Frankencarb?

[Shiny]

I am going through my carb (again) to address nozzle drip/flooding that happened the last two times I tried to get the car running.

Hoping I may finally have some insight...

Reading this thread:  https://cliffshighperformance.com/simplemachinesforum/index.php/topic,4696.0.html, I noticed another member with the same carb says it had NO BYPASS AIR in stock form.

So..

1.  Does a stock 7029240 have bypass air?

Mine (29240 MD) has the holes circled in the image, which I assume is for bypass air.

2.  How would I know if my carb has the wrong baseplate?

The only number I could find is incomplete - attached image.

The only reference I have for the part numbers is a Delco Bulletin.  It lists 7036596 for the baseplate assembly.

3.  Is my understanding correct?  I think if there was too much idle air, I would have to add more fuel to achieve an idle.  It is possible then, that I had to increase the idle speed screw and opened the primary blade too much, causing my nozzle drip.

4.  If all this is explaining my issue, can I just plug these bypass holes or should I get the "correct" baseplate?

Mike

[Shiny]

Woops...

I posted the wrong baseplate number.  It is clearly 38752.

[Shiny]

I found an eBay listing that shows a baseplate without bypass air holes:

https://www.ebay.com/itm/354145578251?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=bhZJb1MCTAe&sssrc=2349624&ssuid=w1_2LomQQYu&var=&widget_ver=artemis&media=COPY

Still hoping someone can tell me if I have the wrong baseplate and need to lose the bypass air.

Image below is from the eBay 7029240

[Shiny]

And another on eBay without bypass air:
https://www.ebay.com/itm/124797535399?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=gE-X7iF_Qlu&sssrc=2349624&ssuid=w1_2LomQQYu&var=&widget_ver=artemis&media=COPY

[Kenth]

I have found Buick startes using bypass air in some 1970 455 and all 1971-74 455 Qjets, none before that, and no 350 Buick uses bypass air.

[Shiny]

Thank you Kenth!

I conclude I have the wrong baseplate and will plug those holes!!

Doh!

[Shiny]

Can anyone tell me what the "correct" number I should find on the 7029240 baseplate?

I have another core, 7040244, and the baseplate on it has a different number (38753) that also doesn't match the Delco reference for the 7040244 baseplate assembly (7036831).  I suspect the numbers on the baseplates may be machined casting part numbers and not assembly numbers but don't know where to look this stuff up.

Would be nice to know if the 7040244 baseplate is actually and magically correct for the 7029240 carb.  But I don't want to deal with it if not... the APT screw was a major hassle the first time and I was lucky to salvage what I now know is the wrong baseplate.

I have also posted on PY and although my "wrong baseplate" has bypass holes, I don't yet know if they actually connect to a vent or other air source.  It is NOT obvious and I haven't yet pulled the baseplate off the body.  It may turn out the holes do nothing and all is well with the idle circuit.

Lots for me to learn!

Thanks,

Mike

[Cliff Ruggles]

Having the wrong baseplate isn't a big deal as far as idle bypass air is concerned.  IF your main body bypass air passages in the voids in the casting aren't drilled you can simply drill them.  You may also need to "notch" the airhorn as well to complete the bypass air system.

Bypass air is used to keep throttle angle down at idle speed and helpful if you have "low" vacuum from a larger than stock camshaft or the engine combination just isn't making a lot of vacuum at idle speed, or anytime you are pulling fuel from the main boosters (nozzle drip) but the carbs idle system is adequate for the application.

You can still get into trouble with the wrong baseplate when it comes to gaskets, so make sure you are using the correct gasket between the main casting and baseplate for the parts that are being used.

There are other potential issues with swapping baseplates from one carb part number to another.  APT height is one, transfer slot location and length is another, not to mention the size of the holes under the idle mixture screws and the size of the idle bypass air holes (if used).

One has to keep in mind that Q-jets were application specific and set up at the factory so all three components worked well together.  If/when we come in and start swapping parts around more times than not it throws things off some as far as the calibration is concerned.  This is why I supplied some "basic" calibrations in my book, but it is impossible to provide exact specifics for any particular carburetor part number being used on a "custom" engine build. 

Even so knowing the "basics" is the first step toward getting a good end result. 

I'll add here that the Buick 350 carburetors made from 1969-1974 are EXCELLENT starting points for a really nice high performance unit.  They don't need a lot of help anyplace either and it is best to start out at or very close to the stock settings for most engine builds they end up on.......

[Shiny]

Cliff, thank you!

I opened it up today and find the body was never drilled for bypass air, consistent with original design.

So even though the mismatched baseplate has idle bypass holes, they weren't doing anything.

