View Full Version : Dieseling after shut off
aarvig
08-19-2021, 10:57 PM
I’ve got a 427w with a 780 CFM QuickFuel carb w/mechanical secondaries. Engine has street cam. I had a problem with dieseling initially and found initial timing to be set at 9 degrees. Readjusted initial to 14 and added limiter straps to the electronic distributor to hold total timing to 30. Leaned idle mixtures a 1/4 turn and bumped idle set screw a bit and the dieseling stopped. Also adjusted choke to slightly leaner. Fast forward 200 miles and I noticed the PCV hose disconnected. Not sure how long it’s been loose (probably was when I previously set mixtures). Reattached it and BAM…car won’t stay idling without help and diesels like the family truckster on National Lampoons Vacation. What happened and how do I fix it??
Gordon Levy
08-20-2021, 01:04 AM
Make sure the PCV valve is actually working the way it should. Second it sounds like you need to go through a do a full setup on the carb. Check the jets, floats, power valve, etc.. and make sure specifically the power valve is the correct size for the cam and isn't blown. This is a common issue. Next is to make sure you aren't making too much fuel pressure, 7 psi is more than enough. If the carb has 4 corner idle screws I can almost guaranty they are maladjusted. Once you go through a bunch of this stuff you can give me a call and we can talk about fine adjustments.
Agree with Gordon and with his help you should get this ironed out.
Back in the olden days of new and tighter smog controls & lean running carbs, engine run-on was a universal issue for all manufactures. All the Big Three used the same crutch to solve the problem -- a idle stop solenoid that when the key was turned off, cut power to the solenoid and allowed the throttle plate to close far enough to kill the engine. Not a elegant fix but it worked. Here's a photo of how Ford did it and back in the 70's when I was a Ford Tech doing tune-ups a smog certs, I adjusted a ton of these.
152436
aarvig
08-21-2021, 09:07 PM
Thanks for the tips guys. I did have the idle mixture screws to lean. I reset the idle mixture screws to 1 1/4 turn from closed. I also reset the idle speed screw. With the new settings it shut off once normally and a second time it dieseled. I don’t have a way to test fuel pressure (probably should buy gauge). It’s got an edelbrock 1725 pump which is rated for 6 psi. It’s new but was sitting for 5 years before running so I suppose it could be bad.
Also, the fuel level in the sight windows is slightly above the center line. Maybe I should readjust floats?
Gordon…how do I check my power valve?
Gordon Levy
08-21-2021, 09:36 PM
You have to take it out and check it. You also need to see if it is a 6.5, 4.5ot 2.5, depending on your can and your vacuum depends on which you use.
Gordon Levy
08-21-2021, 09:38 PM
I also have a different way to set up 4 corner idle settings. I start at 1.5 on the front and 1/2 on the rear. I go in or out on the front until I get the rpm I want then go in or out in the rear to smooth out the idle. Works better for me than 4 equal settings.
aarvig
08-22-2021, 08:51 AM
Where do you set your initial idle speed screw? I backed the idle speed adjust screw all the way out until the throttle stopped moving then turned it in just until it contacted the throttle arm and then from there turned it a full turn. Does this sound like a good place to leave it and then adjust the idle mixture screws from there?
Check to make sure oil isn't being sucked in through your pcv. The oil will cause dieseling. It did on my 427w. Had to install air/oil separator. Remove some spark plugs and check for fouling. Mine had oil on them.
Not saying this is your problem, but it's easy to check.
Gordon Levy
08-22-2021, 11:32 AM
I think you might be talking about the throttle blade adjuster. Base adjustment on that usually isn't more than 1/2 turn of so.
johnnybgoode
08-22-2021, 12:05 PM
I find my motor (414W) is more prone to diesel if the curb idle is too high and it's hot. 1100-1200 and it can diesel, 1000-900 and it shuts off clean. My 2 cents. Scott
aarvig
08-22-2021, 12:29 PM
Check to make sure oil isn't being sucked in through your pcv. The oil will cause dieseling. It did on my 427w. Had to install air/oil separator. Remove some spark plugs and check for fouling. Mine had oil on them.
Not saying this is your problem, but it's easy to check.
Interesting! There is a barrier below the PCV in the valve cover but I will have to check to be sure. I did notice there was oil residue in the PCV hose.
aarvig
08-22-2021, 12:30 PM
I think you might be talking about the throttle blade adjuster. Base adjustment on that usually isn't more than 1/2 turn of so.
Yes, you are correct. I was thinking the throttle blade. So, I’ll set that first, start it, warm it, then adjust idle mixture screws as you described.
Thanks Gordon!
aarvig
08-22-2021, 02:51 PM
So I pulled out the number 1 plug and here’s what I see:
152511
Gordon Levy
08-22-2021, 03:32 PM
Pretty fat
aarvig
08-22-2021, 05:02 PM
Pretty fat
Yep, I agree. Is this a jetting issue? Haven’t checked the power valve yet?
Gordon Levy
08-22-2021, 06:46 PM
probably a jetting issue but could be related to the power valve.
aarvig
08-22-2021, 07:01 PM
Ok, I’ll pull that out and check it next.
Railroad
08-22-2021, 07:44 PM
With the pcv port open, the idle vacuum and all throttle positions would be low, I think. This could allow your power valve to be open all the time or prematurely. This would cause your plugs to be as you posted. I would not jump to conclusions your power valve is wrong. Look up the specs on your carb and see what power valve they use. Put a vacuum gauge on your intake vacuum reading at idle. Post the info and I will be able to advise you, or you can google how to chose the correct # power valve.
thought I would save you some trouble
https://forums.holley.com/entry.php?430-Holley-Power-Valve-Tuning
aarvig
08-23-2021, 06:42 AM
With the pcv port open, the idle vacuum and all throttle positions would be low, I think. This could allow your power valve to be open all the time or prematurely. This would cause your plugs to be as you posted. I would not jump to conclusions your power valve is wrong. Look up the specs on your carb and see what power valve they use. Put a vacuum gauge on your intake vacuum reading at idle. Post the info and I will be able to advise you, or you can google how to chose the correct # power valve.
thought I would save you some trouble
https://forums.holley.com/entry.php?430-Holley-Power-Valve-Tuning
Railroad-great advice. Thanks for the link. I didn’t know I could test this on the car. I’ll check this in the next couple days and get back to you.