Edelbrock Carb Gurus- Run-on (dieseling) question

[barronj]

Well, the truck generally runs like a scalded ass ape when I'm driving, but at shut-off, it tends to diesel. Lately, it's been doing it really bad. It doesn't do it when cold, only when up to temps (190, never hotter, always constant (mechanical gauge)). I will admit that my fuel line (rubber) runs from the DS to the PS, laying across the intake manifold (clear fuel filter mid stream), and a fuel pressure gauge lays on my passenger side valve cover. Not sure if these hoses are getting too hot & "boiling" the gas or creating substantial pressures to push them in to the manifold where they're consumed.

All new motor, low 9:1 compression range, aluminum headed 351.

There was 3 month old summer gas in the tank (87), I've since driven 1/2 of that off, added higher octane (93) and some octane booster, hoping this would fix it, but it has yet to.

Timing is at 10 degrees, down from 12 in an effort to curb the dieseling. Some argue that too much timing can cause it, so changing timing to see result is a quick event. It seems to run fantastic at 12.

I took the carb off tonight to see if the primaries weren't all the way closed, and they weren't. The screw that I turned to fix it was the fast idle set screw, the one that the fast idle cam establishes itself off of. I'm going to assume that my fast idle will be lower now, not sure how low. It was the only thing I could positively say was keeping the butterflies open, and backing off 1 full turn got them closed. Will this fix it?

Although the heads were new when installed, along w/ a fresh bottom end, it did run extremely rich for a while, and all the plugs would be black from idling. I've since dialed the carb back 3 stages leaner on the cruise mode & 1 leaner on the power mode. I would have to look at the step-up springs & metering rods to tell you exactly what's in it. Plugs now look normal (off-white, tan) when pulled after idling. Not sure what a WOT cut-off would yield on the ceramic, but that's another day.

I could run some sea-foam or a dribble a large cup of water through the carb & see what happens after.

Anyone have a solid, works every time fix, short of putting a holley on there?

 

[DirtDonk]

your butterflies need to be adjusted with the choke fully open, and that fast-idle cam all the way past the point where the fast-idle screw even touches it.
You are correct in assuming that with that screw now turned down, your fast idle will be less. If I'm reading you correctly, it will now be virtually non-existent. Your high-idle on the choke will now be virtually the same as your normal idle.

If you're dieseling with the choke all the way open, first make sure that the fast-idle cam is falling all the way out of the way (it's a gravity thing) and not sticking.
Then adjust your curb-idle down to between 600 and 700 rpm to see what happens.
I would turn that screw back up a bit so that your choke helps the idle rise up.

Do you have a tach? If so, what rpm is your engine running at with the choke on? And off?

Paul

 

[DirtDonk]

Oh, and if your fast-idle cam does not retract when the choke is all the way open, spray some WD-40 or similar thin lubricant in that area.
Yes, a lube would tend to attract dust, but this carb does not particularly like being completely dry either. I'd rather clean and re-lube it every so often than run it dry and have it nice and clean, but not releasing.

Paul

 

[broncnaz]

What RPM is your normal idle at? While the high idle setting can affect shutdown if your only idling at 800 RPM or less it shouldnt have any effect. If the high idle is not kicking off once the choke is open then you may need to adjust the linkage a bit.

 

[barronj]

your butterflies need to be adjusted with the choke fully open, and that fast-idle cam all the way past the point where the fast-idle screw even touches it.
You are correct in assuming that with that screw now turned down, your fast idle will be less. If I'm reading you correctly, it will now be virtually non-existent. Your high-idle on the choke will now be virtually the same as your normal idle.

If you're dieseling with the choke all the way open, first make sure that the fast-idle cam is falling all the way out of the way (it's a gravity thing) and not sticking.
Then adjust your curb-idle down to between 600 and 700 rpm to see what happens.
I would turn that screw back up a bit so that your choke helps the idle rise up.


Do you have a tach? If so, what rpm is your engine running at with the choke on? And off?

Paul

Electric choke. Idles around 750/800.

I'm a little confused, being in front of the computer & not in front of the carb (in the garage), about making the adjustments with a fully open choke. Do I prop it open somehow, since it's electric? I know I can manipulate it.

Also, I know that chokes can also be out of adjustment, even coming from the factory like that on occasion. I may be in over my head again.

 

[DirtDonk]

Yes, you can just prop it open momentarily against it's own spring pressure, and blip the throttle lever to release the cam.
If the cam drops down, great. If not, lube it.

With the cam all the way down and your choke all the way open, your high-idle screw does nothing. At that point everything is on the main idle screw.

At even 800 rpm, some engines will run-on under certain circumstances. Maybe your old gas, high idle, and maybe one or two other factors are conspiring to temporarily become a "perfect storm" of possibilities.

