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: www.newagemetal.com
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: http://newagemetal.com/pages/Chevrolet/71/index.htm.
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 (http://newagemetal.com/pages/Chevrolet/71/pages/63-69Chevy0112_jpg.htm) 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.