From The Secret Life of Spark Plugs {LINK}

Different lengths of insulator nose result in varied characteristics and different heat ranges. Heat is dissipated from the insulator nose through the center electrode and the inner seal ring to the spark-plug shell. If the insulator nose is long, the heat-transfer point formed by the seal ring is further away from the hottest point on the insulator nose than with a short one. So it follows that spark plugs with a long insulator nose can absorb more heat than plugs with a short insulator nose.

Quoted material is on page 4 , the section starts on page 3.

Thanks for the info. I'll have to look at the plugs I have and figure out what their heat range is.

 

[Wart]

Part of this is I have my doubts the plugs are glowing hot enough to diesel,

But I'm even more doubtful an engine with 6k miles has caked on carbon deposits.

Some say advance timing, others say retard the timing, some say set it to idle rich, others say lean. aparently there is no clear cut answer.

I suspect the real problem is the fuel. Is there a carburator installed on any consumer vehicle these days? Hell no. Engine management corrects for 'bad fuel' and when power is removed the injectors just don't pass fuel so no run on.

You could have gotten a bad batch of fuel. Or your using fuel from the wrong refiner, back in the day some people would swear by Sohio/Boron/BP, others swore at it.

If you go to a colder plug and the problem goes away you have found the ignition source, unfortunately you then have to find a plug thats hot to self clean without dieseling.

 

[jikelly]

I played around with my car a bit again and still can't find anything really wrong. I readjusted the carb at idle and it's pretty rich, advanced the timing from 4 to 8 degrees initial and still the run on remains.

Could be my carb is still out of whack, but I really don't think so.

My plugs are Autolight 25s I found the following info on another forum.

Autolite Stock Heads

Autolite 25 Naturally Aspirated
Autolite 24 1 Heat Range Colder
Autolite 23 2 Heat Ranges Colder

Spark Plug FAQ.

factory heat range:
Motorcraft = ASF42C
Autolite = 25

one step colder:
Motorcraft = ASF32C
Autolite = 24

two steps colder:
Motorcraft = ASF22C
Autolite = 23

Spark Plug Heat range - Ford Mustang Forums

 

[jcode68]

I readjusted the carb at idle and it's pretty rich, advanced the timing from 4 to 8 degrees initial and still the run on remains.

Your timing seems too low based on everything I have ever read about what Ford small blocks like. Should be in the 12 to 16 degree initial timing based on engine mods etc. Have you tried bringing it up to 12?

 

[jikelly]

Your timing seems too low based on everything I have ever read about what Ford small blocks like. Should be in the 12 to 16 degree initial timing based on engine mods etc. Have you tried bringing it up to 12?

Oh wow, really? Hum, I'll try that. I need to do some serious tuning on my engine this weekend. Course I've got to help my brother move out though. Sad days.