Carbs popping under load

[hiredwrench]

Hello. New to the forum here, and it looks like one of the "better" ones! Too many "I'm better tan you" ones out there!
I'm hoping you can provide some help. This is a bit lengthy, but I'd thought I'd give all the details.
I currently service a pair of straight 8's (and a 56 Caddy) for a client. His newest toy is a 1940 50 series 4 door in maroon(?). He purchased it on ebay, and it was delivered a month ago. Overall a nice car, 3sp, with a 12V conversion already done. It drove nicely. Better than his '49, but it had a nasty exhaust leak, so we set out to find the cause. After discovering that flexpipe was used to make the entire exhaust, the main cause was a cracked exhaust manifold.
I removed the intake/exhaust manifold and found that it had been welded. They probably never preheated the piece before welding as it had cracked right along the side of the welds. So the owner sourced a "new" manifold and pilot and got them in last week. (Not sure how he got them so quickly, but he's a broker and that's what they do best...) I resinstalled the manifold and I must say it looked real NICE being brand new and all. But enough with admiration, let's see how quiet it is.
When it started up there was no sound except the mechanical noise of the valvetrain whirring away. Real nice. (There's quite a bit of vapor coming out of the breather cap though, is this normal? There is no PCV in the system of course)
The trouble started when we put it gear. The carb started popping bad like it had jumped timing or something. The only items removed while doing the job was the carb, the manifolds, and the exhast pipe. I put a light on it and the timing mark is right where it should be. Could this be a stuck valve or something?
Please help me out if you can, it would be greatly appreciated!
Rick

[jerry burger]

take the vaacum hose loose and put a guage on. it should pull 20 psi.it could have a pin hole in the manifold.JB

[320Special]

This is just a thought but is it possible that the increased back pressure occasioned by the new exhaust manifold has leaned out the carb? Buicks tended to crack their manifolds rather early in life and seldom got replaced, but made do with makeshift repairs. Tune ups were done over this, and the carbs set up accordingly (i.e. with low back pressure in the motor).

Try a richer fuel mixture (what have you got to lose). In my Model "A", I could always get it to run better with a little more gas (laugh).

The increased crank case smoke from the breather is the result of higher backpressure, and consequent blow-by via the rings on an old motor (no?).

[guyopel]

I would agree with jb do a vacuum test & report back . It does sound like
a stuck valve and the vacuum test should confirm,
Guyopel

[Great Balls of Fire]

Do a vacuum check. Also check your Distributor vacuum line. I have a freshly rebuilt motor that pulls 20" of vacuum and 120PSI on compression. It has ALL no inner parts and I still get a lot of oil vapor out of the breather. I'm thinking of putting a plate underneath the oil filler hole to block oil splashing off the rockers..