Hello, I am close to done gathering parts for my Speedster Project. Will be beginning tear down of the Doner during Christmas Week, (I get the week off) In my planning I envision a vintage looking dual carb setup. I have a few options, but will be running a stock motor. I am planning duel carbs and a header with dual exhaust just to make it breath a little better. It will be an open side hood, so the dual carbs will be visible. Anyway, I have several Zenith Carbs in stock for 8N Ford Tractors. Since the 8N is just over half of the displacement of a stock Model A engine it seems to me that they would be a good candidate for a dual carb updraft setup. I welcome thoughts.
BTT, 108 good folks read this, and no one has any thoughts? I cannot believe I am the first one to think of putting tractor carbs on a street car.
Will they bolt to your intake? carbs for all intents and purposes are determined by CFM rating and although your logic is good in real world applications it does not always work as planned.
Your logic makes sense.... go for it, try it out, that's hot rodding !! (And report back on the result ....)
Those carbs are one step above an old Briggs & Stratton lawn mower carburetor. A**** other things, they don't have an accelerator pump. Driveability on a car would not be what most would want. The only fix would be to set the carb up to run on the rich side all the time. Not saying it can't be done, just pointing out it's not an outstanding alternative. As to the carb size; on an inline four the cylinders pull on the carb one after the other. There is no overlap between intake strokes. As a result, a carb size that's correct for one cylinder is the same size that's needed to supply all four cylinders. It also means the carb flow CFM needs to match the engines CFM flow requirements. But, if individual carbs are put on each cylinder or pair of cylinders, the carb flow needs to match the peak flow of each cylinder. That means each cylinder needs the same size carb the the entire engine would need it it had a common manifold. So, if you have two carbs that equal the flow of one bigger one, putting one of those smaller carbs on each siamese port will supply only half of the carb flow each cylinder actually needs. Putting two of the smaller carbs on each port, or connecting all ports and two of the smaller carbs on a common manifold, is what you would need to do. What I just described is why an engine that has Weber carbs with individual barrels for each cylinder might have 3,000+ CFM of total flow capability, but effectively be doing what a 750 CFM 4-barrel provides on a divided 180 degree dual plane intake manifold. Although horsepower is a good indicator of flow requirement, as Porky indicated, different applications, different engines, and different manifold designs, all effect an engine's ideal carb size.
I'm doing a pair of zeniths' on a chevy 235 inline 6cyl. These have replaceable venturii so the cfm rating can be changed and externally adjustable main jet - is it the same with yours? These have accelerator pumps.
Thank You for the replies, The 8N Zeniths do not have an accelerator pump, true. But neither does the existing single Zenith that Henry installed. If I did this I would build a log updraft manifold, and run both carbs. One in between each set of cylinders. I have no idea what the CFM rating of the 8N Zenith is. Nor do I know what the CFM rating is on the Model A Carbs. I have been searching but not able to find the data. I resort to the gut feeling of the 8N being half the displacement of the A engine. It just seems right. Any other thoughts feel free to share.
I am pretty sure they do not. I will check though, good point. Tractor Carbs are pretty basic. They do however have easy to adjust main jet and idle jets
You dont really need dual carbs unless you're going to bump the compression, change the cam, etc to up the power output. Going to two smaller carbs that would equal the one stock one that you started with seems to be an awful lot of trouble for no more power.
Rex, Let me preface what I have to say with it is no reflextion on the OP or his intent with the engine in question. This is just a general observation. More often than not multiple carbs have nothing to do with performance and everything to do with eye appeal. A car that looks the part. Using the type of car in question as an example, thow things are important here, one is that the zenioth is a nice looking period type of carb and the other is that multiple carbs are what we sould think of when someone mentions speedster. So with 2 Zenith carbs a car of this type would look the part. Most cars are built with looks in mind first and performance is secondary. Like I said this is no reflection on '68 C20 whatsoever it is just a general observation. I must say that using myself as an example I would dealry love to build a rocket powered '34 coupe, complete with a brace of the bent over oldsmobile carbs. I do know that the same engine could be made to perform way better with a pair of 4 bbls or even a single 4 bbl but it would just be cool with the obsolete bent over carbs.
I'm not disagreeing with you...it just seems to me if you were going to this much effort you'd use the "A" carbs or maybe "B" carbs and just limit the throttle travel....that way later on you could change the cam and heads and pull out the stops and be set to go faster w/o re-engineering the carb setup.
Porkn******, you are exactly right on the mark. As I said in my first post, the Carbs would be visible as the side of the hood will be open. Lets face it, there are damn few Model A's with stock motors that are "performers" and I am not expecting this one to be any different. I am also thinking though that their would be some gain in that the motor would breath a bit easier with dual carbs and dual exhaust through cast iron headers. But lets face it, it is mostly the cool factor of showing it off, which (IMWO) is the reason most of us do anything with cars. But it will take some time to build a good looking log intake, I am not new to the process, but if there is a reason not to try it with these carbs that I have not thought of, then I don't want to waste my time. I am happy to break new ground, but would prefer to tread on the path blazed by others who know more than me.
