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Converting to down draft carb on 28 Buick sport roadster?


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I am seriously considering converting to a down draft carb. (I will be retaining all the stock parts to convert it back). I figure I will need a replacement gasket for when I remove and reverse the manifold. What is the recommended carb to go with? The engine is the "standard six".

i'm thinking of using a "choke cable" with a return spring for the throttle. What other parts and bits might you guys who have done this recommend. I am open to all ideas. To counter all the naysayers who are sure to pop up, I'm doing this because I don't like the power i'm getting or should I say lack of power. I've got 3 Model A's that even the weakest of which will out accelerate this buick at any time. Not at all what I was expecting.

 

Rod

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Rod, 

    I am going to give you some options here.  My personal opinion is that the easiest thing to do is to pull the Marvel carburetor and rebuild it.  You likely need to repair or replace the pot metal venturi at the base of the air valve as it grows over time and prevents the air valve from operating properly.  My 1925 Marvel works great and my friends model A which is also in good tune has trouble keeping up with me.    At one time I had thought of converting my car, until I went thru the carburetor and fixed the problems.  Once they were fixed I was glad that I had not spent the energy to do a conversion.     Hugh

 

https://forums.aaca.org/topic/322950-1927-buick-carb-removal/

 

Option 2 is to use a more modern updraft.  You may find this easier to install and connect than a down draft.  

 

https://forums.aaca.org/topic/318382-1925-buick-standard-marvel-carburetor-replacement/?tab=comments#comment-1808063

 

Option 3 is down draft.  If you go this route I suggest it is connected to the gas pedal and not the choke cable.  

 

 

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The ultimate alternative to fixing your Marvel carb is to flip the intake manifold and bolt on a Rochester carburetor from a "Stovebolt Six" GM engine. There are several models of this carburetor with and without automatic choke. If you keep all the original parts, this modification can easily be reversed if you or the next owner prefers to show the car.

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Mark,

Yes, that is what I plan on doing this winter. I am just looking to see if anyone can point me in the direction of a writeup by someone who has done this. I can probably forge ahead on my own as it seems pretty straight forward. Is an adapter needed? How is the throttle linkage managed? An automatic choke would solve the choke cable issue but that's probably not a problem anyway. Which model# carb is generally used?

See, lots of questions.

One more: If I decided to have the Marvel rebuilt, Who is recommended?

 

Rod

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Rod - I would like to offer some information:

 

Using the three options offered by Hubert:

 

(1) Rebuild the Marvel. In 1928, Marvel was transitioning from a brass bowl to a zinc alloy (pot metal) bowl. If yours is the zinc alloy, either find one of the brass bowl versions to rebuild, or bypass this option. If you choose this option, the brass bowl carbs have the same casting number (10 over 103) as your existing carburetor.

 

(2) Replacement of the Marvel with a more modern (read better ;) ) updraft. The listed reference thread is for a 1925 Buick. By 1926, Marvel had changed to a proprietary mounting flange. There are no modern carbs (maybe no other carbs) that have that mounting flange. Thus one would require machining of an adapter to adapt the modern updraft to the Buick intake. This will lower the carburetor by the thickness of the flange. The lower the carburetor, the more negative pressure must be created by piston movement to allow fuel flow from the carburetor to the engine.

 

(3) Downdraft - Mark, I rarely disagree with you on anything, but in this case I must do so, with apologies. The Rochester B carburetor, if anything, is more problematic and less reliable than the original Marvel! The only thing the B has going for it is price! We have been trying to GIVE them away for 15 years!!! They are quite common, as most of the Stovebolt folks (not all) have given up on them and replaced them with Carters. They have major body warpage issues, in a couple of areas, either of which will cause the power valve to be open at all times.

 

Reference:

 

Free carburetors

 

The 1928 Buick Standard Six is 207 CID, so the sizing of the Rochester B for the Chevy 216 is correct; but much better (opinion) carburetors are available. My choice would be the Carter W-1 tag numbers 574s, 483s, or 420s. These were all used as original equipment on the Chevrolet 6 in various years. They are relatively common, and ALL internal parts are readily available. Other possibilities would include aftermarket replacement carburetors produced by Carter (YF), Stromberg (BXOV series), and Zenith (28, 228 series) specifically as replacement for the Chevrolet 216.

 

Jon.

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Rod,

Here is a really novel idea.

Since you appear to be a mechanic, take a 90 degree bend in the right diameter, weld on two flanges, and bolt on an old S.U. carb from and old Jaguar.

That is the closest thing to fuel injection you can get, and would really confuse anyone who looked under the hood.

Plus they polish up nicely.

 

I used to put them on old Harley's and they really woke up those old knuckleheads.

 

Mike in Colorado

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On 10/9/2020 at 9:45 AM, carbking said:

Downdraft - Mark, I rarely disagree with you on anything, but in this case I must do so, with apologies.

