It wants to stall upon fast braking and on downhill slopes...

[Kern Dog]

Hello,
I'm closing in on the point to start the car after a rebuild. Once I get it up and running, I'll need to do some tuning since I changed a few things while the car was apart.
The combination is as follows:

1970 Charger. 440 based 495 with 9.8 compression. Edelbrock heads, Lunati solid flat tappet cam,2" headers, 3" exhaust, Demon 850 Vacuum secondary 4 barrel with 85 primary jets, 92 secondaries. 6.5 power valve. 3 psi of fuel pressure at idle, 7 psi at WOT. Carter fuel pump registered 11 psi at idle before I put in an Aeromotive bypass regulator. There is no vapor lock and the car runs great in all temperatures.
The problem that I have had is that under hard deceleration, the engine tends to stall. The same happens when descending a steep hill. I suspect a fuel level issue. This carburetor has glass sights in the primary bowls and on level ground, the fuel is right in the middle of the glass like it is supposed to be.
I don't recall if the stalling issue has only happened when I'm low on fuel where the fuel pickup could be uncovered but I don't think that is the cause.
Any suggestions as to where else I should look?

 

[AR67GTX]

So Demon says fuel level should be in the middle of the sight glass? Do they use a baffle at the vent opening in the fuel bowl like Holley? Sounds characteristic of the secondary bowl fuel level being to high but I don’t know anything specific about Demons.

 

[Kern Dog]

The Demon carburetors are very similar to the Holley models. I never thought about the secondary floats being a possible cause. I'll check those too.
Thank you.

 

[1967coronet]

The Demon carburetors are very similar to the Holley models. I never thought about the secondary floats being a possible cause. I'll check those too.
Thank you.

May be coming up and out of the vent tubes . Then dropping in the intake.
Just for a test add a short piece of fuel hose on the vents.
Some of the circle track guys have to.

 

[AR67GTX]

Engine flooding on hard acceleration - look at the primary fuel bowl and vent flooding. Engine flooding on hard breaking look at the rear fuel bowl and vent flooding. The idea of adding some vacuum hose over the vent tub to elevate their opening is a good test.

I thought Hooey put vent opening baffles in all of their carbs to slightly lessen the chance of fuel spilling over into the passage system st the top of the metering block and main body. But I’ve been working on a friends 670 Street Avenger carb and surprise - no baffles. Not sure if they were not originally installed or if someone previously removed them.

 

[6872n73]

A/F meter?

 

[MoparMur]

X 3 or four for fuel being pulled out of the vent tubes. I'd be interested to know what you run for an air cleaner. I had a 440 '70 Challenger that would do the same thing (on a hot day). Had a mild 440 that would always run cool but on a hot day I'd pin it and maybe 10 seconds later the engine would load up and stall, and would be horribly flooded. A circle track guy filled me in about fuel being pulled out of the vent tubes.

 

[Kern Dog]

This is the carburetor.....

I've used this air cleaner since 2002.

I had the carburetor rebuilt late 2019. This problem started in late 2020.

 

[WileERobby]

1) Your car is gorgeous. 2) 3 psi at idle ?

 

[eldubb440]

to clarify; OP says the issue is on hard DEcelleration, not acceleration........that would pull fuel thru the vents?

 

[Geoff 2]

I would lower the pri & sec fuel levels 0.100"

 

[Kern Dog]

1) Your car is gorgeous. 2) 3 psi at idle ?

Thank you!
3 psi. That is what the pressure settled at with the regulator in place. It increases with throttle application. I did try to adjust it to raise the idle pressure but it made no difference. I've had this regulator for several years. It cured my vapor lock problem. This stalling issue only started after the rebuild though I can't specifically blame that, it just never did this before then.
Float levels...that is worth a try. I have to get the car started and start adjusting things.

 

[Kern Dog]

to clarify; OP says the issue is on hard DEcelleration, not acceleration........that would pull fuel thru the vents?

