57 Chevy 3200. Recently rebuilt straight 6 250, turbo 350 transmission. One barrel carb, power brakes. My truck bogs/stalls a little when I first release the brake at stop sign, once it rolls a few feet it picks up and goes well. If I rush the gas it bogs. Is it a vacuum problem? I noticed sitting in driveway when I push the brakes I can feel motor idle down slightly? Not a seasoned mechanic, learning as I go Any help appreciated, if this is a dumb question, I only need to hear it once.
It's only dumb to not ask. Vacuum leak? Yes, probably. Power brakes? Usual way to find a vacuum leak is to spray around with some carb cleaner or starter fluid when the engine is running. Listen for a change in RPM. Track that sucker down. That might work for you. The vacuum hose from the intake manifold to the brake may be leaking. But the diaphragm inside the lung might have a leak. And I don't think the spray will help you diagnose it. But the fact that the RPM changes when you press the brakes is a real good indication.
If the vacuum hose is leaking where it enters the brake booster, you might not even have to use any spray. If you simply move the fitting around a little, you might be able to actually hear the hiss of the vacuum leak.
Process of elimination. First, either pull the hose off the booster and plug the hose or pull the hose off the intake and use a short plugged hose and fire it up and take a short test ride and see what it does. If that fixes the problem you have an issue with the booster or the connection to it, If nothing happens and the issue is the same you either have and acclerator pump issue or a low speed circuit issue. DO NOT CHANGE MORE THAN ONE THING AT A TIME when you are trouble shooting.
The power booster has a vac check valve that holds vac in. You also may have a bad diaphragm. I have unhooked vac lines from the booster even an hour or two after engine last run and had vac present. Check valve is most likely in the press in fitting that goes into the booster housing on vac line. Taking the line off this fitting and vacuuming the booster down it should hold for awhile. Putting a gauge on to monitor intake vacuum can help find the issue. They are available for around 20.00. Booster maybe ok just not enough vacuum to begin with from the engine or a leak somewhere else. Timing etc.
While it could be a vacuum leak, there are a lot of other things that could cause your problem. I would probably check the accelerator pump in the carb first. With the engine not running, take off the air cleaner, make sure the choke is open, and look down the carb throat. Move the throttle linkage to full throttle and watch the accelerator pump squirters. You should get a good, steady squirt for the full movement of the throttle. If you don't get a full steady squirt, that's the first thing I'd address.
Close the idle circuit needle valves all the way. If you don’t stall or at least drop idle you have a vacuum leak.
Look over the booster check valve carefully as a small crack at the base of the nipple which the hose is connected to can go virtually undetected. A new check valve solved the problem after finally making the discovery.
Sounds to me like it could be an accelerator pump problem also. If the pump doesn't begin to give a shot of gas as soon as the pedal is depressed the engine will try to stall or bog, and then as it gets fuel it catches and runs fine.
From the Googlewebs: "A vacuum leak from the brake booster can cause unregulated air in your induction system and create a lean fuel mix in the engine. You'll also notice a drop in engine RPM and possible engine stall." From me: Keep your booster hose as simple as possible to minimize possible leaks. Don't splice anything into the booster hose. Your safety depends on nothing compromising your braking ability. ..
The check valve usually is the fitting that connects the hose to the booster or is installed in the vacuum line from the manifold to the booster. It acts like a residual valve for vacuum in that it helps hold enough vacuum to be able to have some power assist on the brakes if the engine quits for stopping but it is only good for pushing the pedal a couple of times. When you don't have it you loose the assist then an there when the engine dies. All of us older guys have driven cars with power brakes that that happened on.
After you go thru checking for vacuum leaks as suggested above, there's one more place you should check. Assuming you have a Rochester MonoJet, check the two big Phillips head screws that attach the float bowl to the throttle body. It wasn't uncommon for them to work loose and cause an occasional vacuum leak. Also make sure the carb is securely attached to the intake manifold. And if you're poking around in the distributor check for excessive side play in the shaft. Wear in the dist. housing bushing was another "weak spot" in the later model Chevy I-6's. It often caused hard starting and stalling at idle speed.
Checked and installed new clamps on a couple of connections, looking it over the plastic connector to the power brake unit spins easy and may be part of the problem. I will get back under it tomorrow and see if I can tighten? When I upgraded the brakes I installed the power unit under the truck so I didn’t have to move the pedal. Almost instant regret
I will check that out, the engine is fresh rebuild but carburetor is not new, distributor is good, upgraded to electronic ignition with rebuild Thanks for the post
Before you do anything , pull the vacuum advance off the distributor and jam a golf tee into the hose and check again [the brakes while running] If the problem goes away , chances are the vacuum advance was connected to the manifold vacuum. Switch it to ported vacuum and check the timing of the engine again
How was the intake manifold installed? The correct procedure for installing is to have the exhaust and intake bolted together but not tight, install both at one time on the head using the studs to keep in place while putting the clamps, washers, and nuts on. Tighten the nuts starting in the center working out both direction while holding the weight of the manifolds up. Once the all the nuts are snug, tighten the three bolts holding the exhaust to the intake. Now finish tightening the clamps starting in the center working your way towards the ends. Even doing it this way doesn't create a good seal if the flange areas are not level. It's very hard to get a good seal if both manifolds are bolted together before installing if they have been apart for cleaning. I have some extra MonoJets if you find you need one, all off 1970's. My 250 will idle with 18 or more inches of vacuum depending on how hot the engine with a stock replacement camshaft. PCV valve is another place to check for leaks. I would check the accelerator pump linkage, when worn out, the throttle can open before the pump moves.
Thank you We definitely took our time on intake/exhaust bolt up, good place to check. I will check on PCV. Thank you for the reply
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