6 Things You May Not Know About Stromberg 97 Carbs
September 7, 2009 by Hechtspeed
Seems that the MyRideisMe.com Bonneville experience never runs out of steam. Hanging out at the Nugget one evening, we bumped into Clive from Stromberg Carburetors. After a lengthy BS session, the conversation turned to carb tech. And to cut a long story short, we asked him to contribute to our ongoing “5 Things” series. Alright, so 5 turned out to be 6 – or as the English say, ‘half a dozen’. Here’s what he had to say:
1. Stromberg Carbs Run Better With The Chokes Left In
We’ve all seen those pics at Bonneville with 97 chokes removed and the kicker linkages brazed onto the base casting. It should make sense. No choke means more air space means more cfm. And you’d be quite correct, too.
Extensive 97 flow tests carried out this year by acknowledged race carb expert Norm Schenck showed that the carb did indeed pick up a little cfm without the choke plate installed. So all those Bonneville racers were right, after all? Well, yes and no. Salt Racers are only interested in WOT. On the street it’s a different matter.
Stromberg authority Jere Jobe told that 97s run better with the chokes in, so we suspected what Norm’s tests would show. Only we forgot to tell him the full story. Here’s what he said:
“I retested the signal curve with the choke butterfly and shaft removed, with somewhat disappointing results. The signal was unstable at most of the test CFM’s, and taking the average signal at each CFM to figure the signal curve showed a much less manageable curve than with the choke parts installed. My conclusion is that the choke butterfly serves as an airflow straightening “vane” that directs the airflow to the area of the boosters with reduced turbulence. Even though the choke parts cause a reduction in flow, it is not a good trade to lose good fuel metering for that CFM gain.”
So there you have it. The same story from two very qualified horse’s mouths.
By the way, if you want to keep your choke plates fixed open, try our Choke Lock Detent kit (Stromberg Part 9537K-L), which replaces the usual round-tipped detent pin in the airhorn to lock the choke plate open.
Retails at $6.95
2. Bigger Stromberg Power Valves Have Smaller Numbers
Stromberg main jets are easy. What you see is what you get. Stock Genuine 97s come with 45s which means 0.045inch. Power by-pass valves (PV) – the ones underneath the accelerator pump — use the old engineering Number and Letter Drill system, devised as a way to fill in the gaps between the 1/64th sizes. And to complicate matters, the bigger the number, the smaller the drill!
And to complicate things even further, changing your PV by one number does not always mean the same change in jet size! We offer everything from #72 up to #60 (note that I said ‘up to’). The #72 is 0.025inch, #71 is 0.026, but #70 is 0.028 (a two thou’ jump), then #69 is 0.292 (WTF!) . The gap between #66 (0.033) and #65 is also 0.002inch before it returns to 1 thou’ per size right up to #57. We didn’t make the rules! But it pays to remember this when you’re trying to rejet.
And while we’re on the subject, remember that the PV only starts to affect the fuel ratio at just after 50% throttle. And when you swap them, cut a slot in the centre of a wide blade screwdriver so you don’t crush the centre pin and spring.
3. Set The Stromberg Float Dry
The float in a Stromberg 97 (and 48, 81, etc.) is supposed to be set so the fuel level (not the float itself) is 15/32 inch (plus or minus 1/32) below the top edge of the casting without a gasket. But to be honest, that’s easier said than done, especially with the engine running and a cigarette on the go.
So for increased customer safety, our Premium Service Kits (9590K-97 and 9590k-81) now recommend that the float is set ‘dry’, as we do at the factory. Rather than a float gauge, our kits now include an extra leaflet about setting the float level. To download a copy, click here. Basically, just get the float so it sits level in the bowl when the inlet valve is shut.
All new Genuine 97 floats are pre-set at the factory, but if you’re rebuilding, you adjust the front ‘tang’ of the hinge to push it nearer or further from the inlet valve. If (and only if !) the carb is empty of gas, hold the bowl section upside down so it closes the valve by its own weight, then eyeball it through.
4. Most Carburetor Problems Are Actually Ignition
I didn’t write this one. I’ll just leave it to the words of our dealer and very knowledgeable Stromberg guy, Bob Baxter at Baxter Ford Parts in Kansas.
