By: Pat Smith

        Among the most asked questions are how the !@#$%^&* do I get these carbs to work right?  I here offer you some ideas which Tom Langdon helped me with a few years ago when I asked him "how the!@#$%^&* do I get my two Rochesters to work right?

        I was suffering from: a long warm-up period, hesitation on light acceleration, poor fuel mileage and percolation during very hot days.  As you would guess the answer was multi faceted.  Many small cures leading to an overall healthy condition.

        The first was to get the manifold warm.  Droplets of gas you don't need, a homogenized mixture of air/fuel you do.  If you always wondered what those pipe plugs in your Fentons were, they are there to run heat to your intake via copper tubing hooked up to a plate as shown Figures H & I.  And I'm here to tell you that helped a lot.

        Nest major problem was the percolation.  The stock Bakelite spacers, found on most cars was installed to insulate the carbs from the hot manifold.  I had to lengthen the carbs from the hot manifold.  I had to lengthen the throttle linkage but that was a small price to pay.   NOTE be sure to use the gaskets which are slotted to provide vacuum to shut off the full power circuit.  This port, which must be left exposed, can be seen on the base of the carburetor in Fig. A.

        I have to admit that was all I did the first year the car was so much better I drove the rest of the summer. BUT...

       The nest year I got little more critical.  The hesitation, although improved, was still there and the mileage was nothing to write home about.  The cure is in tuning the various circuits of the carb to provide the proper, or at least close to the proper air/fuel ratios.  This is all necessary because with multiple carbs you are decreasing the vacuum signal to each carb.   What that means is you have to make it easier to pull fuel out.  You have to enrich the mixture by removing restrictions.

        The main circuits you need to modify are the 1)  "Off idle" or part throttle which is usually incorporated into the idle circuit see figures A and A1 I also see diagram.)  2)   The high speed circuit which is controlled by the main jet; and the 3)  High speed, full power circuit (power valve) which enriches the mixture under low vacuum (heavy load or acceleration).  If you are running jets larger than the low 50s you are probably getting poor mileage.  You are probably running them because the car ran better.  What you are actually doing is covering up the problem!

        So lets start:

                If they haven't been rebuilt start there.  Better yet start by rounding up a few old Rochesters for parts.  They are all over just look.  When everything is soaked clean start matching parts.

               1.   Look at all your jet stands (letter D in Fig. B, also Fig. C) you will see a small passage running between the power valve opening and the main jet opening. This controls the air/fuel mixture for the full power (power valve) circuit.  This was small on 216 and early 235 carbs but was enlarged considerably on later carbs.  I didn't have anything small enough to measure these holes so I just installed the later, larger ones.   What this gives you is a richer mixture under acceleration so that you can run a leaner main jet for normal cruising.  More mileage, same power when you need it.   Good trade off Ay!

                2.   All the power valve springs should be the same (letter F in Fig. B).  No I don't know how to measure a spring that light but if the wire diameter is the same and they have the same free length and the same number of coils it is the same.

                3.   The power valve pistons should be the same (letter E Fig. B).  This may be somewhat difficult as there were many different styles.  I finally found one which I could turn down to match another!

                4.   Be sure the check ball for the powervalve (letter C Fig. B) is the smaller one in the kit otherwise it will hang up and leave the power valve always open.  The same thing will happen if you transpose spring B with ball C.

                5.   The U shaped accelerator pump linkages should also have the same configuration.   So that all pumps work in unison.  Don't make the mistake of lengthening the linkage as that doesn't allow the accel. pump to operate until the throttles are already partially opened.  Bring the longer ones down to the shortest length.

                6.   The last match should be the jets.  Try starting with around a 51 jet.   Use factory jets or drill them all.  A mix match of factory and drilled jets will not work properly as they will not flow the same.

                7.   Put the carbs together with the floats set to factory specs.  If you don't have neoprene needle and seat assemblies go get some.

                8.   Drive the car and note where the shortcomings are.  Be sure to drive the car quietly as well as jumping on it.  I suspect your hesitation will be worse, especially under easy acceleration (like trying to keep the headers quiet while John Law is watching!).

                9.   Now for the fun.  Take off the tops and remove all fragile parts.  Flip the top upside down and with a small punch knock out the small lead plug on the linkage side of the carb.  See Fig. D

               10.   With a small knife blade work out the small aluminum plug located just behind the lead plug.  See Fig. E

               11.   Start with your smallest number drill and try pushing it through the restriction located under the alum. plug.  Go up one drill size at a time until you find the largest that will go into the restriction.  See Fig. F.  In my carbs that was a .070" drill.  Now take the very next drill size larger, place it in a pin vise (cheap), and drill out the restriction.  See Fig. G.  Go far enough so that you see the end of the drill in the center vent hole.  Put everything back together and drive it again.  You may or may not see an improvement.  If not, or if its not enough, go through it all again with the next size bigger drill bit.  Keep going until the hesitation is fixed.  I ended up using a .078" which worked quite well on my duals and on another triple set up.  Both were 261 engines.  Remember this effects only the off idle under gentile acceleration.  It also enriches the idle mixture so that a needle adjustment will be necessary.

       I am happy to report that with the modifications above I have great power, the engine is extremely smooth and I am getting 20 MPG.  The jets are #50s.  The car weighs 3300 lbs. and has a 3.55 gear.


FIG. A  A Rochester upside down.  The hole exposed with the throttle closed is the idle passage.  Those exposed when the throttle is cracked are the "off idle" passages.

FIG. B  Be sure parts are assembled in this order.  Be sure that ball C slips
in and out of plug A and that all carbs have the same power valve pistons E
and springs F

FIG. C  Another area to look at is the small hole between the power valve and the main jet tube.   Later carbs are bigger.

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FIG. D  A small punch is used to knock out the lead plug.


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  FIG E  A knife blade is used to work out the  
  alum. plug.



FIG F  Number bits are tried to deternine the current size


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  FIG G  A pin vise is used to hold the larger bit.  Go up in size  

FIG H  Use the heat riser gasket as template to make a plate out of 1/4" stock

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  FIG I   Install brass fittings so that copper tubing can be
  attached to two different header pipes.


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