engine and drivetrain Hard braking kills engine - diagnosis?

[KFunk]

This has probably been discussed, but I can't figure out the right search terms to find it.

The problem is that when I push hard on the brake pedal, the engine stalls and dies. It'll do it when coming to a 'panic' stop, or just when I'm sitting still and push on the brake pedal hard.

Several months ago I had the same probleme and suspected the brake booster hose. I put in a new hose with check valve, tightened it down good, and its been fine ever since. The parts store didn't have the little elbow/grommet in stock, so I didn't get a new one of those.

So now its doing it again, pretty regularly. All hose clamps are tight, and no obvious leaks. I suspected that elbow might be loose, so I put some silicone around it to seal it, and no change.

Could it be something in the brake booster? I'm wondering though if it might have something to do with the carb tuning, as I'd been fiddling with it a lot before this problem happened again.

I'm just wondering, because Karl and Jim are going to a junkyard on Friday to harvest some parts, and I might have them grab the brake booster if I need it.

Oh, and its a '74 with 32/36 DGV, ATE super blue fluid, SS lines, stock ATE pads, and recently replaced M/C.

Thanks,

Kevin

[winstontj]

could also be your float. if it's off (too low) then upon braking it could dry up and starve the motor. For the brakes, let the engine idle. When you press the pedal in is there a surge at all? Even if you really sharply wail on the pedal? if yes then it's booster realted - if not then look elsewehre and I'd look at fuel.

Also check your plug leads & coil lead, as well as your idle solonoid's lead on the carb. Double check all the electrical is tight and not shorting/shaking loose under certain conditions and then look to fuel again. Bet you that your float is too low or something.

HTH,

TJW

[Napes]

Just don't use brakes.

[winstontj]

Just don't use brakes.

HA!

[TobyB]

It'll do it when coming to a 'panic' stop, or just when I'm sitting still and push on the brake pedal hard.

2 things-

unhook and plug the booster (plug the manifold, really!)

Then try it. If this fixes it you have a leak in the booster or valve.

If it still does it, make sure your idle screw on the carb is actually setting the idle-

it's possible (but really unlikely) that the pedal return position is acting as idle- and when you nail the brakes, it moves enough to close the throttle a bit more...

hth,

t

[Arizona02]

(nt)

[KFunk]

OK-

No real sign of idle surging when hitting the brakes.

Although before when I had the problems months ago, the idle speed increased after I leaned it out. After I changed the brake booster hose and check valves, it quit doing both (surging and/or stalling upon brake pressure).

So, I tried playing with the mixture: a couple of turns lean, and a couple of turns rich. Still the same stalling when I stab the brakes, no real change (although possibly less resistant to stalling when richer, but not certain).

I haven't yet tried plugging the booster hose, since I haven't found a suitable plug. But even then, I'm not sure how that would prove something. If you disconnect that hose, there should be no connection between the engine and the braking system. So, when I hit the brakes, of course it won't affect the idle speed. Is there another connection I'm missing?

The idle speed screw is resting against the tab on the throttle, so I assume its setting the idle speed. The screw should should prevent the throttle butterfly from spinning any farther back, right?

But, I should also note that my whole throttle linkage could be put together wrong somewhere. I've taken it apart several times to regrease things here and there, and have been known to put things back together slightly wrong. I'm pretty sure its back together right this time, though. I can take pictures if you want.

As for the float level, it might be the next thing to try. I've had the top off the carb several times in the past couple weeks due to problems with my secondary barrel, and perhaps I could've knocked something out of whack with the float one of those times. It's just a hassle to take it apart, and its cold outside, so I haven't tried that yet (doing easier tests first). I've got an unregulated Carter P4070 pump though, which pumps a really high volume, so maybe float level might not matter too much? Perhaps I'm misunderstanding the mechanism, but if the float dropped really low, the high volume of fuel would just pour on in and take care of stuff.

I'm new to carbs, so correct me if I'm way off.

