Tri power rochester end carbs once and for all

Help me answer the end carb accelerator pump question once and for all. Ive looked everywhere and all I find are contradictions. A definite answer would be great.

I have a 3x2 setup on my ford 390 - Bought it this way and dont know if it ever ran right. Its not stock, someone built it on a Elderbrock F380 manifold with 3 GM 2 jet rochester carbs.

It is progressive, running off the middle carb and opening the ends at about 70% throttle

It idles great with choke closed, dies at slight choke open and bogs at 1/2 throttle.

All the carbs have a manual choke(open on the ends) and accelerator pumps.

Should I remove the pumps from the end carbs or not? if so can I just disconnect the pump linkage?

Thanks for settling this for me.

 

Some times when you get one of those good looking cobble jobs it's best to put it on a shelf and admire it. Then get something that works. My $.02

 

Believe me, Ive been tossin and turnin the past few nights with that very idea. But they have challenged me and I decided I wont be beat!

 

I'd start by cleaning and setting the floats. You may have to rebuild the carbs or replace a few parts but there god carbs once you get'em right.

 

You need to run the pumps. Sounds like you have a vacuum leak somewhere if you have to choke it to make it idle. Check the base gaskets for leaks and make sure there are no vacuum ports unpluged.

 

I'll bet that you can solve this problem.
First you need to get it to idle, so if you would give us some more of the basics,
What do a new/clean set plugs look like, going around the engine to check..

How many and how big are the vacuum ports on the manifold and carb?
Are they closed/plugged/connected ?

When you say it "idles great with the choke closed" is the engine under load when you try to idle ?

Do the carbs look like they have some miles on them ?

I wouldn't mess with the pumps just yet.

 

The factory Pontiac Tri-Power setups all used the Rochester carbs with accelerator pumps in all 3 carbs.

The Pontiac engineers wouldn't have put accelerator pumps in the end carbs if they weren't needed.

I know some guys don't use them in the end carbs, but I'll go with the engineers on this.

 

And make sure at idle the end carbs are CLOSED. Make sure the butterfly's are seating good when they are closed. Vacuum leaks on the end carbs at idle are your enemy. Then if you are SURE the end carbs are ok at idle and it still won't idle without the choke being closed on the center carb, the center carb is the problem. Look for vacuum leaks on the center carb. Lippy

 

Draggin I agree with you. If you stomp the throttle at 4,000 rpm, fine, no accelerator pumps needed, but if you hit it at 1,500, It needs fuel to overcome all that air when you open six barrels fast. JMO. Lippy

 

A good way to troubleshoot any Tri-Power that's acting up is to remove the end carbs and install blockoff plates.

Get it running good on the center carb only and solve any idling issues first, then put the outer carbs on.

 

Bingo. Lippy

 

dragginGTO and Lippy have this back on course...

but I need some terminology calibration here:

"It idles great with choke closed, dies at slight choke open and bogs at 1/2 throttle."
WHAT?? When you use the word "choke", do you actually mean "throttle"?


"All the carbs have a manual choke (open on the ends) and accelerator pumps."
All carbs have a MANUAL choke? So you have 3 choke levers in the passenger compartment? Or do you mean that all carbs have a choke flap on each one's airhorn?

Again, please clarify...words mean things!

 

This is exactly where I am on mine. I have been trying to sort out the same exact set of 2G carbs on an edelbrock manifold myself. Also, this engine had not been cranked with this setup, so I had timing issues.

I made some block off plates from some steel plate. I used the gaskets to draw out the shape, then cut them out with my plasma cutter. Ground the edges then drill press to make the bolt holes.

Once those were on there, I was able to adjust the center carb idle screws and set the timing and tweak it from there. That is where I am now. Needs more adjustment, but after that I am not sure whether to swap another one into the center position and try to adjust it to where it runs fine then the last one and then try all three at once, or just go straight to all 3.

Also, I read that you want to block off the power valves in the front and rear carbs. I have not done anything to the other carbs at all at this point.

The engine does run well with the one carb.

I'd say do this method - at least you will have one carb that you know is running correctly.

 

You might PM dickster27 on the board. He worked wonders on my 3x2 setup and is very knowlegable.

 

running three complete carbs simultaneous will be a challenge. The vacuum signal needed to get them all three idling properly and together is the killer. Build the tri power end bases with no idle circuits, and make sure they don't leak air with the throttle fully closed (as mentioned above)

to the o.p.- get rid of all the choke stuff out of the end carbs, and keep the accelerator pumps.

 

No, no, no, no. Your thinking about this wrong I think. Think of the engine being a two barrel whatever engine then at 3/4 throttle throwing more carbs at it. ie, throwing more fuel at it. It does NOT idle on all three carbs. Only on the center carb. Progressive linkage idles on the center carb. You push the throttle and to 3/4 or so throttle it's on the center carb, at that point it starts to open the end carbs. When the linkage is engineered correctly it SNAPS the end carbs open at a fast rate, so the end carbs are on the MAIN circuit. All the carbs end up wide open at the same time. The end carbs do NOT have idle circuits they are plugged. You do not balance the carbs, you just make sure at Wide open throttle they are ALL wide open. I do agree, you need accelerator pumps on all three carbs for low speed response. But thats it, NO idle circuits on the end carbs. though you could maybe run carbs with idle circuits on the end as long as the butterflys on the end carbs were CLOSED completely at idle. I've never tried to run idle circuits on the end carbs. The factories didn't do it ,why would I? Lippy

 

Thanks for all the info guys. I think I got the idle circuit and the accelerator pump confused . I do have the idle adjust screws in all the way on the end carbs .

