My Rochester 2G Runs Too Rich

At the risk of becoming overly repetitive, I’ll deal with another common carburetor problem.

Typical inquiry: “My Rochester 2GC runs too rich; I suspect the jets have been changed to a larger size.”

This does happen from time to time.  If your carburetor looks like it has missing pieces or has otherwise been modified, there is a good chance that someone has monkeyed with the internals.

Years ago, carb rebuilders would mostly find that 4-barrels had been messed with – QJets, Holleys and so on.

In recent years, since 2 barrel carbs have become socially acceptable, especially in the form of Tri-Powers and other multi-carb set ups, we in the carb business have found more and more “funny” Rochester 2 bbls in circulation.

What to do?

Go to our website with the Rochester carburetor manual:

Here you can look up the specifications for your 1932-1979-ish Rocky carb.  For example, if you had a ‘71 Chevelle, you would go here:

You’ll find scanned pages from the manual with part numbers listed against carb numbers.  If you want to know what jet is correct for your ‘71 Chevelle with a 7041114 carburetor, you would find ( that the part number for the jet is 7002658.  The last two digits of the jet part number are the size in thousandths of an inch.

From the factory, the installed jets varied a bit for a variety of reasons. For the most part, 2Jet carbs should be within 1 or 2 thousandths of the size in the manual. Sometimes there were mid-year changes in calibration that weren’t recorded in the published manual, and in some cases it had to do with the nominal size and flow.

Rochester jets are calibrated against a standard jet.  In other words, there is a master jet with an orifice of exactly 58 thousandths to which every other jet is compared. The idea is that every jet labelled as ‘58′ will flow the same as the master jet.  This is why you might measure the hole in a ‘58′ jet and find that it’s smaller or larger.  The ‘58′ refers to the nominal flow, not the size.

Once in a while a jet with a different marked size would get installed in place of the nominal jet.  In other words, you might find a ‘60′ where you  expect a ‘58′ but that might be because the ‘60′ really flowed ‘58′.  Follow me?

In any case, if you have a single Rochester 2-Jet with a reasonable jet size and it runs too rich, you should check the following:

1) Ignition.  Always blame the carburetor last.  There is more on this here.

Check the timing.  If you have a Chevy engine with a harmonic balancer, be aware that the timing mark on the balancer may not be accurate.  As the rubber ring in the balancer ages, the outside hub will mover (“precess”, technically) so that setting the timing with a light gives you retarded timing. I’ve seen these be out by 20 degrees!

If you have points, check the dwell.  This should be 29-31 degrees on a V-8.  Also, check the play in the distributor shaft.  If it’s noticeable, you won’t get good ignition.

At this point, most folks these days go out and buy a new electronic distributor.  It’s not necessary, though, as all you need to do is rebuild the distributor with new bushings.

2) Vacuum.  A vacuum gauge is mandatory when working on old iron.  If your stock Chevy small block doesn’t pull close to 18″ of steady vacuum, you have a problem.  This is a subject for another long post!

3) Carburetor stuff.  If the jets are the right size and it still runs too rich there are a few more things to check on a 2G.

- float: on all carbs, a sinking float will cause flooding.  If the float is brass, shake the float to check for gas sloshing around inside.  If that’s the case, get a new one. If it’s a plastic float, replace it with a brass one.  The black plastic (nitrophyl) floats last 10-20 years and then are done.

-needle/seat: if the needle shows a scoring line where it fits into the seat, get a carb kit.

-power valve: rarely, the 2G power valve will leak.  If your power valve looks like it’s been butchered with a dull screwdriver, you may want to get a new one.  These are not part of a carb kit.  You can order them at here.

-accelerator pump check ball: if the accelerator pump discharge check ball is missing, or if it doesn’t seat, the 2G carb will sometimes siphon fuel through the accelerator pump circuit.  This ball is under the venturi cluster.

-wrong throttle body gasket: in most 2G’s from the late 50’s to the mid 60’s the gasket between the float bowl and the throttle body should have vent slots. Without these the carb might have a hot-soak flooding condition.  Note that marine carbs never use these.

I think that pretty much covers it for single carb applications.

Differences Between Rochester 2G, 2GC, 2GV and 2GE Carburetors

At The Old Car Manual Project, we get quite a few questions about technical matters, specifications, historical information and so on.

