The Stovebolt Site Forums The Shop Area Engine Shop A Few More 261 Carb Questions

On another resent thread I left off by needing to double check that my carburetor was the actual size for a 261 and not a 235. Pre68 Dave gave me sizes to check against and heres what I came up with: at the throttle plate I'm getting 1.600" or about 1 5/8" dia while another 235 carb I have measures out right at 1 1/2". So the question here is does that extra 60 thousands make my carb the correct 261 rochester model? 60k doesn't seem like much.

Secondly on an otherwise bone stock 261 will a 2 barrel progessive carb like a Rochester or Weber give me a little more oomph at the top end at my 7,000 ft. plus elevation or would it just be over carbureting the motor

Which Rochester (2GC ? which one)?

Which Weber?

Recent post on this topic.

I am going to try a Holley AA1, using an adapter plate with the original intake. If that is not satisfactory, I'll try a Zenith for a 270 (I am not sure if an adapter plate is needed).

Tim that would have been my next questions, if I go that route I'll need schooling and advice on that! Also I should have said Holley not Rochester but I'm open to any brand that will work and work well.

_Mike-

Progressive two-barrels were designed for high RPM (6000 or more) four cylinder engines. The small primary allows higher air velocity at low RPM for good efficiency; and the secondary allows for more CFM at higher RPM's.

Under no condition will a two-barrel of any kind work better (or even as well) on a one-barrel manifold than the one-barrel of maximum size for the manifold (the hole in the manifold is the limiting factor).

So two important considerations:

(1) Do you plan on 6000 plus RPM with your 261 (progressive 2-barrel)
(2) Do you have a two-barrel manifold for your 261 (any 2-barrel)

Your comment about using a Zenith 1-barrel from a GMC 270 would be a better way to go.

Jon.

The carb you describe must be a Roch. model. model B. The larger bore carb must be from a 261 & should work good on it. I put one onb a 235 one time & it ran like a V-8.

Jon,

No I'm not looking to run it at high RPM and I do not have a two barrel manifold but the hole in the intake is 1/4" bigger in diameter than my current carb and I thought that an adapter would be able to take advantage of that bigger size. Again I'm on the steep end of the learning curve with this but thought two stage would give me good low end performance and economy and a little more help on the top end when it was ocasionally needed.

-Mike-

Mike - if you have a carb from a 270 GMC, try it. If you would like to continue the conversation, give me a call at 573-392-7378 (9-4 Mon-Wed central time).

Jon.

Jon,

Thanks I'll give you a shout. I don't have a 270 carb but I think I should be able to find one.

-Mike-