Bogging/stalling when braking

[blown240]

Let is know how it works out.

[thelawdoc]

Gentlemen,
Regrettably, even though the bolt thread repair went easy and was successful, the stall-on-braking remains.

I was reviewing in my mind everything that I did to my truck between the time it ran well and the time it stalls at braking. I wonder about the following:

When I acquired this truck it ran perfect. I mean puuurfect. Recently,when I happen to post photos of my engine bay, a member with seemingly impressive knowledge noticed that my vacuum configuration was not correct. That is, I had the pcv sharing vacuum with the brake booster, and I had the tranny on a private vacuum line, and I had the dizzy on a private vacuum line. He said I must change that. That the pcv should have a private vacuum line; the brake booster should have a private vacuum line, and the dizzy can share vacuum with the tranny. So I rearranged it all to conform with his advice.

I have not yet gone back to check whether the initial configuration will solve my issue because it is rather tedious and everyone I have spoke to has endorsed the new configuration as correct. But it is a fact that, since rearranging the vacuums (and swapping out a bad balancer on the timing gears), I have this problem. Note that I do have 16" of vacuum.

My options at the moment seem to be either:
a) go back to the old vacuum arrangement and see what happens,
or
b) perform the following procedure suggested by a knowledgeable mechanic:

Setting the idle for the maximum vacuum will result in an idle where the throttle valve is closed (since vacuum is measured below the valve, highest vacuum will occur when the valve is closed). This generally results is a very low (rich) A/F ratio at idle.

When the engine is run at higer RPM's, the A/F mixture leans out significantly. Then under braking conditions, the throttle linkage with the PowerGlide allows the carburetor basically to go back to idle, and stalls because of the leaned out "run" condition.

Setting the idle mixture control mixture screw at an initial leaner setting AND setting the throttle valve for more initial opening (only a few thousanths) can make a significant difference in the stalling.

(Some vehicles use an "anti-stall throttle dashpot" to slow the closing of the throttle valve under braking. I do not know if your vehicle uses such a valve.)

As far as the sequence of adjustment:

(1) Dwell
(2) Timing
(3) Carburetor



Any thoughts would be most welcome.

[chevy_mike]

Personally, if it ran perfect before doing any work, I am of the believe of "if it ain't broke, don't fix it". Put it all back the way you had it originally and see if you can get it back running as before.

Then you have two options, leave it and enjoy driving the truck OR make ONE change at a time and see if you start having problems. The shotgun approach to troubleshooting (i.e. a lot of changes at once) is never a good idea. Change one thing and see if you can get it to work as well as prior to the change. Do this until you have an issue then either figure out that one problem or go back one step and leave it.

Personally get back to square one and enjoy driving it. Life it too short to be under the hood all the time. My .02 worth.

[blown240]

Sounds like maybe the next step is to go back to the way the vacuum lines were. But I don't think that would matter. Unless the trans needs a stronger vacuum signal than it has?

The thoughts of the mechanic do make sense too...

[thelawdoc]

Guys, Yes, I will reset dwell, timing, and carb by the manual, re-do the vacuum lines to where they were previously, and see what happens. There are no other options that I can see. But if that does not work, I am going to end up in a mental hospital.

[TJ's Chevy]

Or...go back to manual brakes....manual trans...and manual everything to eliminate as many vacuum lines as possible. lol Dang man.

[blown240]

I actually think your truck us telling you it wants a V8!

[TJ's Chevy]

lol. Good one blown...but 6 in a row makes it go...never had issues with a 6, but I have had more issues with v8's...sad truth I know...My dad's 496 v8 has the same symptoms as lawdoc's motor...so....its a disease that affect any. lol

[Jimal]

I think we are WAY overthinking this. If the truck was running great to begin with, timing and dwell can be eliminated unless someone has tinkered with either. I'm still going with the sediment in the carburetor, a dirty/damaged needle and seat or a hanging float.

[thelawdoc]

Jimal, the carb is immaculate. It cannot be the carb itself. An adjustment, yes. But the physical carb, no. The changes entailed the balancer and the vacuum configuration. And yes, we tinkered a lot with the timing as we tried to remedy this stall-on-braking scene.