From time to time I’ll try to answer a few of these here.

Cristobal from Chile wrote:

I have a oldsmobile omega 78′ with a V6 buick 231 engine, and a
rochester carburator 2GE model. I am trying to find the difference
between the 2GE and the 2G, 2GC and 2GV carburators, but i can’t find
it. The only difference that i could see, is that my carburetor uses a
solenoid and no automatic choke. Could you respond this doubt?

I sent you a pictures of the carburetor.

1978 Omega Rochester 2GV carb, Chile

Good question!  This is the source of some confusion among Rochester owners.

The answer:

  • 2G is the model of the basic Rochester Two-Jet carburetor with a manual choke (cable operated)
  • 2GV is the same carb with an automatic choke when the choke coil is mounted on the manifold (a so-called remote choke)
  • 2GC is this carb  with an automatic choke where the choke thermostat is attached to the carb and is heated with hot air from a tube coming from the manifold (heated by exhaust gas)
  • 2GE is an automatic choke carb with the choke being heated electrically

There is more information and illustrations of the 2G family here. The complete Rochester manual up to 1979 is here.

What about the carb in the picture from Chile?  It’s hard to tell – I would guess it’s a 2GE with a missing choke housing, since there is no choke assembly visible.  In fact, in ‘78, at least in US production, GM cars used only 2GC or 2GE carbs.

Rochester 2GC carbThe arrow in this picture is pointing to the 2GC choke housing.

In a 2GE carb, the only difference is that the housing has an electrical connector and there is no pipe fitting (for hot air) at the front.

Regarding the solenoid in the Omega carburetor above – this is an idle speed control (or idle stop solenoid), usually used to speed up the idle on air conditioned cars or for emission control purposes.  It’s not related to the operation of the choke.

In some cases, especially on smaller engines, the idle is set with the idle solenoid energized.  When the engine shuts down, the solenoid retracts allowing the throttle to close fully.  This prevents run-on when a high idle speed is required.

Tuning up classic cars and carburetors

Motor Tune-Up and Carburetor ManualThe old “Hygrade Motor and Tune Up and Carburetor Manual” is required reading for anyone who owns or services mid-1950’s or older cars with one or two barrel carburetors.

It covers the classic ways of tuning up the cars using simple tools and delves into the details of the typical Carter, Stromberg and Zenith one and two barrel carburetors of the day.

Some of the topics include:

  • How to use a vacuum gauge, including for setting ignition timing
  • Use of a compression gauge
  • Testing the ignition circuit

Also, it has practical advice on setting up Buick Compound Carburetion, the quirky dual two-barrel set up used on the Buick straight-eight.

Find the full text here:

Restoring chromate finishes on carburetors

Rochester 7040200 Quadrajet WIth Restored FinishesLet’s say you have a typical, reasonable condition, not-seized muscle car Holley or Rochester Quadrajet. The bodies of these carbs are green chromate-finished zinc; the throttle bodies are natural aluminum. In my old carburetor shop (now closed) we would remove the throttle plates, choke plate and all shafts and fittings before cleaning.

For your own sake, remember that I’m presenting this information only for reference purposes – you must use your own judgment and be sure that you are aware of any hazards present if you’re going to try any of these operations. Some of the chemicals and procedures used below are dangerous and are for professional use only. ‘Nuff said.

Here is a typical cleaning sequence for a fully disassembled carb:

