Thread: Multiple carb tuning (don't run away! lol)

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Originally Posted by Roger/Sacto

I had a 305 SBC built with a comp cam and 2.02 valves in the heads. I installed a new 3x2 rochester setup that has progressive linkage. Also have a MSD ignition with the middle carb vacumn line hooked up to the distributor. It is installed in a 32 Tudor with 350 transmission. It runs ok but doesn't have a lot of power and the plugs are black. Not sure what jets are installed. Anybody have any ideas?

Check your compression first and check/replace the plugs, if OK then check the initial timing and set it to 10 degrees advance with the vacuum advance disconnected. Then confirm that you have at least 35 degrees advance at the crank of mechanical advance (vacuum disconnected). Then confirm that the end carbs are seated fully closed at idle. You can't "sync" your progressive Rochester carbs because they don't have idle circuits in the end carbs. Then adjust the idle mixture screws on the center carb. These numbers won't be exact, but it will put you in the ball park.

Now that you have confirmed and adjusted all of the above you can "start" jetting the center carb. If the plugs were "sooty black" and all of the above settings were close you need to drop the (center carb) jet size down a couple of sizes and see what happens. Basically for a street motor you want to lean out the jetting till it starts to surge (at light throttle, steady speed say 50MPH) then richen it back up a little bit till it runs good on the center carb,(not with your foot in it). I'm not sure of the jetting starting point because I'm still building my set up (waiting for some parts to arrive, and the carb bodies back from powder coating). Eelco is a good place to find reasonable priced Rochester 2G jets, IMO. If you start with jets TOO SMALL you can inlarge with jet drills progressively until your close in size to save some money. But, I would still buy new correct sized jets after you have experimented by drilling.

Hope fully someone will volunteer their 3x2 Rochester jet sizing for us "sea level" guys to get us started.

Have fun