“I bet 75% of the carburetor problems people call about end up being ignition. If you ever get people asking about hard starting problems once the engine is warm, ask them if they’re running 12V. Then ask if they have a wire running direct from the starter solenoid to the coil. Most don’t. Also, suppression wire with 6V is a no no. And if an original 6V coil has been replaced with a modern 6V coil, the ignition resistor might need to be bypassed.”
5. Progressive Linkage Is What You Make It
Having just launched our new Superlink multi-carb throttle arm (9091K), we’re developing our own progressive 3×3 Stromberg linkage system next, and as usual asking all the same questions hot rodders were asking back in the 1940’s. Here are a few things to consider.
First off, with a typical Stromberg progressive, if you want all three carbs to hit WOT at the same time, you’ll find that the primary hits about 50% max before it starts to open the secondaries. Stromberg throttles rotate about 80 degrees so aim for 40 degrees each side of vertical for the best geometry.
Next: Offenhauser and Edelbrock intakes have different carb centres, so the same progressive linkage will open the secondaries at different points on different manifolds. And also depending on whether the primary pulls the front carb or pulls the rear one. Ideally, in a light car, you want to run your flathead on the freeway on the centre carb only, so tyre size and gearing all play a part.
Next: Unless you’re running a monster flathead, you probably don’t need all three carbs to hit WOT. So you can actually set it up so the secondaries come in later, but never hit WOT. Bob Baxter has even set flatheads up with a dummy in the centre and a direct linkage front to back. Or with the outers as primaries and a centre secondary.
6. Stromberg CFM Flow Rates Were Quoted In The 4-Barrel Measure
This is an argument we’ve had over and over again. Back in the day, max flow (cfm) rates were measured at 3 inches of Mercury (inHg) for two barrel carbs and 1.5inHg for 4bbl carbs. And there is a conversion factor between them – ie. a 600cfm 4-bbl Holley is not the same as 600cfm worth of 2-bbl carbs.
So when people read the old published flow rates: 150cfm for a 97, 175cfm for a 48 and 130cfm for the 81, it’s assumed that these are at the 3inHg, 2bbl measure. But that assumption is wrong. For some reason, those figures are actually 1.5inHg, 4-bbl type figures. Which is incidentally why, four 97s work great on a 350 Chevy (work it out).
Finally, our own flow test results put the new Genuine 97 at 162cfm. A Speedway carb made 154cfm. And (to avoid any bias) the Barry Grant 98 made 162cfm too. All measured at 1.5inHg.
Look for more how-to’s and tech info from Stromberg Carburetors in future articles at MyRideisMe.com
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Just What I Needed For My 97.
THANK YOU .
Ron B.
I am trying to set up a 2×6 manifold for my sbc.I am going to run 2 primary carbs 2 secondary carbs and 2 dummys,the active carbs are at the four corners.I am looking for any info or tips.The carbs are ford 94′s.
thanks
mike
Hi Mike, Clive here at Stromberg Carburetor. Whether you are using 97s or Holley 94′s, (the cfm is about the same) four is a good number for a SBC – adds up to just over 600cfm. Yes, blank the centres (we have a kit) but I’d keep the other 4 all the same. The 4 corner carbs will be virtually above the chevy port areas. These intakes were designed for racing so the mixture will take the nearest path. If you have just 2 as primaries they will always be a long way from some ports. The other issue is linkage. We know because were currently finishing off our new Stromberg 6×2 and 4×2 linkage sets, that setting it up with say the front 2 as primaries and the back 2 as secondaries is very tricky all off one back bar linked to your pedal. And as for primaries at opposite corners, forget it! You will have to experiment with jetting and accelerator pumps too. You may not need all 4 pumping, but then again with these intakes it takes a while to get the air moving then you gas it, so extra pump gas might help. 97s are a little easier to manage in that respect. Hope this helps, Clive
What setup to your recommend for two Strombergs on a 1931 Model A Ford?