I'm also kind of unfamiliar with brake boosters, so thats why I'm asking for help. I'm trying to learn here: http://auto.howstuffworks.com/power-brake2.htm

Now, I've noticed on other thing. When I push down on the brakes, I hear a hissing sound of air from somewhere around the brake booster. When I release my foot, the hissing slowly stops.

Is this normal, or is it that diaphragm inside the brake booster leaking, and pulling more vacuum than it needs? (thus stalling the engine)

[winstontj]

OK-

No real sign of idle surging when hitting the brakes.

Although before when I had the problems months ago, the idle speed increased after I leaned it out. After I changed the brake booster hose and check valves, it quit doing both (surging and/or stalling upon brake pressure).

So, I tried playing with the mixture: a couple of turns lean, and a couple of turns rich. Still the same stalling when I stab the brakes, no real change (although possibly less resistant to stalling when richer, but not certain).

I haven't yet tried plugging the booster hose, since I haven't found a suitable plug. But even then, I'm not sure how that would prove something. If you disconnect that hose, there should be no connection between the engine and the braking system. So, when I hit the brakes, of course it won't affect the idle speed. Is there another connection I'm missing?

The idle speed screw is resting against the tab on the throttle, so I assume its setting the idle speed. The screw should should prevent the throttle butterfly from spinning any farther back, right?

But, I should also note that my whole throttle linkage could be put together wrong somewhere. I've taken it apart several times to regrease things here and there, and have been known to put things back together slightly wrong. I'm pretty sure its back together right this time, though. I can take pictures if you want.

As for the float level, it might be the next thing to try. I've had the top off the carb several times in the past couple weeks due to problems with my secondary barrel, and perhaps I could've knocked something out of whack with the float one of those times. It's just a hassle to take it apart, and its cold outside, so I haven't tried that yet (doing easier tests first). I've got an unregulated Carter P4070 pump though, which pumps a really high volume, so maybe float level might not matter too much? Perhaps I'm misunderstanding the mechanism, but if the float dropped really low, the high volume of fuel would just pour on in and take care of stuff.

I'm new to carbs, so correct me if I'm way off.

I'm also kind of unfamiliar with brake boosters, so thats why I'm asking for help. I'm trying to learn here: http://auto.howstuffworks.com/power-brake2.htm

Now, I've noticed on other thing. When I push down on the brakes, I hear a hissing sound of air from somewhere around the brake booster. When I release my foot, the hissing slowly stops.

Is this normal, or is it that diaphragm inside the brake booster leaking, and pulling more vacuum than it needs? (thus stalling the engine)

Lots of stuff here. First of all is it cold compared to Hawaii or just cold - somehow I don't want to give you ANY sympathy!

Toby said plug the booster so you can isolate things. there is throttle & linkage, there is fuel and then there is vacuum. Isolate variables so you can compare things and get a good baseline - exactly how you would with a sterio system (head unit, amps, pre amp, equalizer, crossovers, etc.) Take what you know to be good and leave it alone. Adjust one thing at a time, then put it back to adjust the next.

OK - plug the booster with a cap to a water bottle. A few nights ago I used a cap from a poland spring bottle that I got at a gas station. Not the screw on kind, the small clear kind from the ones that pop up. I litterally put it over the elbow and taped the SNOT out of it with electrical tape. Leave a tail so you can undo it. By plugging off the entire vacuum system (except the distributor if applicable) you will isolate that system completely. Now CAREFULLY, because it's a bastard to stop the car w/o a booster, drive and wail on the brakes. Anything happen?

If yes then it's not your vacuum system or the booster because you just ruled that out. So now it's either your linkage, throttle or brake, or both if something is rubbing or binding, or your fuel. 32/36 dgv and you just mentioned that you have a carter fuel pump??? pumping at what PSI and flow rate? That's odd to me - and another variable to check. Are the electric fuel pump connections OK? do you have a swirl bowl? Fuel filter? Dirt in a filter that could be plugging? I assumed you had stock fuel delivery.

