Engine stall on hard braking (demon carb)- HELP

[DAVID GAGNARD]

Originally Posted by Grubby

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

I don't see how the timing has anything to do with it...It has to be a fuel issue. Under hard braking the fuel will slosh to the front of the carb, front/primary bowl will be away from the jets, rear/secondary bowl will be to the jets.....
Between that and vacum or lack of vacum, is where I think the problem lies... As said earlier, my 25 year old Holley DP almost never does this.........has to be in the design of the carb......

My front float is about middle of the sight glass and rear float is about 3/4 of the sight glass, I've not noticed any difference with different float setting.......

Be careful when advancing your timing, mine is set at 10 degrees intial and 32 degrees total, as I found out on the engine dyno and chasis dyno, any more on my engine and I'll probably destroy it to engine knock.......adding from 4 to 6 degrees more of timing can and will destroy a motor in seconds,depending on how much timing your motor needs. I've seen the dyno results and "most" small block Fords including strokers usually do the best with no more than 30 to 32 degrees total timing..My 331 did the best at 32 degrees, we tried adding 2 degrees at a time and found for every 2 degrees added, the motor lost from 8 to 12 hp and torque!!!!!!!!!!!!

David

[Jac Mac]

Whenever I get the chance I fit an electric solenoid to use for a faster running idle speed up around 900/1000 rpm depending on the cam specs etc. In this way there is less chance of stalling the motor under braking/spin out conditions & when you kill the ignition the throttle goes back to a 'true' idle setting to reduce the chance of run on etc on shutdown. Might be all you need to 'fix' your problem. Try it, set your idle speed @ 900/1000 temporarily & do the brake test, if it fix's it you know what to do.

[Grubby]

I raised the front float level to the top of the sight glass. Now some fuel comes out under hard braking. The rear float is about 1/4.

I advanced base timing 2 degrees at a time and road tested. It is now at 18 deg with about 38 deg total (all in by 3500 rpm). I am using vac advance from ported vacuum. The car actually runs much better and I didn't hear or feel any detonation. It pulls noticeably harder and idles better. I put it in a high load by trying to accelerate in 3rd gear at 1200 rpm. Do you guys think there could be detonation that is not heard or felt? The side pipes are plenty loud. I am nervous about the timing. This is my first Ford.

I did not build this engine. I bought the car partially done with a running engine. It has Edelbrock Performer RPM intake and heads. The previous owner said it had the matched Edelbrock cam and 9.0 compression. It idles at 900 rpm with 12 inches of vac.

The added timing makes it less likely to stall on hard braking. It requires a longer hard stop to kill it. I attribute this to the better idle quality.

I went through the Demon installation guide and it looks like everything was and is set correctly. I read my spark plugs and they were all the expected tan color.

This is the last issue to work out with this car.

John

[ZOERA-SC7XX]

In my case all I needed to do was to richen the idle mixture just a hair (1/8 turn). My timing has nothing to do with it as I'm at 36 degrees total. I won't F with the float levels, Holley engineers know what they're doing. If you raise the float levels too much you'll flood the carb, lower it and it'll starve for fuel. Like someone above said, you don't need to re-invent the wheel. These carbs are quite close to perfect out of the box. Just a little tweak should do the trick.

[jhv48]

I did pretty much what you did. Bumped the timing to 18 degrees at idle. Your 12 mm/Hg at 900 RPM is a little lower than I would have expected for your engine. Mine is around 17 mm/Hg at 900 RPM.

That said, I lowered the float level to the bottom line and haven't had any problems at normal speeds.

However, If I do have to get on the brakes really hard, there is still a bit of a stall, but by then, I will have hit something and the stalled engine will be the least of my concerns.

[Grubby]

Jim,
Can you tell me the specifics on your engine? Cam, Intake, heads, compression? How much total timing do you have? I assume you lowered the rear float to the bottom line.

I played with the base plate adjustments some more tonight. Per Demon Tech I exposed a little more of the secondary transfer slots. Then, I went through the idle adjustment again and got 15 mm/Hg. I confirmed my base timing is 18 degrees and the total is at 38 degrees. No sign of detonation.

The stall is 50% better, but not gone. It sounds about like yours.

John

[Jerry Clayton]

does your carb have the idle eze adjustment in the center of the base plate?
On many carbs I find that there is a burr inside the transfer slot cutout, and the throttle plates aren't adjusted properly---they should be opened to the point that the exposed slot looks like a square hole, not covered and not opened excessively to where the slot looks long. Sometimes I will use a small drill and pin vise to make the exposed holes even by closeing the blades and broaching the hole to .025 by pulling the drill thru the hole.
If both the primaries and secondaries are done this way and you don't have an idle eze adjustment , you can drill a 3/32 hole in the center plug of the base plate---on Holley carbs, I first tap this hole for a 1/4 npt plug and drill it from 3/32 up to maybe as much as 1/8 on big motors.

[jhv48]

Originally Posted by Grubby

Jim,
Can you tell me the specifics on your engine? Cam, Intake, heads, compression? How much total timing do you have? I assume you lowered the rear float to the bottom line.