I will scrutinize the gasket and walk myself through all the circuits tomorrow to make sure the mismatch isn't hurting anything.  All the gaskets and parts I've replaced came from you, so I expect they match the body.  Just need to make sure the body and baseplate passages are aligned.

I will likely ask again about the tuning before I re-assemble...

Also good to know the Buick 350 carbs are decent, as I have a 7040244 core that might need to be rebuilt to replace this frustrating POS I'm battling.  The 7040244 is rough and missing a lot of parts, but maybe they'll all transfer from the 29240 MD Frankencarb if I decide to toss the one I've got.

Kenth also posted on PY that the transfer slots are different between this Buick 430 carb and a Buick 350 carb.  Is that a big concern for use with my mostly-stock 1970 350 Pontiac?

The "never-drilled" holes are circled in the photo.

Mike

[Cliff Ruggles]

Transfer slot length and location can be an issue with some set-ups.  If you are exposing a LOT of transfer slot I've seen scenarios where you can completely seat the idle mixture screws and still WAY too rich at idle......

[Shiny]

Thank you again.

I will re-visit my primary blade seating and ask for guidance on this If I'm concerned.  I can compare the slots on 2 or 3 bases I have laying around to see if this 29240 MD slot design is obviously more liberal than others.

[Shiny]

After following Cliff's book to systematically inspect every circuit, I think the carb might be usable in spite of being a rebuilder mash-up.

I found the airhorn has also been swapped.  With eBay being my only reference, I think the carb originally had an external vent but now does not.

I am ignorant and inexperienced, but I could find no missing features, no extra features, and all circuits seem to be connected correctly from baseplate to airhorn.

Givan I have no source for "original" measurements, I figured a comparison with Cliff's "Recipe 1" would be the best shot at tuning.

Please look at the attached comparison and tell me if I need to change anything.  It is going on a stock (mild) 1970 Pontiac 350, Summit 2800 cam, dual exhaust, 71 intake, 3.55 rear, TH350, at 5800' elevation.

The only "big" difference I see is the main air bleed in the airhorn.  Mine is 0.036", Cliff recommends 0.070".  Do I need to open this up?

I did find some debris in the idle circuit and improved the primary blade seating.  The blades now fully cover the transition slots when closed.

I'd like your recommendations before I call Cliff and order gaskets.  If I need to change parts, I'd like to know first.

Thanks for your help,

Mike

[Kenth]

Here is the recipe i have found in several 7029240 units, give or take .001" on measurements.
      Buick 430
Carb #      7029240
jets      70
prim.rods      45B
idle channels       .050
idle tubes      .032
upper idle air      .054
lower idle air      .080
idle air bypass      0
idle discharge      .086
main air horn      .040
main air body      .069
main air nozzle      .054
sec rod / hanger      AY     /     N
sec acc./ pri. acc.      .054  /   .028
sec air tube / Fill      .025  /   .029
idle screw taper       S     
sec air valves      1.30

Using this Buick 430 Qjet on a Pontiac 350 w/2800 cam i would start with open the idle tubes to .035"-.036", use .073" main jets and keep the original main airbleeds.
Usually there is no need to enlarge main jets keeping the original main airbleeds, but due to the smaller/shorter, less exposed, brass nozzles in center booster ring larger jets is required to cure off-idle hesitation and keep up fuel delivery for the Pontiac V8.
Idle bypass air is optional, if more than .040" transfer slots are exposed at proper idle speed, you may add .050"-.080".

Hope this makes some sense.

[Shiny]

Hi Kenth,

Yes, it makes sense and I greatly appreciate your help.

It seems most of my carb is "as new", which is good news for me since the airhorn is not "correct".

Please forgive my inexperience, but:

-  My idle tubes are 0.037".  Is this too much?  Worth dialing back to 0.035"-0.036"?
-  My main air restriction is 0.036".  Worth drilling to 0.040"?
-  My jets are 70, rods are 44B.  Worth changing to 73/45B given I live at 5800' altitude?
-  My secondary rods/hangers are CH/O.  Worth changing to AY/N?  CH is short, AY is long.  Is altitude a consideration?

I think I'm close to feeling OK about this confusion, thanks to you and Cliff.

Mike

[Kenth]

In 1972-74 Pontiac used some Q-jets for "Altitude" usage.
F.e. i have found the main difference between a "Regular" and an "Altitude" 1974 unit is a weaker power piston spring, 7037851 (yellow) in the Alt. units instead of the Reg. 7029529 (purple) and a couple of thousands larger idle tubes, that´s it.

If you put in a weak power piston spring like the 7037851 in your carb as is i think it would work just fine at your altitude.

HTH