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Stalls during brakeing
Well I'm finally on the road and thrilled not to be dealing with failed distributor gears, leaking heads, and oil pan problems, at least for now:MECOOL: The car runs cool with good oil press. starts easy but stalls under heavy brakeing or turning while in neutral or with the clutch in. It drives great otherwise. It is a 351W. mech. fuel pump, 750HollyDP, 70 and 83 jets, 4.5power v alve, steady 850rpm idle with 9in. vac. and a victor junior intake. If anyone can point me in the right direction I'd be greatful as usual. Thanks:)
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I had the exact same problem with my Holley carb. I finally figured out that the float level was set too low. I adjusted them to where they should have been and the problem disappeared. You may want to check yours.
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This won't help at all but I'm terribly proud to announce that the fix with the Holley Projection is to take the Idle Air Decel Position to 10.
(Slowly, painfully learning the in's and out's of the Holley ProJection system!) |
Holley has 2 fixes for this problem. I have used both and my problem has gone away. The first is to make sure the primary float is exact. Then set secondary float slightly under the bottom of the site glass or plug.
If needed add a longer vent whistle to to the secondary float bowl. Mike |
Thanks guys, I set the floats to leak out at a fast idle but maybe I was not exact enough. I didn't really like all that gas coming around the adjustment nut during the procedure.%/
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brake booster
I dont know if you have a brake booster fitted , but if you have then it could be the "oneway valve" that pushes into your booster and connects to your intake manifold.
If you have a booster and this valve is faulty then when you apply the brakes the booster will drop vacuum and draw vacuum from your intake manifold, if this happens then there will be no vacuum at the carbie to draw the fuel out. |
I was actually going to ask a very similar question. I have a 302 w/650 Edelbrock carb. I noticed it after my first autox, and now on the street. If I get on the gas hard, then let off quickly and put the car in neutral (and/or hit the brakes) it will stall. Fires right back up. It only does this if I'm on the gas hard, then suddenly in neutral. Same problem?
Steve |
I have a different opinion as to why my car, a custom stroked Windsor, with a Holley 750, properly set up, will try to stall as i brake at low rpm. I am accustomed to a touch of "heel and toe" to slightly blip the throttle, until i can apply more throttle, or until the car comes to a compete stop.
Here is my opinion. A "built" engine does not have much vacuum at idle. When coming to a stop at low rpm, the gas in the primary bowl wants to flow to the front of the bowl, away from the primary jets. Then, with the very low vacuum at near idle, there just isn't enough engine vacuum to signal via the "boosters" that some gas is still needed to run the car. Which has to pull some gas back rearward into the primary jets. Assuming your carb is set up properly, as the posts above suggest, the only real cure is to increase your idle rpm a bit. I personally like the low intimidting idle of about 700 rpm, so i leave it set there. So this is why in a highly cammed engine, with lots of valve overlap, this tendency to die at low rpm stops occurs. At least it makes sense to me. |
The best way to set the float level (I've had this same problem myself) is to have the car idle and reduce the float level so that the gas is at the bottom of the sight level but NOT coming out. You should only see gas spill out if you "rock" the car side to side a bit. That is where you want it.
Cheers. |
Thanks again, I do not have a brake booster. If Hal's theory is correct maybe jet extentions would help. I have seen instructions for setting the level with the motor off as well as running. Any thoughts? I really hate gas pouring all over with the motor running.
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float bowl vents
I bet it's because the float bowl vents dont have the good "whistle" style baffles in them. Under braking, the gas runs up and out of the bowl vent, flooding the motor. If the motor is rich upon refire this is it. Take the bowls off, get two of the white plastic Holley bowl vent baffles, and you're most of the way there. Set the float height to the bottom of the sight plug, so the gas dribbles out if you rock the car by the fender. I doubt that it's a going lean condition, or youd probubly hear a "pop"as the motor stalls.
