The Stovebolt Site Forums The Shop Area Engine Shop Engine bogs/stalls when braking.

Gentlemen, I have a '66 C10 250 w/2 spd auto PowerGlide and Rochester B carb, Delco dizzy/P&C. 65k miles.

I just did a tune-up. And I just rebuilt the carb. She has new: battery; wires; plugs; condenser; point; rotor; air filter; fuel filters; harmonic balancer; rebuilt carb; rebuilt tranny; new pcv; and fluids.

She has nearly new: timing gears; distributor; belts; muffler; and heavy gauge wires.

She starts up with 2 second push on button. She warms up fast. She runs very nice -very sweet. Good power. I have 16 inches of vacuum and proper fuel pressure. Exhaust color is good. All plugs firing and all valves dancing properly. A real cream puff.

Alas, out of nowhere, she suddenly has a blemish. Since doing the tune-up and rebuilding carb, when I brake, she bogs and stalls. The harder I brake, the quicker the stall. The easier I brake, the easier the stall. Forward or reverse. But not when in park or neutral. Only in drive and I have to move a couple inches. If I brake and give some gas, I can overcome the stall most of the time. I also have to have her idle set at around 900 to get her to idle smooth. I set her timing by ear and I think it is pretty good based on how she starts and runs. No amount of adjusting the idle and mixture seems to help. Oddly, after I drove her for an hour, the bog/stalling on braking seemed somewhat alleviated. It still happens, but not like it did initially.

Buddies are telling me to re-examine my carb, that there must be some grit in there. Others say to replace the carb. I took off the carb and had a quick look, and it seems perfectly clean to me, needle looks fine, etc., but today I will dismantle it and take it down to components and do a real deep cleaning again. I plugged the brake booster: no change. I see no loose wires, though suddenly my heater blower motor does not work.

Has anyone experienced this?

Float adjustment?

Don, well I did check the float and it seemed good to me, but I do intend on rechecking it again with a machinist's rule when I dismantle the carb. She does start, idle, and run really nice though.

yep, float too high

lefty1, Thank you. I am going to check the drop & height first thing.

Gentlemen, Ok, so today I dismantled the carb. I found the float height at 1-8/32" instead of the specified 1-9/32". I cannot know yet whether this was the cause of the symptoms I mentioned in my original post because when reassembling the carb I found that one of the bolts that holds down the air-horn will not grab the float bowl. I will have to tap it out. Maybe I will re-install the carb as it is just to check it.

As for tapping the float bowl, the bolt appears to me to be a 7/32" x 28tpi. What would be the proper next size larger to tap? (I have available what appears to me to be a 1/4" x 20tpi machine bolt.)

If you could not tighten all of the bolts securely prior to you earlier test runs, you are wasting your time trying to diagnose anything else until you resolve the bolt problem. With only four bolts, one loose on will likely cause horrible problems.
Carl

52Carl, so maybe I have finally found the ghost hiding in my truck. Praise the Lord. This carb confounded a bunch of veteran mechanics this week. I wonder how none of them mentioned to me that this bolt needs some attention.

Before you jump on your veteran mechanics, I would never suspect a stripped bolt hole. I would suspect a warped air horn which would give you very similar symptoms as a stripped bolt hole.
Good luck and make sure you get back to us after you figure it out.
Carl

1/32 float error will not do that. It could be a tranny/dash pot/throttle/down shift thing. If it's a loose screw, then that makes two of us.

52Carl, Bartamos, Nah, I am not jumping on the veteran mechanics -they are buddies of mine. I am but a grasshopper next to them. But the fact remains that they were opening up and closing up the carb trying to figure out the cause of my symptoms and did not notice that one of the clamping bolts spins forever. It certainly was not stripped when I rebuilt the carb- that is something that even I would have known cannot be good. Now, is it the cause of my symptoms? I do not know. I will re-tap all four bolts out to 1/4" x 20 tpi from the current 7/32" x 28 tpi and see what results. There is no warpage between my air horn and float bowl. I checked that with a straight edge. No leaking, touch wood. I also found that the power piston had a larger check ball than specified by the manual. I changed that and I cleaned the entire carb again even though it was still squeaky clean from my rebuild. So we shall see...

It was advised earlier not to open the holes to 1/4 x 20, that will be to big and cause more problems.

Don, I did not see such an advisory. Could you tell me in which thread? Though I am but an amateur machinist, and a novice at engine repair, I do not see how the 1/4" bolts cannot satisfy the engineering specs of the carb as well as the 7/32". Also, since the carb is cast from Zamak or some other recipe of zinc and alum, I do not see any problem in my going from the fine thread to the coarse, though I do understand that this could be an issue in other, more critical, fastening situations. I had no leaking issues while I was missing the holding contribution of one bolt, so I do not expect any leaks now that I am bringing back the fourth bolt. I am sure that I have done similar type repairs on some of the vintage Zamak machines I have restored or worked on, and I cannot recall ever being sorry. So far, I only see that I need to exercise the usual cautions applicable to tapping. But please feel free to opine otherwise. I do not like to contradict those who have more experience than I will ever have time to gather in my lifetime.