Good luck.

Paul

 

[broncnaz]

The last edelbrock I messed with had a issue with the fast idle not kicking down. It wasnt due to sticking linkage it was the linkage need to be bent to allow it to kick off. not saying that it couldnt be sticking but your carb looks pretty clean. The linkage that may need to be adjusted is on the other side of the carb.(choke side) Since the carb is off the engine mess with it and watch how everything functions or doesnt function then you may see the issue.
Also I would never expect any carb to be set perfectly out of the box. while it doesn happen its rare in most cases. If you have the instructions for the carb they will walk you through all the settings. If not download them off edelbrocks site. I dont think your in over your head your just learning.

 

[John Marinan]

Call the Edelbrock tech line they have given me the correct solution every time I couldn't figure it out. The solutions were always stuff I never would have considered.

 

[barronj]

That's what I'll do tomorrow. Thanks

 

[DirtDonk]

Is it back on the truck yet, and have you tried it since you first started messing with it?

Paul

 

[barronj]

I haven't done anything since we last talked about it. Full day, kid duty at end of the work day, lucky to be lurking on this site now! I will probably call edelbrock tomorrow & bolt it back up tomorrow night.

Honestly though, the only change I made was backing off the fast idle screw one full turn. It will get fixed, thanks to all the help.

 

[joshd1971bronco]

I'll chime in with my experience. Timing, like you said can be an issue, but you've already backed it off some. Try going to 8 degrees. I know the motor won't run as well, but try it just to see. Also, try turning your idle down just a bit. Too high of an idle with all the other 'perfect storm' of things can create enough crankshaft momentum at shutdown to keep it going.

The high idle choke linkage SHOULDN'T be affecting things since you say it does it when it warms up and your choke / high idle linkage should be disengaged by then. Make sure it is disengaging like others say.

One thing that hasn't been mentioned - what kind of ignition system are you running - coil and distributor? If you are running an aftermarket distributor and or ignition system or coil, that may be your culprit depending on how you have it wired up. Some ignition boxes come with a diode (like an electrical check valve) to keep current from backfeeding through the ignition components on shutdown and causing the engine run-on or dieseling. Just another thought to consider.

I feel your pain. I hate chasing down these 'simple' issues.

 

[barronj]

I'll chime in with my experience. Timing, like you said can be an issue, but you've already backed it off some. Try going to 8 degrees. I know the motor won't run as well, but try it just to see. Also, try turning your idle down just a bit. Too high of an idle with all the other 'perfect storm' of things can create enough crankshaft momentum at shutdown to keep it going.

The high idle choke linkage SHOULDN'T be affecting things since you say it does it when it warms up and your choke / high idle linkage should be disengaged by then. Make sure it is disengaging like others say.

One thing that hasn't been mentioned - what kind of ignition system are you running - coil and distributor? If you are running an aftermarket distributor and or ignition system or coil, that may be your culprit depending on how you have it wired up. Some ignition boxes come with a diode (like an electrical check valve) to keep current from backfeeding through the ignition components on shutdown and causing the engine run-on or dieseling. Just another thought to consider.

I feel your pain. I hate chasing down these 'simple' issues.

I'm hoping it's a carb mechanical issue, like a choke linkage.

For ignition, I have an MSD box & coil, distributor out of some later model bronco or truck. I just asked the parts counter for a 351 dizzy w/ vacuum advance w/ a steel gear, I really don't remember.

I called the Edelbrock tech line this morning & they suggested I check my float levels, and that heat-soak might be a possibility, forcing gas in after it's shut off.

 

[70_Steve]

It seems Edelbrock has improved the manufacturing quality of these carbs, since they were Carter AFBs. But I did have similar problems with two different Carter carbs, one used to be on my Bronco.

The problem was that the secondary throttle plates weren't exactly centered in the bores. When the carb was warm, the secondarys stuck open just a very little, causing a higher than normal idle, and dieseling when shut off. I "messed with" the throttle plates and got them a bit more centered so they didn't hang up when warm, and resolved both problems.

 

[barronj]

This beatrice was new when I put it on.

I checked everything over, couldn't find anything that was sticking/hanging.

I did shorten my fuel line & ensured that it wasn't resting on any part of the motor.

Other than that, I may take it from 10 to 8 on timing... or I may slap a holley on it if anyone will respond to their craigslist posts...

Tell y'all how it goes.

 

[70_Steve]

This beatrice was new when I put it on.

I checked everything over, couldn't find anything that was sticking/hanging.

My Carter AFB was only 6 months old when I bought the Bronco, and I noticed that problem almost immediately. It would only happen when it was warm. You'd never see it on the bench.