[blown240]

and tune up parts...

[thelawdoc]

Blown, it cannot be the tune-up parts. Truck starts too nicely and runs too well for it to be tune-up parts. I think, based on the symptoms and the checks performed until now, that we can rule out all suspects except the vacuum arrangement and settings.

[Jimal]

Originally Posted by thelawdoc

Jimal, the carb is immaculate. It cannot be the carb itself. An adjustment, yes. But the physical carb, no. The changes entailed the balancer and the vacuum configuration. And yes, we tinkered a lot with the timing as we tried to remedy this stall-on-braking scene.

Immaculate inside and out? What you are describing doesn't seem like a timing or a vacuum issue to me. Everything points to something in the fuel system.

[thelawdoc]

Jimal, fuel pressure is perfect. Filters are all new. Unless the gods are playing with us, it cannot be the fuel delivery.

[Jimal]

Originally Posted by thelawdoc

Jimal, fuel pressure is perfect. Filters are all new. Unless the gods are playing with us, it cannot be the fuel delivery.

Have you driven the truck with a fuel pressure gauge attached? Static Fuel pressure only tells you the pump is good, and another possibility is that you have pickup problems in your tank that only manifest themselves under braking. A new filter is good, but stuff can get into carburetors through vents or through disassembly; more than just through the fuel filter. The specificity of your issue, that it only stumbles and stalls under braking but otherwise runs fine, eliminates vacuum and timing issues in my mind.

What your describing is either fuel starvation or possibly flooding, but it is definitely a fuel issue. A weird fuel issue, but a fuel issue nonetheless.

[thelawdoc]

Jimal, The truck ran perfect up to immediately before I changed the balancer and rebuilt the carb and rearranged the vacuums. She still runs perfect and idles smoothly. The fuel tank is new. The pickup inside the tank is new. The fuel line is new. My Roch B does not flood. The float is new and set with a machinist's rule. The needle is new, as are all consumables within the carb. But we will know better after I put the vacuum lines back to where they were and tune accordingly. While I am but a novice with engines, it seems to be a universal axiom that if you change something and a ghostly bug appears, the procedure is to change back and see what happens.

[cg285]

well no good mechanic sets a float with a machinist's rule. eyeball usually. so that's not your problem unless you can't read it.

you could cap all your vacuum lines real easy, for a test, and you wouldn't have to reroute everything.
i had one once that idled fine and ran at speed fine but hesitated on acceleration. being the careful driver that i am i rarely brake hard - just accelerate hard
turned out it had a point set with a clip on condenser connection (vs screw on) and the way the chinese made it the terminal would not stay in. it would pop out but still touch the connection - at times.
it's sometimes difficult to determine between ignition and fuel

[ACK]

Originally Posted by thelawdoc

Jimal, The truck ran perfect up to immediately before I changed the balancer and rebuilt the carb and rearranged the vacuums. She still runs perfect and idles smoothly. The fuel tank is new. The pickup inside the tank is new. The fuel line is new. My Roch B does not flood. The float is new and set with a machinist's rule. The needle is new, as are all consumables within the carb. But we will know better after I put the vacuum lines back to where they were and tune accordingly. While I am but a novice with engines, it seems to be a universal axiom that if you change something and a ghostly bug appears, the procedure is to change back and see what happens.

Here's a long shot ... GM made 4.1L balancers with marks at 12 o'clock, 2, and 2:30, so you might be top dead center on the #1, but your timing mark could be pointing someplace crazy if it was replaced with the incorrect year. I wouldn't think bad timing would cause the stalling, but it's easy to check and at least you can eliminate the balancer from the list of potential causes. One other thing, did you blow out the ports in the carb with high pressure air? Could be some crud in there making it act up. If you didn't and take the carb apart again, use eye protection when blowing out the carb, you don't know what's going to shoot out of where.

[chevy_mike]

Originally Posted by ACK

Here's a long shot ... GM made 4.1L balancers with marks at 12 o'clock, 2, and 2:30, so you might be top dead center on the #1, but your timing mark could be pointing someplace crazy if it was replaced with the incorrect year.

I must have missed the balancer replacement in the thread. Yeah, I was thinking the same thing too. When hitting the brakes, the vacuum is dipping and with the timing not being correct, it's dipping too low and stalling the engine.

