100-4 Dieseling?

[mjobrien]

Noticed a new behavior today when I was trouble shooting a oil pressure spike….

Car was warmed up after a little idling – pressure 55 psi
I rev’ed the engine a little 2,500 rpm’s – pressure 60 psi
All seemed OK so I turned the key off

Then the behavior….

Engine almost stopped
Went into what I thought was called “dieseling”
Even seemed “I’m not too sure” that is might have ran backwards?

I can prevent this if I let it idle for a minute prior to turning it off. I assume the engine catches up with the carb’s fuel flow. Timing seems to be right on as it runs really well and all the plugs are nice and light gray and very even.


Any pointers, insights or recommendations on an exorcists?

 

[Dave Russell]

It's usually caused by a chunk of glowing carbon in a chamber which provides "glow plug" ignition after the fact. It could in fact run backward with much bucking & jumping.

Idling for a minute before shutting down allows the glow to cool enough to prevent dieseling/glow assisted compression ignition.

Frequently, just small changes in ignition timing, fuel mixture, or idle speed will prevent it, as will higher octane fuel.

Some carb systems pull a vacuum on the float chamber on shutdown to stop fuel flow. Fuel injection automatically stops fuel flow at turn off.

If the problem is persistent, most folks just leave the car in gear & engage the clutch as the engine is switched off. This is the method I use when it happens about one out of 10 shutdowns.
D

 

[mjobrien]

Dave,

Thanks, Funny I just put about 5 gal's of regular fuel in and previous had used High Test.... Might be one of the reasons, I'll play around with slight adjustments.

Thanks again!
Michael.

 

[BUNDYRUM]

Hello mjobrien

What idle speed does your car run at when it is hot?

If it is too high it will "run on" as it is doing.

I would reccommend about 800 rpm and not much more.

What compression do you have ? Has it been raised by a lot?.

If the compression is high it won't like running on regular.

Ignition timing is important but if it starts alright with no kick back then it will be pretty close.

Regards

bundyrum.

 

[mjobrien]

Good insights - thanks...

It's all stock, so std. pistons doing 160-170 lbs.

Idle is about 1,000 but I may need to get it a little lower.

Michael.

 

[pan]

Thanks Bundyrum,
I think Michael's 1,000rpm idle might be the culprit. It is amazing how many times idling speed is overlooked as the cause of "running on". It also helps if you allow the engine to slow down to idle before switching off, instead of doing so immediately when stopping after a drive.

 

[mjobrien]

Just a update, I was able to get some high octane gas in the 100 today and that fixed the run on issue. Also found a manifold leak that helped lower the ideal a little as well.

So better gas a air tight connections - who would have thought?


Thanks for the pointers...
Michael.