No apology necessary. 

I wrote that as part of an article for the Prewar Buick Div. newsletter many years ago for several members who had given up on the original Marvel carb. and wanted a cheap alternative.  I only suggested the Rochester because it was sized about right.

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ok guys let me see if I have this conversion to a down draft carburetor correct (please see photo for reference).

 

  1. I remove the two bolts as indicated by the two upper red lines, as well as whatever is holding on the exhaust heat mechanism? This should remove everything below the short red line.
  2. Now remove the intake manifold, reverse it and with a new gasket, re-install it.
  3. Install an appropriate carb (Would this work?).
  4. Figure out throttle and choke linkage.
  5. Start the car🤣

Rod

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Something very important I did not see mentioned above is fuel pump.  An electric fuel pump will be needed.

The original vacuum powered fuel pump is the correct height to supply the original Marvel carburetor and will not supply a downdraft mounted 10 inches higher.   A clamp will be needed to mount the horn to the manifold or else find another place to hang it. 

I mounted a single barrel Rochester carb of late 50's vintage GM pickup (which used a manual choke) to eliminate one variable for the first startup of my Master motor.  Intake manifold is flipped, blanking plate mounted over the exhaust port for the heat riser tube, and the bolt pattern of intake and Rochester were identical but it rotated the carb by 45 degrees which might complicate a linkage for permanent installation. I still intend to try getting the Marvel to work once the car progresses a little more.  A cable for the choke would be easy to get set up as the Rochester was designed for cable operated choke.

Kevin

rochester carb.jpg

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kevin

Thanks for the response and photo, It's just what I was looking for.

The car has an electric fuel pump with pressure regulator set to 1 psi and works great with the Marvel. I suspect that I will have the same 45 degree problem  and will have to work out a solution for the throttle (Lokar with return spring?).

 Where did you get your intake manifold gasket? I looked on 'Bobs" but nothing on there web site.

Would you have a photo showing the exhaust blanking plate?

 

Thanks again

Rod

 

 

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Rod, 

   You pointed out the 2 bolts on top of the heat riser, but there are 2 bolts on the back side as well.    Once you remove the 4 bolts, the heat riser and everything below will come loose.   Hugh

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Blanking plate is circled. This covers up the exhaust flange that mates up to the upper  heat riser that Hugh pointed out.

I see what you mean about Bob's.  Manifold gaskets are not listed as separate items but are sold as part of $$ full gasket set.

Should be some other sources that will sell manifold gasket for Buick. 

I can recall my dad telling me that Model A manifold gaskets had the same hole spacing as the 26 Buick Standard engine that he worked on.  Distance between holes measures around 3-3/16. Check and see if that will fit. 

You may still need to fabricate an adapter to mate a Rochester carb up to the intake manifold. Mine is a Master which has a larger intake bore and probably wider bolt pattern, your Standard might have 1/8 to 1/4 narrower bolt pattern.   I also was a bit uneasy with the Rochester fuel inlet fitting located directly over the exhaust manifold.  If I was to continue using this carb I would see if linkage could work after rotating it clockwise enough to get the fuel fitting off to the side where a drop of gasoline would fall to the ground instead of a hot manifold. 

Kevin

 

blanking plate.jpg

standard gasket set.jpg

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Kevin brings up one of the advantages of the updraft carburetor.   With the engine pans in place, any leaking fuel is directed by the drain hole to not drip on the exhaust pipe.  The exhaust pipe  is significantly cooler than the exhaust manifold.  If the downdraft carburetor floods over.   hmm   

Olsons gaskets will sell the individual gaskets.    Hugh

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Hugh:

 

This is what a fire, caused by gas falling on the exhaust pipe, looks like.  It was caused by the Dole Primer pump's connection to the gas line leaking out just above the exhaust pipe.  Managed to get fire out before it destroyed the car.

 

Bill McLaughlin

1929 McLaughlin Buick Roadster

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Bill, 

    You are very fortunate that the fire was caught in time.   One other note I would like to make about what Buick did to minimize the event of a fire.  The original mufflers were against the inner frame on the left side of the car.  The exhaust down pipe did not route straight down and then back.  The down pipe was rotated in the flange by a few degrees to get the exhaust left to within about 5" from the frame.  Where I have labeled "bend" is where the pipe actually runs parallel to the frame.   This "jog" in the piping was to keep the exhaust pipe from running under the carburetor.    Most of our exhaust systems have been replaced.  These may not have been configured correctly.   If your down pipe runs straight back under the carburetor. a modification to your exhaust system is the cheapest insurance you will ever buy.   Buick owners should check that they have this critical bend in the exhaust system to prevent a fire. 

