Just woke up this morning and re-read this - man... that's some good stuff, right there. I am curious about turbulence - does the small lip created by the JB Weld "shelf" mess up airflow in the bore enough to make any noticable difference at all? ~Jason
I understand that a carb works on engine vaccum, I should have said I am not a carb expert, however, I DO understand the theory. I do noy have a lot of experience with multi-carb set-ups, nor am I a "super-tuner". Other than dividing the signal amongst three carbs, why is this way of doing it "wrong" as I'm told by everyone. If it idles, it enrichens the mixture when the secondary carbs come in, they are at wot at precisely the same time as the primary carb, and they return to idle, why is it a problem? I'm wondering if the disadvantages ( potential ones, such as with a long duration cam, or long, big runners? ) exist, or at least are noteworthy anyway, with a mild motor, with a typical tri-power intake, which is not exact cavernous and free flowing.
I am going to take a stab at an answer. Basically the motor will get the right air/fuel ratio to idle from one carb. When you have all three idling the engine, you've got 3 times the air and gas, and the vacuum cut to each carb. Those rochesters rely on the vacuum to pull the idle circuit fuel out of little tubes in the carb. Cut the vacuum, cut getting any fuel on the idle circuit. Then you end up opening the throttle blades and end up with way too much air. People can get them to run, but they idle way rich and probably aren't really idling on the idle circuit anymore. Its just simple to do it the same way GM did it.
I cannot take credit for this trick as Ross (Ross Racing Engines) in Ohio taught it to me over the phone. I used Lacquer thinner and a very small paintbrush. First I closed the blades on the base and traced with a sharpie where they sit flush. Then I mixed the jb and thinner on a piece of cardboard and painted a very thin layer over my pen marks, I also did a test spot on the cardboard to check for tackiness. What you want is just a little soft but not sticky. I also applied a very thin layer of Marvel Mystery oil to the blades to help keep the jb/thinner mixture from sticking. After it no longer felt sticky, (this will take awhile, thinning it out seems to increase hardening time) I forced the blades closed making a nice clean impression, then opened it and left town for two days. When I came home it was hard and I could barely see a spec of light next to the shafts looking through the base (Don't get it on the shafts) It may not completely seal but it worked well enough! My car now idles at 900rpm with a lopey Isky cam, (down from 1200rpm). Honestly this took 4 tries till I got it right, it was a pain in the ass! takes a lot of patience and do a test swab of the mixture at as close to the same thickness on your base as possible then test it before closing the blades, (my failures were due to sticking on the blades due to my impatience, as well as applying way too much the first two times). As far as impeding performance, there was no noticeable change in acceleration at all. You want it as thin and smooth as possible! and thin means very little turbulence or impedance of air flow. Apparently they did this from the factory back in the day with some mystery compound. Funny thing is, I have the really thick blades but I bought the long shafts for progressive linkage and they don't fit in them.
Sorry BellM, I hadn't read this until now. Thanks for the offer, I am leaning towards the 4 barrel even though it doesn't look as cool. I am still studying though. GV, thanks for the offer. I may need to get a copy if possible. I am going to look at hastings and see if they have it yet.
O.K....my gears are turning, but perhaps they are turning backwards... Now, if I have too little a vaccum signal to pull on the idle circuits on my secondary carbs, and the throttle plates are shut ( I mean TRULY shut ), then I am not adding fuel, nor am I adding air, at least until the plates open, so: Aren't they just sitting there dormant until my linkage reaches the point of opening the throttles on the secondaries? I would not expect a rich idle, unless they are adding fuel. If they are, aren't they responding to the vaccum, then? I would also think that since vaccum signal decreases with the opening of the throttle, that until the secondary carbs are opening, nothing is being added in the way of air/fuel either, until the point of them opening via the linkage, and getting into the accelerator pump, No? Yes? I'm missing something, huh?
C'mon some carb guru straighten me out and make me look stupid!!! lets go here, I know I'm missing something simple... Why is it "WRONG"?
