Let's see some suggestions on how to STOP an engine from "dieseling" during shutdown.

[toolmanmike]

Yes. I figured that it is a throttle blade issue.
I welcome opposing opinions though since I am not that experienced with this.

Well, good luck. I think you already have found a solution for your issue by lowering the idle speed.

 

[Wietse]

If a solenoid would energize to open the throttle lever once the ignition is ON but then allow it to close further when the ignition is OFF, that seems like a fix for this.

I fear that would not help, because whatever burns is already in there regardless of the throttle being closed more.
If that solenoid would close it more prior to shutting down i think would help more/better.
Think that stuff does not cost much and is worth a try.
Did you do any tinkering on the idle fueling, enrich/lean it out?

 

[66 Sat]

You had me until the Freiburger comment....

View attachment 2073132

I watched all of the Roadkill Garage episodes and shook my head at some of the stuff they did. I don't think he is stupid, just that he doesn't build most of his cars to last a long time.
Thank you.

I only mentioned Freiburger because you mentioned Ehrenburg, to show that different people with lots of "followers" have different opinions. I actually only saw the Freiburger comment on MVA last week when I was watching him trying to tune his carburetor. I initially tried MVA after reading what Geoff had said on here, then doing research, and then trying it myself.

There's some resistance to it on here because it's not what Ma Mopar did at the factory. Who knows the reasoning behind this when GM did use it. Maybe the Head Engineer was stubborn? Maybe Ported is better? I only know what I have experienced.

MVA makes sense to me from an engineering standpoint, it's free, and will probably take 20 minutes to do the whole test. You can always put it back to Ported. The idle solenoid is going to cost you money and probably more time in buying it and fitting it.

 

[Kern Dog]

I fear that would not help, because whatever burns is already in there regardless of the throttle being closed more.
If that solenoid would close it more prior to shutting down i think would help more/better.
Think that stuff does not cost much and is worth a try.
Did you do any tinkering on the idle fueling, enrich/lean it out?

No carburetor adjustments in quite a while.
I put in an air/fuel gauge years ago and am able to read the numbers as the engine is running. It idles near 14.0, give or take a few tenths. I was told that a 10% ethanol blend does not need to be the common 14.7 number but instead High 13s to low 14s.

 

[Kern Dog]

I picked this one from @DeltaV

 

[Kern Dog]

Well, good luck. I think you already have found a solution for your issue by lowering the idle speed.

Thank you.
The manifold vacuum advance idea may have made a difference but since I like how well sorted the engine is, I wanted to leave the distributor and advance setup as it is. Yesterday, I did get the idle stop solenoid and bracket.

That is the closed position. Next is with the ignition ON.

It is hard to measure with my crude tools but it looks like the plunger moves about .170” +/- when activated. The brass bolt is adjustable.

This explains why no instructions were included. It looks as simple as falling off a log.
Maybe this week I can get to work on this.

 

[Kern Dog]

It looked so easy.

I had the bracket and the solenoid. It looked like they would naturally just fit and work.

I do have to make sure it does not sit too high and interfere with the underside of the air cleaner but so far, that isn't the issue. First, I tried to slip the bracket in place. It was too wide between the mount and the throttle lever.

Look again. The black arrows point to the mounting screws, the blue arrow points to where I had to cut it just to fit.

The width between the blue lines below put the edge of the bracket out too far, interfering with the throttle lever. The bracket will have to be no wider than the space between the red lines...

Even then, the top of the bracket may be too high to clear the air cleaner. Note that the bracket has a hole for the solenoid with a flat spot to keep the solenoid in one position. My goal is to place the solenoid so the electrical connection was at 6:00 for a cleaner routing of the wire.

 

[Kern Dog]

There is more.
The solenoid has to be mounted at the angle of the throttle lever at rest to be effective. The body of the solenoid is big so it needs room. There are obstacles....

Note the robust lever for the accelerator pump. It is right in the way.

Now look at how the solenoid plunger misses the throttle lever even though it can't go toward the carburetor any more. What is the fix?

I mocked up a section of steel plate to serve as an extension and vice gripped it to the lever...

Now I just need to make some sort of bracket to bridge between the carburetor baseplate and where the solenoid will fit and work.

 

[Geoff 2]

It depends how creative you are & the tools available.....
I see a large & a small hole in the throttle lever. The holes can be used to bolt on a lever/bracket that might provide a more convenient place to mount the sol.

 

[pnora]

I wonder if BG used a different carb base on the Demon to avoid law suits.

 

[pnora]

This might use the carb to intake mounting bolts. Esp if yours has two holes the tab might locate into the other hole and bolt down on the one.

 

[68-500]

KD, I think you’re making more work for yourself. If you just plug that vacuum advance into the manifold and adjust your idle accordingly I think your problem is resolved. You don’t need to adjust the timing or the advance curve. The only difference between manifold and ported is at an idle. Everywhere else it works the same. JMO

 

[Kern Dog]

This might use the carb to intake mounting bolts. Esp if yours has two holes the tab might locate into the other hole and bolt down on the one.

