Engine stall on hard braking (demon carb)- HELP

Grubby · #1 (**permalink**) 05-10-2009, 12:23 PM

My Cobra has a 351W with Edelbrock heads, Performer cam and intake with Demon carb. It runs really good and was dyno tuned by the previous owner. I think it is a speed demon with vac secondaries.

During hard braking the rpm drops and stalls. I suspected fuel sloshing from the rear bowl and flooding the engine, so I connected both bowl vents with a piece of fuel line. No change.

I have set the float level from bottom of the sight glass to 3/4 with no apparent change.

The car runs fantastic in all rpm ranges. The only issue is stalling.

Any suggestions?

John

PurpleVenom · #2 (**permalink**) 05-10-2009, 02:14 PM

I'll be following this with some interest as I have the same problem with the same type of carb on a 460

round_midnight · #3 (**permalink**) 05-10-2009, 02:15 PM

Do you have a vacuum brake booster on the car? My BDR had the same problem. It was the brake booster sucking all the air out of the manifold when braking hard with a closed throttle. Solution was to add a vacuum reservoir.

Spoooky · #4 (**permalink**) 05-10-2009, 02:18 PM

Guarantee it's the lack of vacuum. I had the exact same problem and the vacuum booster did the trick.

Grubby · #5 (**permalink**) 05-10-2009, 02:31 PM

The ONLY items hooked up to vacuum are the PVC, Heater (does not matter if on or off) and Vac advance. No power brakes on this car.

I am a pretty seasoned car guy and totally stumpped.

My gut feel is that it runs out of fuel instead of gets flooded out. It fires right back up with no black smoke and no need to touch the gas.

John

djslicer · #6 (**permalink**) 05-10-2009, 03:13 PM

I have the same problem with a Demon 825 on an Edelbrock 71054 intake manifold. 427 FE with alum block with Edelbrock heads and Holley fuel pump.

Lowered the secondary float level which seemed to help alittle, but still will die under hard braking.

?????????????

Jerry Clayton · #7 (**permalink**) 05-10-2009, 04:25 PM

push the clutch in a little sooner===you are probably coming close to locking the rear brakes, pulling the engine rpm below idle speed

DAVID GAGNARD · #8 (**permalink**) 05-10-2009, 04:28 PM

Quote:

Originally Posted by Grubby

My Cobra has a 351W with Edelbrock heads, Performer cam and intake with Demon carb. It runs really good and was dyno tuned by the previous owner. I think it is a speed demon with vac secondaries.

During hard braking the rpm drops and stalls. I suspected fuel sloshing from the rear bowl and flooding the engine, so I connected both bowl vents with a piece of fuel line. No change.

I have set the float level from bottom of the sight glass to 3/4 with no apparent change.

The car runs fantastic in all rpm ranges. The only issue is stalling.

Any suggestions?

John

No suggestions to your problem, but my road race car with a 650 Demon has done the exact same thing from day one, I've tried re-setting the floats and a few other things and nothing really helps..... the only thing I do is downshift and brake while the clutch is out so the engine keeps running.....

BTW: my street car,65 fastback with a 600 DP Holley DOES NOT do this!!!!!!!!!!!!!!!!!!!!!!

I have no clue or answer...........

David

Grubby · #9 (**permalink**) 05-10-2009, 05:03 PM

It is interesting that several others have the same problem.

David,
I have been doing what you do. I shift into lower gears and keep the rpms up to keep from stalling it. That really gets to me. I will get the problem fixed. If I have to buy a Holley, I will.

Note; I am not pulling the engine down by not pushing the clutch in. I was doing a lot of testing today with various float settings. During much of the testing I was getting the car to 45 mph and then putting in neutral before the hard stop.

I plan to call Demon Tech help line on Monday. If they give me anything worth mentioning, I will post here.

John

DAVID GAGNARD · #10 (**permalink**) 05-10-2009, 07:14 PM

John;

I like the Demon carb, I think it is a better carb in some ways than the Holley and easier to work on and tune, that's why I have one on the race car and a Holley on the street car.....

The Demon carb was/is supposed to have some of the things like the extended vents,better floats/better needle/seats and such to prevent the same thing we are expirencings, I made those changes myself to the Holley some time back....

I've tried re-setting the float levels and this seems to make no difference, I've tried a few other little things and have gotten the same results....If you find the problem or figure it out, you'll be both rich and famous!!!!!!!!!!!!!!!!!!!!!!!!

Keep me posted...........

