[Singleton]

Pictured is a cluster from a 65 tri-power center carb.

Circled in red are the idle air bleeds, white is the pump discharge ports.

Am I correct in saying the blue circles are the main well air bleeds?
If so, what are the holes circled in yellow??

I'm running slightly rich at cruise speeds, (12.5-13 afr), with sooty plugs. Even though I have dropped the main jets to 62 from 64, it didn't affect the afr very much if at all. I'm thinking that I could open the main air bleeds a little to allow more air to mix with the fuel to lean out the mixture more.

Motor is a 9.5cr 440 with ported 6X, 65 tri-power, and a crower sft cam that is really too big, (.586/.571, 262/252 @.050, 110. 71 overlap).
Motor runs well at wot, just trying to dial in the fuel mixture at part throttle.
Have 78 jets in the end carbs, wide open afr is good at 12.5-13
Tony

[Singleton]

bump

[Radman]

What vacume do you get at idle and cruse RPM? You could have your power valve dumping extra fuel all of the time. The 2bbl is backwards to most carbs. The vacume holds the valve closed, low vacume opens it.

[Singleton]

Idles about 11" of vacuum, don't know about cruise.
I've removed about 10 coils from the power piston spring, and I believe it's working about right. To test it, I removed the spring and drove the car. The afr didn't change from before when the spring was on.
Can't figure out why it runs so rich with the 62 jets, since the stock 389 came with 63's, and I'm running a 440.
I'm thinking the main air bleeds need to be opened a little

[Cliff R]

The type of booster used on the 65 center carbs has a major impact on the end result. They are quite a bit more sensitive than other types and the ones used in the larger bore 2GC's.

It's simply trial and error to get the fuel curve where you want it. Fuel pressure, inlet seat size and float level play a minor role here as well, as does the type of booster and main airbleed size.

Don't be afraid to open up the MAB's. I used to install removable bleeds in 2GC's when they were required for lower classes of circle track racing. We blocked off the power valve and ran them straight off the jets as well, then "trimmed" up the fuel curve if/as needed with the jets and MAB's........Cliff

[Singleton]

What do you think would be a good starting point for the MAB size? Stock was around .026, and I have taken them to .033 but haven't tried it yet because of the rainy weather down here. Do you think I should go larger, say .038-.040 as a starting point?

I have a Robbmc 550 pump, 4 psi fuel pressure, disc type inlets from Wasson, level at 21/32. End carbs throttle blades were adjusted for best seal, then DAG216 epoxy was used to seal them completely, so no issue with leaking by.

Any idea of what the holes circled in yellow are for in the pic above?

Should the emulsion tube holes be modified in any way?

What type of removable air bleeds would work here cliff?

-Tony

[Singleton]

Originally Posted by Cliff R

Don't be afraid to open up the MAB's. I used to install removable bleeds in 2GC's when they were required for lower classes of circle track racing. We blocked off the power valve and ran them straight off the jets as well, then "trimmed" up the fuel curve if/as needed with the jets and MAB's........Cliff

Would blocking the PV be useful, or something that I would want to do on a street engine?

[Cliff R]

I don't recommend blocking the PV for what you are doing. The smaller 2GC's are pretty sensitive at the boosters, and your engine will enjoy a leaner mixture when the engine load is light (PV closed). It may take a few rounds of testing to nail down the jet/airbleed combo that works best, but well worth the effort. I do the exactly same thing with Q-jets, and to this day we still experiment with different airbleed and jet/metering rod combinations to nail down the best tune for different models......Cliff

[Tom Vaught]

Originally Posted by Singleton

Any idea of what the holes circled in yellow are for in the pic above? -Tony

I looked at about 40 Rochester 2 BBL carbs (some normal but most Tri-power 64-66 versions and none had the holes circled by yellow in the cluster.

I also did an internet search and several known carb builders never mentioned (or showed photos with a cluster with those holes). Good luck on this question.

Tom Vaught

[Bertone]

1965 center carbs on the tri powers have the extra holes that you have circled in yellow. High Performance Pontiac Magazine March 2003 shows great detail of those extra holes.

[Kenth]

Yes, the blue circles are the main well air bleeds.
I don´t know why they there but the yellow circles are not drilled thru.

[Singleton]

The extra holes are definitely drilled thru into the cavity as shown. Maybe some kind of atmospheric vent?

[Singleton]

Ok, so today I opened the main air bleeds to .046, (from stock .026). Still running rich at constant cruise speed, with afr 12.4-12.8, plugs still sooty.
Idles great at 14.5-15.0, wide open afr is good at 12.5-13.0.

Opening the main air bleeds that much should have made a change right?
Running 62 jets now, could go down to 58-60, but I find it hard to believe that this 440 cube motor would want jet sizes that were smaller than the stock 389 back in the day, (63 jet).
Could it be that todays 10% ethanol gas needs smaller jets than the older fuel?

Any ideas?

Tony

[Tom Vaught]

Originally Posted by Singleton

The extra holes are definitely drilled thru into the cavity as shown. Maybe some kind of atmospheric vent?

That drilled location would vent the main well area to atmosphere for sure.

Some are saying that all 65 center carb TRI-POWER clusters have that drilling.
Thanks for the new info.

Tom Vaught

[Kenth]

Guy´s, you are totally correct!

Been some time since i´ve last did a 1965 unit and my memory ain´t what it used to be or is it the christmas brew?

I rechecked my calibration notes and these Pontiac small base units DO have "upper" main air bleeds, size .027", as well as the regular main air bleeds, size .025".

[Tom Vaught]

Typically a Main Well Air Bleed will be supplying air to the Main Well Cluster Emulsion Tubes which is what one set of the holes do. The "Yellow" set seem to be more of a vent vs another air bleed. The largest 850 cfm Holleys Main Well Air Bleeds are only in the .025"-.028" range. Strange calibration for sure.

If you open up the "yellow" holes and do not get a Air/Fuel ratio change on your UEGO sensor then the holes you opened up were not air bleeds. More like vents.

Tom Vaught

[Singleton]

I opened the holes, (circled in blue in 1st pic), from stock .025, to .0465, and no change was noted, which I think is very strange. AFR should have gone leaner but it didn't.

[Kenth]

In the Delco Carburetor Models 2G-2GC-2GV Manual 9D-3 May 1973 pages 20 Fig 35 and 24 Fig 40, the yellow holes are refered to as "Top Idle Bleed".

[chrisp]

Are you sure end carbs are sealed & not sucking air / fuel ? Take them off install block off plates and see what happens .

[Singleton]

Originally Posted by chrisp

Are you sure end carbs are sealed & not sucking air / fuel ? Take them off install block off plates and see what happens .

I'm about 95% sure they're sealed. I did remove the fuel lines from the end carbs and emptied the bowls. Drove the car and the afr was the same as before. I would think that if the end carbs were leaking by, but didn't have fuel, that it would cause a lean condition instead of the rich condition that I have.

I'm wondering if my power valve, that screws into the bottom of the bowl, could be bad and leaking fuel into the main well? I don't know if it is a item that can be bought new anymore? I may try to find another one just to rule that out.