Maybe during a hard stop or a downhill decent, a high float level causes fuel to spill out of the rear vent, essentially overflowing into the secondary venturis ?

 

[hunt2elk]

Do you have an A/F meter in the car? Just asking because your main and secondary jets are quite a bit bigger than I have had to use on my 850's, although that has nothing to do with your current issue. On my Quick Fuel carbs, I have been running the float levels at the bottom of the sight glasses. I had stalling on hard braking once when the float level was higher, figured fuel was coming over from the rear bowl. Shouldn't your fuel pressure be around 5 psi at the carb inlet? Just a couple things I would check.....

 

[1 Wild R/T]

try this... By looping the vent & leaving a hole at the top the vents work but fuel can't slosh out of the vent...

 

[AR67GTX]

to clarify; OP says the issue is on hard DEcelleration, not acceleration........that would pull fuel thru the vents?

It doesn’t pull fuel through the vents. On hard breaking the fuel splashes forward in the bowl (no baffles in there) and stacks up against the side of the metering block. If the fuel level is set high enough, and the stop is hard enough, it stacks up until it reaches the window in the top of the metering block that leads To the brass tube vent, and out into the throttle bore. Holley (sometimes?) puts a small baffle over the metering block window to lessen the risk of this. Don’t know if Demon installs them or not. If not, they are cheap and easy to add.

For reasons of E10 fuel characteristics, I’ve developed the theory that it’s best today to run slightly lower float settings than Holly and Carter recommended, mostly from 60s and 70s technology. On Holleys, I’ve been setting mine to factory and then lowering them 2-flats of the adjusting nut and seems to work well in reducing percolation into the carb throats on a hot shutdown. I’ve been setting Carters 1/16” lower approximately. This is just shade tree stuff - other than it seems to work for me I can’t technically defend it.

 

[hunt2elk]

try this... By looping the vent & leaving a hole at the top the vents work but fuel can't slosh out of the vent...

View attachment 1365740

I always wanted to try that, but couldn't get the hose buried under the air cleaner lid.

 

[AR67GTX]

Some of the new Holleys have the vent cast into the carb body without an exposed tube. Bot might be able to find a piece of thin wall vacuum tube and push down inside the vent to either loop them or extend them higher.

 

[Kern Dog]

Do you have an A/F meter in the car? Just asking because your main and secondary jets are quite a bit bigger than I have had to use on my 850's, although that has nothing to do with your current issue. On my Quick Fuel carbs, I have been running the float levels at the bottom of the sight glasses. I had stalling on hard braking once when the float level was higher, figured fuel was coming over from the rear bowl. Shouldn't your fuel pressure be around 5 psi at the carb inlet? Just a couple things I would check.....

The jets are the sizes that came with the carb. I have tried other jets when tuning. Yes, I have an AEM A/F meter with the oxygen sensor in the RH header collector. I had aimed for the mid 14s for cruise and mid 12s for WOT until I read that those target numbers are for true gasoline. The ethanol blends tend to run richer to get the same power. I have since installed a Quickfuel primary metering block to tune even finer, yet have not been able to get even and consistent numbers. I settled on low 13s for cruise and idle and high 11s for WOT. If I jet it lower, it loses power.

I like the idea of the hose connecting the two bowl vents. I could test it and report back once I'm up and running.

 

[steve340]

The other thing you can fit are float bowl vent baffles. The Holley ones are the best. I do a dirt car engine and we use the old hose trick as well as internal vent baffles and it works pretty well for no money. You do need around 1/2 and inch between the hole in the tube and the air cleaner lid. If you do not "vent" the float bowls the carb cannot work correctly. The hole cut in the rubber tube only needs to be about 6 mm. If pushed for height I will do several small nicks on the top side.
You can make little tube "stumps" out of 5/16 copper tube and epoxy them in to the carb body.
If you want PM me and I can email some pics of what I do.