I've got a freshly rebuilt 1960 235 with duel Rochester small base Bs from 216s. It has a very bad tip in stumble, so bad it kills the engine on take off unless I give it about five quick pumps, and again after every shift. I don't know what jets they have but it runs great once its rolling. Reading threads here sounds like it"ss too much carb. Not sure I want to start drilling additional holes in accelerator pump which seems to be the solution most used. Has anyone had good luck with this combo with out customizing carb?
I don't think you you want to set them up progresive, as that would lean out half the cylinders. I might try disconnecting one of the accelerator pumps and see if that helps at all.
The "B" carb uses a power piston (power valve) and it sound like from your description that they are not working properly. Like other power valves, they open under low vacuum conditons. Using two carbs will lower your manifold vacuum and can casue the power valves to stay closed. Changing to a weaker spring may help. Open the carbs up and check the power valves, also check your float levels.
sounds promising. I know the carb rebuilder told me he cut off one coil off the springs. Does the power valve do anything for the accelerator pump or just fortify off idle fuel supply?
Get yourself a vacuum gauge first off and connect it to read manifold vacuum. This will give you your base line at idle and let you know if you may have additional vac leaks. Dial in the carbs air/fuel mixtures for max. vacuum. If you still have stumble issues, its probably fuel delivery, power valves as mentioned, floats or the pumps need adjustment. Yea and read those plugs!
make sure that your carbon gaskets (the thick one) theres a notch at the bottom you think it matches with your carbs.(Does not) make sure the notch is one the same side facing the intake manifold.
Thanks for your replies. It's got 17" of vacuum. The gaskets are installed correctly. when I unhook linkage from front carb it seems much better. I'm going to replace leather on power valve with rubber. It doesn't squirt as quick or strong as I would like. I did bend up the linkage a little. Seems to be getting better.
In my experience the "adjust by bending" linkage is what prevents them from singing in harmony and staying there. Here's a useful article: http://www.thecarburetorshop.com/RochestermultipleB.htm
I'm having the exact same issue with my dual carbs. I have tried everything but cutting down the spring and nothing has worked yet. She idles and can accelerate if i work into it but as soon as i stomp on it, it just stumbles. Going to watch the thread to hopefully find more info on how to work it out.
i am also very interested in what you can come up with. i am working on a 50 with a 57 built 235. it seems to run out of fuel at about 4000 rpm. it also stumbles and has a big flat spot on midrange accell. the car starts and idle perfectly but will not pull as hard as it should PLEASE!!!! keep us posted.
For starters, the carbs need to be identical in bore and venturi size. As to leather or rubber eccelerator pumps, that depends on the year of the carb, I don't believe you can arbitrarily swap them. If they were rebuilt correctly, and you have one of each, then I doubt you have properly matched carbs. I would suggest running just one carb, then swap it for the other carb. That way you can more easily determine which carb has a problem (possibly both). Once you know each carb works correctly on its own, then try setting them up together. Make sure the hole in the bottom of the carb isn't blocked off. That is what gives a signal to the power valve. Make sure the power valves are moving freely in there bores. When setting them up as a pair, disconnect the linkage and use one of those vac signal tools on the carb air horn and adjust the idle and mixture until they are both the same, then attach the linkage.
Running a pair of Carter W-1 from a 46-48 Chevy would probably work better, as they do not have a vac operated power vavle. They are also the small base bolt pattern, so you would want to run an intake intended for a 216. This can be used on a 235 or 261 by using a set of intake port adapters. You should also be running a set of headers with it, or else you will need to change to a 216 exhaust manifold, as the mating surface between the manifolds isn't the same on a 216 and 235.
Not trying to take over the thread but my discussions might help find a solution. Both of mine have the same numbers stamped on the outside of the carb by the air filter neck. I will take a look today and see if the power valve port is not blocked by the base gasket or riser.