Isolate the vacuum and report back. If it's not the vacuum/booster than it's gotta be fuel related somehow. Either your linkage or your float/fuel delivery. The float opens and closes a valve, a needle valve, just like an air valve on a bicycle tube. As the float drops the valve opens, as it goes up, the valve closes. If this is off for some reason you could be flooding/starving your motor. If you have a bad float (hole in it so it fills up) then it could slosh around more/less and be an issue too.

Carbs always seemed really scary to me till I took one apart - like a sub/speaker which also seemed really unknown to me till I saw what was inside. Your 32/36 is SIMPLE and can be rebuilt in under 2 hours.

Rule out vacuum via isolating the booster/plugging the hose, then rule out fuel but really going through the fuel pump, inspecting connections, relays, swirl bowls, filters, etc.

If you rule out everything up to the carb then it's gotta be in there. Probably then it's your needle valve, float or could be as toby said, your throttle. Try adjusting your idle up to 1200rpms with the screw and drive it. does it happen that way?

hope all this helps...

[TobyB]

is that it can create a hellish air leak into the engine.

Try it- unhook the booster hose, and watch what happens.

Then stick your thumb over the inlet to the manifold.

really....

t

[winstontj]

Then stick your tounge on the inlet to the manifold.

really....

t

????

[SHEN]

in the hose from your booster to valve cover.

[KFunk]

It was 41 degrees outside! Thats too cold to tear apart a carb. I'll play outside tomorrow if its warm.

The problem occurs when I'm at a dead stop, sitting in the driveway, not touching anything, and I push the brake pedal hard.

If I took the brake booster hose off and plugged stuff up, and did the same thing, how would the pushing of my foot on the brake pedal affect anything with the motor? I'm not using the throttle linkage all, the wheels aren't spinning, the car isn't moving.

The problem is caused by the vacuum created by the booster, because there's no other connection between my foot on the brake and the engine speed.

I've driven cars without boosters connected before, its not that bad, but I don't see how that could prove anything in this case.

To me, it seems like the brake booster's diaphragm is leaking somewhere, and when I push down the brake pedal, it sucks the air on out of the booster, but due to the leak it forces it to suck a lot more to overcompensate, and stalls the engine in the process.

Oh, and if my brake booster hose were connected to the valve cover, that sure wouldn't work right! j/k, that was probably a typo.

That check valve is just a few months old, and was bought from an import parts store that always sells me the best quality stuff.

[KFunk]

Oh, and the fuel pump chugs along just fine. It's rotary pump with a free-flow rate of 72 gph. I can hear it running just fine throughout. But even if I cut off the fuel pump completely, the car can keep running off the fuel in the float bowl for like 5-10 seconds. When I stab the brakes, its a much quicker cut-off than can be explained by a fuel pump issue.

[winstontj]

Oh, and the fuel pump chugs along just fine. It's rotary pump with a free-flow rate of 72 gph. I can hear it running just fine throughout. But even if I cut off the fuel pump completely, the car can keep running off the fuel in the float bowl for like 5-10 seconds. When I stab the brakes, its a much quicker cut-off than can be explained by a fuel pump issue.

Time to start looking for a good booster then! I'm going to be doing a delete sometime soon (I hope) so I'll offer up mine when the job is done.

[KFunk]

Fixed!

'New' booster is in it, thanks to Karl picking it up from a junk 76 in Xenia.

It was a fight to get old one out and new one in, but I managed to do it without having to bleed the brakes again, and without bending stuff too much.

I haven't gotten the chance to tear apart the old one yet to see the real problem. I did notice its much easier to push it in and out by hand, whereas I wasn't able to do that very easily with the 'new' one.

While the booster was out, I had easy access to clean up my drivers side frame rail, so I blasted all the gunk off it, and slapped some POR-15 on it. There was some slight surface rust thanks to previously spilled brake fluid.

The brakes seem to work fine now, with no sign of engine speed changing when I push the brakes. So all is well I hope. Plus my frame rail is nice and clean and rust protected.