Several years ago there was a post on 'end carbs' for a 3-carb set-up. I can't find it (using the normal phrases for 3-carbs turns up hundreds of posts). In this post it was explained that you should seal the end carbs and not use the idle circuit and that the throttle plates should be 'lapped' to make as tight a seal as possible when closed. Anybody remember what he used? I think it was some type of chrome polish. Or what else would work? Regular valve lapping compound is too thick.
I don't remember hearing about the throttle plates being lapped and I'm not sure how you would even do that. Are you sure you're not thinking of the sealer that was used on the Rochester end carbs? It was called DAG-213 or something like that.
Thanks for the tip on the Holley 94 forum - I'll look there For some reason I saved a picture but not a link; this is what the stuff looked like, not PAG123
It's standard practice. With a progressive linkage set-up, the two end carbs act strictly as secondaries and don't even open until about 2/3 throttle or so. If the throttle plates don't seal perfectly and if you don't remove the idle circuits, you're going to have a helluva time trying to get your system tuned right.
I just used a piece of 400 grit paper on a piece of thick steel, and sanded the imperfections out of them. Then installed carefully, and used a flashlight to look at the seal to the bores. Kept lightly tapping them until I got a good seal, then tightened them up. If you don't do this, you have a vacuum leak, as no fuel is added to the air leaking past the butterflies (with the correct secondary carb base plates.)
on factory tri power set-ups, there were no idle circuits, just not needed because they don't come into the picture at idle, the bases had no idle screwa at all
OK, then why aren't 4 barrel carbs secondarys sealed/no idle circuts? I got a friends 286 to idle on all three carbs, all three passed the same amount of air at idle and all six mixture screws work, end carb opening is smooth whether you just get by set opening by a little or a lot..Killing the secondarys isn't done on other tri-power applications..Wouldn't need them carb syncroizers they sell..
One of the reasons to seal is that 6 idle screws would have to be either very lightly open, passing almost no fuel each as you have 200% MORE idle jet area, or just having the idle richer than preferred; having just the centre carb idling makes it far easier to get an appropriately lean idle. There has been an argument for idle on the outside carbs for to circulate the fuel in the outer carbs, though the very performance nature of the beast would seem to argue that regular, normal (performance-oriented) operation would open the outer carbs enough to circulate the fuel adequately. I'd seal the outers. Cosmo P.S. The opposite for dual or triple singles on an inline: the fuel flow on these with just a single carb favors the inner cylinders; with triples, the outer cylinders can then be fed more evenly at all speeds, idle and above.
Carb syncronizers are only needed if all carbs work on an idle. With progressive linkage only the center card is used untill 1/2 throttle or so. Usually foreign cars are the only ones that had all carbs working all the time, old Jags, etc
Yes, but if progressive linkage is used, and it is on most, the center carb is still the main feature till secondary carb tip in.. Ok, its not written in stone..I have done it several times and have not had any issues, maybe lucky [oh no, not me] or a accumulation of errors that let things work ..Some day I'll find some one who has killed the end carbs and let him take me for a ride so I can feel what the tip in is like when the linkage bumps them open a tad and the end carbs have to go from no air/no fuel to slightly open where is the fuel going to come from? I don't think the accel pumps can cover and if there is no idle gas then part throttle [if that position is maintained] gets gas from where if there is not enough air flow to be on the main jets? Thoughts?
Progressive linkage is normally not used on inline engines. The goal is not so much performance (though it is part) but to even out the fueling throughout the engine. Cosmo
When I lapped my butterflies on my 2G 6x2 setup. I just used standard valve lapping compound. I also set them a little with a punch and hammer. Mine seal up good and they're sitting on top of a blower. Fwiw it's worth I run all 6 of my idle circuits and I have NA and blown. It wasn't until I added the blower that I had leakage issues. All though if you run the outer circuits my guess is you'll have to change the size of the air bleeds, I did. Here's a good link that shows how to convert non tri power carbs. http://www.hotrod.com/how-to/engine/hrdp-1210-rochester-tri-power-tuning-secrets-1932-ford-coupe/ Posted using the Full Custom H.A.M.B. App!