Dirty Thirty, you've already answered your own question. The secondaries are often called "slaves" or "dumpers". At idle the primary carb does not feed gas though the venturi, it feeds on tiny holes (idle circuit) below the blades which are exposed to a tuneable amount of vacuum (idle mixture screws). When you open the throttle you engage the venturi (powered by vacuum) and the accelerator pump (mechanical) squirts extra gas directly into the body. If you floor it, you also engage the power circuit (opens at vacuum drop) adding additional fuel into the body through yet another set of holes. The secondary carbs are inactive until they are exposed to manifold vacuum. Until the blades open there is no vacuum signal and therefore no gas being drawn through the venturi. When the secondary blades open the venturi feed gas via vacuum, you don't even need accelerator pumps or power circuits in most applications, in fact for street use keeping accelerator pumps often makes for too rich a mixture causing bad transition and bog. Racing applications sometimes idle all three carbs or a 4bbl with "four corner idle" does the same but this is only practical for a vehicle that will be run almost exclusively at peak power. A typical 4bbl carb has primaries and secondaries, only the primaries have idle circuits, then when you reach approximately 60% throttle they open, dumping more fuel/air just like a 3x2 setup does. The reason all three cannot idle is it would be too rich and it will take too much vacuum away from the primary carb resulting in poor air/fuel mixture and velocity at low rpms causing poor efficiency and performance. It is true that a large, modern 4bbl with a good flowing intake manifold can out-perform a 3x2, but that does not mean 3x2s cannot be high-performance, dependable, and even economical. Its really quite simple, my rocket is a two barrel until I press the pedal more than about 60% to the floor, so if I drive casually I get mileage like a 2bbl, when I get aggressive I have 6bbl that works exactly like a mechanical 4bbl does. If you keep idle circuits in your secondaries you also need to have direct linkage making it a 6bbl all the time or all the different circuits of air/fuel in the carbs will not work right together.
O.K....to get past any further explanation on carb theory, which, I admit I do not know everything about, but, I am always grapplin'...let me get right out here with it: I'm looking for straight-up science. Not ( and no offense to anyone, none at all...I'm just very inquisitive! ) quotes from books, or manuals. somebody tell me why my tri-power set up works, AND works well I might add, with idle air bleeds. It has a progressive linkage. It idles at 500/600 rpm... It is not overly rich. The throttle plates on both secondary carbs shut fully. The end carbs reach WOT at the exact same time as the center one. It has excellent throttle response, both off-idle, and above, with or without a load. It has functioning accelerator pumps...in fact, I removed the large spring ( the one that resists the onset of the pump plunger, to allow for more fuel, because the motor was stumbling when the throttle was cracked enough for the secondaries to come in ) since then, with the addition of more fuel when the secondaries open, it runs like a swiss watch. No lean stumble, just clean, quick rpm... Why??? In fact...it runs G-R-E-A-T...I am to assume then, that it would run just a little bit better with no idle circuits in the secondaries??? p.s... it is a reasonably mild motor, also, by my estimation...10.5:1, 228/234 duration hydraulic cam, 2.02 valve heads...no port work. Basic as it gets...
If the secondary carb butterflies are "shut fully" at idle - they aren't leaking vacuum or contributing gas to the idle. I assume the center carb plates are slightly open at idle if the engine idles. You said it runs good. Exactly. What's the question?
Thats what I'm thinking...thank you! My question, though, really was: Why does everyone tell me ya' can't do it that way. I'm also sorry to hi-jack...I would have started another thread, but, Root seemed to be doing a general investigation into the wonders of tri-power, so I figured I'd join in the knowledge hunt.
Not to hijack...but has or does anyone run the Offenhauser 3x2 with the larger base Rochesters? Their catalog says it's for generally "hotter" engines...but I don't know what is realistic.
If you close the mixture screws all the way down in the secondarys will that eliminate the idle circuit thus allowing the use of unmodified carbs?
the key is the butterflies on the secondaries being shut or not. If they are shut sealed off and you leave the idle circuit in tact - they aren't doing anything when the butterflies are shut. If you hae regular 2 barrels used as the end carbs witch cracked open butterflies, then you are idling on three carbs. Way too much air, gas, and not enough vacuum signal. I am not sorry if anything was hijacked. Just kidding.
Yeah, man. If the outboard carbs are shut all the way, then they're sitting pretty much dead-to-the-world at idle. A carb that's not at least partially open is a brick, and the motor acts as if it isn't there. A carb won't atomize gas without the motor sucking air through it. Closed butterflies = no air OR fuel at idle. A bunch of folks setting up 3x2s have problems sealing the end carbs. Some folks never figure out that this is their actual problem and give up after misguided jetting and fuel regulator and/or pump swapouts. ~Jason
nope because the butterflies will be open and leaking air/vacuum killing the idle fuel mixture the center carb is letting in.
Exactly. You've got to kill both the fuel AND air in the outboard carbs at idle if you want it to idle at all. That's why I was so impressed with the JB Weld thing above. You can pull the bleeder screws and epoxy the holes to cut off the fuel in the outboard carbs at idle, but the butterflies will still be open at idle, seeping air like sieves, throwing the idle mixture off. This just nullifies the great job that the unmodified center carb is trying to do idling the car (as you can imagine, with three carbs' worth of air at idle and only one carbs' worth of fuel). The JB Weld thing is a way to make those end carbs seal really well at idle. And if they're sealed really well, they won't suck any air at idle. And if they're not sucking any air at idle, then you may as well leave the bleeder screws in and not mod the carbs any more - no air sucked in equals no fuel being sucked in either. Idle problem solved. I had tried to solve it by thinning out the edges of the butterflies, surgically, and with a file - it takes a long time, but eventually it does work. I'll never do that again, thanks to JB Weld! ~Jason
This is what I was driving at...exactly. I was starting to feel like filling the idle orifice's on the end carbs was not exactly treating the actual problem, or at least kind of being a fail safe, which is fine too, in the event that the butterflies don't fully seal, but it seemed like it was hardly the only "right" way. I know if I put my hand over a running engines carb, and there is no place else for it to draw from, it dies. The guy's with dual quads, they idle both carbs., and they have four corner idle circuits, in a lot of instances, due to the difficulty experienced with getting a motor to idle with runner configuration/cam profiles common to high rpm motors.