Alden B, Hotrod Lincoln mentioned it in the third post down.
https://www.stovebolt.com/ubbthread...at&Number=1073264&gonew=1#UNREAD
Brian

Alden, it is not the end of the world to tap a 1/4-20. The reason for finer threads IS the material. The 1/4 carries a larger head screw and may not land. You need 3-4 threads to be effective. If you work all that out, install screw with two finger torque. You have at least two posts on this, please be sure you read every word of advise. Missing the Hotrod post, or any other, can cause you grief. There are many options, none are easy and have some risk and take some skills. I'm sure you will be fine at whatever you decide. It's not a decision of no return, you can get another bowl.

55Shaker, Bartamos, Shoot! I knew I had that thread, but somehow I neglected to mark it for watching and so I have been in the dark for those responses. When you mentioned that Hotrod Lincoln was helping me, I thought you mis-spoke. Bartamos, the double posting was accidental -I sincerely apologize. I have been a little stressed out lately and I was not mindful to mark the thread for watching. Thanks guys.

Your symptoms can be caused by using a vacuum gauge to set the idle mixture control screw for the highest vacuum.

Set the timing per the service manual first.

Then reset the idle mixture control screw.

Since you have the P/G, make sure you have the correct spacer and gasket under the carb.

Jon.

It's not double post and is not a problem, just was pointing out two places to watch. You posted in two different forums, perfect! Good luck and let us know.

Carbking, Thank you. I had always thought that we are supposed to use the vacuum gauge to set idle and to seek the highest vacuum. I have the manual but I am a novice at timing. I have been doing it by ear during these days when I had to get the carb in order. I just remounted the carb. I believe the carb is now perfect. So I guess I have to go back to timing, right? First timing, then carb? I had to have the idle rpm's at around 850-900 to help me overcome the stalling issue. Hopefully, she will not stall anymore, and I can reset the rpms to around 450-500.

Carbking, could you elaborate as to the significance of my PG vis-a-vis the 'spacer'/gasket? I have only a gasket and it looks like just a standard gasket.

Bartamos, Thanks. If you had not mentioned it, I would have completely missed out on the advice given there.

Do you have booster brakes? Sometimes my 57 will almost kill the engine if you apply the brakes at an idle. IE the booster is taking away my vaccum too quick.

According to Rochester there is a 1/2 inch spacer used under the carb that insures correct vacuum supplied to the carburetor.

Setting the idle for the maximum vacuum will result in a idle where the throttle valve is close (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 mixtures leans out significantly. Then under braking conditions, the throttle linkage with the P/G 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

Jon.

Old GMC Gold, I have a booster. I just remounted my carb after a very thorough cleaning and exam. But she still stalled on me when I gave hard brakes. I am finding it hard to believe that my carb is not good. I do not think that a carb 'goes' overnight like that, precisely when you do a tune-up. Maybe the booster went suddenly? I happen to have a brand new booster but it is a double pot and I have a single pot right now. That is, I have add a line. I can't just swap it in real fast. But I am going nuts already about this stall-on-braking. Never had it before. Don't know what I did to bring it on.

Carbking, Thank you. I am beginning to wonder whether I am having that situation. But I am not familiar with an 'anti stall throttle dashpot'. I have to check whether my timing light does dwell also. I think I will have to let my mechanic buddy read your post and see whether your idea is the one eluding us. I have no 1/2" spacer under the carb and I do not think I ever did, even when the truck ran great. Thank you.

Carbking, You know, as I was tossing and turning in bed last night about this headache, I was giving some thought to your post above. As a novice, I have given little thought to the interplay of the tranny with the carb. 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. And with your post in mind, I wonder about the following:

When I acquired this truck it ran perfect. I mean puuurfect. Recently, on another forum, 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. Carbking, do you agree with the foregoing, or do you have a different opinion? I have not yet gone back to the initial configuration because it is rather tedious, but it is a fact that, since rearranging the vacuums (and swapping out a bad balancer on the timing gears), I have this problem.

Alden, If it was my truck I would unhook the vacuum line that connects to your vacuum advance on the distributor and temporarily plug it. Then take the truck for a ride and see how it acts. I believe that the distributor should have ported vacuum to it and that the transmission should have straight vacuum.
The brake booster line should have a check valve in it also.
What site did you post the pictures at? I'd like to take a look at them.
Good Luck,
Don

Don, I did plug the dizzy vac adv but I was still able to stall her with a hard brake right in the driveway. So I did not bother to go for any test drive. Yes, the dizzy must have ported vacuum according to my understanding since that regulates the weighted arms. As far as the tranny having straight vacuum from the manifold, that is what I used to have before the stall issue and that is what I plan on going back to in the morning. I will make the pcv share vacuum with the dizzy's ported vacuum. I do have a new check valve on the booster. Here is a photo (if I did it right) link:
http://www1.snapfish.com/snapfish/t...SHR/otsi=SALBlink/COBRAND_NAME=snapfish/
In the photo you can see original configuration that had no stalling, the dizzy hose going into the base of the carb, the pcv and booster sharing a manifold port, and the tranny going into the manifold's side port.

