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

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Hey guys,

How do you guys manage to tune multiple carburetors? I know how to balance them but not tune them. I have tuned carbs before but not the multiple set up.

I plan on getting a '66 GTO in the distant future (Tri-Power of course, 3 smaller carbs looks cooler than one big ol' carb in my opinion)

-CJP

I use a sync tube left over from my motorcycle wrenchin' days. With the Rochester carbs, usually works good to use the big carb in the center and the smaller one's on the ends with progressive linkage. I know if I ever run multiple carbs again, the first tuning tool I would install is O2 sensors in both collectors and a pair of fuel/air gauges in the dash!!!!!

Think of a "progressive" Rochester 3x2 set up is nothing more than a "Two Barrel" (with secondaries, LOL) as far a tuning is concerned. The end carbs (the secondaries) are completly closed at idle with no idle air adjustments, or idle fuel flow. Tune the center carb first, confirm that you have no air flow in the secondaries (linkage adjustment) and tune away just like normal. After your satisfied with the low end, then start jetting the secondaries just like on a four barrel carb. It's no big deal, just looks a little different.

ah, ok i see now. Thanks Dave and John! What is the CFM on the center carb? How about the CFM when they are all open? I heard that if you want to run in the 11s in the 1/4 the Rochesters won't cut it. Is that true?

-CJP

Don't remember what they were rated at stock, somewhere around 300 CFM for the big one, maybe 250 for the ends. Still more then enough CFM to support a 400 or 455 Poncho.... Back a zillion years ago, Arnie Toftland and a few others ran NHRA Super Stock with the tri-powers, they were in the high 10's, low 11's way back then.... Some of the circle burner carb builders have some modified Rochester 2 bbls. that will flow enough to run a single carb on alcohol though....

Don't know if you can get an archive version, but Stacey David on "GEARZ" TV did a whole show recently on how to set up tri-power from scratch using junk yard carbs, including special kits from Speedway Motors for new base plates for the outer carbs with no adjustment ports, no power valves and no accelerator pump/nozzles. I thought he said his were ~250cfm on center, and 300 outers -maybe bigger, for economy below 1/2 throttle, and then the outers start opening for the "Hold On Baby".

Originally Posted by rspears

Don't know if you can get an archive version, but Stacey David on "GEARZ" TV did a whole show recently on how to set up tri-power from scratch using junk yard carbs, including special kits from Speedway Motors for new base plates for the outer carbs with no adjustment ports, no power valves and no accelerator pump/nozzles. I thought he said his were ~250cfm on center, and 300 outers -maybe bigger, for economy below 1/2 throttle, and then the outers start opening for the "Hold On Baby".

yea I saw that show. It was GREAT!

-CJP

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?

What plugs are you running? Are they the right ones for your application? The fouling could be due to the heat range of the plugs or getting to much fuel. Have your carbs been balanced? How is the progressive linkage set up? Is it coming on too early, and putting excess fuel into the system?

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

I've had good luck starting with .046 at my elevation (4500FT), try .048 as a starting point.

Thanks John Palmer
I will pursue your advice this weekend.
Roger

I talked with Marty at Eelco. He said use #52 jets instead of the #55 that I have and maybe the accelerator pump is too narrow. They have different sizes. Waiting for parts. They are very helpful up their.