If you don't seal the secondaries, they are a source of a vacuum leak. Since there are no idle circuits, air enters, and isn't "paired" with the right amount of fuel to promote combustion. The reason it isn't very important on lots of old engines (i.e. Stromberg equipt) is that they run so rich anyway, a small vacuum leak isn't an issue. And smaller carbs like Strombergs are made for smaller engines, so multiples on a larger engine, you can still idle them down. A larger, more sophisticated carb, like a Rochester 2 bbl, might be more difficult to idle with 3 carbs. So they made 2 on a Tripower, secondary. Best of both worlds...easy tuning/idling, and lots of available power. AND a leak on both ends of a Tripower, would definitely lean out the end cylinders. That's why a lot of set-up like this run rough at idle, and the only compensation is running the center carb over rich at idle. Don't believe? Put an air flow gauge over the end carbs of a Rochester Tripower set up that isn't dialed in with fitted butterflies, and see how much air they suck in.
When GM got into the business of 3 multiple 2bbl carbs, they used special and designated end carbs. Because of that, tuning was simplified provided one single caveat, that is the end carbs throttle plates be sealed. Easy peazy. There are a handful of differences in the end carbs, and linkages but the principle remains the same. Hot Rodders can and will do anything they want but B I G auto manufactures can't. So, if you want your Rochester tri-power set up to work as designed, follow their rules. If not you're free to do what you want. Some guys speak "carburetor" very fluently and can tune a 1,2,3,4,6,or 8 carbs induction system with no trouble. Some guys can't speak it but profess from the highest mountains that all tri power set ups are hard to tune. It's just not a wise investment of time to explain that it's identical to tuning a 2 bbl. As far as tri-power tip in goes, it's legendary. quite a unique kick in the pants and an auditory experience. Rolling into it is a bit more dramatic that flat footing the loud pedal off a light. Since their time, guys make more power and go faster with a single carb....
What is the best way to remove the idle circuits on the end carbs without spending $340? I know you need to buy thicker throttle blades, power valve plugs and idle screw plugs.
It's a bit more than just removing the idle circuits. The base is dimensionally taller, the throttle blades are thicker with more chamfer and more elliptical in shape to seal in an oval shape within the thicker base deeper throttle bores. It's done like this so the blades open and close without getting stuck
Seb, There are two schools of thought on the subject. The idea of sealing off your end carbs and having no idle circuit is the same basic idea as a 4 bbl with no secondary idle circuit. Your secondary carbs become the same as the secondary barrels on a 4bbl carb with out a 4 corner idle circuit. on a tri power setup without secondary idle circuits you don't want the excess air and the fuel it would pull out of the float bowl in your intake until the secondary carbs come open. My thoughts on it is that if you run no idle circuits you also run power valve delete for simplicities sake, most carb builders don't do that, so to properly tune you still need to match your power valves to the vacuum. In theory that would be vacuum when the carb is coming open not at idle. The second school of thought is that if you have the secondary carbs with functional idle circuits it is easier to tune. That is a big yes and no at the same time. Like a 4bbl carb with a 4 corner idle circuit it all has to be balanced out. This can in reality be a tuners nightmare, especially if you are not a tuner. Neither method is incorrect or more correct then the other it is just a matter of what you are willing to deal with. Each comes equipped with its own headaches.
So to do it the right way, you must buy new throttle bodies? Which are better: aluminum or cast ? Best deal around?
You can use the original throttle plates and custom fit butterflies (blades). The throttle blades for an end carb idle delete will be thicker and elliptical so that when at an angle it will completely close off the air flow to the intake. I prefer a steel throttle plate (carb base) when I can get one. It is just a personal preference I think.
to set the throttle plates on the secondary sides we would loosen the screws a tad ( not real loose but enough to hold the plates in place and still without needing alot of effort to move them then snap shut the assy 2-3 times and let the springs pull them closed then check with a light to see if they set in the bore perfectly then tighten the screws down ( go side to side til they are tight ) and stake them ( or use a drop of locktite green on the screw ) 9 times out of ten they centered and not hung up against the sides .
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