The Stovebolt Site Forums The Shop Area General Truck Talk Question For Carbking

Carbking,
I just installed a tri-power w/Offy manifold & Rochester 2G's on my small block (350). Any advice on primary carb main metering jets?? They came with .056's, am I wrong in thinking this may be to lean?? Should I go to .058? Or, maybe .060?
The 350 is a 9.5:1 comp ratio, rated 255 h.p @ 4100 rpm stock w/single exhaust. I am running hooker comps w/dual exhaust, 4:11 diff ratio.
Any advice would be greatly appreciated.
Thanks,

Jim Sgrig

Jim - very little advice on trying to make a tripower work on a small block (we would have advised using dual quads).

Best I can tell you is think of the tripower as a four barrel with remote secondaries. In other words, block off the outer two and attempt to calibrate the center for idle and high vacuum. Then attempt to calibrate the outer two for WOT.

An exhaust gas analyzer would be a wonderful help in calibration.

And still the best advice I can give is replace it with a dual quad.

Jon.

I'm using progressive linkage.The secondary carbs don't open until 60% throttle. The secondary carbs have no idle, or power valve circuits, basically dumpers. I run on the primary carb only until I drop the hammer. No problem here. This thing really wakes up. With the .056's, there is an off idle/cruising speed hesitation/surge. No problem with idle, or hard acceleration (opening the secondaries). I've opened the primary jets up to .061, this eliminates the hesitation/surge. It seems to be running "fat" though. Maybe I'll go to .058 and see how it runs there. I thought you may have had some experience with the 2G's. I should have kept the original 2G that was on the motor when I got it. At least I would have a reference point. I just have to play with it a little. I'm happy with the performance overall. Just have to find that "happy medium" between the hesitation concern, and fuel mileage.
Thanks for your input,

Jim Sgrig

Jim - I have lots of experience with the 2-G series Rochesters, and have probably rebuilt between 1500 and 2000 factory (and aftermarket) tripowers. However, we (as did the factory) reccommend using tripowers only on "torquer" engines, not "screamer" engines. "Screamer" engines work much better with dual quads due to the eddy currents created at the "join" at higher RPM's.

Rochester made literally hundreds of different 2-G carburetors. These vary in internal venturi size, fuel jet size, air jet size, etc.; so impossible to give a reccommendation on jets.

Often, an off idle hesitation is caused by using a vacuum guage to adjust the idle mixture, and adjusting for maximum vacuum (creats a "puddling" condition).

Cruise surge may be due to too lean jets; it may also be due to a camshaft mismatch to the vacuum requirements of the primary carburetor; and the usual cause of a cruise surge is because of some of the M/M aftermarket progressive linkage, which creates a minor vacuum leak at either the front or rear carb.

Finally, in the "food for thought" category: generally, if a PERFECTLY tuned tripower on a Chevy 350 is capable of a 13 second quarter-mile, then then same engine/vehicle with a proper 4-barrel would turn 12.50's; and with proper dual quads would turn 12.20's. This is why we don't like to play with tripower on a SBC.

If you really want to discuss this, feel free to call during normal office hours (see bio).

Jon.