Bogging/stalling when braking

[geezer#99]

If you're all stock, does your heat riser still work?

[thelawdoc]

Geezer, I think you refer to the knob that I have under my dash, much like the manual choke knob. I pull it when starting a cold engine and I push it back in when the engine is warm. Last time I checked, the cable appears to properly swivel the gate that is inside the manifold. The truck starts with just a couple seconds push of the button. Real sweet. She runs great. I mean, really, really nice. The only trouble is the bogging/stall when applying brakes. The harder I hit the brakes, the quicker the stall. If I do not use brakes, I can start her up on an ice cold morning and drive across the country.

[geezer#99]

So then you don't actually have a heat riser then!
What does it do if you back up and hit the brakes?

[thelawdoc]

Same exact stall.

[chevy_mike]

Does it stall if you are not moving and you hit the brakes or is it only if moving (forward or back) and hitting the brakes?

[thelawdoc]

ChevyMike, It does not stall when I apply brakes in park, nor does it stall as I put her into drive and move my foot from brake to accelerator. It does stall, however, as soon as I give some gas and then go to brake. The harder I brake, the quicker the stall. The easier I brake, the more bog I get before the inevitable stall. This is so whether I am moving forward or rearward. I need only go a few inches and I can stall her with a hard braking. She starts right up again, even cold, and I can stall her at will. But if I give a little gas while braking, that is, use two feet, I can avoid the stall. My idle is set at around 800 to help drive somewhere with less stalling, but it does not prevent all stalling when braking.

[chevy_mike]

Wow, that is definitely a weird one.

It was hard to figure out reading above but have you blocked off the vacuum port to the booster and tried it without any power brake?

[thelawdoc]

Chevy Mike, Yep. Look at how long this thread is. The forum has been great. I have, by now, done everything. Yesterday evening I sent the truck to a 'specialist' -an older guy who works out of his garage on these ol' babes. He checked her out and told me last night that the only reasonable thing to do is swap out the carb and see whether there is something going on with it that the rebuild either caused or did not remedy. I gave him the go ahead. He is waiting to receive a rebuilt Rochester B to swap with mine. I am kind of hoping that the issue is due to the carb because if not, then the truck is haunted and we are just not going to find the problem. Between the guys here on the forum and a couple of talented, old school guys with me, hands-on, we are just not seeing it. As I said earlier, if it be the carb, I must have done something wrong when I rebuilt the carb. At this point I have to pray that it is the carb, because everything else checks out. I mean everything else is new under the hood, except the carb. And really, we are just talking about a large lawnmower here. I am perplexed at how this bog/stall on braking has come out of nowhere and stumped the best of the best.

[blown240]

Does the brake pedal somehow pinch a wire when you press on the brake? Its a long shot, but....

[thelawdoc]

Blown, I did not consider that. But it is hard for me to imagine a) what wire might be getting pinched there, and b) how that might cause the symptoms I have. I do not have the truck with me at the moment, but I feel pretty confident that there is no wire near the brake pedal. And, if there be some wire there that is stalling her on braking, how is it that I can sometimes overcome a stall by giving gas while braking?

[blown240]

Its hard for me o imagine too. At this point I think its almost has to be a carb issue, but still trying to think of other ideas...

[thelawdoc]

Thanks, Blown. I was thinking about checking the torque on the wheel lugs, emptying out the ashtray, etc.. Never had a situation like this before.

[blown240]

Drain the blinker fluid first.

[oem4me]

This IS puzzling. Like the horse/zebra story suggests, it's probably something simple you've overlooked. Given the fact you've done exhaustive brake/vacuum tests and come out ok, I say it's related to interrupted ignition.
Somewhere is a loose wire, or nicked insulation, or a flaky condenser, or SOMETHING that moves when braking and interrupts proper ignition.

[thelawdoc]

oem4me, I will tell you why I am so frustrated. The truck drove like a sweet babe for years. Whatever went wrong with her was always fixed without need for major research. I had one 'headache' with a tranny rebuild shop, but they refunded my money and I had the tranny rebuilt elsewhere. I go and do a simple tune-up 'by the book' and I end up in the doo-doo. I am confused and losing confidence in myself to own this truck. I mean, there is no simpler truck than mine. I have a stock 250 I6; 2 spd auto PowerGlide, Rochester B and P&C. I always believed a 12 year old could service this truck.

[blown240]

I dont suppose you have the old tune up parts still? Could be one of the parts you changed in the distributor is bad. Or maybe has come loose and is grounding itself during braking...

[thelawdoc]

Blown240, I guess so. But over the last few days that we have been working on this issue, we have been to the dizzy several times. She is new, clean, and everything looks just fine. I am still waiting to hear back from the shop regarding the swap of my carb. They were supposed to call me. My guess is that they swapped the carb and there was no change. But I am just in a sour mood. If I got a bad tune-up part, like a condenser for example, it seems to me it would reveal itself more than just when I brake. But I don't know anything at this point. Edit: No, actually I tested the condenser before installation. It was good.

[oem4me]

Originally Posted by thelawdoc

oem4me, I will tell you why I am so frustrated. The truck drove like a sweet babe for years. Whatever went wrong with her was always fixed without need for major research. I had one 'headache' with a tranny rebuild shop, but they refunded my money and I had the tranny rebuilt elsewhere. I go and do a simple tune-up 'by the book' and I end up in the doo-doo. I am confused and losing confidence in myself to own this truck. I mean, there is no simpler truck than mine. I have a stock 250 I6; 2 spd auto PowerGlide, Rochester B and P&C. I always believed a 12 year old could service this truck.