View attachment 2078713

The carburetor I have is a Barry Grant 850 Speed Demon. It does have mostly "Holley" like components. I'd have to look again but I think the rear of the baseplate does have two mounting holes but the front only has one per side. The bracket above would not work.

KD, I think you’re making more work for yourself. If you just plug that vacuum advance into the manifold and adjust your idle accordingly I think your problem is resolved. You don’t need to adjust the timing or the advance curve. The only difference between manifold and ported is at an idle. Everywhere else it works the same. JMO

A fellow FBBO member uses manifold vacuum and has the same dieseling condition that I do.
Using manifold vacuum seems really strange to me. There is what.....maybe 35 degrees of advance at idle with my 20 degrees initial and 15 degrees of vacuum advance, I crack the throttle to around 1500 rpm and it backs off to mechanical only 28. How is that going to make a smooth power curve?

I don't understand how that would work.

 

[68-500]

KD, if your va canister has say 13* advance, with your 20* initial you will have 33* idle manifold vacuum. When you open the throttle slightly manifold vacuum won’t change much neither will va timing.
Ported, 20* initial and 13* va you will have 20* idle. When you open the throttle slightly you expose ported vacuum port to manifold vacuum, now your timing jumps up to 33*. The rest of the throttle sweep is the same between both methods.
So really manifold vacuum is smoother than ported as you don’t get the initial spike with throttle opening.
Now when you mash the throttle from idle ported will just go straight to centrifugal curve. Manifold will drop from 33* to 20* initial then go to centrifugal curve.
I don’t drive your car but, mine on the street sees more of the gradual throttle opening than just mashing it unless I want a smoke show.

 

[Kern Dog]

I drive the car casually and aggressively. The tune as it is now is great. No hesitation and throttle response is fantastic.
As far as making more work for myself....Hey man, I'm retired! I have time to tinker. I actually enjoy making brackets and modifying stuff.

 

[68-500]

KD, getter dun then!

 

[66 Sat]

I drive the car casually and aggressively. The tune as it is now is great. No hesitation and throttle response is fantastic.
As far as making more work for myself....Hey man, I'm retired! I have time to tinker. I actually enjoy making brackets and modifying stuff.

But no time to tinker with Ported vs Manifold Vacuum Advance?
You could be just the guy to come up with the definitive answer - you have the skills and the time to try both and report back, and people on here value your opinion.

I'm not talking about changing the curve in the distributor or altering the timing (although that may be necessary in some cases). I'm suggesting just swapping the vacuum hose, lowering your idle and going for a drive. If you've got the idle solenoid working you could disconnect the power to it briefly to see if the MVA also stops the dieseling.

 

[Kern Dog]

Yeah, I see the point. It costs nothing to try it.

 

[Kern Dog]

We all know that often times, a project includes a few obstacles along the way. A few hiccups are to be expected and I do expect them.
The 20-9 Holley bracket looked like it would be a direct fit.
It was not.
The base of the bracket fit the carburetor but the shape just didn’t fit and clear the throttle lever or accelerator pump so…. I cut, shaped, tack welded and test fitted….

Keep in mind, the bracket is just tacked together and not finished…

The solenoid fits and is aimed correctly as far as I can tell.



I mocked up an extension on the throttle shaft so the solenoid would have a place to press against. Again, this is mock up stage.

The air cleaner does not sit down all the way.

The body of the solenoid sticks up too high and the underside of the base plate hits it. The bracket clears, the throttle shaft clears but the bulbous solenoid sticks up too high. I can't lower it down from it's current spot because the accelerator pump is about 3/16" below it.
This means I’d need to move the solenoid out further and down lower to clear. Yeah, I can do that.
I hooked up wires to test the solenoid just to see it open and push the throttle open like it would in normal operation. I grounded it and put power to it but got nothing. I keep trying…nothing. It turns out this solenoid isn’t strong enough to overcome the throttle return springs. Huh?? Yeah. The solenoid works but just isn’t strong enough. It isn’t like I have a real firm pedal either. Maybe the solenoid is defective?
There is more though. The way I made the bracket, it blocks the idle mixture screw.

I could drill a hole in the bracket for access but for now I’m stepping back from this. I only bought this Holley solenoid because Holley also had a bracket to make it work. The dang thing was $140 when other generic solenoids were much cheaper.
Maybe I’ll try manifold vacuum after all…..

 

[Kern Dog]

Another option is this:

I have one of these FBO plates in the distributor now, set to the 16 degree mark. The way it is, it actually resulted in a 15 degree spread from a 20 degree initial to 35 total. If the "manifold vacuum" results in a bump in timing at idle to increase idle speed, the same could be achieved by rotating this plate to the 12 or the 10 number. This would result in an increase base timing if I set the total back to 35. That would increase the idle speed, allowing me to back down the curb idle screw and hopefully, stop the run on.
Still, for you guys that seem to love that manifold vacuum, I will try that first. Maybe my apprehension to it is unfounded.