The Stovebolt Site Forums The Shop Area The HiPo Shop tri-power on a 350 crate engine

I have a small block chevy in my 55 second series. I was running it with a 650 four barrel and it was running fine. My bud had a newer tri-power set up with newer Rochester carbs and an Offenhauser intake and I bought it from him. I installed the tri-power and I had it running a couple times. It seemed to be running OK. I recently changed the intake manifold gasket to use one that closed the air port in the middle of the intake. Since then I can not reduce the iddle below 2,250 RPM's.
I found a large vacuum leak in the front carb and a minor leak in the rear. I think the butterflies won't close tight. Can someone please advise me how I can adjust those butterflies? I'm new to tri-power and have not had to deal with vacuum leaks in a carb.

Thanks, Pete

I'd use an 8-lb. sledgehammer to "adjust" all three carbs and then go back to a single 4-BBL setup.
Jerry

Pete,
Your problem sounds similar to one I had on my '55 several years ago. I installed a tri-power on the bored 327 and actually got them running quite well, but not completely satisfied with the performance, I swapped to a single 600 four barrel carb and immediately gained several horsepower, had better throttle response, and the gas mileage improved. To this day, I still like the looks of three twos on a small block chevy, I'm just not crazy about the performance.

I had a similar problem with the 348 in my 1957 NAPCO. It ended up being one of the end carbs needing to be rebuilt. I determined that was the problem by blocking off the two end carbs and disconnecting them from the linkage. You could stuff a wet/damp rag in the air horn of the potentially offending carbs. I would just do that for a "quick test." At idle you normally run on the center carb anyway and it should go to normal idle. Anyway, that was what my problem was with that symptom..
don

Some reading:

http://www.thecarburetorshop.com/Multiplecarburetion.htm

http://www.thecarburetorshop.com/Tripowertuning.htm

http://www.thecarburetorshop.com/Aftermarkettripowersetups.htm

When installing/initial tuning of a tripower; ALWAYS put block-off plates under the front and rear carbs. Once you get the center dialed in, introduce the ends, one at a time.

Jon.

Also suspect the intake gaskets, it wasn't running this bad before you swapped them was it?

A good set of carbs will run as good any four barrel if set up right. Dad's '65 Pontiac Safari wagon came with trip-power and ran for 25 years without any problems. It's all come downs to the setup and proper carbs. The big Pontiac's used vacuum operated end carburetors, either on or off, no part throttle. GTO's used progressive linkage.

If you are trying to use standard 2GC carbs on the ends, it's not going to work. The throttle shafts and butterfly valves are special to end carbs to help in sealing. You can replace standard ones with the correct ones from online venders. Google trip-power throttle shafts. True tri-power shafts and butterflies and much thicker so the can be forced shut for a true 100% seal.

If yours runs correct with the outers blocked off, ( solid gasket under each one ), thens it's just a matter of sealing. If it still acts up with the ends blocked, you have intake or center carb problems.

Don't try to work with all three carbs and intake at one time!

Joe

If you really want it to run good, leave them there and run on the center carb. The gawkers at the car show will have something to marvel at, and you'll get better performance and gas mileage!
Jerry

If done correctly, with correct carburetors, it can be made to work. Never as well as a four-barrel (Pontiac actually paid Carter to sabotage the Carter AFB 4-barrel so the less expensive 4-barrel engine would not outrun the tri-power engine).

Take your time, do it right, and you can make it work.

Once you get them dialed in, they will be as reliable as any other modern carburetor. See the Joe H. post. I had tripower on my GTO (lost a full second from the 4-barrel, but they looked great ) for many years with no issues.

Jon

Jon, this is a little off-topic, but it still deals with multiple carbs. I'm considering using three Keihin side drafts on a 261 stroker (close to 300 cubic inches) so each carb only has to supply one cylinder in the firing order on each intake stroke. The Keihin is a Harley Davidson application with the vacuum-operated variable venturi piston like an old S-U carb. Assuming I get it dialed in properly on mixture, what's your opinion of using a triple side draft setup on an inline six? It seems to make more sense than duals. Obviously, I'll have to fabricate my own manifolds.
Jerry

The Corvette 427 engines with a 4bbl had 25 MORE horse power than the three carb 427 and with 1/10 the headaches.

I went to a car show in Kansas City when I was stationed there. A guy had a V8 with three carbs in his truck (don't remember the truck or the engine model). I commented on the three carbs and asked him how much trouble it was to get all three synched and working properly. He sort of glanced around quick to make sure no one was listening. Then he looks at his shoes and tells me that the only one getting fuel was the middle carb. They were all chromed out and looked great. Must have cost a fortune for useless eye candy.

3 dueces are neat and if mechanical linkage can be adjusted to run 60 mph or so on the middle duece in high gear. Or whenever you want. Does the intake ports match up with the head ports. They probably do but Edelbrock made a 355 for 265 cubic inch sbc that had smaller ports and will not match up with later heads --- air leaks. They also made a 357 that did. Not sure if Offenhauser had the same diff manifolds.

Your problem may be air leaks or too much gas. As said above, block off the front and back, unfasten linkage and see how it runs on just the middle. If not fine, check for air leaks or carb problems. If ok, then unblock one of the front or rear, leave linkage off and close port. Narrow down which carb is the problem. Rebuilds on 2Gs or 2 GCs are easy once they are clean.

It should only choke and idle on the middle duece. The ends are just dumpers unless someone has 3 centers on it. If so go back to the 4 barrel or make dumpers out of the end ones. So synching them is not as much an issue.

Thank you for all the comments and recommendations. I appreciate everything suggested and I will use it to make up my mind.
Thanks again! Pete

I have built 3 of the Chev. V-8 3-2 setups. 2 I sold before I used them. I have one on my 56 now. I figured it would be a tinker toy but it worked great right from the start. I did have a problem with the accel. pumps. I got some bad kits. I used the kit from Speedway which has replacement throttle bodies for the end carbs & long shaft for the center carb. I seldom need to crack open the end carbs but a guy needs to every so often to keep them cleaned out.

My mildly modded 350 crate W/Rochester tripower has run perfectly since the first time it was fired up - 2 years running so far. Most of of the credit goes to Ron at..,

http://www.chevytripower.com/complete.shtml

http://i36.photobucket.com/albums/e22/riverwork/Public%20stuff/Truck%20Parts/Fintrk9.jpg

...it was bolt on and when fired up, just a couple minor adjustments on the center carb. I would call Ron & possibly send your carbs to him. They can be flakey if the outer (dumpers) are not done right to begin with. When done correctly, Rochester trips are a reliable, old school road trip that gets lots of looks & conversation at shows.