428 Stalling when braking or low rpm.

[mj_duell]

Hey Guys,
I am having a difficult time with my 428. The timing is set to 32 degrees total and the plugs are a perfect light brown color. It will idle (very lopy) and if you gas it you get tons of power. I have about 6 pounds fuel pressure (measured). I have been to 4secondsflat.com and tuned my Speed Demon 750 with the instruction provided. This resulted in great pedal response. However at slow rpms (under load) or during braking the engine stalls. FYI - I have a Comp Cam 292/.560 (244@.50)). I am not sure at this point what the issue is. I called Barry Grant before ordering the carb and I went with this one as they suggested. The only thing I have not played with to much is the floats. Any help would be greatly appreciated.




--Mike

[Ron61]

Mike,

It could be your float level as this seems to be a common problem at times. Also the fact the engine lugs,(stalls) at low RPM when you get on it would indicate to much gas. Your cam may require more RPm than you are running at low speed before you open it up. I had this problem on my 850 double pumber but just on braking. Finally kept playing with it and I have the clear site plugs so I can set the floats without taking the plugs out. I noticed gas was on top of the carb on both ends and it could have only came out the two little vent pipes that stick up. So I checked the level for the 100 th time and it showed just at the bottom of the site plugs so I lowered it another tad. That helped some but didn't clear the problem completely. I am thinking of putting a rubber tube over the end of the vents to extend them a little higher and see what that does. Your Carb.may have something completely different from what I have found so far.

Ron

[mj_duell]

Thanks Ron,
When I first put pressure to the Demon (just out of the box) I was spitting gas out of the primary vent. I adjusted the level doen until it was half way across the site-glass. My secondary has always been half way across. I'm thinking both secondary and primary float levels need to come up. My Idle is between 1000 and 1200 depending on adjustments. When I did the four corner adjustments, I found that it engine kept looking for more fuel. It was satisfied when the screws were about 3/4 all the way out. I think the answer might be more fuel (higher float). What are your thoughts? Oh, what Idle RPM are you running?



--Mike

[Excaliber]

Hmmmm,,,,, I don't know about higher floats, half way up the glass should be totally fine. It DOES sound like a carb problem though! 427, radical cam and 1200rpm (or higher) idle here to.

Very difficult to get a "decent" idle adjustment on my carbs (dual fours) because the vacuum is SO low and the cam SO radical it really screws up the idle circuits in the carbs. I run an idle rpm that just high enough to keep the motor running without stalling. BUT, if I stop to quick it WILL stall.

[mj_duell]

Just got off the phone with Barry Grant. I was told to try lowering the fuel in the site-glass (Float adjustment) and maybe raising the Idle RPM. Also They want me to double check my timing. I have a feeling the floats may be the culprits. I'll let you guys know. If anyone else has some input, please let me know. The vacuum is the biggest problem with the cam, but MAN WHEN YOU STEP ON IT, IT's WORTH IT!!!!!!



--Mike

[Ron61]

Mike,

I have a rather radical cam also and vacuum is a problem at times. But when I talked to a Holly Tech. he told me that the gas should just be right at the bottom of the site plugs and that if I take them out it should just barely splash out at idle. My engine idles at 1,000/1,100 RPM. Let me know what you find that cures your braking problem as I still have some but not as bad as before and as I said, I am thinking of putting a small hose on the vents to raise the tops some.

Ernie, did you get yours to stop killing the engine under braking, and if so was it your carb adjustments?

Ron

[mj_duell]

O.K. here is the latest. I talked with Don at 4secondsflat.com via email. It seems that I my need a 750 Mighty Demon or Larger to accommodate my cam. It seems that the 720 Speed Demon thinks I need more fuel during off throttle conditions so I'm flooding out and stalling. Secondly, the four corner screws should not have to be adjusted almost all the way out to get enough fuel for the idle settings. He said this is typical of a Carb / Cam mismatch. I had answered the questions the techs at an on-line race shop (Who shall remain nameless because they advertize here) had asked pertaining to the cam and other important variables before ordering the 750 Speed Demon. Guess they didn't know what they were talking about or there was some miscomunnication. Anyway, Don says they may be able to work out the problems with the Speed Demon with a few additions and adjustments, but a carb more in spec with my cam is what is needed. Anyone want to buy a Brand New 750 Speed Demon? I will keep you informed. More input is always appreciated.




--Mike

[mj_duell]

Hello again,
Here is were the problem stands. I have spent a couple of days talking to Don Gould at www.4secondsflat.com and I have decided to go with his suggestions. First off, a 750 Speed Demon is not the correct carb for a cam with the dimensetions .244@.50 / .560 , so I am getting a blueprinted 750 Mighty Demon. He is also going to recurve my MotorSport distributor to 22 degrees initial. Lastly, I am going to purchase a coil that covers both high and low range spark for Don. He has these units specially built and they are a very good price. This should help with low idle and high lope issues. The cost of these changes wasn't too bad and after going through the numbers a few times, I believe my problems should be over. If anyone needs any help with thier FE, they should get ahold of Don. www.4secondsflat.com . Hope everyone is having a great summer. I'm so close to my first real drive that I can hardly sleep at night.



