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Converting to down draft carb on 28 Buick sport roadster?


Rod L
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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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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

Edited by Oregon Desert model 45
update (see edit history)
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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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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  

 

918062583_1925BuickStandardexhaustpipedrawing.thumb.JPG.2ef61edcaaa6b9100fa7a8d85a90cb3c.JPG

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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

100_2163.JPG

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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Is there supposed to be a gasket between the blanking plate and the heat riser? There wasn't one when I removed the heat riser. I suppose a little high temp exhaust silicone would suffice?

 I now have the 3/8th aluminum plate, "bell crank" and an assortment of bolts to start fabricating the throttle linkage support. The carburetor is arriving tomorrow, so I expect to have the engine back up and running by Monday if the creek don't rise and the wife  doesn't have too many honey-do's.

 

 

DSCN5828.JPG

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Got it installed and it runs.

Using a re-manufactured 1941-1948 Chevrolet Carter W1 Carburetor on top of a reversed existing intake manifold and a fabricated throttle support made from 3/8" solid aluminum, a Bell crank,  brass all thread rod and  Mr. Gasket 3811G Carb Link Ball Joint Quick Release fittings, the whole thing came together in about 3 hours. The hardest part was cutting the aluminum support plate (I needed extra space to get behind it to get the nuts on it).

 I haven't had a chance to drive it yet, but it started right up, idles super smooth and rev's up as expected. I still have to get a choke cable for it, but starts up easily without it (inside heated garage though). The bell crank needed to have the holes enlarged to accommodate the linkage fittings but works fine now. With the bell crank and the threaded rod and fittings there are endless adjustments to the linkage.

I did not use an adapter so the carb sits on a 45* angle from the manifold, it did not pose  a problem as the linkage

i'm using is very much adjustable and swivels nicely, also, with that 45* turn it swings the fuel intake further from the exhaust manifold reducing the likely hood of a fuel leak causing problems (I'm still going to put a deflector in that location).

 

 I am very pleased so far. Here's a breakdown on costs as of this date:

bell crank...$ 34.54   threaded rod and quick release fittings $ 36.44   aluminum plate $ 19.48  carburetor $ 395.00 Choke cable 20.95

Total: $506.11

 

Here's some photos: (more to come later).

 

RodDSCN5853.JPG.1a34c7581de70764fbbaa3b8fc992364.JPGDSCN5853.JPG.1a34c7581de70764fbbaa3b8fc992364.JPG

 

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3 hours ago, Dave_B said:

Lookin' good. So how does it run?

 

Dave

Well Dave, as i mentioned in the above post, it started right up and the idle is near perfect plus it revs  up (while stationary) very nicely. However, I got it out of the garage for a test drive and it was anything but perfect.  Missing, no power and the occasional exhaust backfire. I found that closing the choke to about a 1/4 opening results in some improvement so I'm thinking i have a vacuum leak causing a lean condition. When I reversed the manifold, the gasket looked good so I didn't replace it (mistake?) I sprayed starter fluid around it and thought I detected an rpm increase (if so, it was very small).

 I only see one external adjustment on this Carter W1 carb and I think that is for idle? The carb is rebuilt and supposedly test run on an engine and adjusted? Guess I'll start over!

 

Rod

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12 hours ago, Oldtech said:

Metering rod. wrong or not adjusted?

 

OK, suppose I take care of the vacuum leaks (gaskets are on the way) and that doesn't fix the problem, how do I  go about checking/adjusting the "Metering rod"? Is there a book or other description of the procedure?

 

Rod

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Well... Proper procedure requires a gage that would come in a carb kit. However, for diagnostic purposes you can try raising the metering rod slightly by bending the L shaped connector rod at the bottom of the L. 

 

I don't know how far you want to get into the carb but if you do, be careful with the metering rod, it is easy to not get it in the jet correctly and break off the small tip.  A Motors or Chilton manual for the late 40's should have instructions. 

