69 427 Dieseling
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
Hard to tell on a Q-jet I know but how far open are the throttle blades? If the blades are open enough to compensate, say, for old leaky vacuum lines...or other type of vacuum leak then it could set the stage for dieseling. The closer you have the throttle blades at idle to being shut.....the less likely it will diesel.
Just something to check........
Jebby
Just something to check........
Jebby
Thread Starter
Advanced
Joined: Jan 2018
Posts: 92
Likes: 0
From: Brisbane, Australia
You don't, nor does anyone else need a solenoid stop on the Holley. You still have other issues that are the cause of dieseling. You need to figure out what your Vac advance is doing at idle and where your timing mark is at idle. Are you using full vacuum port on the Holley not timed port? That Holley should have two small vac ports in the front. Put a Vac gauge on each one, one at a time. One should have no vac at idle (timed) and the other is full vac.
Is the lack of vacuum advance causing the engine to run hotter promoting dieseling?
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
Not enough total timing can make an engine run hotter at cruising speed. But that's a different issue with your dieseling.
You really should be running Vac Adv on the street. And from a full manifold vac port. You get better throttle response, better MPG, run cooler and starts better.
Your dizzy advance has been curved mechanically. And that's fine. But now you need to tune the Vac Adv also.
There's two parts to timing advance: mechanical & vacuum. You are only half way done.
Google Lars Papers on carb / timing / tuning. Print them out, go from there.
You really should be running Vac Adv on the street. And from a full manifold vac port. You get better throttle response, better MPG, run cooler and starts better.
Your dizzy advance has been curved mechanically. And that's fine. But now you need to tune the Vac Adv also.
There's two parts to timing advance: mechanical & vacuum. You are only half way done.
Google Lars Papers on carb / timing / tuning. Print them out, go from there.
Last edited by HeadsU.P.; Sep 2, 2018 at 07:29 PM.
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
changing the timing can change your combustion chamber temperatures, but if you have a hotspot in there from carbon buildup or a sharp metal Edge you have a hotspot. You could try the junkyard trick of spraying a little water down the intake as you rev the engine. That quite frequently takes out some carbon buildup. First time you see people doing it it's like WTF?
Last edited by derekderek; Sep 2, 2018 at 08:47 PM.
Le Mans Master
Joined: Jan 2006
Posts: 6,506
Likes: 139
From: Butler Pa
I don't believe it's possible to replicate the effect of vacuum advance with the mechanical advance.
Tech Contributor
Joined: Jun 2004
Posts: 20,910
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From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
Pro
Joined: Oct 2009
Posts: 664
Likes: 89
From: Marysville Ohio
I would look to Jebby's post as a primary place to start - most often dieseling is due to throttle blades open too far.
An idle Solenoid can be used to control the opening at idle while ensuring it closes at shutdown - but it may be a band aid (Been there, done that).
I found that adding more timing at idle allowed me to close both primary and secondary throttle blades (keep them equally adjusted, and keep some of the idle transition slot on the primaries exposed) and get idle speed down where I wanted it. The combination of more initial timing and less throttle blade opening successfully eliminated my problems with dieseling.
It took me several adjustments in tiny increments to get it right.
An idle Solenoid can be used to control the opening at idle while ensuring it closes at shutdown - but it may be a band aid (Been there, done that).
I found that adding more timing at idle allowed me to close both primary and secondary throttle blades (keep them equally adjusted, and keep some of the idle transition slot on the primaries exposed) and get idle speed down where I wanted it. The combination of more initial timing and less throttle blade opening successfully eliminated my problems with dieseling.
It took me several adjustments in tiny increments to get it right.
Last edited by cooper9811; Sep 12, 2018 at 06:00 AM.
Racer
Joined: Oct 2006
Posts: 412
Likes: 1
From: St. John's Newfoundland
I spent a whole summer battling the dieseling problem. I run a 383 stroker with a 770 holley dual feed carb with comp XE274 flat tappet cam. I found I was paying too much attention to what other people said my initial timing should be. I ended up using a vacuum gauge to adjust my idle circuit on the holley, gradually turning them in until i got my highest vacuum reading, which slowed the idle down. I would increased the timing and dial my idle screw out until it would almost stall, then increase my timing a little more. I had it to the point the ole girl would chug over at 500rpm but would stall in drive when I put my foot on the break. Increased my idle until it wouldn't stall in drive with your foot on the brake and took her out for a drive and found it had a little ping on hard acceleration. Checked my timing and was at 21* initial. Dialed it back to 18* initial, set the dist to 36* total all in at 3000rpm. I have a 6al box so no vacuum advance. It took a lot of playing around and ended up with a idle in park of about 750 rpm and in drive with foot on the brake around 600 rpm. It starts well now and shuts off right away. Just my 2 cents.
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