84 C20 stalling when braking hard in drive; not reverse

[84 C20]

ive got an 84 C20 with a 350/350. the engine has a Carter AFB that's been rebuilt recently and put back to factory needles/jets. The floats are at factory specs. Initial advance is 10 degrees and overall is about 16. I have replaced all of the vacuum lines and check valves on the engine (i did bench test the PCV but it's working correctly) and transmission. The dizzy is a streetfire with new wires. The fuel pump is less than a year old, the fuel filter is clean and the tank is full. The idle is around 820-850 and strong (she sets off car alarms, much to my amusement.) The only thing left is the brake booster. I think. Maybe.
with the truck not running I've pumped the brakes a few times and started the engine. the brake pedal goes down what feels like an inch or two but nothing near where my 2013 sonata goes down when I do the same thing in it.
I can cruise around, come to slow stops and take off like a scalded dog but if I hit the brakes hard going forward the engine just stops. No sputtering or slowly idling down, just stops. The electrics are still working when this happens, it is just the engine. I put the truck in reverse to see what would happen. No stalling, just locking up of the front wheels.
It seems to me that if the brake booster was the culprit the engine should stall in either direction when the brake is pressed hard.

I'm at a loss as to why the truck is stalling. She idles great, accelerates well and I'm not smelling any richness in the mix. I've even tried cussing and flipping her off but that's not helping either. Does anyone have any ideas I haven't tried, aside from shelling out for a brake booster, that could be the issue? Any help is much appreciated!

 

[1987 GMC Jimmy]

If you had a hand vacuum pump w/ a gauge, you could disconnect the line from the check valve, hook up the pump, pump it up to 18-20 inches of Hg, and see if it holds vacuum. If it doesn't, then bye bye to that booster. Usually, booster failure is accompanied by a loud whoosh when pressing the brakes, enhanced pedal travel, loss of fluid in the master cylinder, and stuff like that. You mentioned that it doesn't do it in reverse so to rule out a transmission factor, I'd make sure the line going to your modulator is okay. I don't know TH350's well at all so it maybe a waste of time to look at the modulator, but I was trying to give you something else.

 

[84 C20]

I'll try that tonight after work. In the process of trying to figure out the problem I replaced the rubber bits going to the transmission and clamped them.

Thanks!

 

[chengny]

TCC (if equipped) not releasing?

Does the speed of the vehicle have any effect? IOW - if you slam the brakes on at 5 mph does it still stall, or does it only happen at higher speeds?

The TH350's that had TCC's were reported to be extremely tight and from what this wiring diagram shows, the clutch was designed to lock-up at 26 mph:

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That's a real stretch though. In order for that to occur, both the brake switch and low vacuum switch would have to be not opening as well.

 

[84 C20]

I can get it to stall out in the driveway as well as out on the street at pretty much any speed. I didn't get a chance to test the brake booster this weekend - had to take some animals away from a *&$$^!@! who was starving them. The plan tonight is to noodle out if the brake booster is going bad.


update: Hooked up the mighty vac and the brake booster is holding vacuum. Is there a possibility i've got the wrong springs for the needles? If they are too weak the manifold vac could slam them "shut" and starve the engine. Now I'm grasping at straws...

 

[chengny]

I don't know if you're up for this, and it might not help you isolate the problem - but then again it could.

Try to induce a stall by using the e-brakes instead of the service brakes. If the engine doesn't stall, it would seem that the issue probably is not related to the lurching motion produced by the stop.

That wouldn't provide you with much to go on, but at least it would let you eliminate the fuel/air intake systems. And you wouldn't have to tear into a perfectly good carburetor.

IDK - just a thought.

 

[84 C20]

I've pulled vac on every connection I can think of and everything is holding. I've got worm clamps on all the metal/rubber connections, replaced the 3/8" fuel line with vac line for the brake booster, played with the timing again, put the 8 inches of Merc springs in and I can still get the truck to stall within the space of my driveway.
The springs in the carb helped, misadjusting the floats has helped as well which leads me to believe its a "sloshing of fuel issue". Using the E brake only locks up the rear tires which is a gradual enough of a stop that there's no stalling. The new theory is that if I go 'rich' in jetting then the abrupt change from metered vac to manifold vac won't cause as much of a disruption in fuel delivery to the cylinders.
Or it could be the initial advance is too much (10 degrees) and with the vac it keeps the overall advance too much (about 22 degrees) so there's no spark to light off the cylinders. or it's a little of both. or neither.

I know its not a flooding issue because she'll start right up immediately after throwing it into park. I don't smell excess fuel either.



or it's all magic and I'm not a Copperfield.

 

[84 C20]

Solved:

After doing all that work I ended up adjusting the floats so they are 3/16" of an inch from the "ceiling" of the float bowls. Then I noticed what started all of this to begin with: moisture on the side of the carb that I thought was fuel. After looking, poking and prodding I decided that it was condensation. I've got a heat separator between the carb and the intake. The passenger side of the carb is noticeably cold even with a warm engine. I didnt see any fuel leaking out or spilling over anywhere. Since I smelled like fuel myself it was a little pointless to sniff the liquid.

now all i gotta do is figure out why all my new vintage gauges arent working when the last time i had the truck running they were. You know, if its not one thing its my mother.

