weird vacuum leak

[429bbf]

I've always been one to love trouble shooting . my explanation for why it happens after wot is that you have done such a good job of rebuilding the carb that it has no place to get air (vacuum )when the engine is trying to decelerate from very high rpms you have created a massive vacuum pump.and with your shaft bushing and throttle plates set perfect it has to try and get air from somewhere . and it has now found the weak point. try this (I'm not fond of sealants )but this will work )take and put a small bead of locktite gasket maker (black )and install between the manifold and gasket and just a dap between base plate and gasket . let set a couple of hours and the problem will be gone .next time you have it apart you can true all the surfaces and fix the problem . fwiw.

[Hillbillyenginering]

I believe that the ice and transition slots do receive the fue through the jets. That's the changing the jets I size,.001, changes the ice quality.
Squirt break clean down the restrictor tube and watch where it comes out.
But I could be wrong

[Marx3]

the idle tubes DO get their fuel supply from the main jet stream, but as far as I know, the idle quality is not affected by the size of the jet/rod combo. The tubes are so small in diameter compared to any main jetting. I am not willing to bet money on this, but I am pretty sure.

429bbf: interesting theory indeed! I am not af fan of sealant in those kinds of places either, but I tried just to be sure. Problem was still there. I am going out to the car tonight to investigate. I'll let you know

[429bbf]

heres just a little info on the locktite si5900. caterpillar has stopped making gaskets for a lot of the equipment . you have a choice ,ether an anerobic sealer or a good gasket maker. the locktite doesn't squeeze down quite as far as the anerobic but it doesn't matter because you using shims to adjust your preload or free load on your bearings. my quincy air compressor blew the head gasket to the high pressure side and would not build over 100 psi. i pulled the head and saw the problem . couldn't find a gasket so i used the locktite on the old head gasket and let it sit for about 10 minutes. i torqued it down slow  . then i let it sit for 12 hours . it still builds 125 psi today and that was over a year ago.so i guarantee you that you can't draw enough vacuum to cause this to leak . i don't know where you live but you won't be able to get over 30 inches at sea level.one other note you didn't get a bad gasket and its pulling vacuum through the gasket?fwiw

[429bbf]

this ones a little out in left field but you didn't get some crud in your bolt hole and you think its tight and the bolt is bottoming out and not pulling the carb down . you can also (if your using the gasket with the plastic bushings )grind .020 or so off the bushing and that will squeeze the gasket tighter.hth

[Marx3]

I'm right at sea level. I used MS Polymer and laid a thin bead around the edge of the sealing area of the gasket, let it sit for 10 minutes and installed it. I haven't had time to get out to the car and check up on things yet. I must say I have absolutely no reason to believe, that the gasket is to blame.

You guys came with some interesting suggestions. I'll let you know as soon as I continue work on this car.

[Marx3]

It turned out to be the secondary throttle plates not centered 100%.
At first glance they seated good and evenly, but after opening the throttle, they would not seat perfectly.
Seemed something was binding or what ever. Re-centered them and the problem went away.

Saw this once before on a Buick. That engine would not idle below 1200.

[Ethan1]

It turned out to be the secondary throttle plates not centered 100%.
At first glance they seated good and evenly, but after opening the throttle, they would not seat perfectly.
Seemed something was binding or what ever. Re-centered them and the problem went away.

Saw this once before on a Buick. That engine would not idle below 1200.

 Glad you got it figured out! 

[Marx3]

Thanks.
Boy, did it drive me crazy. :-)