 

One other note, the down pipe also has a long radius bend.  This is to facilitate fitting the engine pans.  Most exhaust system people do not have the bending dies to make this type of bend, and this section likely needs to be a section that is mig or tig welded in.  Having the engine pans would help prevent a fire as well.      Hugh  

 

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I will testify to the need to check the correct routing of that down pipe! The cobbled mess they had on my 1925-25 had the drain hole directly above the pipe. DSCF7062.thumb.JPG.e56c8c805123d3b657ef9023291ae3c1.JPGDSCF7065.thumb.JPG.04f0c60e558ee828b1e4a89268ad4372.JPGDSCF7061.thumb.JPG.39b3591bb4158e3f0aaf96b4e89efd36.JPGWhat was on my car. You can see the orange stain on the pipe where the gas had been leaking. 

Thanks again to Hugh for the above illustrations so I could get the correct configuration for my installation. 

 

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Great information in this thread, thanks to all who have contributed so far.

if an adapter was fabricated to turn the carburetor 45 degrees so that the linkage would be better accommodated, the carburetors

i looked at all have the fuel inlet on the back side. To reduce the danger of fire from fuel leakage you could use a deflector to route any dripping fuel away from the manifold and down pipe, i think i would also sandwich some insulation wrap between the deflector and the manifold. 

I have checked the exhaust "down pipe" and it is not in the path of any possible dripping fuel.

Based on what I have garnered from this thread, I am going to proceed with mounting a downdraft carb on the car. I will keep a detailed record both written and pictorial and then post it. I intend to use this carb  https://newcarburetors.com/?product=y200-universal-carburetor

and a gasket from Olsons gaskets if the model A's don't work, I have a bunch of those lying around here to try. (thanks for the tips Hugh and Kevin).

I'm sure I will run into issues and will need the expertise of this group to guide me.

Thanks,

Rod

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14 minutes ago, carbking said:

Since you have made your decision, no recommendation from me is necessary.

 

Jon.

 

Jon,

Nothing is set in concrete, and until I actually make the purchase, I am open to any and all recommendations.

Rod

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Rod - in post 5, I suggested several different Carter, Stromberg, and Zenith carburetors.

 

ALL of these:

 

Used carbs are readily available (read inexpensive). Rebuilts, and even new old stock (mostly) are not overly scarce.

Were made in the USA.

Use S.A.E. sizes.

Calibration specifications are readily available

Spare parts (kits, calibration parts, even to a certain extent, castings) are readily available.

Each are calibrated specifically for a 216 CID 6 cylinder engine. The calibration includes venturi size (air flow), jetting (metered fuel), and air bleeds (metered air).

 

As to what I have available? That is best done off the board. My answers are to share information gleaned over a few years (rebuilt my first carburetor in 1959), NOT to advertise my business. I gave you actual part numbers, which you may shop around. The only advertising is in my signature block.

 

Jon.

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Here are a few pictures of a car I'm working on. I flipped the intake and set up the carburetor linkage. This is a popular way of setting up the linkage. The carb is a Carter BB1 bolted directly to the manifold without an adapter. I picked up the air filter at Hershey last year. I think it is a Plymouth or a Chevy. If anyone recognizes it please chime in.

 

 

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14 hours ago, Dave_B said:

Here are a few pictures of a car I'm working on. I flipped the intake and set up the carburetor linkage. This is a popular way of setting up the linkage. The carb is a Carter BB1 bolted directly to the manifold without an adapter. I picked up the air filter at Hershey last year. I think it is a Plymouth or a Chevy. If anyone recognizes it please chime in.

 

 

Dave B, Thanks for the great photos. Do you have one showing the bracket I circled, from a different angle that would show how the 'post" supportig the swivel is attached? I'm assuming it has some onnectio to the angle bracket attached to the blanking plate?

These photos are very helpful, just priceless for anyone contemplting this modification. Many thanks.

 

14 hours ago, Dave_B said:

 

 

InkedIMG_4537.JPG.c118667c367395cc51d6df5d69b5c676_LI.jpg

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After studying Daves photos of the blanking plate and the bracket attached to it, I think it might be possible to just fabricate a longer blanking plate with a "bell crank" in place of the angle bracket. Dave, thanks again for the photos they certainly are a huge aid in figuring out how to set his up. i've got a 1941-1948 Chevrolet Carter W1 Carburetor on order and will probably make up an adapter to swing it 45 degrees, though leaving it is also a possibility.

 I never would have thought to use the blanking plate for support of a bell crank without those photos!!!!😀

 

Rod

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25 minutes ago, Dave_B said:

Rod,

 

Here is someone else's idea. Maybe this is what you're thinking?

 

Dave

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Dave, yes that is along the same lines. I just ordered a 3/8" x  8" x 10" piece of solid aluminum which should do the trick.

I've already reversed the Manifold (very easy and straight forward). It looks like the horn can use the same mounting holes in the manifold but with thinner and longer thru bolts and I may have to reverse the bracket on the horn to mount the carb (still waiting for the carb to arrive).

 

Rod

 

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