Engine Run On/detonation

[cdog]

I have been having difficulty with my 514 BB, as when I turn the ignition off, the car runs on for 10-15 seconds, culminating in a loud detonation/pinging type noise.

Sometimes white smoke puffs out through the carb.

I have a 975 Race Demon RS on the car. Any ideas?

Thanks.

[Ron Randall]

Cdog
Its sounds like a low octane problem, dieseling is caused by un durnt fuel in the chambers. I have a 428 tweeked and the engine builder reccomended no larger than a 750 cfm carb and no less than 98 octane.

[Oafus]

My 427 started to do this as soon as CA lowered the octane rating to 91. (Without warning or cause, I might add!)

Craig

[dalola]

cdog,
While your problem is in fact most likely octane related, as Ron and Craig stated, if that doesn't solve the problem, you may want to check timing, and also check for voltage "leaks" from accessories such as electric fans, fuel pumps, etc.. If it is the later, the instalation of a diode will usually cure it.
Good luck!
Dave
P.S. I'm sure you're aware of how extremely damaging to an engine this condition is!

[Richard Hudgins]

Hi Cdog,

You might wish to check your idle mixture. If it is rich by just a little bit, you will get run on.

Try leaning out the idle mixture just a bit.

This was a big problem in the 70's and 80's with emission motors and the factories went to a lot of trouble to try and stop the problem with various solenoids, etc. None of them worked very well.

this was during the fight to keep NOX down.

[Rick Parker]

As mentioned previously this is a very damaging condition if allowed to continue. I too recently notice the reduction to 91 at the pumps does anyone have an explanation of why? Fortunatly
the are a couple of Union 76 pumps at selected stations have 100 octane unleaded. Check the heat range on your plugs, go one step colder if your ignition will accept it and you might put in a fuel system cleaner in the fuel in a attempt to rid the combustion chambers of any deposits from an overly rich condition. They will form carbon and somtimes continue to glow and cause what you are experiencing. good luck

Rick

[427sharpe]

Three simple things to ck:
1: Timing, if in doubt retard the timing slightly & see if it helps
2: Fuel mixture. I run a 535 w/ a Holley 750, and I am not jetted to max. Sounds like you are WAY overcarbed! Make a WOT run, shut off & coast to a stop. Pull 1 plug (prefer #4) and ck--if it has raw gas on it or is sooty black, jet way down or go to a smaller carb.
3: Throttle plates. Make sure throttle plates are not binding OPEN on intake--this would cause a rich condition and run-on. Also ck carb for a blown power valve caused by backfire.
I really think you are too big on carb--if you have a smaller one, it might be easier just to bolt it up and see what happens before you grenade the motor from detonation. Good hunting, and please post results/further info, OK??

[aeroace]

LOWER YOUR IDLE SPEED

The CFM rating of your carb doesn't matter(750 vs 850). Idle speed does. If your plugs are too hot, or too much carbon buildup in the combustion chamber, these could be the cause. These form hot spots that act like glow plugs and ignite the mixture. Don't over richen the idle mixture to try and correct; it'll cause more carbon buildup.

If nothing else works

DRAG THE CLUTCH WHEN KILLING THE MOTOR

LEAVE AUTOMATICS IN GEAR UNTIL DEAD, THEN SHIFT TO PARK

[Dave Samson]

Are you running a multi spark ignition, eg, an MSD box ?
If so, a 1 amp 100 volt diode needs to be installed on the field wire of the voltage regulator. If that doesn't fix it, try installing a dual ballast resistor to the small red wire on the MSD box.

[Cobrabill]

One more simple thing to check.Make sure the accelerator pump arms have 10 to 12 thou.clearance.This CAN cause run-on.

[khall]

Dave,
I notice that Superformance includes a ballast resistor with their big block installation kits. What exactly does it do? Why create more heat in an overheated engine compartment?
Keith

[Dominik]

CDOG,
If the throttle plates are just too wide open at idle (more than 3/4 of a turn) your mixture may also get overrich.

In the 70s they used a solenoid to hold the throttle plate open during idle and shut the TP closed at ignition off.

Ingnition for these engines was too retarded to keep them idling with a small throttle plate gap.

Also, your power valves may be incorrect for the used cam causing open power valves at idle = overrich.
As does an incorrect float level. Can you see little drops in the carb at idle? - Watch out for backfire while peering inside!

I also think the carb is a little too big, but that has not much to say at idle. I used a 850 for my 496.

Dominik

[Dave Samson]

Keith,
You are right, there will be additional heat but inconsequential compared with the engine and radiator heat.
Rather than I scratch my head for an explanation, I thought I'd refer you to www.vtr.org/maintain/ballast.html where someone has already done the hard work.