Ford did not use Zenith carbs on the 8N, so whatever you have is going to be a modern replacement. This may (or may not, depending on your idea of vintage) negate the concept of vintage carbs. Most of the carbs designed for tractors and/or industrial applications do not have an accelerator pump (some do). This probably will not be an issue with the low compression Ford A engine. After all, more than a few model A's were sold, and none came from the factory with an accelerator pump. Will they be large enough?? Don't know, but probably not. CFM in a multiple carburetor application is NOT additive! When selecting O.E.M. style carbs for a dual carb setup for basically stock engines, we suggest that each carburetor comes from an engine that is from 3/4 to 7/8 the displacement of the new engine and exactly the same by identification number. No, it is not a hard and fast rule, but it generally works pretty well. As others have posted in this thread, to know for sure you will have to try it; but I do believe the mentioned carbs will be too small for the application. I would agree with Rex about using original Ford A Zenith carbs. They are readily available, inexpensive, and (if you ask) genuine USA-made parts are readily available (as are some import parts). And ******, if the desire ever overwhelms you on your Olds, look for the Carter carbs rather than the Rochesters! Jon.
Good observation, and on the mark, but to me it is overshadowed by the desire to do something a little different. Of course if the Carb flange mount on the 8N is the same size as the flange on the "A" carb, then I could kill both birds at once. Try the 8N's if they they don't work, go to twin B's and limit the throttle travel. I will do some measuring today, I will have to pull the carb on the "A", unless someone here happens to know the key measurements of the flange?....
You are right of course, the factory used Marvin Schebler's. Zenith made a replacement carb and that is what I have. Either the Zeniths or the Marvin Shebler's have, in my opinion, the right "look". Thank you for the informative reply, I guess I have some more research to do. Feel free to point me in the right direction. Thx.
If you are happy with the look, try it However, the original Ford A carbs were cast iron, as were the original Marvel Schebler TSX series used on the Ford 8N. The replacements are probably diecast. The mounting bolts on both the Ford A and the Ford 8N carbs are 2 7/16 inches center to center. One thing to consider in the fabrication of your manifold: make the height as short as possible. When using updraft carburetors, initial starting requires engine vacuum to pull the air/fuel mixture against gravity. This means one always uses a choke to increase the air velocity in the carburetor for starting on an updraft setup. With multiple carburetors, the air volume supplied by each carb will be reduced by one half, which means the velocity will also be decreased. Jon.
****** - there is a reason the Rochesters are easier to come by. Think of the old saying about voting in Chicago "vote early, vote often" In the case of the Rochester AA series it is "fail early, fail often". The best two things I can say about the Rochester AA series are: (1) Rochester killed them after only 2 years, and they were only used by Olds. (2) As far as I am aware, we are the only source for rebuilding kits Jon.
Thank you for this note, I have a couple TSX's perhaps I will start with them. Iff'n I end up ****ing wind then no loss to convert over. Thank you for the note on the short manifold as well. Makes good sense.
Thanks for posting this info, it should come in handy down the road. Good Luck with building your manifold, '68 C20.
Live and learn time, yet again. I never had this occur to me, and I now understand why I never could get a progressive secondary 2305 Holley to work right on stock manifold on a 2.3L Ford Ranger truck back in the 80's. Damn! If I'd only had the internet back then............... Tom S. in Tn.
If I follow the logic from CutawayAl, then i wonder, If the Carb is sized to one cylinder, then would i run two stock Model A Carbs and leave the intake open in between them so as to equalize them, or seal the manifold so each Carbs can only feed two cylinders?
All the ones I have ever seen left them all tied together, whether they were updraft or downdraft. Most also used that center section tied to the exhaust to help with pre-heating the mixture also. There is a guy, Purdy Swoft, over on the Fordbarn that has posted some pics of a manifold similar to the one you are planning that was a cast iron piece made years ago. Not sure if he is here on the HAMB or not.
If you run two "A" carbs, putting one of those on each port would be the same as feeding all four cylinders with just one of those carbs. Connecting the two port feeds with a balance tube would, in theory, be the same as using a carb twice the size the engine originally had. However, balance tubes don't always behave as one would expect. They usually flow well under the amount you would anticipate, and sometimes they cause weird tuning issues. All you can do it try it and see what happens. I suspect the original "A" carb was smaller than needed for max power, so upping the carb CFMs might be a good thing.
Thank you for the reply, I understand a little more now than I did. I am sure I will be back with more questions. I think I will build the manifold, load it with two 8N Carbs and see what happens. Since the 8N Carbs use the same flange size as the "A" Zeniths then the experiment is really a no brainier. To be continued...
Al your logic is spot on. The effectivness of a ballance tube is dependent on the size and length of the tube. A ballance tube is not intended to take the place of a common plenum it is just there to equalize the pressure between two or multiple plenums. All it is for is to make ballancing the carbs a little easier. If I am going to run one progressive I run a common plenum for obvious reasons, if I am not I run my plenums seperate and a ballance tube between each. If I have large plenums I use either multple ballance tubes. If I have more then two plenums I daisey chain them.
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