Dieseling

[Chevynation]

Hello, I have a quadrajet that runs on when I have the idle set at 800 which is where the engine likes to idle. If I lower the idle to close the throttle blades then it will not run on but the idle will be too low at that point. I increased my initial timing but it’s still not enough. No vacuum leaks and I even tried using manifold vacuum on the vacuum advance. Idle mixture screws have full affect and I can’t see any dripping from the boosters. I read about idle bypass air on my carb model but how can I increase it so I can close my throttle blades to prevent run on at shutdown? I have a manual transmission as well.

[Chevynation]

Correction, I know I can increase them by opening the size of the holes on the base plate. Depending on the size of the holes how much do I need to increase by in order to see a difference? I shouldn’t need to increase idle fuel correct? The carb was rebuilt and runs great all around. I’m just trying to slightly tweak it to correct this issue run on issue that’s all. I know another option is an anti dieseling solenoid but I prefer not to run it.

[Chevynation]

Correction, I know I can increase them by opening the size of the holes on the base plate. Depending on the size of the holes how much do I need to increase by in order to see a difference? I shouldn’t need to increase idle fuel correct? Cliff rebuilt the carb years ago and it runs great all around. I’m just trying to slightly tweak it to correct this run on issue that’s all. I know another option is an anti dieseling solenoid but I prefer not to run it.

[77cruiser]

What are the specs of the engine? How much timing are you running at idle with vac. advance?

[Chevynation]

383 288HR cam. Duration @ 0.006":288° / 294°Duration @ 0.050":236° / 242°
Max Lift w/ 1.5RR: .520" / .540" Lobe Separation:110°. I have 1.6 rockers. Dual plane edelbrock performer rpm manifold. AFR 195 heads 10.5:1. Had 18 initial and 28 with advance plugged in. I limited the vacuum advance to 10 degrees. I currently have 22 initial and the total is 34 degrees which the engine likes best with the efficient chamber heads.

[Mudsport96]

No expert but 22 initial is a whole lot of base timing. My inefficient heads don't need that much. Start by backing the timing down around 14-16 initial with no vacuum. And try resetting idle screws.

[Chevynation]

Since I don’t have enough idle bypass air when I decrease my initial I have to open the curb idle screw too far and expose too much transfer slot so it starts dieseling. The cam really likes 18 initial but I just bumped it up to 22 for a test. I took the carb off to see how much transfer slot is exposed and it’s too much.

[77cruiser]

Does the idle speed change much when you increase the timing from 18 - 22 - 26?

[Chevynation]

Absolutely. At 18 I could only get it to not run on at about 600 rpm or so and at 22 I can get it to idle at 750 without running on. 750 isn’t bad but it’s a little low at times especially with heat on. It likes 800 but it will run on. So I just need a small amount of additional air so I don’t have to have the idle screw open as far. I also want to see why my full manifold vacuum port on the side of the carb doesn’t have any vacuum. Neither of my ports have vacuum but the carb has a different main body than it originally had. Hopefully it’s just the gasket that needs a hole drilled but it shouldn’t be too difficult to fix. I’m not worried about the ported vacuum port so that will just remain inoperable. Will .010 increased idle bypass size be sufficient in your opinion? Depending on how the size of hole in the baseplate is I’ll either install a brass set screw so I can drill to the size I want but if the hole is small and just has epoxy then I’ll just drill it a little larger.

[77cruiser]

Have you tried more timing just to see what happens?
What size is your IAB now? Guessing that can you probably have 10-11 in.  of vac at idle.

[Chevynation]

Not sure what the current IAB size is. Carb was done two years ago if I remember correctly. 12” vacuum at idle. If I increase initial then yes it won’t run on but it won’t run on all the time. It doesn’t run on if I decrease the idle either but it’ll just be idling too low. If this was a double pumper I could simply open the secondary curb idle screw to add some air while keeping the correct amount of transfer slot exposure on the primaries.

[Kenth]

What carb-#?

[Chevynation]

7045213 it originally had a 7040203 body but it was replaced.

[quadrajam]

I had the same problem with my previous 350. Tried every trick in the book and
then some. Even tried different carbs and distributor. Ended up using an anti deiseling
solenoid which worked. Problem with mine was horrible quench distance combined with
factory dished pistons. New motor has .036 quench and flat tops. no more deiseling,
even with exhaust heat under the intake and a 195 degree stat.

IAB mods probably not going to help as you will still be letting the same amount of
air into the engine for the same 800 rpm just from a different path.

The idle solenoid may be the best solution for now especially since it is running fine otherwise.

QJ

[Chevynation]

Doesn’t run on with a different carb. My quench is tight. No carbon buildup on pistons. Why would you have to change the distributor? If I increase the idle bypass air by .010 then how would the engine be getting the same amount of air? I should be able to increase the idle bypass air to the point that the engine has a fast idle that can’t be lowered other than reducing the idle bypass air hole size.