Bogging/stalling when braking

[thelawdoc]

Gentlemen, I just did a tune-up on my '66 C10 250/2spd PowerGlide/Rochester B carburetor/P&C dizzy. She starts nice. Idles nice. Drives nice. But: when I brake she bogs and stalls. Not intermitently. If I do not brake, she runs just fine at all speeds. As soon as I brake, I get bogging and a stall. I can sort of "get around" the impending bogging/stalling if I give throttle immediately.

I adjusted timing. I adjusted carb idle & mixture. Fuel pump is fine. Plugs new and clean. Wires new. P&C and rotor new. Condenser new. Fuel filters new. No vacuum leaks. Carb just rebuilt. New balancer. Timing gears new.

I have on hand a new coil and a new vac advance which I did not yet install, but I somehow feel that these are not causing my problem. I have a vacuum gauge but did not check vacuum because I have a nice idle and running.

I suppose there are many possibilities/suspects for bogging. This morning I was going to take off the carb and check the needle for dirt. Any other ideas?

[blown240]

Maybe a loose wire that moves forward a bit under braking? Coil wire?

Do you have an electric fuel pump? I had one that would move just a bit when turning and it was just enough to pinch the soft line. I had to mount it more solidly and the issue went away.

[thelawdoc]

Blown240, Well all wires seem pretty tight on this babe. A mechanic buddy of mine is convinced that it is something in the carb that I just rebuilt. Maybe the needle seat. I don't know. I am really scratching my head on this. I will take off the carb and double check all of my work. I am a little frustrated at this point.

[TJ's Chevy]

I have a slightly similar issue with my 496/turbo400....Idles around 850 rpm and in gear around 500 rpm...little low for the cam in it. I had it where I'll be coming to a stop and it'll struggle to idle while I'm sitting...but with a little pressure on the pedal to keep the idle up a little its fine...I've noticed that people with 6 cylinders and auto transmissions leave the idle fairly high in park cause when it goes into gear the idle drops a noticeable amount...try setting your idle to 900 and see what happens.

[thelawdoc]

TJ, I did pick up the idle a bit, That was my first suspect, but the bogging and stalling still happens every time I brake.

[blown240]

This may sound stupid, but how is your fuel pickup in the tank? I had one on a 66 Suburban that acted crazy for a bit until it broke off and fell into the tank.

[thelawdoc]

Blown, I replaced the fuel float/pickup in the tank about 6 months ago. My recollection is that the sock was fine, the fuel is fresh, and I was on a 400 mile run with the truck before I did this tune-up without any issues. Plus, I checked fuel flow at the carb and it is good.

[TJ's Chevy]

Originally Posted by thelawdoc

TJ, I did pick up the idle a bit, That was my first suspect, but the bogging and stalling still happens every time I brake.

Are you running power brakes or manual?

[blown240]

Good Call TJ. A bad vacuum booster will do exactly this.

[thelawdoc]

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. I did that on the recommendation of this forum. I am pretty sure there are no vacuum leaks.

[blown240]

I would suggest unhooking the booster and capping the port on the manifold and see if it still stalls. You won't have power brakes while testing, but it will tell you if the booster is bad.


ALSO - do you have a check valve on the booster? Not sure if that matters, but I'm pretty sure you have to have one for good brakes...

[thelawdoc]

Blown, well that is easy enough to try. I will do that. But in general, the brakes seem just fine.

[TJ's Chevy]

Well...reason I mention it is cause I have an 84 C30 with power/vacuum assist brakes...And power steering...Mine bogs just like yours...and I have a manual trans...I did some tests...and in neutral and not moving, I pushed on the brake pedal as hard as I could and I almost stalled the engine...which shouldn't happen...only slight rpm drop should occur....I'm betting on vacuum hear...or bad booster...disconnecting it and capping off the manifold from the booster or carb...which ever its hooked to should tell you if its the booster...but you'll have to drive it and beware of no power brakes...so the pedal will be a bit firmer.

[thelawdoc]

TJ, I understand. When I hold the brakes and put her in drive, she does not stall. But if I go forward just a bit and hit brakes, she dies.

[TJ's Chevy]

Originally Posted by thelawdoc

TJ, I understand. When I hold the brakes and put her in drive, she does not stall. But if I go forward just a bit and hit brakes, she dies.

Almost seems like yer tranny or something...like yer torque converter or something isn't separating to allow you to stop...like slamming yer brakes on in gear with a manual trans...engine will die cause the clutch is engaged and tires won't move...I don't know a darn thing about auto transmissions though. lol But that seems another possible issue is tranny.

[thelawdoc]

TJ, yeah, I wondered about that too. Maybe I should put the vacuum lines back to the way they were, i.e., dizzy direct to manifold, and pcv sharing with brake booster, and tranny direct to manifold?

[blown240]

Check the booster first, thats easy. Then reroute the vacuum lines if you want. Then move to trans...

[thelawdoc]

Blown, yeah, thanks. I think I will it that way. First the easy stuff, before I take off the carb and dismantle it...again.

[ls60apache]

maybe float level too high flooding out do you notice any black smoke when you restart?

[thelawdoc]

Ls60apache, well I haven't noticed any black smoke. Plugs are clean. My Rochester B carb not leaking as it would if flooding.

[blown240]

Have you had a chance to check the booster?

[thelawdoc]

Blown, Unfortunately, I did not get a chance to do anything today on the truck because, without the truck, I am on the buses and trains and that is really killing my schedules and routines. I have to do some of the easy checks suggested above, and hopefully I will not have to take off the carb to dig into the needle seat. Because if that needle seat is good, I am really out of ideas. I would give it to a shop, but I am not sure that today's shops will be able to figure out this one. They would likely just start throwing parts at it I am afraid.

[blown240]

I agree. Most new shops I wouldn't trust. Let us know what your tests show.

[thelawdoc]

Tomorrow I will start at the top of my checklist and just do it by the numbers. I was able to spend some time today online, but by this point, I think I have all the points that need to be checked. So, hopefully I will find something that I did wrong and get back my wheels again. I think the only part that I might throw at it is a carb, but I really hope I don't have to. I will go over my carb real slow and with twenty eyes to see whats going on in there.

[ACK]

Just a thought ... drive the truck up to a telephone pole, leave it in gear and give it some gas, then hit the brakes and see if it stalls. If it stalls, maybe it is a vacuum issue. Hook up your vacuum gauge to the different components and test one by one with it against the telephone pole, maybe this will help track down the problems origin. Oh, and if it doesn't stall, maybe it's a carb issue. Like I said, just a thought. Good luck, hope you figure it out sooner rather than later.