Engine run on.
Drifting
Joined: Jul 2015
Posts: 1,765
Likes: 253
From: Catlett VA
The NOM 454 in my 68 convert diesels on warm days. Adjustments to timing and other fixes suggested here did not work, so I leave it in first gear and let out the clutch when I turn off the ignition key (foot on brake). Works every time.
Tech Contributor
Joined: Jun 2004
Posts: 20,779
Likes: 957
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
Where's your vacuum advance hooked up to?
Race Director
Joined: Apr 2000
Posts: 17,648
Likes: 189
From: Who says "Nothing is impossible" ? I've been doing nothing for years.
I think it may have been mentioned, but the most common cause of run-on is insufficient timing, usually followed by idle speed set too high (or a combination of the two). Often, they are both in play, as insufficient timing can lead to idle speed being too high.
The throttle blades in in your Holley may be set too far open, allowing more air in and encouraging run-on when you shut down. Make sure your primary transfer slots are set up correctly, and that the throttle blades between primary and secondary bores are set up to allow approximately the same amount of air through.
If advancing timing (which can also be achieved by running manifold vacuum advance) then lowering idle speed to the slowest acceptable idle does not cure it, at that point an idle stop solenoid may be required. Sometimes engine combinations are just prone to this.
Most often, it can be cured by finding the right initial timing and/or idle settings.
As an example of a more difficult case to solve, in one of my motors, I initially had pretty bad run-on. After adjusting timing to achieve enough advance at idle, it stopped most of it, but after a long run it may still happen occasionally.
I removed my carb, adjusted primaries and checked that the transfer slots were set correctly. Then I held it up to light, and adjusted the secondary openings to match the primaries. I used the light as a visual indicator to help me estimate the secondary blade position (I adjusted to allow an equal amount of light through all 4 bores).
Reinstalled carb and adjust idle slowly to 850-900 RPM by tweaking both primary and secondary blades equally. This helped and did improve the symptom, but still had an occasional run on.
At this point, I added an idle stop solenoid. To set this up, I removed carb and rechecked throttle blade settings. I adjusted the solenoid plunger to keep primary blades open to the correct position when energized (square transfer slots). Then I set the primary blades to close completely when the solenoid is not energized - in short, the solenoid sets my idle speed when on, but when off it completely shuts off the flow of air.
This solved my run on completely. Most don't need to go this far, but in this case, it was the best solution for my combination.
AS mentioned above, a milder motor like yours may only need timing the initial adjustments to resolve the problem. I just thought my experience may be useful to someone else out there.
The throttle blades in in your Holley may be set too far open, allowing more air in and encouraging run-on when you shut down. Make sure your primary transfer slots are set up correctly, and that the throttle blades between primary and secondary bores are set up to allow approximately the same amount of air through.
If advancing timing (which can also be achieved by running manifold vacuum advance) then lowering idle speed to the slowest acceptable idle does not cure it, at that point an idle stop solenoid may be required. Sometimes engine combinations are just prone to this.
Most often, it can be cured by finding the right initial timing and/or idle settings.
As an example of a more difficult case to solve, in one of my motors, I initially had pretty bad run-on. After adjusting timing to achieve enough advance at idle, it stopped most of it, but after a long run it may still happen occasionally.
I removed my carb, adjusted primaries and checked that the transfer slots were set correctly. Then I held it up to light, and adjusted the secondary openings to match the primaries. I used the light as a visual indicator to help me estimate the secondary blade position (I adjusted to allow an equal amount of light through all 4 bores).
Reinstalled carb and adjust idle slowly to 850-900 RPM by tweaking both primary and secondary blades equally. This helped and did improve the symptom, but still had an occasional run on.
At this point, I added an idle stop solenoid. To set this up, I removed carb and rechecked throttle blade settings. I adjusted the solenoid plunger to keep primary blades open to the correct position when energized (square transfer slots). Then I set the primary blades to close completely when the solenoid is not energized - in short, the solenoid sets my idle speed when on, but when off it completely shuts off the flow of air.
This solved my run on completely. Most don't need to go this far, but in this case, it was the best solution for my combination.
AS mentioned above, a milder motor like yours may only need timing the initial adjustments to resolve the problem. I just thought my experience may be useful to someone else out there.
Pretty much what I said in a lot less lines 
Initial timing gets too retarded for one reason or another and you turn the idle screw too high to compensate.
First step is to take the carb off and get the transition slots square ( .040" ). Put it back on and disconnect the vacuum advance.
Rev the engine to 3000RPM and try and get ~ 34degs. Let the base timing stay where it is. At this point set the idle screw to where you want it at idle. You should not have a problem after this.
Race Director
Joined: Jan 2000
Posts: 12,399
Likes: 247
From: San Jose CA
I agree. With manifold vacuum to vacuum advance you can get the timing screw down to essentially nothing.
