[Joel Koontz]

I have a .035" over 455 with SD Performance Ported/Chamber Mod 330 CFM Eheads(11:1 Compression Ratio) headers, 242/248/110 Solid Roller Cam and Auto Trans.

Street use only. Elevation is about 1200'.

I know it is not ideal for maximum performance, but I am running a 66 Tri-Power and want to tune it for better performance.

I have stock Tri-Power End Carbs and The Center Carb is of unknown origin.

I am running K&N Air Filters.

What modifications would you recommend?

What jets in center carb?

What jets in end carbs?

What power valve?

What power valve spring/or what mod/where to buy?

What modifications to idle circuit? Please provide photos of exactly where to drill, if possible.

[steve25]

Cliff will likely post up and help ya out, but it would be nice to know the lift of the cam at the valve and the compression ratio?

[Tom Vaught]

I would go to a family that is actually running very quick with a tri-power on 2 cars:
SLOWBIRD. Talk to Brian.

69GrandPrix 462 tri-power 11.81 @ 112.24mph
65GTO 9.50 1/4et at 138.88 1/4mph tri-power dragcar 505cid

9.50s at 138 with a large cubic inch engine fits all of your questions.

Tom V.

[Singleton]

I'm running a 65 tri-power on a 9.5cr 440 inch motor, SD ported 6x, 262/252 solid flat tappet, 4 speed.

Jet size - 73/66/73
Idle tubes opened to .038.
idle and main air bleeds opened slightly (can't remember size).
Drill .080-.095 diameter hole in each center carb throttle blade so that enough idle air passes without having the throttle opened past the off-idle slots.

I recommend a AFR gauge so that you can monitor whats going on at all times, saved me a lot of time chasing my tail.

Mines not perfect, still runs slightly rich at cruise, (12.5-13.5) but idles well, and wot is good at 12.8-13.0 afr.

[Joel Koontz]

Thanks for the info.

I don't recall the exact lift, but IIRC the cam has about .650" lift with the 1.65 HS Rockers that I am running.

I'm pretty sure I know which ones are the Idle Tubes.

Can someone please post a photo showing exactly where the Idle and Main Air Bleeds are.

I feel sure that I have read(but don't recall where) that Quadrajet Jets can be used in 2G Carbs (but not the other way around) Can anyone affirm or dis-affirm this?

[Tom Vaught]

This thread should help you out joel.

http://forums.maxperformanceinc.com/...d.php?t=754100

Idle air bleeds are directly above the idle tubes on the top side of the cluster.

Tom V.

[Joel Koontz]

Thanks Tom, the photos in that thread should definitely be helpful.

[Joel Koontz]

The Center Carb I am using is #7041060(1271VA1) which appears to be from a 71 Pontiac.

Is this an OK carb to use/modify?

If so, what modifications would you suggest?

[Tom Vaught]

Measure the venturi bores with the cluster removed. With that info you can determine if you have one of the 1-3/8" venturi carbs or a smaller venturi one. With the big venturi you can tune the thing much like a 750 Holley carb. Some actually tap the passages to use a Holley jet with a lot better jetting assortment.

Tom Vaught

[Joel Koontz]

The venturi bores are just over 1.26"(I removed casting flash and polished) so it is not the 1 3/8" venturi version.

Is it OK to use this carb or should I shop for one with 1 3/8" venturis?

If I should upgrade to the 1 3/8" version, any suggestions on what donor vehicles I should look for?

[Tom Vaught]

Stock 64 GTO Tri-Power came with the 1.250" venturi vs the smaller one for the other engines but the 1-3/8" venturi carb mainbody would really help on an engine with "Big Motor and Big cam", Joel. Save the original GTO carb for a restoration.

Tom Vaught

[Joel Koontz]

I don't have an original GTO/Tri-Power Carb(at least I don't think I do), but instead of using the 1 1/4 Venturi Carb that I currently have on the car, I have ordered a 1 3/8 Venturi Carb(at least the seller says it is) on Ebay. It is supposed to be off of a 71 Chevy Truck # 7041123 with a date code 0071 DE1. Hopefully this will make a good center carb for me to modify/use.

Can Venturi Clusters be readily interchanged?
Example: If the Idle Circuits were working well on my current carb, could I take the Venturi Cluster off of it and put it on the "new" carb and get similar Idle Performance"?

[Tom Vaught]

You will want to keep the cluster and main body together.

You can tweak the 71 cluster to give you all of the performance you need, and it is calibrated for the 1-3/8" venturi signal from the mainbody.

So it should drive great just like the 71 Chevy truck did.
Except you will open up the idle feed restrictions in the tubes and possibly some jetting changes. An ideal center carb would be a 455 PONTIAC 2 BBL from that engine, but the chebby carb can be made to work.

Tom V.

[Dick Boneske]

I think the '71 Chev truck carb would be the large Rochester 2GC. This won't fit on your '65 manifold. I'm not aware of a small Rochester 2 bbl. with larger than 1 1/4" venturis.

Keep in mind, the '66 Tripower "large" center carb had 1/3/16" venturis---smaller than the "small" Rochester you have.

The info in the thread referenced above has very good info on idle circuit mods. Just remember---drill in small increments because once you drill too large, there is no way to correct the problem. In any case, .038" idle tubes would be a good start with idle channel restrictors at about .048-.050". The suggestion from Singleton to drill the throttle plates is a good one, also. The factory did this on automatic Tripower cars.

Be sure you have the power valve with .070 holes. I've never seen an advantage with larger than .073" jets in the end carbs. Stock center carb jets were .062-.064". You're close to right with .066" and your cam.

[Tom Vaught]

Dick, the title of the thread says he has a 66 tri-power.
"Tuning 66 Tri-Power for Big Motor/Big Cam"

So I do think the Chebby Carb will work.

Tom V.

[Dick Boneske]

Sorry, I read '65 Tripower in Singleton's post and got that stuck in my thick head!

The '66 "correct center carb" has venturi diameter at 1.170". The casting number of that float bowl is 7031575. Many, such as the '67/'68 vintage Rochester 2 bbls. have venturi of 1.230". But, there are some that have the same venturi diameter as the end carbs at 1.312" or 1 5/16". I've attached a picture of one of the large ones with another showing the casting number 7009834. I don't know what the largest 2GC's venturi sizes were.

Yes, the Chev carb will work, but you may want to use your old throttle base and airhorn. You've got to be careful with the gasket between the venturi cluster and the airhorn---the later ones have some extra idle passages.