rumble seat (Oct 23 1999 8:07AM)
RE: 2X2 vs 3X2 Intake
BillB From my experience I recommend using 2X2. A 3X2 is really neat looking and can only be beat by a 4X2, but could be a driveability problem. They're for Bonneville, drags, or circle track in my opinion. If you use an Offy 3X2 you should use Stombergs since good old Uncle Fred (Offenhauser) designed his 3 carb manifold using them only. (Holleys front screw in the air horn hit the carb in front of it. Some guys just grind the screw off.) Other 3X2 manifolds I've worked on don't have this problem. Anyway, I got out the old pocket spiral note book I've entered notes, spec's, etc. in since 1955 or thereabouts and checked on carb cfm's and a formula to determine total carburetor cfm for street. Remember cfm is only concerned with bore, stroke, number of cylinders, and rpm's. Nothing else affects cfm. total cfm = (rpm)(CID) divided by 3456 For an example I'll assume the max rpm you'll turn this is 3800. Your engine will have 276 cubes. Then: cfm =(3800)(276) divided by 3456 = 303 cfm. This is what I'd recommend for the street. And I'd use a 2X2 'racing' (or 'super') type manifold... one that the carbs are located directly over the intake ports. You'll have to run an offset generator on a head. They will give you a good 4% increase in power over the 'street' manifold. The street manifolds have the carbs located high and close together so you can run a generator in front. The long runners make for very smooth throttle response but squelch power somewhat. The Thickstun hi-rise (street) was one of the best in producing smoothness and making the best horsepower. And I would not run a manifold on the street that doesn't have heat risers and an equalizer chamber. Now you're wondering what the different carbs have for cfm aren't you? First I'll list the Stombergs.... The model 81 is rated at 135 cfm. The model 48 is rated at 150 cfm. The model 97 is rated at 135 cfm. The Chandler-Grove/Holley cfm's.... The model 92 is rated at 142 cfm. The model 94 is rated at 155 cfm. The model 8BA is rated at 162 cfm. The model ECG is rated at 185 cfm. 3X2 manifolds should use progressive linkage unless you're a carb freak like I am. They make determining jetting a lot easier. I use only Stromberg 97's for ALL multi-carb installations. If you're curious, I would be glad to explain why. Good luck..... rumble seat

