Rochester Tri-Power

Been getting my self into trouble recently dreaming up wild looking mods to my new toy. 3x2 setups are cheap to do, at least for me, because the local boneyards will practically bury me in 2g cores for very little money, and it's a no sweat deal plugging off the idle circuits and accelerator pumps on the outboard carbs. Hey, anyone with a few tools and half a brain can remove a choke butterfly and it's linkage - that's the easiest part.

There's a question of throttle linkage, though. The typical all out race setups involve direct linkage, all three carbs is unison - and that's not what I want.

The first factory type was mechanical progressive. The outboard carbs would be brought on line after the throttle was opened x percent via mechanically linked rods. This type was what was seen on your average '66 4-4-2, gto tri-power, 3x2 corvette etc.

The other type was vacuum actuated, as seen on 50's J-2 Rocket v8 engines. This is really attractive to me, since you can control the outboard carbs via engine load vs throttle position. That means better overall "automatic" control of fuel delivery. It looks like it works on the same principals as a vacuum secondary 4 barrel. I know very little about these setups, since I've never seen one in person. Can anyone help me out here?

The vacuum systems were finnicky. Back in the day, if you ordered a 3x2 setup, streetability wasn't your goal. After a few years of the 3x2 being popular they adopted the option of the vacuum operated secondaries. You would have to use a combination of factory parts to actuate the outboard carbs at the right time based on cam and vacuum signals. Its a no-brainer using a stock engine, but might be tough to recreate on a modified engine. I have to think that the aftermarket has solved the issue with adjustable vacuum pots, but its something I haven't really investigated.

But, there is no reason why you couldn't get a reliable street setup by using a progressive mechanical linkage. It would be a little tougher to get spot-on tune, but its usually easier than trying to engineer a vacuum operated system from scratch.

I guess that might be the way to go. It seems every reliable source for the vacuum pods has fallen off the end of the earth. Trying to rig something using a cruise control modulator or something would be a nightmare.

Be advised that adjusting the end carbs to full closed may lead to the butterflys sticking shut over time. Had that problem with a 4 carbed Corvair and bother-in-law had that problem with a tri-power Vette. However if you leave it with "6 corner idle" it may have some drivibility problems. You have to experiment.