David

ZOERA-SC7XX · #11 (**permalink**) 05-10-2009, 07:29 PM

I had the same issue...not stalling, but dropping almost to a stall while braking for a corner with the clutch in. I saw this question addressed a couple of weeks ago and someone suggested (I have a 750 Holley vac secondary, 428 FE) turning the two idle mixture adjustment screws our 1/8 turn each. I tried it and it worked perfectly.

PurpleVenom · #12 (**permalink**) 05-10-2009, 07:39 PM

I have two vacuum taps - one for the vacuum advance and one for the newly installed PCV (problem was happening prior to the PCV being added). Under hard braking, the fuel can slosh or move only in the tank and the fuel bowls. That being said, could the fuel pump pickup not be getting fuel? However, the carb should burn the fuel in the bowls. If the fuel in the bowls "sloshes" away from the metering blocks, could that starve fuel creating a lean condition? My thought is the fuel would move away from the primary metering block, and in to the secondary metering block. So maybe a higher float level in the primary bowl would be better? I'm sure you have it, but here is the link to the manual http://www.barrygrant.com/fromBarryG...n%20Manual.pdf

ray humbel · #13 (**permalink**) 05-10-2009, 07:41 PM

had same problem with my 428 with 750 deamon, stall at hard breaking also at slow stop down steep hill. it didn't die, but tried. i had to push the clutch in and hit gas peadle a couple of times. deamon tech said to raise float level. i raised it to top of glass and it helped. ask deamon if longer pickup tubes will help. i think it's starving.

CHANMADD · #14 (**permalink**) 05-10-2009, 08:02 PM

How about the fuel,in the carb fuel bowl, rushing to the front of the car and leaving the jets uncovered by gas.....Lean!!..instantly!
The jet extensions are designed to overcome that. They extend the jet intake towards the front of the fuel bowl keeping them covered. I always blip the throttle and downshift when slowwing down...well 90% of the time.Makes everything smooth.
I think, One has to always remember that when driving a Cobra most things happen a little quicker than in most other cars.

John

Rick Parker · #15 (**permalink**) 05-10-2009, 08:13 PM

Have you tried tried putting jet extensions on the jets in the primary bowl. Under braking the fuel is obviously forced to the front of the bowl away from the jets. They may temporarily be sucking air or heavily emulsified mixture (with air) until the fuel re-levels itself within the bowl.

FWB · #16 (**permalink**) 05-11-2009, 05:10 AM

its not vacuum, its not the floats........i have 18 inches of vacuum supplied from a electric vacuum pump separated from the motor for the booster, to stop the stalling when you are letting off the throttle, install a dashpot.
(spring loaded plunger for throttle lever to hit at idle position)
the wheel doesn't need to be reinvented. the round wheels work fine.

Jerry Clayton · #17 (**permalink**) 05-11-2009, 06:34 AM

I have several customers running both Demon and Holley carbs-----there are no big differences

Many manifolds are cut at an angle and you carbs are tipped toward the front--if so , raise the float level to the top of the glass
Do not, I repeat, do not put jet extensions on your primary jets---you will lean your car out and can demolish the engine in a short run--

I find more carb problems in the base plate/transfer slot area than any where else--many times plates are cranked open to set idle and transfer circuit is out of the picture.

But, on these cars-----you can't just go for a RIDE, someone must DRIVE----there are three pedals and you have to use them all

Grubby · #18 (**permalink**) 05-11-2009, 08:01 AM

I just got off the phone with a Demon Tech Rep. We talked about float level. I have tried several settings ranging from bottom of the glass to top on both the front and back. It is currently set with 2/3 on the front bowl and 1/4 on the rear.

The suggestion is to advance my base timing from 12 deg to 16-18 deg. I will try this tonight and post the results. I am concerned about detonation and will watch for it.

I liked the suggestion about backing the idle mixture screws out 1/8 turn. If the timing does not fix my problem; that is next.

This thread is interesting.

John

ray humbel · #19 (**permalink**) 05-11-2009, 10:22 AM

it can't hurt to just raise the levels to the top of the glass, it is easy to do and you can lower them if it doesn't help

Jerry Clayton · #20 (**permalink**) 05-11-2009, 01:13 PM

Demon's answering system should say advance the timing as it seems they tell that to everybody who calls in about anything.
IF YOU DO ADVANCE the base or initial timing be very aware that you will need to limit the total by recurving your distributor!!!!!!
Failure to do this can cause several very serious consequences!!!!

Most Holley and Demon problems are in the base plate area, not floats, not timing, but simple setup of idle speed and transfer slot parameters along with accerator pumps and pump cams