Bogging/stalling when braking

[thelawdoc]

Guys, well we checked vacuum today and it was good -16 inches. Brake booster is good. Took off the carb and opened her up and all was good in there too. But she still bogs and stalls when braking. So I don't know. She starts up right away even cold, runs nice, but give brakes and she bogs and stalls. Every time. I raised idle rpms to 900 and it still bogs/stalls. I am out of ideas at this point. I am considering throwing a carburetor at her.

[TJ's Chevy]

If its a carb issue such as a vacuum leak it would cause problems braking or not...When a carb develops a leak it'll show mostly in acceleration...I had a leak in my old rochester on my 292 and when I'd punch it the engine would die...simply because of the leak...If you have a way of putting the truck under load that would tell you if ya had carb issues...have you had any hesitation upon hard acceleration? Kinda like the carb sucks in air before then engine revs? Yer timing may be to retarded as well...8-10 at idle without vacuum is good. So if you keep a little bit of pressure on the gas pedal when stopping the engine is fine?

[joedoh]

did you take the booster line off and plug it and repeat the test? or just check for a leak?

also, there should be a check valve on your power brake booster, when it goes bad it can cause the engine to bog when you step on the brakes. it will not exhibit any other symptoms because it is literally a simple one way valve.

[thelawdoc]

Joedoh, I checked the vacuum for the booster by pulling its hose from the manifold. I got about 16". I then pulled off the manifold hose at the tee for the dizzy and tranny. I got about 16" again. I figured that if I have 16" of vacuum at those two points, then I likely do not have any vacuum leaks to search for. I am also inclined to suspect the booster, since the truck bogs and stalls only when applying brakes. The harder I apply the brakes, the quicker the stall. The easier I apply the brakes, the softer the bog/stall. But my buddy, a retired old school mechanic, says the booster is good. I don't know, can it be that I have good vacuum and then, when I apply the brakes, I lose vacuum? Should I check vacuums again while applying and holding brakes? I mean, if the booster gives 16" of vacuum without applying brakes, can it happen that only when applying brakes I lose vacuum and that causes the bogging and stalling? As I said, the truck starts up fast and easy, and runs very nicely -as long as I do not brake. And the brakes seem pretty good to me for a truck that is all drums.

I was also suspecting a leak at the carb mixture screw, because when idling, if I take hold of the head of the screw and try to move it up and down, I get a whistling sound. But after I checked out the carb, that screw seems to be threading fine and no whistling sound. When I set the idle higher, around 900, she bogs without necessarily stalling, although if I apply brakes hard, she stalls even at that idle speed.

At this moment, I am not sure exactly where I am at as far as advancing/retarding timing. We have been doing that by ear but finding no remedy by either advancing or retarding. I think at this moment, she is retarded a few degrees.

What has me most frustrated is that I never had this issue before doing the tune-up. So it seems that I must have done something that is making her unhappy. Something in the timing or in the carb.

[blown240]

It's possible that the vacuum booster only has a leak when you press on it. There is a rubber membrane in it that that vacuum pulls again when you press your brake pedal. To really test it you need to unhook the vacuum line from it and drive your truck. You just won't have power brakes for the test.

[thelawdoc]

Blown, thank you. I have a new booster that I intended on swapping in. I think I will swap the check valve. I will do the drive test as you suggest. BTW, I can bog/stall the truck without even driving much. That is, I put her in drive, go a couple inches, brake, and she bogs/stalls.

[joedoh]

Originally Posted by thelawdoc

Blown, thank you. I have a new booster that I intended on swapping in. I think I will swap the check valve. I will do the drive test as you suggest. BTW, I can bog/stall the truck without even driving much. That is, I put her in drive, go a couple inches, brake, and she bogs/stalls.

Occam's razor suggests the simplest solution is the most likely. Put another way, when you hear hoofbeats behind you and you live in the USA, do you think of zebras? or horses? You may be surprised to find out the hoofbeats belong to zebras, but the likelihood is very small.


since the only interconnection that those two systems, the brakes and the carburetor, have is the brake booster, it makes sense to investigate that before looking for "zebras", the more complicated and therefore according to Occam statistically less likely. Anyone can see the effect a vacuum booster has on running condition in any car or truck, modern or old, by rapidly pumping the brakes at a stop and in park, the engine will bog slightly and computer controlled cars will rev slightly after bogging.

repeating the test with the brake booster disconnected will be the real test, especially since you say it does not take much braking effort to notice the bog. It may not be the problem, but it makes sense to try this free test before throwing parts at it or, worst case, fixing it with a road flare in the seat. (thats a joke)

[thelawdoc]

Joedoh, your words make sense to me. I am hoping that it be the brake booster. Not just because I have a new one on hand, but because I do not believe I will find the gremlin if it not be the booster and I will have to just give the truck to a shop -if I can even find a shop who can troubleshoot such a simple engine. Thanks, Joedoh. I will come back with results.

