The Stovebolt Site Forums The Shop Area Engine Shop How can identify the size of my holley 2brl carb??

I am rebuilding a GMC 302, and will keep it stock with one exception. I am building custom tubular headers, and duel exhaust. The carb that was on the motor is a holley 2 barrel. I was thinking of looking for a Mcgurk or CLifford triple carb intake setup. If I go this route, what size carbs should I be looking for? Or, should I go with a single carb, 4 barrel say 500- 600 CFM??

3 carbs look sharp, sometimes the inner carb fouls the fender brace rod.A single 4bbl is easier to do.If your engine is modified ,about 500-600 CMF should be ok for street use.Keeping thing equal,rating all carbs for flow like 4bbls, the Holley Weber 2bbls flow 200cfm each, Rochester and similar sized 1 bbls about 125-150 cfm.Your Holley is the original style carb?

Tony,
Yes the holley 2 barrels are original style, in fact I now have two of them. My intake manifold is no good, and was thinking of fabricating a new manifold. Not sure if I should make it out of Stainless or aluminum. I would prefer stainless since I will have polished stainless custom headers and duel 2" exhaust. Or, if I buy, who sells nice quality intake manifolds for single or triple carbs?? SOunds like I could use three of the holley two barrels (200 cfm's each) for the triple if I go that route??

I have seen 3 of those Holley Webers on several inline engines, the owners claimed they worked quite well.Personally, I don't care for the Holley Webers,but they are the right size and available for less than silly money.Also seen 600 cfm Holleys and Edelbrocks on big Jimmy's, Sixpack, who posts here, has such a setup.You'll supposedly get better mixture distribution from a multi carb set up VS a single 4 bbl., but from my experience on street engines,the advantage of multi carbs is just bench racing stories.
If you want to buy an intake, check around on the Inliners site for info,most GMC stuff is kinda rare.Building your own is a great idea,with 3 carbs relatively close to the intake ports, a heated manifold is probably not necessary unless you plan to drive in real cold weather.Another carb to consider is the Holley single barrel 1900 series as found on 6 cylinder Mopar and Fomoco engines.Or the Holley 2bbl "Bug spray", designed for WV's, they are on Ebay, new ones for reasonable money.Another choice is the common two barrel 2300 holley, available in 350 and 500 cfm ratings, which flow around 250 and 330 cfm when rated like four barrels.Many of these carbs will require some tuning to set up properly and linkage to be fabricated.

Tony, Thanks again for the great info. You seemed to like the fact that I can fabricate a new 3-carb intake. You mentioned, keep the manifold close to the intake port, and heat may not be needed. I dont plan on driving it in the winter, but if the fall is a great time to drive, and could still be a little cool since I live in CT. If I fab up an intake, for three carbs, should I make one long manifold with risers for the carbs, or should I make individual intakes for each carb, therefore not connecting all three, but keeping them individual? I have not seen anyone doing this, must be a good reason. If I do fab the manifolds, what's your opinion on the material choice, IE stainless or aluminum? I prefer stainless since it cleans and polished up nicely. ONe last question, do you think I will get enough heat transfering from the headers, to help pre-heat the intakes without having to rout hot air, or coolant.

On a street engine, a balance tube between each carb is necessary for smooth lower speed running.You can make 3 individual small manifolds and hook them together with like one inch hose or you can fab up a log type manifold. Stainless will probably be easier to work with, the tubing is readily available.Keeping the runners short generally aids power and response in the 1500-3000 rpm range, where a street engine operates most of the time.In addition the short runners have better airflow,keeping the fuel air mix atomized, especially if you don't heat the manifold.I think heat won't be necessary for your situation, and if you need heat, a simple plenum sheet metal cover between the intake and exhaust will help heat the intake.I saw the sheet metal baffle on the same 261 Chevy engine with the 3 Holley Webers.
You also might consider other carburetors,the Holley Webers are butt ugly looking. I like the idea of side draft carbs, saw a GMC with 3 Jaguar SU's , ran very nice,looked fantastic, but clearance can be problem.There's always the DCOE Webers if you got a large wallet or my favorite, 3 Harley S&S carbs, which can be had for about 50 bucks each at swap meets.
I do know the Holley Webers will work fine without a lot of tuning, the other carbs may require a lot of messing with jetting.
What ever you decide on ,make sure the carbs clear the fender braces or anything else in the way, good luck, let us know how you make out.

I am thinking of using a tight bent tube, to come off of the face of the head, and up 90 deg. Then tig weld on a mounting flange, and transistion the flange to the tube for proper flow. I like the side-draft carb idea, do you konw of anyone that has done that that I can contact. I manage a precision metal fab shop, so making the difficult, will be fine with me since we bend tubes, sheetmetal, wire, cnc machining etc. Regarding the crossover tube, should that connectced to the tube, or to the carb mounting plate? Lastly, you refer to re-jetting carbs. How is this done? Is this something that I could do, or would I have to have an expert do this? I am really not familiar with any carb modifications, but I am concerned that this may limit which direction to go in, if it's going to not be feasible.

Formula1

Slightly off topic, but I just stuck a rebuilt 302 in my 51 GMC 1-ton. It has a 2 bbl that I assume is a Holley but don't know for sure. The carb sits directly on the manifold, on a thin gasket. Do you know whether a carborator spacer (insulator) is necessary? Will I vapor lock or boil the gas out of the bowl without it?

Thanks

You can look up the factory as-built specs on any Holley carb at this link:

http://www.holley.com/HiOctn/ProdLine/CarbList.pdf

You'll need the "List" number of the carb. On 4-bb's it's stamped into the front of the choke air horn. It's usually a four or five digit number, sometimes with a dash number behind it. And a "0-" to the front and look it up at the above link.

Example, a 4777-2 would be a 0-4777-2 on the list. It shows to be a 650cfm 4150 series double pumper. Note the column that says secondary nozzle size or spring color. A secondary nozzle is a double pumper squirter. A color here denotes which vacuum secondary spring is used. Just thought I'd toss that in for the 4-bbl guys.

This is a very handy tool for those buying used Holley carbs. You can "reset" the carb back to factory sets and start from a known baseline when initially tuning them.

Formula,most balance tubes are located nearer the carb that the head, but I don't think the location is as important as as the tube itself.
Carb jetting is a trial and error affair,requires some knowledge of how the strength of the mixture affects driveability, and yes it's best to start with the stock jetting and go from there.For instance if you need to lean out the part throttle mixture, you change the main jets down a few sizes until the engine stumbles slightly on light acceleration, then go up a size.But many guys forget that also changes the full throttle mixture,so now you may have to increase the restriction size on the so called power valve which provides the necessary richer mixture for high load or wide open throttle.Generally, on a multi carb set up, if using carbs originally designed for a single carb intake, you will have to lean down the power valve since now you have 3 instead of one high load enrichening circuits.The Holley Weber was designed to enrichen a 120 cubic inch engine,3 of them on a 302 cube engine may work out fine with no jetting changes. You might want to get some carburetor books,Jeff Williams has some Holley manuals,you can pick them up used on Half Dot Com or buy them new.
I have a number of Holley spec books,listing the jet numbers and such. When you decide on a carb,email me and I can give you the info.