Does it stall on hard braking if the automatic transmission is in neutral? That would eliminate the possibility of drag from the transmission, and if the stalling continues, you can eliminate the transmission from the possibilities of the cause.
Carl

52Carl, There is no stalling in neutral or park. Only in D or R and then only if I give some gas and move a few inches and give a hard brake. Soft brake does not stall her. I can drive ok so long as I put her in neutral before braking, or, if braking slowly/softly, no need to go into neutral.

Don, did you see photo link? I finally got the link right. See it in my above post.

I had a "dash pot" go bad,on a '65 G.M.C. pick-up I had. That caused the same problems as mentioned.(It also had the 2-speed "Slush-Box"). There are a couple of "vid's",on "Youtube",on how to "Straighten-out" a warped Roch."B".(Make sure all gasoline is evaporated,before attempting to heat halves with a torch)! Mine was warped,and I was lucky enough to find a "freebie".

Wetwilly5757, My float bowl has no warping. I checked both the air horn and the float bowl with a straight edge. Indeed, my Roch B does not leak unless it is flooded. Again, this is a problem that appeared virtually overnight. It has to be something that I did, imo. Today, after work, I will change back the vacuum lines to be as they were when I got the truck and she was perfect. If she is still unhappy, I will swap out the carb.

What is a 'dash pot'?

Gentlemen, well this morning I put the vacuums back to the arrangement I had when the truck was good. No change. The stall remains. I do not see any other choice now except to throw a carb at her. But for the life of me, I cannot figure what might be bad in my carb. If I could, I would send it down to Virginia and ask the FBI to examine it. Now, my nightmare is that I swap out the carb and still have the stall

I still think that buying another carb is premature. As I said earlier, I would disconnect all the vacuum lines and cap the source for each. That will rule out all of the components that are operated with vacuum. If the stall remained maybe you should put a dwell meter on and watch and see if the ignition is failing when the stall occurs. Another idea is possibly something hanging up with the transmission or torque converter? I'm not a tranny guy but it may be worth talking to some tranny people. I'd also take some time and look the little things over, make sure you don't have a wire pinched someplace that when your braking hard things move and it causes the failure but if you brake easy things don't move? Spending money and throwing parts as a hail mary to fix things usually only irritate a person rather than solving issues.

Don, if I disconnect the tranny's vacuum line, is it still permissible to put her in drive to see if the stall remains?

Yes Alden.

Don, I isolated the tranny, but the stall remains. Can I take that to mean that there is no issue in the tranny that might be causing the stall? Ditto re the booster and ditto re pcv and ditto re the vac adv.

I know the mechanic put a dwell meter on her, but I do not recall that it revealed anything relevant. I will ask the mechanic what he read.

I don't like the Hail Mary play. But isn't a Hail Mary play done when there are no other options left?

There is no joy in Mudville today.

Just unhooking the vacuum to the transmission only rules out the vacuum as the cause of the stall. It still could be other issues with the transmission or torque converter, but not likely. The idea behind putting the dwell meter on is to see if it shuts off when the bog/stall happens, thus indicating that you are loosing spark. You should try raising the idle speed to say 1000 rpm's, then see if it bogs/stalls out when hitting the brakes hard. One last thing that comes to my mind is the condition of the fuel supply hose from the tank, have you checked it for cracks? I wonder if the pickup in the gas tank is loose or has something blocking it off? Some of the ideas are long shots but keep digging until you find it. Good Luck, Don

The simple check for transmission-related problems should be obvious- - - -SHIFT TO NEUTRAL BEFORE STOPPING! If the bog remains, you definitely don't have transmission problems. If it goes away, or there's a noticeable improvement, try tinkering with the idle speed. If you're running a 350C or a 700R4 with a torque converter lockup clutch, the clutch might be dragging or failing to release fully as the vehicle stops. 700R4's were notorious for converter clutches failing to release and causing engine stalls at stoplights.
Jerry

The truck does not stall in Park or Neutral. When I drive her and I have to stop, I put her in neutral and I brake without her stalling. She only stalls in Drive or Reverse, and only if I brake hard, and only if I moved at least a few inches. In other words, if I put her in Drive and brake hard without having moved, she does not stall. If I give just a bit of gas and move a few inches and then brake hard, she stalls. All driving, including on highway, is fine. She has power. She runs great. Light braking does not bring on the stall.

I tried raising the idle to 900 rpm. The stall remains the same. So I put it back to 500.

I have the 2 speed auto PowerGlide.

We did a check with the dwell meter, but I did not stall her during that check. I will do dwell again tomorrow and stall her and see what the meter says.

The fuel tank, fuel line, and filters are new.

Hotrod Lincoln, Regarding the tranny test that you mention, she does not stall when I go into neutral and brake while driving. And she does not stall if I am sitting in place, in neutral, and braking. Only in Drive or Reverse. Also, she does not stall at all if I brake easily and come to a gradual stop. Can I rule out the tranny?