I still can't tell, since there are so many posts, did you ever try driving it with all the vacuum ports closed and with the power booster and distributor disconnected? Did you have the same problem? If so, not a vacuum issue.

[blown240]

He unhooked the booster last week to see if it was bad. That made no difference. But I don't think he has removed ALL the vacuum lines yet.

[thelawdoc]

Guys, thank you for the attention. Yes, I isolated the booster, pcv and dizzy with no change. As for the balancer, the timing mark on the Dormans was not where it should have been when 1st cylinder was brought up by hand. And balancer goes on with a key, so there are no possible adjustments in that regard. We brought the #1 cylinder to tdc using our finger/screwdriver and then set rotor, ignoring groove on balancer. Timing seems good in that she starts right up and idles nicely at 500, and runs really nice. As for the carb, I blew out the ports. I went through a can of air on them. Today I took off the carb again and flipped the throttle valve. No change. All I can do now is swap out the carb. AutoZone has a reman Roch B that they sell under the name of Autoline. $200. I can give the money to AutoZone or a psychiatrist.

Giving hard brakes in Park does not bring the stall. Nor in Neutral. Nor in drive, unless I first give some gas and move a little. In fact, I can drive the truck and put her in neutral when braking and thus avoid the stalls.

[Jimal]

Originally Posted by thelawdoc

Guys, thank you for the attention. Yes, I isolated the booster, pcv and dizzy with no change. As for the balancer, the timing mark on the Dormans was not where it should have been when 1st cylinder was brought up by hand. And balancer goes on with a key, so there are no possible adjustments in that regard. We brought the #1 cylinder to tdc using our finger/screwdriver and then set rotor, ignoring groove on balancer. Timing seems good in that she starts right up and idles nicely at 500, and runs really nice. As for the carb, I blew out the ports. I went through a can of air on them. Today I took off the carb again and flipped the throttle valve. No change. All I can do now is swap out the carb. AutoZone has a reman Roch B that they sell under the name of Autoline. $200. I can give the mone to AutoZone or a psychiatrist.

Giving hard brakes in Park does not bring the stall. Nor in Neutral. In fact, I can drive the truck and put her in neutral when braking and thus avoid the stalls.

To me, if the timing is that far off, the truck won't idle, or at least will die when put into gear. And if it were a vacuum leak, the problems wouldn't occur under braking. The vacuum systems on these trucks are really rather simple, even with the automatic transmission. I would try swapping out the carb.

[thelawdoc]

Jimal, yes, I am at the point where there is nothing more to check. Swapping out the carb is all that is left. I am ordering a carb today. AutoZone said they will take it back if it turn out that it does not solve my issue. So that is nice. If it does solve the issue, I will go the rest of my life wondering what the heck was wrong with my carb.

[blown240]

long shot, but could it be that your torque converter is doing something strange? and when you put it in neutral i takes that out of the equation?

or maybe something happened in the trans when you changed the vacuum lines around. like before it had timed vacuum, and not it doesn't...

[thelawdoc]

Blown, I keep telling my mechanic buddies that it must be the new arrangement of the vacuums. And they are insisting that it is a Newtonian impossibility. They are saying that I will not find a mechanic on the planet earth who will agree that what I had is proper. But I will tell you a secret: today they left and told me to call them back when I get my new carb from AutoZone. Well, if it is, indeed, my carb, I won't need them if the new carb solves the issue. But before I order that carb, tomorrow morning I am going to switch back the vacuums to the way they were when I did not have this +&%#$ braking stall. And let me tell you, if that braking stall disappears.... Well, I prefer not to say anything more, lol. And, if the new carb does not solve the issue? I have that one figured out: I am going to change all the bulbs in the truck.

How would the tranny have timed vacuum by sharing vacuum with the dizzy (as it is set up now)? Their hoses meet at a tee that goes into the base of the carb. I do not see how the tranny's vacuum would be timed. The previous arrangement, when the tranny had a direct vacuum is the set up I had when the truck ran perfect, no stalls. As stated, that previous configuration had the booster sharing vacuum with the pcv -which everyone tells me is a no-no.