The Stovebolt Site Forums The Shop Area Engine Shop stalling on power brake application

I put an ECI power brake booster on my new dual circuit master cylinder (same size as on old corvette) and it works great...but...when I push the pedal down very much, it seems to use so much vaccum that the engine tries to stall. I have the vaccum line going straight from the booster to my 292's manifold fitting just below the carb.

Any suggestions, O River of Knowledge?

I called ECI and they said I was using the wrong line (a 3/8 heater hose). NAPA sold me a fuel line and this one is 5/16 so it fits real tight on the barbs. I know its getting vaccum because the brakes are easy to apply.

Seems like mebe I could put an extra vaccum can on or something like you would use on a turbo motor??? :confused:

I would try richening the air/fuel mixture. The power brake booster dumps air into the intake manifold you may be too lean to handle it.

Are you using vaccum hose? Not all hose is rated for vaccum systems. Just a thought: work in some steel tubing where possible to keep hose from being sucked flat. Also, you should check out the check valve on the line to the booster. These sometimes will fail, even got my 421 Pontiac v-8 to stumble!

You may want to check the booster to make sure there isn't a hole in the diaphram in the booster causing the vacuum to continue and not let it build up when ever you step on the brakes. What you describe is the symptoms of a hole in the booster diaphram.

hmmmm....its a brand new booster! Never been used before. Mebe a check valve would work....

Install a check valve and see what happens. The 292 should be able to handle the booster with no problem, most all newer trucks used power brakes. The line won't have any effect on the engine side, booster won't work if its to small or sucked shut, the engine won't know the difference. Joe

talked to the guys at ECI, and they had a good suggestion. Remove the booster from the circuit, hit the brakes, and see what happens.

And...

its not the booster. with the booster off the vaccum line the same problem is there. Stalls. I hooked a vaccum gauge up and watched it, but I don't think its a vaccum issue.

So my timing light needs replacement, but I am going to go get another one, and then put it on and drive the truck and watch the flash as I go down the alley to see if it cuts out under braking.

Dunno what could be causing it at this point.

Engine has rebuilt carb and new fuel pump. Also has new spark plugs, wires, cap, and rotor. Wiring harness is brand new Painless Wiring 18 circuit universal (which I recommend). So in other words....tain't obvious at this point.

:confused:

timing light didn't reveal anything. I tried disconnect brake lights just in case that circuit was causing a problem. No difference. I tried just moving it about in driveway making the truck rock. Both back and forward make it cough. And that's with and without the brake booster on manifold vaccum. Without I put a piece of line on with a round punch jammed into it so the engine doesn't lean out.

I am kinda stumped, but will keep monkeying with it.

Any ideas, no matter how odd (relating to this problem ) would be appreciated.

last test (least its nice outside here in texas - in 60s) I went in with a long socket extension and poked wires - all of them. No changes. I also made sure the battery cables were tight. No changes.

Is it possible that fuel is sloshing about in the carb bowl causing the problem? I just had the carb rebuilt.

This is a 1969 292 with a rochester one barrel #70045xx (xx = unreadable) and looks like:

blury carb photo

Remote (really!) possibility: Engine ground strap loose? You may be losing groound under deceleration. Carb is also a possibility. Maybe the float is set wrong and the port is uncovered while braking. This would occur almost immediately (I think). If the float is allowing too much fuel in during braking, there would probably be a slight lag between brake application and stalling.

Good luck! and Merry Christmas (or Happy Holidays, as the case may be)

too much fuel? I was thinking perhaps too little, as it would let the jet go dry. But, I can't see in there. I might call the carb shop and ask them what it should be set at, or open the top of the carb and see what the fuel level is. I suppose I could go both ways with it and see if I can get the thing to run right.

I have triple checked all the wiring, including the ground strap, and they are all tight and right.

hmmm,,,,, Have you looked at the wiring on the ignition switch under the dash?
Any loose "HOT" wires hanging around under there?
-
OH,,, disconnect the vacuum advance and plug the vacuum line and see what happens.

>> Is it possible that fuel is sloshing about in the carb bowl causing the problem? I just had the carb rebuilt.

This sounds logical to me - the float level in the carb may be set incorrectly.

>> I tried just moving it about in driveway making the truck rock. Both back and forward make it cough.

When you do this, of course the gas in the carb bowl is sloshing.

To check the float level, you can just pull the top half off the carb without pulling the whole carb. I'd suggest buying a carb rebuild kit (about $14) - the kit has instructions for checking and setting the float level, and even includes a small ruler to measure the level.

Good Luck!

thanks boiz, no loose wires anywhere. I went and tugged on all of them.

Carb was rebuilt for me (off the truck) by a local shop that only does carbs. Going to run the truck over there as they want to see what's going on.

I just don't have a listing to know what the level should be.

and the answer is....

CARB FLOATS SET TOO HIGH!

Tadah! Stump the chump. (Just like with the tappet brothers)

Took the old dog to the carb shop and after a couple of dissassemblies of the carb, the floats got set right. Fuel was sloshing about too much in there and mucking up the engine.

So file that one away for future reference. Truck runs like a champ now. I am very pleased.

Ahh haaa! It was sounding like that might be da culprit.

Glad to hear you got it solved!

John

yup, give a chimp a banana and he eats for a day...

give a chimp a 59 1 ton, and he can get his own durn bananas....

Good to hear the truck is fixed and running healthy again