VCCA Home Forums TECHNICAL - GENERAL Personalized Modifications Rochester 2 barrel on a 235 motor

wondering if anybody has experience with running a Rochester 2 barrel on a 235 six using a carb adapter. Do you get as much power and performance as two one barrels without having to match two carburetors??

Not happy with my 52 1 barrel on my 54 motor.
At the least, figure I might have to rig up some new carb to throttle arm linkage and some new fuel lines.

Any power increase would be at higher engine speeds. like over 3000 RPM or 60 MPH.

Use the 1954 Corvette engine VS the 1956 truck 235 engine. Both used the same engine and camshaft. The truck had 140 HP at 4000 RPM and the Corvette had 150 or 155 (depending where you look) at 4200 RPM. Horse power and torque almost the same at 3000 RPM.

In giving this more thought. If the engine is properly tuned and timed is the carburetor opening fully? The 1951 and 1954 engines have completely different linkage.
Was the rear axle ratio changed to a 3.55? The 3.55 will give no better performance under 50 MPH than a 216 with a 4.11.
At what speed do you feel the power is lacking. In high gear or in the gears?

Adding to Gene's comment:

Acquire a helper. With the engine off, have the helper push the foot-feed to the floor, while you observe the throttle shaft on the carburetor. Does it open fully?

As to the two barrel -> 1 barrel manifold: think NASCAR restrictor plates. It might not run as well.

Jon.

As s follow onfrom carbking's comment, it is no use opening the carb throat up if the manifold opening isnt capable of the increased flow. You will probabley need to open the manifold out instead of the adaptor.
Tony

OR, if you really think more carburetion is necessary, use a different 1 barrel with a larger venturi.

Jon.

Rick - you mentioned dual 1 barrel carbs.

It is true that, if done perfectly, it is possible to acquire a performance gain from dual 1 barrel carbs, possibly as much as 8~10 percent. It is also true that a less than perfect installation can LOSE power!

The Chevrolet 216, 235, and 261 cylinder head design does not permit a larger gain from dual carbs. The folks that report much larger gains are either replacing a defective original single carb, or doing a complete electrical rebuild at the same time, and the gain is from these items.

Why?

The reason for multiple carbs on virtually any street engine is to improve average cylinder fill. The Chevrolet head has three sets of two intake ports. So the average cylinder fill density is still not as good as it can be using dual carbs.

If one really wishes to optimize the carburetion setup, then triple 1 barrel carbs (smaller carbs) will allow the cylinder fill density to be very close to the same on all cylinders. But the kicker: on many applications, the rear carb on a triple setup hits the firewall.

So:

Easiest and least expensive method of picking up SOME power is a larger single carb (think of one from a 300 CID six from that make is the same as the last name of an old country singer from Tennessee!)

The largest power increase would be from a properly designed and tuned triple setup.

The dual setup, IF DONE PERFECTLY, will give a very modest increase in real power, and a huge increase in "parking lot" power!

Jon.

To add to the above the camshaft would have to be exchanged on a 1954 235 solid lifter engine to improve high RPM
breathing to actually increase power,. The 1954 solid lifter cam was very consertive. the increase would be only at high engine speeds.
Without knowing where the owner wants more power,engine speed wise, makes a coorrect reply next to impossible.

Thanks to all for their input. I wasn't looking for a radical change vis-a-vis swapping in a Corvette 6 or running a multi-carb intake set up. Just looking for better performance. Running a recently rebuilt 1 barrel Rochester ( 1000 miles) on a 54 235 motor with 3 speed, stock 4:11 gears. Lack of power more evident on hills in second gear. Removing my oil bath air cleaner and running with nothing on the carb seems to improve it, so the thought is my old air cleaner may be gummed up and restricting air flow. May try an aftermarket cleaner to see. Have adjusted mixture by feel and driving and feel it's pretty much where it needs to be despite running pump gas with corn dogs added. LOL

Reading feedback on the H.A.M.B. forum yields some positive comments running either a Rochester 2 barrel with the adapter plate or Strombergs. Just trying for a little more driveability. Throttle is wide open with pedal to the floor and have adjusted mixture as best I can. Last thought may be this corn-squeezins gas I have to run here in Paradise. I see there is even a 4 barrel intake available, but that may be too much carb.

If this is in 2nd gear below 30 MPH more carbureation will not help. Try advancing the timing via the octane selector. AND DO NOT BLAME THE GAS.

Adjusting the mixture is ONLY effective for idle mixture; maybe 800 RPM at the maximum. Above that, the mixture is controlled by the main jet and the power jet.

With all due respect, it may be time to not think about carburetor changes, and try to determine why the vehicle isn't running as you think it should. Even with the Rochester, that truck should pull a small trailer up a decent hill in second gear with no problems.

Jon.

That lite little 1951 coupe with a 235 and 4.11 rear end should be able to easily start off in 2nd gear and pull any hill with ease in high gear, not to mention 2nd gear.

Replaced my badly clogged oil bath air cleaner with an aftermarket air cleaner and success!! Runs like it picked up 30 HP!!!
Sometime it's the simple things that are the solution.

I figured I was on the right track when I ran the car without an air cleaner and she ZOOOOMED!!!