REPLY rumble seat (Oct 23 1999 8:10PM)
RE: 2X2 vs 3X2 Intake
The Stromberg 97 was used in '36 thru '38 Ford cars and in trucks in '36 and '37. It replaced the 48 model due to complaints from customers concerning poor mileage and winter drivability. One reason was the 48 has only a single knob on the butterfly shaft for the accelerator link to connect to, and this was a problem that the 97 corrected. The 97 has two knobs... one for winter when more of an initial shot of fuel is needed to prevent an accelerator flat spot and one for summer driving. The 97 has a 0.969 inch venturi and is rated at 150 cfm. They came with a 0.043 main jet and a #63 power valve (or high speed jet as they were sometimes called). The main jets are sized according to the diameter of the hole. Thus a 0.043 jet is bigger in diameter by 0.002 than a 0.041 jet. The power valve is sized like numbered drills. Thus a #63 power valve has the same diameter hole as a #63 drill bit. This means that a #63 power valve has LARGER holes than a #61 power since numbered drill bits get smaller as the number increases. Main jets are drilled to within 0.002 inches of the desired size. Then they are reamed to the final diameter. This is because drilling leaves an egg shaped hole to some degree and reaming leaves a round hole. I've found that a change in jets seldom exceeds 0.003 inch (unless you're running alky or nitro). Power valves seldom change more than 3 numbers. Float level is adjusted wet. Measure 15/32 inch to the GAS level from the top of the float. This is done with the engine running but swinging the air horn assembly aside. This has caused lots of fire because of sloshing of fuel and the presence of sparks on the armature communtator quite close to the front of the carb. These carbs were designed to be run with a max of 2-1/2 psi of fuel pressure. The stock Ford fuel pump could often make 3-3/4 psi. The design of the float system isn't as well designed as the Chandler-Grove/Holley carbs and the excess pressure overwhelmed the needle assembly. Thus flooding the carb and dumping raw gas onto the engine, and in the proximity of the sparking armature. This gave Strombergs a bad rep. All that is needed is to use a fuel pressure regulator and set if at 2-1/2 psi and it won't happen. I set the float level dry at 5/16 inch. This is measured 1/4 inch from the front of the float and from the top of the float to the top of the float chamber (no gasket). If it runs fine (and 90% do), I don't mess with them. If it stumbles on acceleration or acts lean, I will set the fuel level with the engine running. One advantage the 97 has over the Holley (I'll just call them Holley instead of Chandler-Grove/Holley from here on) is the butterfly arrangement. Strombergs fit much better and don't stick coming off idle anywhere like most Holleys do. The biggest reason I like Strombergs is their power valve arrangement. It's all mechanical and is operated mechanically by the accelerator pump. Whereas the Holley is vacuum operated. The Holley is designed to open at 7 inches Hg (mercury). Most flathead engines in good shape running a single carb may make 18 inches Hg at idle. Change to dual carbs and the vacuum is about 12 inches Hg... probably less if you're running a cam. At 5000 feet elevation, 10 inches Hg is about all you can get with a single carb and most are in the neighborhood of 8 inches Hg! Just touch the throttle and the vacuum drops to near zero in a hurry in all of them. When the power valve opens it changes the fuel/air ratio from 14.5 : 1 to 12.5 : 1. This was designed to provide fuel enough to run a 250 cubic inch engine wide open. Just imagine what happens when you're running dual carbs. That's right.... it enriches the system enough to run a 500 inch engine!!!! Think about 3 of them.... 750 inches! No wonder the plugs show rich and the car won't rpm like it should. Changing main jets won't affect this either since it's all vacuum controlled. The only fix is to replace the vacuum power valve with one that operates with less inches of vacuum.... or plug the holes to make the power valve inoperable (an old trick we tried). Jerre Jobe of Vintage Carburetion in Calif. has them in stock if you're so inclined. Keep in mind that if your engine backfires just one time, it'll likely blow the neoprene in the power valve.... so you should order a couple of changes for each carb. One advantage the Holley has over the 97 is main jets. Holley is still making jets for their OHV carbs and they are the same as used in the older Holleys. However, no such luck with Strombergs. And you have to have a special jet wrench to change them. The power valves are not made anymore either. Flathead Jack and others still offer main jets. Power valves can be soldered shut since there is only a single orfice. I redrill with a pin drill using the drill size I want some distance away from the solder. Not the best solution, but sometimes we can't have everything the way we'd like. Something that may be of help to you. I always remove the throttle base to check the fit of the butterflys. I hold them up to a light. If I can see light around them, I loosen the 4 tiny screws holding the butterflys about a turn or so and hold the throttle tightly shut while I tap on the end of the shaft with a screwdriver handle and tap inside the throttle bores. This usually centers things and gets rid of the gap around the butterflys. Tighten the screws on the butterflys while you continue holding the throttle shut tight. Whenever you can see light around them, the multi-carbs won't idle down and will probably stick coming off idle. Single carbs aren't affected much if they show light around them. A big advantage Strombergs have over Holleys is the three straight slotted screws that hold the main body to the throttle body. They can all be reached with a screwdriver from the top. The Holley has the front straight slotted screw come up from the bottom. When I use Holleys, I replace the front screws with bolts. Reason being is I bolt the throttle bases tightly on the manifold and connect the linkage between them loosely. Then I install the rest of the carbs. After the carbs are synchronized, I tighten the linkages up. Now if I have to work on the carbs, I don't disturb the throttle bases. I remove only the main body and air horn as one piece. When I'm through rebuilding the carbs or whatever I'm doing, I reinstall them on the undisturbed throttle bases. Thus I don't have to re-synchronize them. Don't know what I've forgotten. Sure been a long lecture. Sorry about that. But whatever it is I've forgotten will come to me after I've posted this. If you need something other than what's here, please contact me.... rumble seat

REPLY RE: 2X2 vs 3X2 Intake
Tom B:
Boy did I blow it big time when I stated jet and power valve sizes on the Stromberg 97! They came with a 45 main jet and a #65 power valve at sea level. In case you're curious, a #65 number drill is 0.0350 inches in diameter, a #66 is 0.0330, a #67 is 0.0320, and a #68 is 0.0310. Hope I didn't screw you up too bad.... rumble seat.

REPLY RE: 3x2 Manifold
Sounds rich timing to me. You might try setting the float level (not the fuel level) to 5/16 inch. If you don't have one, pop for an adjustable fuel regulator and set it at 2-1/2 lbs pressure. Be sure it's installed BETWEEN the fuel pump and the carbs. The 45 jets are probably a good place to start assuming you're at, or near, sea level. I find a lot of jets aren't the size they are marked. Sometimes they're so far off that you can see the difference in jets with the same number! Verify the actual size with numbered drill bits. Set the pump stroke on the leanest ball (marked S for summer). Make sure your distributor is timed at 4-5 degrees BTDC. Don't know what plugs you're running, but too cold of a plug will foul quickly and make you think it's too rich. Start with Champion H-10 or H-12 and go from there according to the plug color after a 10 mile run and a clean shut off. Hope this helps some. rumble seat