I played with the base plate adjustments some more tonight. Per Demon Tech I exposed a little more of the secondary transfer slots. Then, I went through the idle adjustment again and got 15 mm/Hg. I confirmed my base timing is 18 degrees and the total is at 38 degrees. No sign of detonation.

The stall is 50% better, but not gone. It sounds about like yours.

John

I've got my throttle blades adjusted so that there is no transfer slot showing on the secondary side and the proper amount on the primary side. I have 18 degrees initial timing and 38 degrees all in at 3200 RPM. Both float levels are at the lower line. I have a mild street cam (224 degrees duration with .514 lift.) 9.5 to one compression and 6 pounds of fuel pressure. A good street engine. I pull 17 pounds of vacuum at idle but it doesn't run right at that setting, so I richened up the idle by an eight of a turn to get rid of an idle circuit stumble on light accelleration.

Start there and then adjust the idle mixture screws to give you the most vacuum. Set the idle speed screw to 900 RPM and take it for a ride. Try to accellerate slowly away from a stop. If you feel a hesitation, (when transferring out of the idle circuit) adjust all four idle screws (I assume you have 4 screws) OUT about another 1/8 th of a turn. (richer). Now go do the accelleration run again. If the hesitation goes away, leave the four screws alone. Your idle is now just where it wants to be for best driveability. Several tuner articles have suggested lowering the float level for street use to the bottom line. It cured about 80% of my problem, but if I really stand on the brakes, it still acts like it wants to stall. But it doesn't actually stall.

Hope that helps somewhat.

PS, the idle eaze is primarily used when you have a radical cam that won't give you enough vacuum to set the idle properly. If you have a street engine, you should tighten the screw into the base and leave it there.

[Tech @ BG]

The timing issue was discussed by the tech not directly related to this issue but as another area that needed to be addressed in the overall set up of the carburetor. Contradictory to some of what has been posted we do find many of the carb issue caused by guys that do not have enough initial or base timing in the motor at idle .When advancing the base timing as mentioned you do need to recurve the distributor so you do not have too much total but raising the base timing and then recurving the total to the same as what you had will not cause engine damage and infact in many cases the car will start easier and be more responsive.

In regards to butterfly positions it is best to start with both the front and the back open the same on these cars [approx .020 of the transfer slot showing] and adjust the idle speed from there. If you find it will not idle with the transfer lsots set square then you will need to either advance the base timing [if low] or open the idle-eze up. basically the idle-eze is there to take pressure off the butterflies if you find the motor will not idle with the transfer slots at .020 or less.

[CHANMADD]

in all honesty these are not fuel injected computer controlled cars, and as such we should not expect them to be as smooth and troublefree as a modern car.
My carb ,I think is setup pretty close to ideal. I do however always change down gears and blip the throttle to clear the cylinders of the excess fuel that sloshed into the intake when I just hopped on the brakes, after accelerating so fast just a few seconds ago that I left huge black stripes on the road.......
You all know what I mean!!!!
john

[Grubby]

Demon Tech support did offer some good advice. The tech spent some time walking me through the base plate adjustments.

I just finished readjusting the carb. It now idles well with both primary and secondary transfer slots exposed about .015. I made sure the primary and secondary transfer slots were equally exposed. I am getting 14-15 inches of vac at about 900.

The Demon carb impresses me with its overall driveability. The only issue has been the stalling.

I am feeling pretty confident that things are now pretty close. Unfortunately it is raining here today and the rest of the week. I will post results when I get to road test it again.

John

[Grubby]

Results from todays road test.

1. Off idle is good. No sign of detonation. I put a vac hose on my stethascope and laid it on the intake for the test drive. WOT is the most powerful I have felt with this engine. Everything works really good. The engine pulls hard to 7000 rpm.

2. During full braking the engine will die. Anything short of a panic stop and it will not stall. I think it is pretty close.

Jim's settings worked pretty well for me.

I now have 16 degrees initial timing with 38 degrees total. My vac advance is hooked to full manifold vac below the throttle plates.

Both floats are set at about 1/4 above bottom of the sight glass. Both transfer slots are exposed about .015". I get 15 inches of vac at 900 rpm idle.

I am some what concerned about the amount of total timing advance. I will call Demon and Edlebrock tech lines on Monday and see if they know how much total advance worked well in their test mules.

John

[jhv48]

Grubby, if it runs good and pulls hard and you don't hear any kaboom noises, then your timing is ok.

I run about 18 degrees on my motor even though the builder set it initially at 15 degrees. Some motors like more timing, others don't.

Check your plugs just in case.

[vector1]

interesting thread. i just put a demon rr carb on a crate motor cobra and i have the butterflies adjusted all the way down and the idle is still too high. i am going to have to retard the timing i think. the front bowl is set just a smidge lower than the rear, although the carb is tilted down at the front cause of the rpm air gap manifold. the fuel pressure gauge says around 8 lbs off a mechanical pump, although i don't know if it is correct, prob just a relative thing. i haven't done any hard braking but did pull some pretty good g's on a round-a-bout intersection and didn't notice any idle drop which i did with a pro systems carb and a marine carb which would both drop idle on hard braking.