I've been considering a multi carb setup on my Jimmy 302. The thought of which carburetor to use, what CFM do i need, how many do I use, how many is too many, is two or three 1bbls better than one or two 2bbls... The list of questions goes on and on...
5 97's on a Howard manifold got shitty mileage but hauls ass. A triple can be set up progressive if you don't use the stock style linkage that pushes up the carb arms. Depending on what your goals are you need matching carbs and 2 or 3 off the latest 270's would be a good start depending on which manifold you use.
I listen to guys like Jon (Carbking) and Dickster here on the HAMB and do what they say. Then I watch for posts of guys having troubles and dont do what they did. It all depends on what you want to do with your ride.
Ideally i want a fun driver. Contemplating building my own intake. I will be splitting my stock exhaust manifold. Although i really do appreciate specific info towards my 302, i was asking in general. Is there a formula or general rule of thumb for multi carb setups? @jimmy six what carbs did the later 270 use?
This subject could easily be the subject for a book! Many individual issues to consider. However, if you are certain you want 3 carburetors, this link might help: http://www.thecarburetorshop.com/Triple1barrels.htm Jon.
A motor that big could easily handle 3 - Holley 94's, its bigger than most flatheads. With progressive linkage it'd be very manageable.
Inline six? I'd like three sidedraught Webers or equivalent Solex/Dellorto etc. of a size appropriate for the engine size (or rather, horsepower). Looks good, sounds good, works well (although get a air flow meter so you can syncronise them yourself). Unless the engine has been extensively tuned you can probably get away with using a relatively small size, meaning you won't have to fight over the same models as the people tuning inline fours.
mild cam, balance the thing, get the ignition perfect, pick the right gears/trans/tire combo for where and how you want to drive along with the carbs/exhaust and you will be fine
I run two, Carter W-1's off 216 cubic inch engines on a 250 inline six. It runs really well and gets great mileage. I had two Rochester Mono-jets, same size as engine came with, that sucked gas but ran great at full throttle, you could watch the fuel gauge move out on the highway! I had three Rochester Model H carburetors off a Corvair for a few years. They could be jetted for power or mileage, but not both. I ran all these combinations as one big carburetor, no progressive linkage. The Carters are by far the best carburetors I have had on the engine. They have multiple step metering rods for tuning, great accelerator pump systems, and were easy to adapt. Look for carburetors off an engine smaller then yours and be sure there are plenty of jets and metering rods available for tuning, you are going to need them! What ever you choose, be sure both or all three are the same number. Rebuild them using dial indicator or calipers so every detail is exact. I used a dial indicator to set the throttle openings just so the linkages were right on the money, same for the float levels. The better you prepare them, the better they will run. Joe
Other than the information I provided in the link in my first post, you might also wish to consider: (1) ALWAYS number 1, the configuration of the cylinder head ports. I am unaware of the port configuration on the GMC 302, but the Chevrolet 235, which is the most common for which we receive question is three Siamese groups of two ports. Because of this configuration, while two carbs can be made to run; three can be made to run better, as you have better cylinder fill density average with each carb feeding one of the Siamese ports. (2) Intake manifold design - is it plenum or individual runner? Individual runner manifolds will require much larger carburetors than a plenum manifold. (3) Will your choice of manifolds fit in your engine compartment? (4) Brand/model of carburetor - this is simple. Pick the brand and model of which you (or your tuner) is most comfortable with modifications. Don't believe that, even if they are picked for you by a professional, that some modification and tuning will not be necessary. (5) How much money you wish to spend. Some carburetors are more in demand (initial greater cost); some carburetors are more difficult to modify, and every modification is going to cost either in money or time. And finally, gasoline engines are quite forgiving. As long as you don't go hog wild in putting too large carburetors on the engine, just about anything can be made to at least run; how well is a different story. Jon.
I could write a book on how many times i screwed up on different multi setups on different engines.. sometimes you get it sometimes you dont until you pull out your hair and have a ah ha moment.
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