It all boils down to more Tinkertoys, and less TV. Yep, you blew it as a kid. There's no going back either. Ha! Just kidding.
My very first truck was simply equipped like yours (simpler actually, it had a three speed) and presented a problem that I could not trace for a looong time. That truck would run perfectly for weeks or months and then totally crap out all at once, usually in traffic on a hill or some other place that got me in trouble.
The problem was fixed with a new condenser, until it would crap out again. Then another new condenser would fix it. Huh??
After many condensers it was discovered that the wire from the coil to the distributor had a nick in it (on the underside, of course) and on rare occasion would shift with vehicle movement and produce a minute gap in the wire. It was explained to me that when the gap opened, the current would arc through and produce a current surge that the condenser could not handle, burning it out or impairing it, with faulty ignition immediately following.
Like you, I thought anyone who could successfully handle play-doh could fix anything on these trucks, but sometimes it's the simple things that are most elusive.
Don't despair, your truck wants to run right, but it must follow the basic rules of physics and something isn't in order.
Let us all know what is is when you nail it, no matter how dumb you might feel. Lol.

[luvbowties]

Originally Posted by 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.

There are several things that can affect your problem: primarily, ignition, compression, & fuel: and secondarily, vacuum. It may be wise to eliminate each, one at a time, in an orderly fashion.

...should you wish to quickly rule out ignition, you might use an inline ballast resistor & wire it inline from +12v battery terminal directly to power side of coil--NOT the points side. Also run a temporary jumper from NEG-battery terminal to the bolt holding your coil in place. "Truck is now 'switched on'". Start it & try to make it stall again: if it does, then ignition is likely ok; if not, you've determined an ign. problem. Attack the problem, rationally, from there. *Remember to remove the tempo +12v wire to switch it off.

...to rule out vacuum, unplug and cap off every vacuum source EXC. trans-modulator [which would be needed to make tranny shift up & down properly]. BE SURE TO INCLUDE BYPASSING VAC TO PCV-VALVE, as it is way too often overlooked during standard tune-ups. Be careful while driving with no power brake assist--dangerously stiff brake pedal. Test drive. Then add back the vac advance if stalling is still present & suspected as caused by lack of advance. Test drive again. While vacuum sources are plugged & capped, spray carb cleaner around possibly leaking vacuum surfaces while idling steadily & check for changes--up or down--in rpm's. Now vary idle speed, 1st up, hold steady, and spray cleaner again to look for variation in rpm's; then let idle speed come down (below starting position), hold steady, & spray cleaner again, noting variation in rpm's. Such surfaces should include carb base, intake-to-head and any surfaces that should seal vacuum leaks. Nothing remarkable here should rule out vac problems.

...the only things left are fuel and compression. Since it idles so good and produces 16" vacuum, compression should be good.

...to test fuel, you may wish to begin by lowering float a tad, marking where you begin. Re-assemble & test drive. Next, raise float a tad above where you started. Re-assemble & test drive. Having gotten to this fuel-testing stage, one of the new float settings should help your problem. Whichever one did help now just needs fine-tuning, changing a bit more [or less?] in the direction that helped. When you get the problem minimized, ensure you have by then replaced the bowl-gasket. Trial-and-error, I know; but after 50 years on the road, sometimes nothing beats such age-old logic of using a process of elimination.

...just thinking in general. Somehow my mind keeps going back to the make-shift method of controlling the heat riser in your exhaust manifold. I picture seeing a correctly-configured one flutter from open to closed and back when instantaneously opening-and-releasing throttle--even after engine has heated up. Just for experimental purposes, after your engine has warmed up sufficiently, tinker with its degree of opening by using your under-dash control cable. Try making it stall with it in various positions. I'm expecting you to see a difference. I'm hearing you say, "But it was controlled that way before." OK, I understand. But if you find that it can cause your problem to greatly lessen, then here is an avenue worth following. I intentionally placed this paragraph/thought in last place. But, in case you read this entire reply before enacting any suggestion herein, this would be easiest to try first--just didn't want to turn you off by hearing this 1st and maybe discounting the logic of the earlier paragraphs.

With sincere hopes for your rectifying this 'ghost'--I think you called it--I will be watching your progress.
Sam

[thelawdoc]

Sam, thank you very much. I do not discount anything that is ever suggested here. I am going to copy and send your note to the mechanic. I already sent him all of the other suggestions I got here. Between the books and this thread, if we don't bust this ghost I do not know what I will do. Thanks.

[tenni126]

It sure seems to me like a float height issue, but if you already checked that then disregard. The Edelbrock on my 350 was uber-sensitive to float height, and would do that also until I got it right.

[thelawdoc]

Tenni126, your float was causing stalling only on braking?

[tenni126]

Yep. The Edelbrock is maybe an apples to orange kinda thing compared to your specific carb, but that was my problem. It was after I swapped to 4 wheel power discs from manual drums, so the added vacuum load kind of exacerbated things. It used to only kind of stumble on braking, then after the brake booster it'd stall completely. IIRC it was that the fuel level was too high, and when I'd brake what was happening (I think) was that it would flood the carb temporarily. It didn't take much to correct it, and I remember being surprised at how sensitive it was to float height. That advice was from the troubleshooting chart in the carb's manual.

[thelawdoc]

Tenni, that sounds interesting. I will have my float checked again. Thanks.

[tenni126]

Yep. The Edelbrock is maybe an apples to orange kinda thing compared to your specific carb, but that was my problem. It was after I swapped to 4 wheel power discs from manual drums, so the added vacuum load kind of exacerbated things. It used to only kind of stumble on braking, then after the brake booster it'd stall completely. IIRC it was that the fuel level was too high, and when I'd brake what was happening (I think) was that it would flood the carb temporarily. It didn't take much to correct it, and I remember being surprised at how sensitive it was to float height. That advice was from the troubleshooting chart in the carb's manual.