The Stovebolt Site Forums The Shop Area Engine Shop Duel carb problem on 235

I have duel single barrel rochester carbs sitting on a fenton intake. When i step on the throttle, it bogs down and will die if i don't don't let up. It idles fine and will run at higher rpms if i slowly depress the throttle or feather it. I have reset the valves 3 times, set the timing to advance and retarded positions, still does the same thing. The carbs are sycronized because i used a uni-syn tool and both are set to the same vacumm. This is driving me crazy, help!!! Cecil........

sounds like an accelerator pump issue to me. Have the carbs been rebuilt?

Is the intake manifold heated?

maybe one accelerator pump isn't working,or too much fuel is going in when you step on it quick

The intake manifold is not heated but i warm up the engine before trying to accelerate.
Both carbs were rebuilt at a carb shop, but that doesn't mean it was done right. I will check the accelerator pumps. Cecil.........

You've got a classic case of over-carburetion. You need to find carbs with smaller venturis, to avoid killing the venturi vacuum as the throttles are opened. The Carter carbs intended for the 216 engine will be better suited for a basically stock 235. Two Rochester carbs intended for a 235 might work if the engine is highly modified, with lots of cylinder head work and a radical cam, or for a somewhat warmed-over 261, but they're WAAAAAY too much carburetor for a stock 235.
Jerry

i have two rochesters on my 235 works just fine but did have the same problem until i added heat. ron

I'd also heat the intake manifold. My dual one-barrel Rochesters had the same hesitation until I heated the intake manifold (this was a 261 - note the comment above, regarding a 235). There is a cooling affect (latent heat of evaporation) that takes place when the fuel is vaporized. The vaporized fuel then cools and condenses. That is why the stock intake manifold was bolted to the exhaust manifold.

What style of exhaust manifold do you have? If you have the stock exhaust manifold attached to the Fenton intake, you're OK.

I have the Fenton duel exhaust manifolds. Maybe a heater is needed. What heater did you guys use? Cecil........

With an Offenhauser intake, I used water from the cooling system. With a Fenton intake, I am using copper tubing from the exhaust manifold to a plate on the intake manifold (Jim Carter sells a kit).

https://www.stovebolt.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=459640#Post459640

https://www.stovebolt.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=421608#Post421608

Thanks to everyone. I'm going to put on a exhaust heater(easier), and i did notice that one carburator appears to be puting out twice as much fuel as the other. I learned a lot tonight and i see light at the end of the tunnel. Cecil.......

Not sure what you mean Cecil, "one carburetor appears to be putting out twice as much fuel as the other", are you referring to the stream you're seeing in the throat from the accelerator pump????

Hey Jerry, I must be having over carburetion problems also, the problem that Cecil describes is exactly right down to the last word the way my 216 acted and now acts even after extensive work with several meticulously rebuilt Rochester 'B's. I know a lot of you guys have seen the detail that I go into with my projects. This summer I switched to a, meticulously rebuilt Carter W-1 and you guessed it, same exact symptoms. All the carburetion that I've tried on this stock engine reacts exactly the same; it runs the best and accelerates the smoothest when the choke is out 3/8-1/2 inch. Don’t need nor want to use the choke at idle though, every thing is normal at idle. The problem is definitely; more air, not enough fuel.
No problem with heat here either, completely stock and the heat riser works like a charm. And for the umpteen billionth time, I’ve completely rebuilt and all new parts in the ignition system and the ignition timing has been tried all over the place and always ends up running best back on the ball.
I know this is getting desperate, but I haven't had the front of the engine off yet so I don't have any idea whether the cam timing may be off some from a worn gear. I wouldn’t expect this to create this symptom but nothing would surprise me at this stage.
Sure wish some one would come up with a logical reason for the hesitation syndrome. Over these past two years that I've been active on the forum, the guesses that I’ve read are all over the board, from soup to nuts.

And,,,,the theory that I had about the 10% blended alcoholic gas causing a lean condition, well, that doesn't come out in the wash either. I finally got a chance to test it out, taking a chance of a $10,000 fine for running leaded gas in a road vehicle here in Illinois, I recently tried 5 gallons of an aviation 98 octane non alcoholic fuel, after 50 or 60 miles of testing out on the highway and in town traffic I find that it still hesitates just as bad and still runs it's best with the choke partly pulled out. And as odd as it may seem I really believe that the Rochester ‘B’ performed a tiny bit better, that is, except for the fuel weeping through the gaskets and staining the carb on a constant basis!

Now I know I'm gonna hear that the accelerator pump isn't working, it is, and that the high speed passages are dirty, they are not, and that the timing should be set to 35°, it's been there and it’s not, and that the float level is not set properly, or the metering rods not set, and on and on but all of that has been attended to with fanatical detail and at great expense.
There isn’t much more that I can come up with and my next step is, over the winter to do a complete rebuild of the short block, bored, new pistons and a new cam and gears just to make sure that the foundation is sound.
The really difficult part for me is, it’s been over 40 years since I’ve ridden in or driven another Chevy 6 so I’ve got nothing to compare to. But from what I can remember from my youth when I had and drove many Chevy inliners, they were pretty smooth runners.
Denny Graham
Sandwich, IL

Cecil, the heat also vaporizes the fuel when it hits the bottom of the intake, just like water hitting the bottom of a frying pan. V8's use the exhaust crossover under the intake plus the heat of the motor, a six with the intake hanging out in the air stream needs as much heat as you can give it. Mine ran terrible till I added heat. Joe

Are the carbs a matched set? I ran into that issue when I was running Carter YFs. Both carbs were Tomco re-mans, supposedly the same application carb, too (part # on the boxes were the same). But internally, they were very different. Turned out that the first carb was a correct 235 application, but the second carb was for a 300 Ford, which runs about double the jet size. I ended up gettin a pair of $20 apiece junkyard Holleys, rebuilt and rejetted them myself, and haven't had any trouble since.