[64speed]

Mines not bad but I do have a little when it’s 100 degrees out. Anything to worry about? Cause and cure?

[Kenth]

Late ignition or/and lean lowspeed circuit causes too far opened primary throttle blades.
Tune ignition and carb, possibly install an antidieseling solenoid.

[PunchT37]

Mine does that too. Didn`t have that problem much with 6x heads on a 455 but more of an issue with Dave`s fast burn chamber eddy heads. Both had 160 stats and 93 octane. I turn mine off in gear [auto] when real heat soaked. Most times the run on isn`t that bad.

Ignition isn`t a problem as the spark is off when this occurs. Heat is causing fuel to light off [like a diesel]. Cooler chambers, better fuel and lower idle help in these things.

[steve25]

If your idle temp is 195 or lower and your idle speed is under 950 and adjusting the Carbs idle mixture screws closed more will cause the idle to go ruff and get lower like such a adjustment should then your fuels Octane level might be right on the edge of being enough and or your plugs are a heat range too hot.

How are your plugs looking in terms of the color change over point as seen on the ground electrode ?


In terms of pump fuel of the same Octane rating some brands will perform better a high air temps then others due to the blend, which of course you have no control of.

At times I have seen lesser Octane fuels perform better at high aimbent air temps then a fuel of 2 points higher in Octane rating.

When all else fails just dump the clutch a little one the E brake is on.

[leeklm]

"then your fuels Octane level might be right on the edge of being enough"

This appeared to be my issue. Regardless of timing, qjet or holley, etc. The only fix was running 93 octane pump gas from my local BP (10% ethanol). Would consistently diesel with 91 ethanol free or even 92 with 10% ethanol. Hard to believe 1 octane point could make a difference, but maybe other fuel additives or quality of fuel was a factor?

Sent from my SM-G965U using Tapatalk

[64speed]

I’m running 93. I got this car running great now or at least by my standards. It’s not bad it just sounds like it sighs and rattles and cuts off. If I cut it off and immediately let out the clutch in 2nd of course it immediately stops. It’s been close to 100 here with a heat index over that.

[64speed]

I’m running 93. I got this car running great now or at least by my standards. It’s not bad it just sounds like it sighs and rattles and cuts off. If I cut it off and immediately let out the clutch in 2nd of course it immediately stops. It’s been close to 100 here with a heat index over that.

[jhein]

I had a run-on issue that just came up out of nowhere with the new motor. It was fairly mild but disappeared with the next tank of gas so in my case it appears to have been fuel related.

[PunchT37]

Originally Posted by jhein

I had a run-on issue that just came up out of nowhere with the new motor. It was fairly mild but disappeared with the next tank of gas so in my case it appears to have been fuel related.

Most of the time it is.


Let`s say you ran 100 octane. That b!tch ain`t running on. Unless you had really high compress in a hot engine.

[jhein]

So here's the question, is run-on dieseling as bad for the engine as detonation while running or is it just an annoyance?

[steve25]

You can't even compare a idle run on condition to anything that occurs under load like preignition and especially detonation.

[Jay S]

Dieseling won’t or shouldn’t cause issues in the engine. But it can catch the carburetor on fire, sometimes it will back fire right before it stops. Keep an eye on it if you want to not start a bad fire.

The Oldsmobile 455s ranon terrible. They often either had to have the idle set really low or a shut off solenoid to shut off proper.

If you don’t run a vacuum advance often running on shows up bad also. The extra advance the idle vacuum provide keeps the timing advanced a little more at idle, then the throttle blades are turned down into the idle idle circuit further.

[geeteeohguy]

You can shut the car off in gear and it won't diesel. But what the other guys said about too fast an idle speed, too much throttle opening, etc.

I haven't been able to buy 93 octane gas in CA in 30 years. 91 is as good as it gets, and it's not good. Boils at less than 140 degrees.

[jhein]

Originally Posted by jhein

I had a run-on issue that just came up out of nowhere with the new motor. It was fairly mild but disappeared with the next tank of gas so in my case it appears to have been fuel related.

I have to correct myself. I misremembered that a bit. I actually did reduce my curb idle when I had that problem which did stop it. But since it did start up out of nowhere, bad fuel likely played a role too.

[Schurkey]

There's thirty potential causes of dieseling; and I suppose like most tragedies, it takes multiple causes to create a problem--on the edge of too much engine heat, too-lean fuel mix, too-little octane rating, too-hot spark plug heat range, too much carbon in the chambers...on and on.

My experience is that the number one cause of shut-off dieseling is that the idle speed is adjusted too damn high. The idle may be adjusted too high, to "compensate" for crappy air/fuel ratio at idle, not enough spark advance, and lots of reversion from a "lumpy" cam that maybe COULD be tuned-out if the rest of the adjustments were closer to what the engine needed, but then the folks at the Dairy Queen wouldn't be so "impressed".

When the idle speed is turned down to "reasonable", the dieseling is usually over.

[Gach]

They change the Flash rate between summer and fall. So its dependent on different fuel companies. No worries tho, just shut it off in gear.

[Kenth]

The use of an anti-dieseling solenoid started 1968 when Pontiac retarded the ignition advance by using a ported source for the vacuum advance.
This was done for reduction of NOX emissions by heating up the cylinder heads, NOT for increased performance.
Switch from ported to full manifold vacuum for the vacuum advance will usually cure the diesling/run-on at shutoff.
Use a vacuum advance unit that fully employes at idle speed.

FWIW

[steve25]

And the above move of having your vacuum advance active at idle will require one to recurve the Distributor.

[Cliff R]

High compression, low octane fuel, and hotter than balls outside when the engine is fully warmed up and too well heat soaked will quickly cause this scenario.

I can be further complicated or enhanced by the tune not being perfect leaving the carburetor throttle plates a tad further open at idle than they should be, or just not enough timing at idle speed to make the engine happy.

The last 455 in my Ventura would do this, but I had to put 87 octane fuel in it and get it so hot you could fry an egg on the intake in 100 degree outside temps to make it happen. It was also 11.3 to 1 compression and none of the previous engines with lower compression ever ran on no matter what I did with them.......

[i82much]

I just let the clutch out and apply the brake whenever I turn my engine off ...