Well I searched but couldn't find what I'm looking for. I took a Rochester 2 barrel off of a '66 Chevy pickup with a 283 and put it on a 1970 350. It seems lean and I don't think I have an air leak so I'm thinking the jets are too small. I can't find any info on jet sizes to know what size to drill them out to if that's the problem...any ideas? Thanks everyone.
Carbking probably has the info. I have not seen him post here recently, but he might spot this. He might want the carb number to be exact He has a website with carb numbers, but I don't recall if it lists jets. I kind of doubt it.
Jet sizes are I think close to wire sizes ( somebody step in if I am wrong). They do not have a straight hole but are venturi's for more efficient flow. Suggest you go up 5% to start. Increase in steps until you get it right.
I don't think you'll want to try drilling the jets unless you have some spares that you want to experiment with. They're a little more of a precision part than they appear to be. If possible, try to find some larger jets to swap out if you have to. But even before doing that make sure the float's good and adjusted properly, the accelerator pump is working and that the power piston and power valve are moving freely. And make sure you haven't covered up a vacuum port on the throttle body with the gasket somehow.
You can try bigger jets but that carb is designed for a 283 and even with larger jets it still might not run like you want it,if you are going to stay with a 2 bbl find a carb for a 350.
It might be a smaller carb. Easy way to tell is the smaller carbs are side feed and the bigger carbs are front feed with a 1" hex filter. smaller: Larger: http://www.circletrack.com/enginetech/ctrp_1101_rochester_2g_carburetors/viewall.html
How does it "seem lean" ? If you open the throttle to 2500 rpm and slowly close the choke a bit does it pick up rpm at some point? If not it isn't lean on the cruise circuit. At the RPM the carb should be about 10 pct lean on the cruise or economy circuit. so they will pick up or flare a few hundred RPM when LIGHTLY choked. If they gain a lt, 4 or 5 hundred you should richen them. That car had a range of about 55, 56, 57 jet size no matter what the application. The 70's 2GC's were bigger carburetors and used a lot in restricted racing. Hoop
They made 2 size carbs, small base and large base they do not interchange. If your carb fits both vehicles it is a small base. Small base had both front and side inlets and flow about 285 cfm. I have jets outside the 55-57 range posted above. Wish I knew where I put them. Jets are made out of a brass alloy and you should be able to drill them with a hand drill, the old fashioned crank kind works fine. Drilling the jets should be a last resort, check everything else first.
The Rochester 2-G from a 1966 Chevrolet 283 in the truck is the so-called small base. THIS UNIT CANNOT BE USED SUCCESSFULLY ON A 1970 350 INTAKE MANIFOLD! The 1970 350 manifold accepts the so-called large base 2-GC. The bolt pattern is different. Slotting the carburetor so that it would fit the manifold introduces a vacuum leak UNDER the carb, as the gaskets for the two applications do not align. Are you trying to use the 350 manifold, or did you put the 283 manifold on the 350? Since manifold interchange is not my area, I do not know if one can successfully place the 283 manifold on the 350. Others on this forum will know. A tag number on the carburetor would certainly help those on this forum to help you with an intelligent answer. Jon.
To follow up on Jon's comments; The original "small top" has center to center stud dimensions of 2” x 3-¼”, and a bore size of 1-7/16”. The "large top" stud dimensions are 2” x 3-¾”, with a bore size of 1-11/16”. If you didn't have to slot the mounting holes then you either have a small 2gc manifold, or a large carb. Here is a chart of the different CFM Ratings Those are 2bbl ratings, multiply by 0.70721357850070721357850070721358 just kidding, by .7072 to get 4bbl equivalent rating. A base vacuum leak usually kills the idle, if you choke it with your hand it will idle faster. I mentioned how to ball park the cruise A/F above. You can get any size jets you want here; http://www.carburetion.com/r2.asp
Hey guys, sorry it took me so long to get back to this to answer everyone's questions. The carb is a small base and it is mounted to a 2 barrel to 4 barrel adapter with slotted holes, which explains why I am not having the mounting bolt issue. That is why my first inclination was to think that I had a vacuum leak, but after some pretty careful checking I don't think that I do. There were some openings at the slotted mounting holes that could leak but I have those plugged. The symptoms when driving it are stumbling on hard acceleration from a low rpm and then a lean surge when cruising. Thanks for the replies so far!
That's the plan eventually, I was trying to get by with something free for now, lol. As soon as funds are available we will probably put an Edelbrock on it but we were simply trying to get the thing going and work out some other bugs by using what was here and free!
So we have a 283 two-barrel trying to feed a 350 through a two-barrel to 4-barrel adapter and a four-barrel manifold??? Wish you had stated this earlier. With all due respect, you could probably do better by hiring a 15-year-old to stand on the running boards and pour gas into the engine out of a leaky boot! All joking aside. The airflow characteristics of the adapter are such that you probably lose another 30 or so percent of the airflow, so now effectively you have a one barrel carb from about a 216 trying to feed the 350. Suggestion: forget trying to dial in engine issues until you are able to install the four-barrel of your choice. In the meantime, take care of tires, brakes, squeaks, stereo, shocks, interior, etc. Good luck with your project. Jon.
Hit a local swapmeet, or hit up your local speed shop, or look in the HAMB classifieds, find what you really want, a good used carb, and a good used manifold to go with it, and THEN start tuning. NOTHING is ever free.
The real issue is not so much that the carb is the smaller of the 2 2 barrels. All the issues are at cruise, so jetting is out the window here because the idle curcuit at the transfer slot is what runs your cruz . The idle circuit is lean to begin with because of it's app, then when you add the fact that todays fuel is leaner than that of yester years it's even leaner. An engine only so much CFM to cruz but if AFR is wrong, it's wrong. The stumble upon acceleration most likely is the accelerator pump.
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