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This is the classic reason for keeping cars stock, or nearly so. Not discussed was the fact the the original carburetors of the era were also a rev limiting restriction. If you over carburetor a early car, you will probably scatter the engine, or burn a hole in a piston. I have seen it countless times. Carbking offered advice.......take it. Looking at all the modifications for linkage.........poorly done. A Buick was a GREAT car when new. Not one issue with reliability or performance. If your Buick wasn’t preforming correctly.........just jam on a down draft carb? I understand some pot metal carbs are junk, but staying with a similar design or a correct aftermarket carb is what should be done. Do you have a five gas exhaust analyzer? An oscilloscope to check the ignition? Stoichiometry is the key to getting the car right.........,and if you can’t understand it, you shouldn’t be trying to modify the car. A early car is what it is.......it has limits, and trying to get it to push past them is an accident waiting to happen. You have brakes and steering from 1928, don’t try and make it run like it’s a 1958. 

Edited by edinmass (see edit history)
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Ed:

 I agree as I have seen this flipped manifold "FIX" on many 1920s Buicks. I see it as an expedient from frustrations as you outlined. On my 1925 Buicks I have spent much time trying to dial in the Marvel cabs.

 As noted in other Marvel posts the most offensive feature was the (at the time) the heat system to allow the low grade fuel to vaporize. The pressed in sheet metal tube in the heat riser was prone to rusting out. A popular aftermarket replacement item when these failed in the 1930s. Just a pinhole makes the car a sluggish mess. I do believe the unit can be made to perform better. But the massive reengineering involved with the downdraft bothered me to do it on my cars. Also the other issue for me was the electric fuel pump. I had one on my 1925 Standard and the pressure regulator could never be dialed in low enough not to provide a flooded condition. (they also had an incorrectly bent exhaust down pipe with the gasoline to pour from the carb on to the hot pipe) All this done by a previous owner to avoid rebuilding the vacuum tank. If all components are properly rebuilt and set up in unison all should be well!

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As to the last two responses, Edinmass & dibarlo, I didn't ask for a critique! Neither of you gentlemen have driven the vehicle in question and have no idea as to the nature of its power, I am not trying to hot rod or even make a "1958" out of it. I have retained the complete Marvel system for the next owner should they chose to use it. The poster prior to my last post, suggested a possible adjustment to the carb I am using, I was merely seeking clarification for his suggestion and NOT someone else's opinion  as to the merits of what I decided to do. If you don't have a directly related answer to the question asked, you are just making noise.

Rod

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Rod.......after you have fifty years of experience in the hobby, I will consider your opinion. You come here, looking for help, and want to make your car more drivable. What you don't realize is that the chances of you damaging the car is high, and getting it to run right are very slim. It’s your car, do to it as you please. Carbking is one of the top experts in the world on carburetors........and you don’t take his advice.........enough said. Everyone here was offering help, just help that you don’t want to take. Funny, I have driven countless miles in pre war early cars............and I don’t have any cars modified from my Ford T to my Pierce 12 and everything in between. My noise......is my car driving correctly, without any problem, and enjoying myself. Feel free to hack your car, after all it’s yours. As for me, I’ll just go jump in a car and take a drive. Have a great weekend.

B2BEFF20-C63D-4880-9241-B522CD8B7289.jpeg

Edited by edinmass (see edit history)
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Ed,

Wow, aren't we spicy this morning! If you were to have read all the posts in this thread you would be aware that in my 2nd post I asked for a recommendation for a re builder of the marvel should I decide to go that route, you would then be aware that NO such recommendations arrived! I then proceeded down the down draft trail. I also did follow Carbking's advice and used the Carter w1 carburetor (the one I am seeking assistance with. Also being 78 years old and while not an expert on these cars, I have modest mechanical abilities and have been at this awhile.. As for all this help I'm being offered, could you point me too it? Only "Oldtech" has answered my question.

 

 

 

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