29204 Throttle Body

[lintmann]

Hey everyone. This is my first post so I hope my question isn't too detailed.

Quite a few years ago I bought a 29204 carb and I'm just now getting ready to rebuild and install it however I've been told that the throttle plate is wrong for that carb part number. After looking into it I see that the throttle lever is in fact wrong so the throttle plate itself may also be wrong.

In addition to the numbers shown in the pic there is also a 10(date code of sort?) stamped on the side of the drivers rear mount lug and a 206 stamped on the aft side of the passenger rear mount lug and a 37925 stamped on the bottom side between the primary bores.

Please have a look at my pic and let me know what you think. Any info is appreciated on what it might be from.

Thanks!

[Cliff Ruggles]

It looks period correct from what I can see of it.

Can you post a pic of the bottom side?

The big change to them came in 1971.  The later throttle bodies will fit the early castings and function correctly if you use the right gaskets so it's pretty common to see later baseplates on early carburetors if the unit was every commercially "remanufactured" at some point.

I would recommend getting one of my rebuild kits for it.  It will have the correct parts and highest quality out there.......tks.....Cliff

[lintmann]

Thanks Cliff, here is a pic of the bottom side, please let me know what you think and do those numbers that I posted earlier help much to ID it?

I have no problem using this throttle plate but was told that it is a 68 plate and won't work with my main body. The guy that told me this says that I can only use a throttle plate from a 69 and that the idle bypass would have to be modified to work properly. I'm having doubts to the credibility of his remarks though because he can't tell me how to positively ID a 69 plate nor how to modify the idle bypass to have it work properly. Does any of this make sense to you?

If I can make this throttle body work would you have or know where I could find a proper 37250 throttle lever?

As for your carb kits I am thinking of getting one but it will be for a 7040273. I should also probably get your carburetor book once this is all sorted out too!

[Cliff Ruggles]

The biggest issue swapping around baseplates on those early divorced choke Chevy carbs is the height of the APT system.  Each one of those carburetors that uses APT in the baseplate sets the Power Piston and the correct height for the carb it was used on.  If you raise or lower the Power Piston on a stock build with stock jets/metering rods in it the calibration is thrown off some. 

Many early units don't have idle bypass air.  The holes coming down are in that baseplate but I can't see from the pics if they are drilled into the bores or not?  In any case it would be easy to add it.

There were several different throttle levers used on the early carbs as well.  Some don't have anything below center aside for a place to hook up a return spring, others, like the one you have are more generous........Cliff

[lintmann]

Thanks Cliff. So it sounds like this throttle plate can be made to work with my carb without too much work then.

I think this piece does have idle bypass ports and I'm told that they would have to be opened up for my big block application, does this sound correct?

Would all the mods required to make this work be covered in your book?

As for the throttle lever itself the correct one has the bottom portion cut off(except for the small tab for the return spring)and is stamped 37250. Would you happen to have one of those for sale in your parts bins?

Thanks again for your help!

[Kenth]

I have found the main difference of 1969 SBC and BBC throttle plates is the throttle lever.
Since CBB uses either M/T or TH400 no provision for a TH350 detent cable needed.
As for idle bypass air i have found CBB Qjets 9200 and 9215 have the same amount as the CSB Qjets 9202, 9203 and 9207, .072" and .080"-.087" idle needle holes.
With a hotter than standard camshaft larger idle bypass air and idle needle holes may be needed.

HTH

[Cliff Ruggles]

Correct, intake clearance is a problem with certain combinations in those years.

As far as the height of the APT it can be a BIG issue depending on the jet/rod combination the carb is using. 

When I build early Chevy and Olds Q-jets that use that system I remove the factory plug over the APT screw, the APT screw, and replace it with one we make here in the lathe.  This has proven to be worth the extra effort as it allows us to not only return the power piston to the same height, it gives us full control of part throttle A/F when custom tuning without taking the carb apart.......Cliff

[lintmann]

Thanks Guys, So here is a pic of the passages in my primary ports. From what I'm starting to learn is that I probably don't have to worry about them with this main body but I still wanted to see what you think about them. Should they work okay with my big block carb?

All the jetting, rods, hangar, etc in this carb check out as correct/original and it's a bone stock restoration so given that, is it possible to set up the correct APT without some special fixture? Cliff is this covered in your book? I like your adjustable one that you have but this is a correct "numbers" resto so I'll need to have that plug covering the adjuster when it's all set up.

[Cliff Ruggles]

If you can get the APT screw to turn then you can set it wherever you want then put a cap over it.

I can't imagine anyone even noticing the external adjustment screw as it's not visible without getting down low enough to look under the main casting......