To answer your question Checker

New plugs are black and suty except for 2.
Only 2 vac ports on the back carb. One to distributor and other to manifold vac and tranny. the center and front have no ports.
The engine idles fine until under load, then it sputters.

What I meant by all have manual chokes is, they all have the choke plates in them- not connected but open. The center carb is run off cable.

I think I got the idea that you had to remove accelerator pumps from this article

http://www.rodandcustommagazine.com/techarticles/0905rc_how_to_increase_carburetor_intake/index.html

 

Since you say you have 3 chokes, it sounds like you dont have true tri power end carbs. Real tri power end carbs have thicker throttle plates that seal to the throttle bores. Ill bet you are leaking air past one or both of the end carbs. With it idling with no air cleaner, put your hand over the airhorn of each of the end carbs. If your idle speed drops, your end carbs are passing air. On a progresssive setup, you only run off the center carb at idle, the end carbs should not pass any air.
You say it idles great with the choke closed? This fattens up the primary carb and is probably compensating for the air leaks at the secondary carbs causing a lean mixture.

 

Ill give it a try- I think your right

 

I have seen regular carbs used as secondaries but they MUST be completely closed when not in use, normally some filing or bending linkage is required to make this happen.
Base plates in your hand with a bright light behind them, and epoxy in the idle circuits.
"Mikes Tripower.com" has the thick bladed baseplates (repro) he seems to know his stuff and he treated me fairly.

Also, make sure all the distributor advances are cool, I spent hours once messing with a Qjet only to find out the vacuum advance has a hole in it (no advance plus vacuum leak ) AND the centrifugal was rusted up.

 

chop32 and Darrin have got it, you have the wrong carbs. The end carbs should be special ones, with NO idle circuits, thicker, angle cut throttle blades, and (I think) no power valve. You shouldn't be trying to idle on all 3, it can be done, but WHY? Use the correct secondary carbs, and life will be MUCH easier!
you can convert regular 2GC's to secondary carbs with a kit fromm Speedway. Not cheap, but it's the best way to go, unless you can find the correct original type carbs...like tripower 348 Chevy, or EARLY Pontiac, even Caddies had trips.
And for the original problem, it's a vacuum leak. 9if you close off the air to a carb, and it runs better, it's too lean, and most times, it's a vacuum leak. Of course with the wrong carbs, you've got air going into the end carbs, probably without enough vacuum signal to add the appropriate amount of gas to it, so it's a vacuum leak, in essence.

 

Here is the one that I did. I got all the parts to convert the end carbs to dumpers from Vintage Speed. He knows what you need and sent the parts quickly. You can buy the bases already done from them or Speedway. I'm a die hard DIYer and followed Charlies directions for the conversion. I did a post on the process when I did it. No I still haven't got it running yet but I hope to get it running in the near future.



You can see the difference in the thickness of the butterflies that seals better and lets the engine idle with out this vacuum leak. I used epoxy to seal off the idle circuits in the carb bases of the end carbs.



I have the same Edelbrock intake.

 

You say the distributor vacuum is connected to the back carb. This is not correct unless you are using manifold vacuum to the advance, not the best.
The dist vac adv should be hooked to the ported vacuum port on the center carb. If you need more information, I am sure it will arrive shortly.
I would stay with the center carb only until I got the engine running perfectly. Adding the outboard carbs with the center not correct will only frustrate you further. The rear carb maybe the center carb and the other two could be the correct ones, since no vacuum ports??

good luck,

I have a factory 3 X 2s on a 390 in a 67 Fairlane GT.

 

Thanks Darrin, From what Ive been researching it sounds like your on to something. Im thinking the secondaries arent closing all the way. Im going to pull them off and check. Ive read about making some sort of rim from JB weld to make a seal, not sure about that though. Also going to make some block off plates and see what happens just on primary carb. Great info!!!

BTW looked for mikestripower.com found out it is now http://www.pontiactripower.com/index.html

thanks again

 

That is my setup exactly. Very cool

 

The ported vacuum port will be located above the throttle plates and give you little or no vacuum at idle, but will increase with the speed of the air flowing thru the carb.
Manifold vacuum ports are located below the throttle plates and will give you high vacuum at idle and little or none at full throttle.

 

My first 2G tri-power looked good from the outside but performed poorly. I messed with it for a long time adjusting it, buying small parts for it etc. and it still was never right.

When I blocked the air from the end carbs and the idle changed I realized I had a leak past the throttle plates and it would never work right if that wasn't fixed.

I then decided to pull all three carbs, disassemble them and see what I had. Each carb ended up being of different vintage with different vintage internal parts inside of them. Basically someone had a box of parts to build the three carbs. I had three pieces of junk.

I went on to get a proper matched set, rebuilt them, and install proper linkage and they worked great.

I guess my message here is you can spend a lot of time trying to debug these setups. But it may be worth your time to dissassemble them to benchmark what you have and see what you have to work with.

It saved me a lot of time and resulted in a nice performing setup.

 

Here's a write up to help DIY'ers:
http://www.clubhotrod.com/forums/showthread.php?t=26886

 

This is not a picture of my carb- same model though- this is where my distributer is connected

 

Awesome! Thanks Dzus. There is a ton of info out there, but its all over the place and gets overwhelming. This helps allot