This is my first three deuce set-up and I can tell you right now, its also my last. They may look cool, but as far as reliability they suck. I'm sure mine need to be fixed or whatever, but I haven't found anybody yet that can tune these things correctly. I talked to Vintage Speed about a new set up for $1100 and they said "you'll still have to tune them". I'll be damned if I give them a $1100 and have the same problems I have now. Put a 4 barrell on it and away I go. The only problem is, I see tons of guys using them and they either don't have the problems I am having or they just lie and say everythings great.
You'll always have to tune them, though, that should be expected, after all you have three carbs, that must work in harmony, and the manufacturer has no way of knowing everything about your specific combination, where you are, what time of year you run it, altitude, etc... I, for one, will NEVER say I've had no problems with mine, it took a LONG time, all last year, in fact, to get them where I was truly happy with them. However, now I have no problems with them...I don't think I'm gonna fuck with 'em again unless they start being bitchy. They quit at around 5500/5800 rpm, but, they STILL look bad-ass.
Yep, thats what I'm running. Ran great when the rocket was stock, and even better now that its more cubes, bigger heads and cam. I posted a picture of it a few pages back.
Its never a matter of can't and always a matter of what works best for the application. As already stated if your secondaries are closed fully, (and sometimes they do seal pretty good with no mods) there is still only one carb idling regardless of circuits in the base because the blade will be closed above where the holes in the base are. By hte time they open your car is already at higher rpm so it matters very little if they bleed gas/air into the manifold. As far as the accelerator pump thing, your car is far lighter than mine so a little richer will run well, whereas on a heavier car leaner is more desirable. Also you could possibly jet your secondaries bigger, lose the pumps and get a longer pull at WOT cause the venturi rely on vacuum to supply fuel and the pumps are done after the squirt, or even keep them and jet it larger maybe? I also wonder if yours runs so nice because it does get a little extra air from the open idle circuit to compensate for the added pump squirt? Of course if it works well, then don't fix it. There are too many variables to have any concrete answers, car weight, cam, gearing, head flow, etc. One fact is, 3x2 always looks cooler!
Ahhh! You know, thats funny...BOTH of those things have been shambling around in my head, too... I actually expected to have to jet it up, when setting it up, but, after my trick with acc. pumps, it roared, so...this set up was on a milder motor, prior to mine. I wonder, though, if I'd get more rpm out of it with jetting, though...hmmm...thanks for any and all input everyone... I was actually thinking that, perhaps, there was something I was missing, and someone would simply lay it on me, and I would go rip my shit apart, do it right, and feel stupid! Tri-power is fun. And, I admit, the butterflies were not closing at first, either...it surged and idles all over the place, until I dressed them, and added a stout return spring on the linkage. I was just feeling around in the dark, and this is what I arrived at...
Man, you guys with 3x2 tuning probs need to take your car to a bike shop. No, not so much a Harley shop, but a Honda or Kawasaki or Yamaha shop. Not only can those guys tell you what to do to get those carbs tuned, but for a beer they can probably do it for you. They usually run one carb per cylinder on the big bikes, and they sync them all the time with a vacuum gauge. It's not too big a stretch to explain how your linkage works and what you're trying to do for them to catch on and be able to help. You may also want to shoot an email to Jere (pronounced "Jerry") Jobe: http://www.vintagecarburetiontech.com/ He's the best Stromberg guy I know, but he's forgotten more about tri-power than most of us will ever know. He can also recommend a Rochester guy you can email back and forth with about tuning these. You know, like when you know all the notes on a piano, you can play any song. ~Jason
Those dual quad guys sync their carbs with a vacuum gauge so they'll idle on both. They're set up to run that way - look at the intake runners, etc. ~Jason
what size jets do i use for the end carbs on a 3x2 setup?? i am putting together a 3x2 setup offy intake 3 2gc carbs, 2 i will be modifying to work as dumpers from talking some people with experience doing the same thing this will all go on 283 CHEV
Just an update. I bought a polished Edelbrock manifold and I guess I will try this on my own. I figure if I can rig a fuel control on a Boeing, I can figure this out.
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