[thelawdoc]

Gentlemen, well the booster is good. The check valve is good. I am just plain out of ideas. I have never been stumped like this before. I have a simple engine, simple carb, simple dizzy/p&c, and I cannot get rid of the stall on braking. Plus, I have to keep her idle rpms at around 800 to run her smooth. I am feeling frustrated at this point, and stupid that we cannot locate the bug in my babe. I am wondering whether I should gamble and swap out the carb.

[Jim53066]

TJ, I have the booster. No power steering. One change I have done since the truck ran well was rearrange the vacuum lines. I got the truck with the PCV line sharing vacuum with the brake booster. I rearranged that so that the PCV line goes direct to manifold, the brake goes direct to manifold, and the dizzy shares vacuum with the tranny.

Did you ever put the vacuum lines back where they were when it was working?
Parts like carbs, and boosters don't usually just break, they wear out and usually slowly over time.

The way you re-routed them, is it possible that you are now starving the vacuum advance when you apply the brake?

Go back to the way it was when operating properly, then change one thing at a time if you feel the need to tinker.

Jim

[thelawdoc]

Jim, thank you. Right now, the vacuum advance shares with the tranny. The pcv is direct to manifold, as is the brake booster. I had intended on going back and having the pcv share with the booster, but guys from this forum and my buddy mechanic all said that my original configuration could not be good. But now, as I am so desperate, I am going to go put them back to the 'wrong' way.

[geezer#99]

I might have missed it, but did you test with the booster disconnecrted and the vac line plugged at the source?

[thelawdoc]

Yes. I tested vacuum at the dizzy/tranny tee and got 16". I then tested vacuum on booster's hose and got 16". I also disconnected booster and the truck still stalls when braking. I gave the truck to a mechanic to test drive, he says carb is causing the problem I have. Even though I just rebuilt the carb, there really is no other option at this time than to swap out the carb and see what the outcome is.

[blown240]

When you rebuilt the carb, did you accidentally forget any parts of it? The first time I rebuilt a carb, I forgot to put the metering rods in.

[thelawdoc]

Blown, I had that carb disassembled on my dining room table for days as I cleaned it and rebuilt it. Every little thing I did was double checked and triple checked, including the large check ball vs the small check balls. I mean, I was really anal about it. But, obviously, either I did something wrong, or the carb just plain wore out on me. I don't know. But if a different carb makes the truck happy, then clearly I screwed up on the rebuild. Note that the truck starts up fast (2 seconds on push button) even when cold, and runs smooth as a baby's ass. Just when I brake, she conks out, and she is demanding like 850 rpm to idle smoothly. Note also that I have the dizzy retarded a bit because when I put it at original position, she is not happy.

[geezer#99]

Speaking of your dizzy, what's your initial timing and where is the vac pot hooked to?

[thelawdoc]

Geezer, dizzy is sharing vacuum with tranny and has 16" of vacuum. I do not know initial timing nbrs because we set it by ear. I know it is somewhat retarded right now to give smoother idle, though that idle is at around 850.

[geezer#99]

Have you tried plugging the vac line to the vac pot to get your idle down?
That high idle might be part of the problem.
The mechanical timing is starting to work at 850.
When you tap the brakes, the vac drops momentarily, the weights pop back to normal, you get a stumble due to no timing, you throttle a bit, rpm comes back up.
Get your idle down to 600 or so with the vac pot plugged and see what it does.

[thelawdoc]

Geezer, I understand what you are saying. At some point, we did plug the adv vac line when we were doing timing. At 500 rpms idle, IIRC, she would not idle smooth without a lot of advanced timing. I mean, the vac adv was up against the block. We could only get her to idle smooth without advance or retard in timing by raising the idle to around 800. Right now, at this moment she is at a shop, the idle is set at around 800; the vac adv is positioned as it was for years when she ran perfect, i.e., no adv or retarding. The mechanic is telling me he has a rebuilt Rochester B to swap in for my Roch B. So I told him, go ahead and let's see what happens. I am waiting for him to call me back.

[geezer#99]

To save me the search, what motor, tranny?

[thelawdoc]

1966 c10 250 Powerglide, Rochester B

[joedoh]

hmm. thats a stumper.


how about watching your ammeter when you step on the brake and see if it dives or goes wonky.

[thelawdoc]

Joedoh, I have a '66 250. It has the generator idiot lamps. But tell me more about your theory.

[joedoh]

its the old adage "its always the ignition"


grounds on these trucks are notorious for problems, and a good way to manifest a ground problem, is the brake circuit which is essentially a short to ground. My theory is, you are stepping on the brake and losing current or voltage or both to the ignition. You could potentially test this theory by turning on the headlights and see what happens.

[thelawdoc]

Joedoh, I will mention this to the guy in the morning before he swaps out my carb. I will ask him to do that test before swapping the carb. But I am not optimistic that this might be happening. For one thing, when I increased the rpms, I can give some brakes without stalling. For another, I do not see exactly how braking with a bad ground somewhere is going to cause the bogging/stalling. My buddy took my truck today and drove it about 15 miles. He avoided the stalls by using both feet at stop lights. On the other hand, I did notice that my heater blower suddenly does not work. I will get back to you tomorrow. Who knows? Stranger things have happened with these trucks, huh? Thanks, Joedoh.