Eric: 1/31/00 - 2:32:57 PM
3 Duece Set-up
Does anyone know the best way to set up 3 carbs on a Offy intake. I currently have 2-stromberg 97's and one 81. I have heard that it is best to place a 81 in the middle. Also, which kind of linkage works the best. Thanks for the help!

philbill: 1/31/00 - 3:09:15 PM RE: 3 Duece Set-up
i personally have never run 3 carbs.however have run 2 many times.never had any sucess with progressive linkage and always ran direct positive two at a time from idle to full tilt.most of my friends who have ran three usually blocked off center carb and ran direct linkage to for and aft carbs with center what you will but 239 cubes has hard time handeling 3 carbs.unless you out there a bit 3 not necessary nor worth the trouble to have operating.only my experiance mind you and to each his/her own.(three look real nice i have to agree)but again clearance troube sometimes.

atout29: 1/31/00 - 4:28:26 PM RE: 3 Duece Set-up
I have not had too much experience with this But i do know that you Will most likely need a pretty beefy Flathead to run three dueces. Again like philbill said, most that I have seen block off the center carb and run off the other two, So in essence the third is usually just for looks.

Don: 1/31/00 - 10:54:51 PM RE: 3 Duece Set-up
I've heard this before. My current project came with a 2-duece offy intake. It'll have a mild cam, aftermarket heads headers, etc. I'm not a flatty expert by any stretch. So, do you 'experienced' rodders truly think that a two-duece set-up is sufficient?

finn34: 2/1/00 - 1:21:49 AM RE: 3 Duece Set-up
I have 3 97īs on a 3 5/16 x 4. It is a little tricky to set up as you cannot allow the outher carbs to close completely or the butterflyes will 'stick'. The idle is difficult to get below 700 rpm. And the simple and cheap progressive linkage is not very good. If the center carb is a 81 then the outer carbs certainly should open a little earlyer. If the engine cannot handle all three there is no need to have the outher carbs open completely. But certainly 2 carbs are mostly simpler to set up.

JD: 2/15/01 - 2:55 PM
Although I admire and agree with most of the sentiments and data posted above, I feel compelled to disagree on a few points.
Those who say a 3 jug or 4 is excessive for the street, run in contradiction to what I have done, and leave out some maybe important factors. With a 3-3/4 +1/8 stroke x 3-5/16 on Offie 3 jug and 97's, there was no sign of excess carburetion. As it was hauling around a 29 roadster is an important factor! Stick that in 51 Merc or something approaching that tonnage and a different picture emerges. With a 3.78 gear and 700x 15's would have been different with a 3.54, for instance.
A 4 jug needed a fairly docile cam, more so than the 3 jug, but the big importance was throttle linkage ratio. Longer arm on the carb linkage to give more pedal per opening was sorta essential, as was absolute zero linkage play uniting the carbs. But carb prep was essential . New throttle shafts were enough to get a fit (then!) and fitting the plates by file and trial and error, to allow closing tight, with no bore bind when closed. Some emery polishing of the bore helped as well. Once you get the plates to close, some detailing of accel pump linkage was necessary, as a squirt has to be immediate with no delay. Bending the ball arms sometimes is necessary, an adjustable link would have been better, as you want all carbs to open the power valve in unison, as well as deliver the same amount of squirt. The heavier the car, the more squirt it needs, and no rules apply except the result. I never had a new float needle leak under 3 pounds, if a good filter was in use, and pump springs were adjusted to get 3 pounds, not more or less, but no idea of the gauge accuracy I used. Floats were set wet and running with the tops off, and glass pipe level guages in the jet plug holes. (huge fire hazard here kids ! keep CO2 bottle handy and watch your hair !) I suppose I'd use a unisyn today, but never had one then; a straw to the ear down in the throttle body, to hear the same "note" was all we had, and mixture was set by ear and smooth idle. There are better ways. 97's were the only way, far as I was concerned, 48's for alky was ok, but on gas they never quite had the response, no matter what, even as a single. Chandler Groves, and I don't know if Holley does better now, were good for wide open, only, and never could be adjusted for response, smooth driving, or anything but full throttle. "Cruise surge" was common, and so was "pop back". The IHC version was even worse, but bigger. 97's could be fixed to do anything, the rest never made it in my experience.
If venturi size to displacement interests you, look at 4:: 40mm venturis on a 2 liter 4 cyl twin cam Porsche? Huge carbs for 122 cu in, and amazing throttle response. The "3 carb" (times 2 ) Solex or Webers on a 911 are another case in point, and I visualized a flattie manifold to mount those. Webers by intention (I think) are never jetted right, anything with a book full of jet choices never will be, I believe. I never had a hanker for "progressive" or an 81 in between two 97's. But never found a reason either. Different strokes for different folks. --JD

Return to Index Page