I been chasing a vacuum problem on my 1984 olds 98 that I swapped a ‘72 350 rocket into. When u press the brakes the car dies. I pulled the brake booster vac line off with the car running and when the line is open the car runs a lot better and if I put my thumb over the hole and plug the line it runs bad again. Is this a normal phenomenon or does this indicate the booster is bad or does this indicate there’s a problem somewhere else in the vacuum circuit


if I put it in gear and hold the brakes and tap the throttle a couple times it still wants to die tho

 

Typically, with the line disconnected and open there’s a massive vacuum leak which should affect the engine operation. Plugging the line with your thumb should make the engine run better. Since the engine runs better with the vacuum leak, there apparently is an issue with the carburetor air/fuel mixture. Letting more air in through the vacuum leak causes the air/fuel mixture to be better than without the vacuum leak. That’s about all I can say from the information presented.

 

If an open vacuum line causes it to run better most likely it's running way too rich. Either disengaging the choke doesn't fully take it off choke or there's some adjustment problem causing not enough air for the amount of fuel. That's my take anyway, not an expert.

 

It doesn’t make sense to me either I’ve taken every vacuum line and put smoke to them. Unplugged them all and capped em. Sprayed around the carb and intake with carb cleaner and still can’t figure out where I’m losing vacuum. Is it possible for a Holley carb to leak vacuum internally?

 

Why do you think there’s a vacuum leak? We’re saying the carb may be running too rich.

 

because the car runs ok until u hit the brakes and use the vacuum up in the booster and then it dies

 

The swapped/installed Olds 350 Rocket has an Olds iron intake manifold or something different? You have the proper Holley square bore>spread bore adapter & correct gasket?

 

But you said the engine runs fine with that hose open, which is a huge vacuum leak, and the engine stalls when you plug it, which is no vacuum leak.

 

yeah that is true but it doesn’t make sense cause either way w the line plugged or plugged into the booster or open if u put it in gear and hold the brake and hit the throttle a couple times it dies
I can drive the car if I press the gas and the brake at the same time when I’m comin to a stop. Otherwise if I’m not on the brake it accelerates and runs like it should

 

You should have a rock steady vacuum between 17"Hg - 21"Hg for a stock 350cid. What is the vacuum reading @ idle (all vacuum hoses connected)?

The 1st test (upper LH side of chart) must be able to be passed flawlessly.

Post #7 answers?



Engine should pass this test flawlessly.

Vacuum gauge reading should be rock steady - no bouncing around - very steady.

 

it’s a edelbrock intake w the correct gaskets

 

I need to get a vacuum guage

 

There was a point in time where this car ran good with this exact configuration

 

Could be the power valve in your Holley carb has a blown diaphragm. When this happens it will be pig rich at idle. Has there been any backfires through the intake before it started running bad. A good backfire is what usually ruptures the diaphragm.

 

no backfires that I know of but I did buy this carb used off marketplace after I realized the 850 double pumper I bought was gonna be too much

 

Rather than guess....which Holley model is installed?

 

it’s a 750 w/ vac secondaries

 

You have a 4160? What is the Model Number? Why is this a difficult question?

 

it’s not a hard question
ill have to look when I go home. I bought it used off fb marketplace so I don’t know the model

 

Fair enough.

 

Should be really simple to determine as the LIST number is right on the air horn.

 

This is what I got

 

https://www.carburetion.com/CarbNumb...Number=R3310-5

Model H4-4160
750 Vacuum Secondary

https://www.carburetion.com/_aspstuf...p?part1=125-65

Power Valve 125-65

 

if I pull the power valve out of the carb how can I tell if it’s bad or not

 

And, as Gary suggested, if the PV diaphragm is faulty, it will run pig rich.

 

I changed the power valve in the carb and no dice

I also noticed if I turn the steering wheel all the way to the lock the car wants to die too so that leads me to believe that it’s the extra load on the engine of the vacuum booster that’s killing the car and not a vacuum leak somewhere

 

I threw the 850 double pumper back up there for ***** and gigs and it no longer dies when u press the brake so it must be something else inside that other carb that’s causing the condition