Demon 850 dieseling when shut off

[coosawjack]

40k Later...

I had the same problem with my 408W and installed an A/C idle solenoid to hold the idle up when running and let it close the throttle completely when the ignition is turned off!!

It dies instantly now but I have since discovered that my electric fan was the culprit!! After the ignition is turned off the fan winds down and generates enough current to keep the ignition hot for a second or two.....VOILA....DIESELING??

You might want to check this out as well before you make too many changes??

Good Luck!!
Coosawjack

[CJ428CJ]

Coosawjak may be on to something. Here's a link to the MSD site that describes how to fix engine run-on caused by current from the charging light backfeeding the MSD unit. http://www.msdignition.com/1troub1.htm

[SCOBRAC]

My reason for asking was in some cases (like mine) too much compression will cause run-on. Nothing you can do about it aside from running 98 octane or big addatives. (or lowering the C/R).

You have two problems. Running rich isn't causing your run on. That is caused by too much air / fuel at shut off. You need to get the butterflies closed to reduce this. Often in the case of big cams or long duration cams in order to keep them idling you inadvertantly open the primaries too far. I'd suggest verifying the static timing, advance it if you can enough to keep the idle at 800 while closing the idle adjustment screw.

Try that. If it doesn't work we'll try something else. Never, never attempt multiple fixes on a carb enless you know what the result will be. It only makes things worse in most cases.

That said the next step would be removing the carb and closing the secondary butterflies slightly to essentially do what we just did on the primary side.

[Bruce Edwards]

Ok beg to differ on this one. If the fuel mix at idle is to lean it will cause run on. In the 70's with the new laws the factory cars all came with anti diesel sol's to control the run on problem brought on by the leaner mix that the new law created. The sol was the idle set not the screw on the carb and the priomary was set to shut all fuel or most fuel of at shut down. Lets not forget that at this time the compression was dropped as the fuel was changed from leaded to unleaded.
Run on can be caused by a few things. Granted one is to high compression with poor fuel however in this case it is reported as a new problem. Thus something has changed.
The trick is figuring that out. For me, after working on cars of all kinds for over 35 years, I find cabs and or spark to be the biggest factor. I have built engines with compression in the 12's on pump fuel with no run on or ping. All a matter of setting it all up so everything works together.

[40k Later]

Thanks for the link to the msd web site run on issue. I will disconnect my fan shortly before I turn the engine off and see if it helps. But it doesn't always diesel now at the current setting that I posted previous.
No matter, the car is still running very rich. The B/G people won't go into trouble shooting from any aspect as they are sticking with the carb is too large approach.
Compression I don't think is a factor at 10.5:1.
Jets are pretty easy to come by so I will try it this weekend perhaps.
An earlier thread mentioned to crack the secondaries a little. I may try that to keep it idling and then turn in the mix screws

[rpred]

I have the same problem with my 850cfm Demon. I have a 460ci with 10:1 c/r. Cast iron ported heads. And a Cam Research 291 duration cam. Timing is set at 10 init and 36 total. Plugs run tan not black. I asked Barry Grant tech line why the "Idle-Eze" circuit wasn't doing much to change the idle speed and allow me to close the primary throttle plates and maintain a decent idle without run-on after shutting off. After two e-mail attempts I still have not received a response. I will try again today.
Rick.

[BIG529]

Just a thought, you keep saying that you adjusted the floats just to the bottom of the site hole, that's the way you adjust the holley carb, I think the BG float level needs to be above that and in the middle of the site glass.


ken

[decooney]

Originally posted by CJ428CJ


Coosawjak may be on to something. Here's a link to the MSD site that describes how to fix engine run-on caused by current from the charging light backfeeding the MSD unit. http://www.msdignition.com/1troub1.htm

Hey Chris,
I was going to ask him if he had an MSD too but you beat me to it. I'm aware of the resistor/MSD mod but have not had the nerve to cut that wire next to the voltage reg box near the ID tag. Ugh, ugly spot for it. Have you added the resistor to yours, and did it work for you?. I too still use the let-out-the-clutch method for now.

[SVO429]

I used to have a falcon with an australian built 351 cleverland ,it had a carter thermoqaud carb on it ,it was factory stock no mods , every time i turned it off it would diesel (run on) to stop it running on, i would push the very bottom of accel pedal down 1/2 an inch with tip of my shoe then slide my foot down off the pedal,this would make pedal bounce up,and close the primary throttle plates a bit further.
If i did this it would never run on if i forgot to do it ,it would run on and become hard to start the next day.
This may sound simplistic but it worked

[CJ428CJ]

Duane,

I haven't tried that yet because I don't think it will do us any good. The ERAs don't have a charging light so I don't think we're backfeeding any voltage to the MSD throught a light. Maybe back through the amp meter? I don't know. I need to get the wiring schematic out and see if that seems possible. Someone up above mentioned that when they shut their motor down, the fans turn into a generator and back feed the MSD. That makes sense but I think that will only cause a problem for cars that feed the fan off the accessory or ignition side of the ignition switch. On our ERAs the fans aren't switched but are fed directly off the battery.

Chris

[badrich]

40k Later: Some of this has been touched upon, but here's how I eliminated my run-on problem. I have 10.6:1 compression and was told I would have no problem running standard [crappy] 91 Octane gas. Wrong. Leaving the car in gear and letting the clutch just enough to lower the RPM a tad does work, but it is embarrassing if I forget (sounds ugly). 100 Octane gas is available, but it is well over $4 a gallon. Tried a couple of additives that said that they would cure the problem. They lied. Then I tried Amsoil Series 2000 Octane Boost. It worked. One bottle at fillup. I became a dealer so I could get a deal when I bought a case. You could do the same, although you might want to try a bottle first to see if it works for you. Good luck. Run-on is nasty and it is really embarrassing sounding when it happens at the filling station next to that hot babe in the Ferrari. Rich