  1. Degrease: I use lacquer thinner, gunwash grade. This stuff is flammable and you don’t want to breath the vapors. Use caution. An overnight soak followed by a thorough brush cleaning is sufficient. Use a wood-handled bristle brush, not a synthetic one (they will fall apart). Work outside or in a fume hood.You can also use commercial carb cleaner. If your cleaner contains dichloromethane or tetracholoroethylene (2-layer type cleaner) DO NOT soak aluminum parts any longer than necessary. They will corrode if left overnight. Many bike carbs and SU’s have been ruined this way.
  2. Air dry thoroughly and blow out with air. Wear eye protection.
  3. Acid soak. I use a commercial metal cleaner which consists of mainly phosphoric acid and detergent. Soak for a couple of minutes until fizzing is evident and ‘white rust’ is removed. Don’t overdo it or your carb will be destroyed! If you don’t have this cleaner or you aren’t restoring the chromate finish later, you can use vinegar instead. It won’t attack the finish as fast, but will remove crud with a bit of brushing. Again, don’t overdo it.
  4. Rinse thoroughly in hot running water.
  5. Air dry and blow out very thoroughly. The carb must be bone dry for the next step. Bake in a 200 F oven for half an hour if unsure.
  6. This and the following steps are only performed if you’re going to restore the original chromate finish.
    Glass bead: One way to get the carb down to bare, shiny white metal is to bead blast using very fine (#8) beads. Use clean beads and do the carb inside and out until it’s a shiny silver color. Use a blast cabinet and wear breathing protection.
  7. Blow out thoroughly with a high pressure blow gun – a ’safety gun’ isn’t powerful enough. Wear full-face and eye protection and hearing protection and be thorough.
  8. Acid rinse: Using a fresh batch of the same type of phosphoric acid/detergent as above, immerse the zinc parts of the carb only for about 30s or so. Aluminum parts don’t need this step, though they can be cleaned this way – no more than a few seconds exposure, or they’ll go dark on you.
  9. Rinse very thoroughly in hot running water until all signs of fizzing are gone.
  10. Immerse in the chromate solution for the appropriate time (see below). This is the trickiest part, since it depends on the concentration of the solution, the temperature and on the type of carb – not all zinc alloys are the same. I will say more about the chromate solution below, but for now, suffice to say that it’s dangerous, expensive and tricky to use. You can immerse both zinc and aluminum parts for corrosion protection, though only the zinc parts will pick up the green colour.
  11. Rinse thoroughly in COLD running water until all traces of coloured chromate solution are gone. Hot water will wreck the finish at this point.
  12. Air dry.  Blow out with clean compressed air, thoroughly as before.

If everything is done right, the zinc parts of the carb will be an iridescent olive-green colour, just like new. Be warned though, it’s tough to get it right.

If you’re doing a backyard resto, you can stop after step 5. Usually, it’s best not to remove the finish (step 6) unless you’re going to redo the chromate. So, I would stop after the vinegar rinse if that’s the case. Now you can invest in a can of Eastwood carb finish and spray the carb. It’ll look OK from a distance, and even better when the hood is closed.

About the chromate solution

This consists mainly of chromium (VI) oxide (aka chromium trioxide) dissolved in water with a bit of nitric acid added. These are dangerous chemicals in untrained hands!

If you want to try, you can obtain the chromate mix (solid) from Atotech. Minimum quantity is 100 lbs – enough for a lifetime. Follow the mixing directions and prepare a solution of about 30 g / L of the solid mix in water. Add about 1 mL / L of concentrated nitric acid.

With this solution, it takes a 10 s soak for a typical Holley or Rochester carb to form the chromate coating, OEM style. The picture above shows a 1970 big-block Quadrajet that was refinished using this procedure.
As I said, this isn’t really practical or advisable for most people.

Replacing the hot air choke on Quadrajet with electric

(originally posted 14 June 2010; recovered from 17 May 2013)

To replace your hot air choke on a M4MC Quadrajet – one that has the choke on the side of the carb – all you need is a later model OEM electric choke thermostat, say from a mid-80’s Quadrajet.

One electric choke that is suitable for all Chevy Quadrajet and Dualjet carburetors is part # CC139, available from The Carburetor Doctor.

The CC139 electric choke replaces the hot air choke exactly. Don’t use a gasket, because the choke is grounded through the carburetor.

The CC139 choke has a notch which is aligned with the front screw on the choke housing on the carburetor. This is a preliminary adjustment, but is usually pretty good for most applications.


The CC139 is an OEM design which contains two heater elements (high and low) and a thermal switch. No external sensors are required.

To wire the CC139 electric choke, connect it to power that is switched with the ignition or that is ‘ON’ when the engine is running. This can be the wiper motor feed, or direct from the alternator brown wire – use a fuse for safety.

That’s about all there is to it.  If you’re using this part in a car which already has electric choke wiring, you can use the original wiring.  Remember that in most 80’s GM cars and trucks the choke is wired through the oil pressure switch, so that if you have a bad switch you may not have power to the choke.