seventysixvette

Guys, I posted this in the performance/tech area but didn't get much figured out. I just thought if I cross posted here a couple more people might have a few more suggestions......





My '76 dies when under hard braking. If you roll up normaly to a stop and brake, it's fine. But if you get caught by a short yellow, or really press on the brake pedal to pull it down, it'll die just as it stops.

This started about two weeks ago. I had a problem with the short piece of rubber fuel line between the fuel pump and and metal line collapsing and cutting the fuel off. I towed the car home and fixed the line.

It might be sheer coincedence, but the problem started about then.

I'm running a Holley 650 4150 4777-7 (dual line, double pumper) with a Edelbrock intake. I've got Hedman headers going into 3" side exhaust. Nothing fancy or exotic, it should be trouble free, and has been perfect the last 2000 miles or so.

I've pulled the carb, checked for any problems like a damaged power valve or gasket. I replaced the base gasket. I carefully checked the floats for any saturation problems and re-adjusted them live with the side level screws. The needle seat assemblies were replaced and seem fine. I'd rejetted eariler when I first got the carb. All seems to be fine internally.

I've also checked the distributor cap for carbon tracks or cracks, looked at the rotor, and made sure all the wires are seated well. Checked the timing and pulled the plugs to check for gap and carbon problems. All looks good.

I thought I might have a problem with the transmission or torque converter, so I ran the car up to 60mph, and kicked it into neutral. I let the idle return to normal and then braked hard with the same same problem. The engine pulls down and dies. Normal braking in neutral and everythings fine. It's a TH350, no lock up converter to worry about.

Years ago I fought a problem just like this with a friends '65 Mustang fastback. It turned out to be his brake vacuum booster was killing the engine. It also died under hard braking, BUT it died even if you were sitting still with the car in neutral. If I stomp the brake pedal in park nothing happens, it idles fine. The car has to be moving forward with the resulting nose-dive for the problem to occour.

I've pulled the vacuum lines off the intake manifold and carb base and plugged the sources up. With no vacuum running to any device on the car it will still die.

I got a long length of vacuum line and put my 6" test vacuum gauge on it. I watched when braking on a back road and didn't see any wild fluctuations in vacuum.

The amout of fuel in the tank doesn"t have an effect. I'd given thought to maybe the intake plugging up in the tank, but the car restarts instantly right after it dies, and runs fine.

This is vexing. I'm simply out of ideas. I hate just being a parts changer, but I'm about ready to throw another carb on and just try it.

I'm more than willing to listen to any ideas or suggestions on my next step. Surely someone else has had this problem before?

Thanks!
John

Duke94

I posted this in your other post.

When it dies, does it start right back up or do you have to crank it? If it start right up, it can't be fuel related. Start looking for electrical issues, loose connection, bad ground.

Does the engine have EGR, A.I.R. or vacuum actuated heat riser valve?

Gary

seventysixvette

It starts quickly. It starts just like I did a normal stop turned the key and killed it, and then just re-started.

If it dies or if I just turn it off with the key and I'm just firing it up it starts the same.

A loose connection, maybe a wire moving around under hard braking is as good an idea as I've heard so far. This might take a while to track down.

John

seventysixvette

No, none of the above

John

Duke94

A long shot, but check the trannie mount to make sure it isn't broken causing the engine to tilt forward. Check HEI connections. Check around the brake pedal under the dash especially the harmonic connector on the steering column which goes to the ignition switch.

Gary

Z-man

Just a couple of thoughts...it seems momentum plays a big part, so not a typical electric problem. Fuel is affected by deceleration however, so I'd check:

Fuel level in bowls? Are you getting any leaks around there? Might need to be raised a bit.

Do you have an idle stop solenoid? (I can't remember for that carb). Perhaps it needs some adjustment.

gerry72

If you don't have the anti-slosh pieces in the carb, the fuel in the rear bowl will flood the carb out of the vent. This is a very common issue in Holley carbs under hard braking. It's the first place you look. It will be either a white plastic "whistle" or a stamped brass diverter at the top of the metering block.

78anniversary

I suspect the vacuum brake booster is bad, causing a big vacuum leak under hard stopping

Duke94

I believe it came out earlier that it did the same thing with the booster disconnected. Can the OP confirm this?

Gary

JimT

Sounds like a vacuum issue to me. Had a similar problem - engine stalling while braking. Had to rebuild the carb - a Holley.

RagTop69

I sure hope you find the cause of this problem because I have the identical problem with a 69 Mustang convertible. Mash the brakes and it dies as you stop. Restarts normally and immediately after that. Mine is a SB Ford with a Road Demon 625 cfm carb.

Lon Wayne

When coming to A stop (hard stop) put the car in nutral rev the engine up like you were warming it up. If it stops and does not die your float bowl needs the drop down adjusted.This will take a couple of times to get the stop correct do it, not worring about what is in front of you .(Mall parking lot).You dont say what its ideling at should be 550ishx10.

seventysixvette

Guys, thanks for all the suggestions.

gerry72, I'm going to pull the carb again tonight and check for sure if those anti-slosh diverters are ther or not. I've never had this problem before with a Holley, but those diverters were probably there with the correct float adjustments on all the past ones.

78anniversary and Gary. Yeah, it's doing the same thing whether I have all vacuum sources capped off or live.

Jimt. I went through the carb, replaced the gaskets and power valve and re-jetted when I first got it. It's been running fine the whole time I've been shaking it down tweaking and adjusting.

Ragtop, just stay tuned. I haven't had a problem yet this forum hasn't helped me figure out.


Ok, here's something I hadn't tried. I had to go right out about an hour ago and try this.

I CAN keep the car running by blipping the throttle as I'm mashing down on the brake. I have to blip pretty good to get through those 4 or 5 seconds it's faltering. It almost seems I have to keep it up in the low speed circuit and out of the idle circuit just those few seconds. A couple hundred RPMs won't do it.

I always had my idle 550-600 rpm before. I've set it much higher right now trying to trouble shoot. I'll lower it back down and readjust the idle mixture when I get this ironed out.

I'm going to pull the carb again and quadruple check everything against a blow-up parts schematic and make sure all's kosher internally with the carb.

Thanks again fellows.

John

gerry72

http://www.holley.com/data/TechServi...ded%20View.pdf

You're looking for item 117, the brass baffle (if you don't have the white plastic whistle).

seventysixvette

I tore the carb down again last night. I do have the plastic whistle type. They're tan/taupe colored. The only other thing I found out of sorts at all was the secondary butterflies hanging open just a tiny bit. I loosened the two screws in each one and reseated them shut and tightened everything up.

The one thing I didn't want to happen has......the car is idling just fine right now. I threw the carb back on and went for a good drive, stopping constantly on a nice stretch of wide highway.

Nothing.....no problems at all. The ambient temperature was cooler yesterday. The engine temp doesn't seem to matter, maybe the outside temp does somehow. I dunno, the only thing worse than ironing out a problem is ironing out a intermittent problem. I'll just have to wait untill it starts acting up again. (at the worst time possible, I'm sure)

Thanks for all the help and suggestions.

John

Brcmpbl

John...this is an old thread I know, but I have this exact problem with my 1970. It has progressed now to the point though where now my car won't start...though I admit I think this is from my friend and I messing with the carb settings the other night. I might have to rebuild my carb now, and it's the original Q-Jet that came with the car.

Did you ever get to a solution with this? Thanks.

Brian

vettebuyer6369

Let's not resurrect a 3 year old thread.