Oh, P.S. new pics are in my gallery.

--Mike

[Chaplin]

MJ-
I also have a 428 with the same cam that you have (comp cams 292). The motor initally had a holley vac. sec. carb on it which I was not happy with. To make a long story short, after talking to Don @ 4secondsflat, I bought a MightyDemon 750 and it is much better. (Although, the carb did need to be jetted up significantly from the stock jetting- I think I have 85s in the primaries and 92s in the secondaries now- have to double check that though). Anyway, the car runs much better now- car will idle all day long at 850 if I let it . Incidently, my timing is set at 18 initial and 36 total all in by 2500 (it made the most power on the chassis dyno at 36 total). You made the right choice buying from Don, he was a big help and went out of his way to help me out.

Also, you will likely have to change the powervalve in the Demon. I think it came with a 6.5 pv, and if your motor is the same as mine, it only makes about 5.5 to 6" of vacuum at idle- so you will need to drop down to a 3.5 pv. But you should measure you vacuum at idle to see what it is and let Don know so that he can set up the carb with the correct powervalve.


Let me know how it works out.

[mj_duell]

Thanks Chaplin!,
I'm going to copy your reply and send it to Don. I'm glad I talked to Don as well. I have been struggling with trying to get the right set up for awhile. I will let you know how it goes.



--Mike

[decooney]

...start with the recommended jetting and power valves first. It does sound a bit like your floats were too high if its running over when you stop. IMO: Your Daemon should be fine if properly jetted/configured. I run a bigger cam, and a Holley competition 830 with no problems. If you are not sure, or tired of hassling with it, bring your car to a local/reputable mechanic/engine tuner and save yourself the extra money and frustration of throwing more parts at it thay may not fix the problem. A good local engine tuner or Dyno person can save you time and money. Good Luck. Sounds like carb configuration and tuning issues to me though.

[decooney]

Mike,
Also, check to make sure you don't have some sort of a vacuum leak. The stopping and rough idle issue just made me wonder a bit. Check manifold, carb base, fittings/lines into the carb, etc... Do you have vacuum assist power brakes by chance? Does this only happen when you apply the brakes? Also, still wondering if fuel is running over from your float boals being set too high.. Let us know.

[mj_duell]

Hey Duane,
Nope no vacuum assisted brakes or leaks. We have really done a lot of testing lately. The problem is most definetly the carb/cam set-up. I called Demon to double check the combination and they had said pretty much what I had heard from Don. With a lift of 560 and duration 244@.50 I set my self up for problems at low RPM's. It's not just the carb however, my timing and coil do need to be a slightly better caliber to handle the idle to WOT of a cam like this. I did think I could get away with a jet and/or power valve change, but the internals on the Mighty Demon are more suited to this set-up. I also have to compensate for a 1/2 spacer under the carb which we needed to make everything in the turkey pan fit. Yes, the floats were way to high at first, but we have run the whole level range without success. I had a Barry Grant tech on the phone one evening for an hour and a half. Can't wait for that phone bill LOL. Anyway, I will keep you up to date. Thank you and everyone else here at CC for all of the help.



--Mike

[downdraft]

U have LOW VACUUM IN MANIFOLD. YOUR BRAKES REQUIRE VACUUM AND TAKES FROM MANIFOLD, THUS NOT LEAVING ENOUGH VACUUM FOR THE CARBURATOR. ALL HIGH LIFT AND MAINELY BIG DURATION NEEDS MORE VACUUM. CONNECT A VACUUM CANISTER TO INTAKE AND BRAKE WITH GAUGE, U WILL HAVE NO STALLING AT IDLE. THE CARB. MAY ONLY NEED A 2 SIZE DOWN JET SIZE ON PRIMARY. BURNOUT. i HAD SAME TROUBLE MY CAR, RUNNING TO RICH AND STALLING WHEN STOPPED. MT CAM HAS A 588 LIFT, 315 DURATION. i PUT A VACUUM CANISTER ON, IT WORKS. IT`S ALL ABOUT VACUUM.

[downdraft]

ABOUT YOUR FLOAT BOWLS. THEY R SET TO HI. GAS SHOULD BE AT THE BOTTOM OF WINDOW, JUST STARTING TO ENTER A COUPLE OF THREADS. THATS RIGHT FROM THE HOLLEY BOOK. BURNOUT.

[mj_duell]

Hey downdraft,
I solved the problem a few weeks ago and "yes"part of it was defiantly the float bowl level being too high. Under hard braking the carb was flooding the engine. I don't have vacuum assisted brakes so that wasn't an issue. After replacing the carb to the correct Demon model (Mighty) and adjusting the float bowl level, the car now runs perfect. I actually runs so well it's almost like it's injected.



--Mike

[downdraft]

Hi, Glad to hear U fix the problem.