Dieseling with carburetors is frequently caused from having too much throttle blade exposed. When you kill spark, the motor doesn't stop instantly - so it pulls fuel through the system and there's likely enough heat left in the cylinders to keep things burning even without a spark.
Adding vacuum advance at idle will keep cylinder temperatures cooler and also decrease the throttle blade used to maintain idle RPM. Both will contribute positively to solving the problem.
Unless this car is solely trailered to drag strips, there's really no reason not to run vacuum advance. It exists on street cars for a reason.
Dieseling with carburetors is frequently caused from having too much throttle blade exposed. When you kill spark, the motor doesn't stop instantly - so it pulls fuel through the system and there's likely enough heat left in the cylinders to keep things burning even without a spark.
Adding vacuum advance at idle will keep cylinder temperatures cooler and also decrease the throttle blade used to maintain idle RPM. Both will contribute positively to solving the problem.
Unless this car is solely trailered to drag strips, there's really no reason not to run vacuum advance. It exists on street cars for a reason.
Le Mans Master
Joined: Mar 2001
Posts: 5,737
Likes: 644
From: Lincoln, CA
Want a good story. When I took my car get dyno tuned I left to get coffee. About an hour.
At that point I was getting aligned and I was all set on the carburetor adjustment.
Coming home I noticed it was really getting hot at idle "220" but cooled down once I was moving.
Got home and I swear it was a 10 minutes run on.
Started playing with it. Broke out the timing light.
10 Degrees at idle??
Switched to ported vacuum??
Drilled holes on my butterflies??
WTH, I was not happy.
Ordered new butterflys and reset everything.
Base timing 18 Degrees, no vacuum
Hooked backup the manifold vacuum.
Reset A/F Ratio and idle to 800RPM.
Checked transfer slots and it was perfect.
No more run on. No more over heating at idle.
At that point I was getting aligned and I was all set on the carburetor adjustment.
Coming home I noticed it was really getting hot at idle "220" but cooled down once I was moving.
Got home and I swear it was a 10 minutes run on.
Started playing with it. Broke out the timing light.
10 Degrees at idle??
Switched to ported vacuum??
Drilled holes on my butterflies??
WTH, I was not happy.
Ordered new butterflys and reset everything.
Base timing 18 Degrees, no vacuum
Hooked backup the manifold vacuum.
Reset A/F Ratio and idle to 800RPM.
Checked transfer slots and it was perfect.
No more run on. No more over heating at idle.
Team Owner
Joined: Oct 2004
Posts: 81,241
Likes: 3,062
From: altered state
St. Jude Donor '05
Adjustments to timing and other fixes suggested here did not work, so I leave it in first gear and let out the clutch when I turn off the ignition key (foot on brake). Works every time.
No vacuum advance hate to lock it out on a street/strip curve is perfect as is. have to fool with it some more. Drilling holes in primaries not recommended kind of band aid.
Drifting
Joined: Jul 2015
Posts: 1,765
Likes: 253
From: Catlett VA
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Drifting
Joined: Jul 2015
Posts: 1,765
Likes: 253
From: Catlett VA
Thanks, it works fine where it is and gives enough vacuum to properly advance the ignition timing . All three small ports (and the large one for crankcase vent) have vacuum at idle and higher rpm. I do not have AC or a heater control valve, so I only need the one for distributor advance. The other two are plugged. Dieseling in my 68 is mainly due to not having an idle cut-off solenoid. Newer cars had one, mine came without and does not create enough problem for me to add one. One less electronic device to go bad. ..... I have a 1993 RX-7 with 12 vacuum solenoids just to control the coordination of the primary and secondary turbos (whenever I get the urge to fool with electronics!)
Race Director
Joined: Apr 2000
Posts: 17,648
Likes: 189
From: Who says "Nothing is impossible" ? I've been doing nothing for years.
My Race Demon came with holes drilled in the primary and secondaries from the factory.
Race Director
Joined: Jan 2000
Posts: 12,399
Likes: 247
From: San Jose CA
Throttle blade holes are necessary when you have a big enough cam that necessitates jacking up the curb idle. But if you're not running manifold vacuum at idle, that should be your first bet. Easier to undo than drilling holes.
Tech Contributor
Joined: Jun 2004
Posts: 20,779
Likes: 957
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
Thanks, it works fine where it is and gives enough vacuum to properly advance the ignition timing . All three small ports (and the large one for crankcase vent) have vacuum at idle and higher rpm. I do not have AC or a heater control valve, so I only need the one for distributor advance. The other two are plugged. Dieseling in my 68 is mainly due to not having an idle cut-off solenoid. Newer cars had one, mine came without and does not create enough problem for me to add one. One less electronic device to go bad. ..... I have a 1993 RX-7 with 12 vacuum solenoids just to control the coordination of the primary and secondary turbos (whenever I get the urge to fool with electronics!)
What's different about your engine that keeps it from operating the same (ie: not dieseling) as the majority of the engines here?