Updraft carburetor

[Soupiov]

What is a decent vacuum reading off the intake manifold for a four cylinder engine running a Carter BB-1 carburetor?

[Mark Shaw]

That measurement will vary depending on RPM, piston & valve seals, etc. 

I suggest you start with a compression test to see if all four cylinders show similar compression.  

[Soupiov]

Compression is good on all cylinders.  Car will run at high rpms with choke full choke but not idle.  Dies when choke is in.

[31Buick96S]

The engine has to be pulling air from somewhere if it is running on full choke. Usually a sign of a vacuum leak. 

[Oldtech]

Could be Idle circuit in carb plugged too. But Check for vacuum leaks first.  What engine are we talking here?

[carbking]

Which engine and which BB-1?

 

Jon

[dodge28]

One simple way to check for vacuum leak. While the engine is running , however you  get it running with choke in or out, use a propane torch ( not lighted ) open half way and point it around the manifold and any place you suspect vacuum leak. The engine will speed up when you hit the offending spot or spots. The vacuum leak at any one point will suck the propane gas .

Another way is to spray Easy Start around but that gas is volatile and dangerous. May cause fire.

[edinmass]

Propane torch is the way to go, I did that test yesterday on a V-16 Cadillac. There is an actual tool with a hose and nozzle that you can control better and direct the HC’s more accurately. I have had mine for decades. They are still available.

Edited April 14 by edinmass (see edit history)

[Soupiov]

Thanks for all the input.  This is a 30-U Plymouth 4 cylinder and I am running a BB-1 carburetor that I believe was supplied on PB Plymouths.  The carburetor has the larger orifice for gravity feed as my car has a vacuum tank.  Last night I swapped out another BB-1 from one of my other cars that was running fine and had the same result so I guess it must be a vacuum leak.  The engine was just rebuilt and came back from the rebuilder fully assembled to include the manifolds so all I did was put the carburetor on.  Guess the manifolds are coming off next but I was thinking about checking with a vacuum gauge first on the off chance that it was something else; that's why I was asking what  a reasonable vacuum reading should be.

[DFeeney]

I believe 30-U's  came with a Carter Carb.  They all have pot metal problems now.  I use the later BB1 on mine.

[dodge28]

If I remember correctly the BB1 use an adapter 180 degrees on the manifold. these adapters are sort of home made. May be the flanges are not sturdy enough or not completely flat for gasket fit. I am having the same problem at present . The idle screw has no effect. I will have to do the propane test when I get the car running.