I have a 350, 1979 z28 , i can't get it to have the right vacuum for anything. Its to low and on the gauge it floats between8-12. I have only 1 vacuum port on the intake and its not really held together good, its an assortment of different screw in vacuum slots. I have the pcv attached to it and the other part is blocked off. I had the motor rebuilt on it and don't know how well of a job was done on it. I've tried spraying brake cleaner on it and redundently it caught on fire for a minute. But it sounds like a vacuum leak in the back around this vacuum assortment or the back of the intake. Also if i give it gas, the vacuum will rise instead of get lower. But being that the vacuum isent steady and it jumps, that has to do with the valves correct? any help would be appreciated.

 

If you want to get a true manifold vacuum reading, I would disconnect everything else on that port except for the gauge. You make it sound like you have a bunch of leakers connected to one vacuum port?
What do mean about the "other part" of the pcv being blocked off, where?
Also make sure all of your vacuum hoses are on good and snug. Replace the hoses, or cut off the stretched out ends and put back on.

 

There are two types of vacuum in an engine. Manifold vacumm and ported vacuum. Ported or "carbuartor vacuum" is cause by the air rushing thru the carb. The faster the motor goes the higher the ported vacuum. Manifold vacuum is in the manifold and is created when the carbs butter fly valves are closes. At idle your butter flys valves are close or near closed. This would be the max manifold vacuum(idle) with the car sitting still. The only time engine vacuum should be higher then idle is when you use the motor to slow the car down (manual down shifting). 8-12 inchs of vacuum is way low for a fresh stock engine at idle. If you are hooked to manifild vacuum you have a major problem.

 

I only have one vacuum port on the manifold itself, i have 2 ports on this one manifold vacuum port, one being the pcv and the other being plugged off. I pull the plugged off piece off and put the vacuum gauge up to it with the pcv valve still connected. and it jumps between 8-12 on the gauge, it never stays in one spot. I can take the pcv valve off the valve cover and plug that end up with my thumb and the idle seems to straighten right out as to where if its in the valve cover it runs rough. And if i give it gas the vacuum goes up not down. It almost sounds to me like i can hear a vacuum leak from the back of the intake. And this thing i use for vacuum ports on the manifold are poorly put together. I've looked for what it had on the old intake to put on which was a vacuum "T". But no one sells them anymore, so i had to rig up somethin that i believe is leaking vacuum.

 

sounds like you determined your own problem

 

I don't know enough about this stuff to know what my problem could be. I'm just taking a guess at it. And my dad helps me out alot and He is a master tech, but doesn't really remember much about these older engines, and he's a ford guy. So, im sure that gives him less motivation to mess with this turd i bought and am trying to get to run right. But it sounds like a whistle to me at the back of the intake, and I was told by another tech at work it could be the gasket under the distributor.

 

yeah it could be that or it could be one of those tiny hoses on the back of the carb that dont do anything but cause vacuum leaks. haha. it usually is one of those.

 

could be a bad pvc valve. if not it sounds like u have an open vacumm port somewhere. try to locate the noise and look for a hole in the manifold or a hose fitting without a hose on it. if nothing then while the motor is running, take some carb cleaner and spray it on were u think the leak is. if the engine speed changes u found a leak. you should post a few pictures of what we are looking at. dist. gasket will not effect vacumm.

 

Well, i was spraying brake cleaner on it this weekend and it caught on fire for a second when i sprayed it around the back of the intake where the assortment of male and female fittings were, maybe i hit the distributor. I don't know but it did catch fire for a minute then went out as the brake cleaner burned quickly. And is there any specific way to tell if the pcv valve is bad, or going bad?

 

Testing the PCV valve

carbureted and Throttle body (TFI) models

To test the PCV valve disconnect the PCV hose from the rocker arm cover on 1991-1995 models or disconnect it from the three way connector on 1989-1990 models. Now start the engine and let it idle. Place your thumb or finger over the open end of the PCV hose to feel for a vacuum(suction). IF you don't feel a vacuum you need to check the hose for clogs or other blockages for example a sharp kink in the hose. If need be just replace the hose with a new one. Once you have established that there are no blockages in the hose turn off your engine. Now you need to find out if the valve it's self is operating properly. What you need to do now is blow air through the PCV hose. Note*Use extreme caution , make sure you do not suck any air through the hose as it contains very toxic gases that can poison you and cause you serious physical injury.When you blow air through the hose you should feel resistance if there is no resistance the PCV valve is stuck open and you need to replace it.