vacuum and crankcase venting

[Orangecrush]

Okay, I'm runnig a 351W. I just recently (finally) got it running right by just buying a new carb. The thing is, it likes to stall in gear at idle when cold/warm. Even when hot it seems like it almost wants to die. It seems like the vacuum is marginal when at idle in gear. The cam isn't too big.....should idle pretty well, more of a torque cam.

Anyway, today I was dicking around trying to test my theory about the unhealthy idle being ralted to vacuum. I know there are no leaks; so, I started testing some things.

-- Both of my breathers are connected to a vacuum source. One is connected to the base of the air cleaner (not full vacuum). The other is connected to FULL VACUUM at the manifold! Is this appropriate?

While it was idling, I clamped the FULL VACUUM hose off completely. The revs immediately dropped from 1000 rpm to 700 rpm (very significant drop due to less air available). Is this normal?? What would happen if I only let the one breather stay connected to the cleaner base and just let the other one vent down and away from the engine?? Wouldn't this tend to allow me to tune the carb much better and smooth out the idle due to higher vacuum and also a cleaner mix??

I know this is long; but, I am sure someone knows what I'm talking about and can give me an education on this one. Also, how much venting should the crankcase be doing through the breather....at idle? witha litle rev?? I can see obvious smoke coming out of the breathers even at idle? Worn rings??

My corvette doesn't even have a vent on the valve covers for venting crankcase gasses?? The only vacuum connection is to the timing advance.

 

[Broncoholics]

You don't need the hose going to the air cleaner from the driverside but its optional (doesn't hurt). You can buy a breather cap so it just lets out air pressure or use a standard sealed oil cap. The passenger side should be connected to the carb and yes it will decrease the idle if disconnected, keep it connected. If you think there is to much positive crank case pressure you might look into the intake manifold gasket. I had this happen. When I tried to use a sealed oil cap and oil started to pump out the dipstick tube. I added a breather type cap and it fixed the problem for a while. Then it got worse. I found that the gasket on the intake manifold was blown out right in the middle (the two bolts that are close to each other) where exhaust enters the intake manifold to help keep the bottom side of the carb warm. Now exhaust was pumping into the engine causing positive crank case pressure and the oil had to go somewhere. Once the gasket was replaced I was able to use a standard sealed cap gain like most engines.

 

[Orangecrush]

You don't need the hose going to the air cleaner from the driverside but its optional (doesn't hurt).  You can buy a breather cap so it just lets out air pressure or use a standard sealed oil cap. The passenger side should be connected to the carb and yes it will decrease the idle if disconnected, keep it connected.  If you think there is to much positive crank case pressure you might look into the intake manifold gasket.  I had this happen. When I tried to use a sealed oil cap and oil started to pump out the dipstick tube. I added a breather type cap and it fixed the problem for a while.  Then it got worse.  I found that the gasket on the intake manifold was blown out right in the middle (the two bolts that are close to each other) where exhaust enters the intake manifold to help keep the bottom side of the carb warm.  Now exhaust was pumping into the engine causing positive crank case pressure and the oil had to go somewhere.  Once the gasket was replaced I was able to use a standard sealed cap gain like most engines.

21961[/snapback]

​

What if I did this:

1) replace the PCV breather caps on both covers with standard ones (no pcv valve in either)

2) run a hose from each breather down to the road for venting/drafting

3) cap off the vacuum source that was connected to the passenger side breather (the one with the pcv).

3) cap off the the ****** at the air cleaner base too

Wouldn't that work best as it would elliminate the vaccum leak (albeit designed) coming from the manifold sucking in lots of air from the crankcase? It seems to me, this would raise the manifold vacuum a lot. Also, it would clean up the mixture getting to the carb (since it's now not pulling in some % of crankcase gas). I just don't know if the crankcase will vent itself enough without a "positive" system ? In the old days, there were no pcv valves and things seemed to work okay.

 

[Broncoholics]

There is no reason not to. Might as well give it a try. The tube that attaches to the carb is only to help burn additional stuff venting out the engine.

 

[stephaner142]

Just disconect the high vacum source off the breather, it has no bisyness being there, that would give you a open vacum. Just plug it. that's why your rpm's droped when you plug the line, you sealed the vacum. If you want to make shure that there is no more leaks use a propane torch and attach a short fuel line to it, run the engine at idle, turn on the torch and with the fuel hose go around all your gaskets and seals. If there's a leak your engine will rev up. If there's no leak check your timing, is your carb adjusted properly, and what type of transmision are you runing, manuel or auto, if its auto it could be your converter. this is my best gess. its hard to diagnose when the truck is not in front of you.