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Holey Holley
I think the ignition is drawing power from the brake lights. Wire the brake lights to the hot side of the coil. I"M KIDDING!!!! The low vacuum theory is sound with a wild cam. I have drilled very small holes in the butterfly's so they have to close to reach an idle. This is the only way to get the idle transfer circuit to work with a low vacuum cam. That's the little slot in the venturi beside the butterfly. Check the position of the butterflys with the car off and the choke open (hot) and the throttle back to the idle stop. Good luck
Cheers Nick |
Does the motor smell rich when you fire it back up? Does it "pop" when stalling? Any other symptoms you've noticed.
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After all is said and done, open your idle screws half a turn......
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What happens if while hard braking, right as it wants to stall, you rap the throttle. Does it choke on too much gas, or pop from almost dying lean. Holleys usually don't lean out bad, they overflow the vents into the intake and choke themselves out rich. Funny incedent while test driving a 2-4bbl 460....tested the brakes hard coming down from 130mph, stalled the motor, as I coast up to the stopsign, I put it into second gear and drop the clutch to bump start it. Motor does not fire, but flames start shooting out the sidepipes. The Honda I pulled up next to can't believe the 1' flames out the sidepipes by the look on his face. Crank it, but flooded too badly. Hold the gas pedal to the floor and crank it again, when it fired off, four foot flames shot out the sidepipes like a nitroburning dragster. Smiled to the guy in the honda and continued the test drive.
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Mr Fixit,
I have the same problem with my 600cfm double pumpers on a 428. When I make a hard left turn, the engine dies or sputters real bad and when I restart it, its very rich -- black smoke bellowing out the pipes. Holley said the floats were set too high and to turn the float adjustment screw in a half a turn. I took their advice and it seemed to help, but I think I may have to give it another half a turn as it tried to do it to me again when I had it on the Talladega Grand Prix Track but it wasn't as severe. I guess you have to keep playing with the float levels until it runs right. I think adjusting it so you have to rock the car to get it to flow out the sight hole is just a good starting place and then you have to play with the adjustments from there. Does this sound like the proper approach to you? |
I bet you have side hung float bowls on those carbs, huh. Do they have the "whistle shaped baffles" in the bowls for the vent tubes? These usually fix that problem. Otherwise, you can get floats that have goofy shapes to compensate for cornering, launching at the drags, etc. Your float setting approach is correct except the screw only serves as a lockscrew, the 5/8" hex below it adjusts the float..
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Mr Fixit,
Yes, I have side hung floats and as I recall when I pulled the float bowl apart, the floats were square shaped with rounded corners - I guess you could say they look sort of like a fat whislte when looking at them from the side. These are Holley 4150 600cfm carbs and I'm running 66 jets. The plugs run a nice tan color. I really meant I adjust the hex screw/nut (whatever they call this hybrid) - just trying to say it simply. I'm not familiar with the term "vent tubes" or "baffles" and tried looking the terms up on the Holley Web site. They have something called "idle discharge passage, idle transfer passage and idle discharge hole." Is this what you are refering to as vents? I couldn't find anything I'd call a whistle shaped baffle. Are baffles some aftermarket thing you stick in the bottom of the float bowl, but aren't normally included as part of the carb when you buy the carb - sort of like the baffles in racing oil pans? The Holley web page showed a baffle as part of the needle valve assembly but it looked like a square box and not like a whistle. I drive it both on public roads and 3-4 times a year on a Grand Prix style track. So I don't want to make mods that are not streetable. Thanks |
The tubes which stick up into the aircleaner about a 1/2" and are usually cut off at a 45 degree angle, those are the bowl vents. You need to get the white plastic baffles for those passages. The white plastic baffle is the "whistle" I was referring to. It's about 1 1/2" long, 1/4" thick plastic that's hollow in the inside. It is inside the fuel bowl not within the vent tube itself. Actually mounts to the metering block, and sticks out horizontal, above the float. It is a factory part and I din't understand why Holley doesn't put them in every carb. Will be entirely unnoticable, except the lack of fuel slosh in corners and braking.
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Thanks. I appreciate the advice. I'll see if I can find some baffles this weekend.
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