Go to the inliners.org site. You will find a very good article on setting up dual carbs on the 235/261. Read it several times and follow it and you will be able to fix all your issues.
ok so i got a bit further ahead. with both carbs still mounted on the intake i got one all set up to work perfect. when i open wide open and fast with the one carb she accelerates fast and perfect. The other carb when opened causes the stumble and backfire. I then swaped the carb locations to just ensure it had nothing to do with fuel delivery favoring a side of the intake. after the location swap the good working carb still worked flawlessly in the other location. The issue has to be with the other carb for sure. I completely rebuilt the faulty carb again for the third time with a new kit and it still did the same stumbling isue. Took both carbs apart at the same time to carefully inspect them side by each and found out that the stumbling carb seems to have a microscopic hole in the side of the POWER PISTON compared to the working carb that has one about .040" in size. Being that i have nothing to lose here i drilled out the hole to match the working carb and will reinstall later today and see what it will do. It also looks like the POWER PISTON overall length on the bad carb is about 1/16" shorter than the working carb. Can this be an issue? The picture now shows the same hole size as the working carb.
Stumble like you describe are from being too lean when you try to take off, there isn't any real airflow so no vacuum...the powervalve would be already open, right? Where i would look is the transfer slot exposure, if the throttleblade angle is too great that slot will be overly exposed and that makes for a poor signal to strat fuel flow. The transfer slot starts right at the closed throttleblade, as the blade opens the slot gets greater exposure to vacuum and fuel then flows, if the slot is overly exposed initially then the vacuum depreesion won't be great enough to start fuel flow. To overcome this lack of fuelflow you are stomping the accelerator pump, right? Have a looksee at the throttleblades, the tip of the transfer slot only should be exposed, it should kinda form a square. If it is greater then that is your problem as you have increased the blade angle to overcome a different problem to get the thing to idle right.
Aside from you carb issues, are you getting any heat into you dual carb manifold? Unless you doing a full race setup, these Chevy sixes need a heated manifold or they will always stumble on acceleration. The factory iron manifold was heated by the exhaust it was bolted to and most aftermarket alum intakes have provision to heat with water once the engine warms up. You can crutch it to some degree throwing more fuel at it on tip-in with a bigger shot from your a/p, but a heated manifold makes all the diff in making them drivable.
Yes i do have heat lines installed from the headers to the bottom of the intake. When the blade is closed the air / fuel mixture port is exposed on the bottom. All the other ports are all exposed on the top of the blade. Actually the 3 lowest holes are covered a fraction by the blade when its shut.
verify that all of the small holes/ports are not blocked internally. That includes the crossbar... make sure the acellerator pump is squirting fuel the instant you move the throttle linkage. Make sure the port at the bottom of the carb isn't blocked off by the gasket, etc... Do you have another carb you can try?
verified all the ports were not blocked yesterday. I have a feeling it has something to do with the blade and the relation of the holes in the bore. I did have the blades out to do a bushing install and I wonder if its not sitting in the right spot now. I will also installed another new power pump yesterday just in case the other new one in it was not sealing correctly. I wish i had another good carb to use.
I have read through the thread, and was wondering weather or not some issues are related to fuel pressure too. I found that fuel pressure of 3-5 lbs works well on mine. The pump with out a fuel regulator is around 7. I used the inliners.org method and have no issues with my set up on the truck with out heated manifold is does start slower but runs great down the road.
The power valve has nothing to do with the accelerator pump or the actual mixture at idle . It's job is to supply extra fuel when the vacuum drops, just off of and above idle, when the throttle starts to open to accelerate the car. I'm no expert on these carbs, but wouldn't cutting one coil off of the power valve spring, increase the spring pressure and make the problem worse? I would think that you's want to decrease the spring pressure to allow the power valve to open sooner. Unless of course the decreased vacuum from using two carbs is giving you too much fuel from the power valves. and they are hanging open even at idle. The guy to talk to here on the HAMB is probability a guy named "Jon" who goes by the name "carbking" He is "the man". Mart3406 ===================
If one carb works and the other doesn't, then its not a fuel pressure issue. If you think the throttle base is the issue, then swap the bases around and see what happens. As to installing throttle shaft bushings, unless you had a perfect original base to use to help build a fixture for drilling out the holes on the bad ones, the only way you would be succesful in getting the shaft centered is by accident.
Relax. Just swap the bases and see. Then you will know if that's the issue or not and then you can proceed with finding another carb base, should that be the issue.
Did a base swap and that one carb still has an issue. Looks like something is wrong with the top and bowl I guess. Need to try to hunt down another carb and see how it works after that.
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