The Stovebolt Site Forums The Shop Area Engine Shop main jet for a 261

Have a fresh 261 strictly stock w/ Rochester 1 bbl carb in a 37 2 dr sedan. Going down the highway @ 50 it seems to hesitate or falter until I pull the choke out about 1/4, then things smooth out very nicely. I'm thinking its a jet prob. Had the carb rebuilt by a pro and he said the marks on the old jet was obliterated so he put in an .053? jet which is giving me the rough run. Had somewhat the same performance issue before I sent the carb out. Does anyone know what the jet specs are for a 261 or what works. Thanks for your response.

Very good chance the original Rochester 'B' was replaced with
one from a 235. This in combination with today's gas formulation
could be the source of your problem. This was a common
replacement because they bolt right on and look the same
It should be a 7005140 stamped 5140 on the lip of the
throttle body flange.

dg

That could also be too much spark.

Two thoughts:

(1) A "pro" would NEVER pay any attention to the markings on the jet without actually checking the orifice size; as many enthusiasts drill out the jets. Nor would he/she consider using the old jet. A "pro" would determine the correct jet for the carb and install a new one.

(2) The original jet in the Rochester B used from 1954~1959 on the 261 was 0.064, not 0.053. There is a "wee" bit of difference However, reread Denny's post. A 64 in a 235 carb is too much. So if the carb is a 235, and you failed to tell him it was for a 261, he/she may have done the correct thing.

Jon.

One other thing he could check Jon, is the throttle body inside
diameter. 1.560" on the 235 and 1.680" on the 261. Venturi is
would be hard to check with the carb assembled but it's 1.340"
on the 235 and 1.460" on the 7005140 261 Rochester.

dg

You can see the differences pretty clearly with it assembled. There are three indications. The actual 261 carb is pretty rare.

http://devestechnet.com/Home/TheVenerable261

Middle of the page.

My carb does have the #5140 stamped on the underside of the throttle body mounting flange and the throttle body bore id measures out at 1.68 so its safe to say this is a 261 carb. Therefore a .064 jet seems appropriate per carbking's comments. Thanks to all for your input