 

[84 C20]

actually i lied. the solution worked only until the truck got a few miles down the road. then it ran for sh@#*(%*#! and died at every chance it got.
found that some fuel was leaking out the side of the carb not near a gasket so I bought a brand new, right out of the box 1406 to replace it with. the thought was that the carb was just broken.
just installed the new carb, adjusted the idle. put in a fuel pressure gauge and a fuel pressure regulator so I know that the carb is getting about 6 psi of fuel. brand new fuel filter too. don't have any of the charcoal lines connected; all ends are capped off. if I'm running down the road at 30 and come to a stop the truck wants to die and tries her best to die. after a few miles she will die turning 90 degrees right but turning left has no effect. she'll die at a stop sign but start right back up immediately and run happily until you drop it into gear. it doesnt seem to matter if the dizzys vac advance is hooked up or not. she will run like #$@()#@ with or without it hooked up after a few miles.
I don't mind saying that I am so frustrated I want to go out, put a few 12g shots into her and set her on fire. I'm missing something but I just don't know what it could be. I've gone through all the trouble shooting items i can come up with plus anything that gets suggested.

baaa. Any thoughts? I'm going to have a beer and pet my pig.

 

[chengny]

Any thoughts? I'm going to have a beer and pet my pig.


Did you ever get a chance to investigate the possibility of an inappropriate TCC lock-up? You might have a TH350C. Google "lock-up problems with a TH350C" - and you'll will see many stories/questions that will sound very familiar to you. Below are a couple of examples - out of many:

A year later, I finally got around to installing a 350 small block. I could get her to idle, but once in gear, I couldn't get her to run without two feet and standing on the gas. I initially thought it was a vacuum leak. As a result, I redid the intake manifold and checked the carb gasket and everything is totally sealed. No leaks and the idle is good.

Still the stalling problem persists. Now after reading on the internet and checking the tranny, I realize that I have a TH350C (I guess you learn something new every day...I didn’t know TH350s came in a lock up version.). None the less, I'm coming to the conclusion that since the common symptom between the old v6 and v8 seem to be that the car won't idle in gear, it must be that the tranny is stuck with the converter engaged in a lock up position.



The "TCC Always On" wiring in the diagram above really means "always on" - the TCC always on unless you step on the brakes. That means you could get an unpleasant surprise if the TCC is locked and you step on the brakes, stop the vehicle, then let off the brakes. At that point the TCC will try to engage wit the vehicle stopped, running, and in gear. The engine will likely stall or at least buck as the TCC will try to lock even though you are not moving yet. "ON" really does mean "ON" in this case! One idea to fix this is to rig up a low-speed switch of some kind to open the circuit below, say, 10 MPH. It would be interesting to wire it into this system and see how it worked. I'd bet acceleration would be sluggish, and the TCC could have longevity issues (just theorizing here, no hard proof), but it would certainly be a nice safety feature for when others might drive your vehicle - aka, the wife. Don't say you weren't warned!

http://www.rowand.net/Shop/Tech/LockupTCCWiring.htm

 

[Rusty Nail]

Pics of the pig?

 

[84 C20]

I haven't followed up on the 350 vs 350C issue. I always thought it was a regular 350 but it could be a 350C. there were a lot of parts laying around in the shop when they built the truck. I'll look tonight when I get home from work.


Mah piggies:

edit: talked with my girlfiend, she built the truck, and she believes it is a 350C because the guy she sourced it from loved those trannys. with that confirmation I'm starting to feel hopeful that Chengnys (repeated, sorry for being obstinate) suggestion is the culprit!

 

[84 C20]

well, I was all excited about having gotten to the root of the problem. Went out last night and searched all over the transmission for the connector so I could rig up a switch but there are no wires or places for wires on the whole thing.
I did the next best thing: I sprayed starter fluid on anything that might pull vac on the transmission, engine, charcoal can, everywhere to no difference in engine speed. I guess I'm going to have to take it somewhere because now all i can come up with is that maybe i put the heads on upside down....

 

[1987 GMC Jimmy]

Upside down?

 

[84 C20]

so, in an effort to share information I'm dragging this back up to the sunlight. after changing out all of the vac lines, adjusting the timing and the valves, changing out gas and lots of glaring at the radiator I finally, accidentally, found the problem. i decided that the carb wasn't seated and that was what the problem was. once I got the carb off and while i was cleaning off the flanges i accidentally knocked the fitting that supplies manifold vac to the transmission. it wasn't even finger tight in there. after putting some rtv on the threads and tightening it up (then putting everything else back together) the truck is running like new. no stalling, no hesitating, no acting up. Passed inspection easily!


In short, it was a vac leak that didn't show up when sprayed with ether. If you can't find a bad hose then try looking at that little fitting behind the carb. Hope this helps someone!