Hi Clive, We have worked closely with Charlie Yapp at Scalded Dog Speed Parts on this question. Charlie runs the Secrets of Speed magazine, dedicated to hopping up Ford 4-banger engines so he is very knowledgable. While there are a number of 2×2 intakes for Model A, not many have a plenum chamber effect allowing you to run a progressive linkage – running on one carb most of the time and bringing the other in at higher revs. According to Charlie: ” My “Equalizer” dual intake is designed with a “mixing plenum box” for equal runner length. Thus you can run one carb and one as a dummy, or two fully operational carbs, or two with progressive (my first choice). Works very good.” Stromberg even made a progressive linkage for it. Part 9248S ($45.95) is direct and 9248PRO ($99.95) is the progressive. You can find them on our website. You can connect one carb to the standard throttle linkage and let our linkage do the rest. Jetting will depend on your engine tune, local gas, elevation, and how you set them up, but you need to start at around 0.043 main jets and small power valves – maybe numbers 69 or even the smaller 71s. This is for Stromberg 97s, by the way. People used to fit two 81s thinking it was less carburetor, but the engine takes what it needs so it won’t know (or matter) whether you have 97s, 81s or whatever on there. It just uses a different part of the fuel curve. If you need more help, email us using our tech@ email address which you’ll find on our website at http://www.stromberg-97.com All the best, (the other) Clive at Stromberg Carb.
Does the Stromberg 97 require a pressurized fuel system (a fuel pump) or can it work on a gravity fed fuel line like on a Model A with a cowl fuel tank?
I have 4 Stromberg 97 EE1 sitting on an Edelbrock maniforld for a Ford flathead. I have had the set up for over 50 years now and used them on my first hot rod build when I was in high school. It’s time to clean house and from what I’ve seen on the internet the prices are all over the place. Can you give me an honest appraisal value and where I should start as an asking price?
Thanks
Mike Stark
I have found so much mis-information and conflicting info on tri power on the Internet that I’ve become completely confused. I have an Edlebrock (3-bolt) 5418 three deuce intake and want to use the Stromberg 97′s. My engine is a 327/330 HP HIGH PERFORMANCE (BALANCED) with a high energy Competition camshaft .454 lift and 268 duration. Will the 97′s provide enough cfm… and run efficiently. Love the 97′s this new engine will in on my 34 coupe.. Thanks, Ray
Ray, Clive again at Stromberg Carb. Sounds like a good strong engine. If you do the recommended math on this, the formula for required cfm is engine cid x max revs divided by 3456 (and then multiplied by the volumetric efficiency which will be less than 1 for most engines.). Each new 97 is 162cfm so you don’t have to be a scientist to see that 486cfm is nothing like the typical Holley 600cfm carb that most people use.
Luckily that’s not the end of the story. The math only tell you about top end cfm requirement and you only need high cfm at high revs. So let’s do the math for your motor: 327ci x 5000rpm/3456 = 468.75. And that’s at max vol efficiency. We said that 3x97s = 486cfm.
Now, the same math would say that a 350ci motor at 5500rpm needs over 550cfm, but I can say from personal experience that we had a hot 355inch Chevy motor on a dyne with three regular 97s and an Edelbrock 3×2 intake and it ran like hell, made 347HP at 5600rpm and 382 ft.lb torque at 3800rpm. Both were more than the 4bbl Holley we baselined. And that was without friction correction or any of the other BS people use to report more dyno HP. We didn’t even change the stock jetting, though we did disable the two outer accelerator pumps because it just didn’t need them.
So why did I calculate to only 5000rpm? Those old dual-plane 3×2 intakes were designed for early Chevy small blocks – 283 etc – which didn’t rev that great, and they are all sharp corners so they stop at 5000rpm pretty much. We saw 344.5HP at 5000rpm and it was almost flat up to 5600 before it starts dropping off. But the truth is, they provide bags of torque.
I think the important point here is that for a lightweight hot rod on the street, what you need for everyday drivability is low torque. If you drive your ’34 ford at over 5000rpm too often you’re a braver man than me! This set-up was making 362ft-lb at just 3000rpm. And on a progressive linkage, you’re only on one carb most of the time so fuel efficiency is good too. Hope this puts your mind at rest!
To Mike Green – We have spoken to Model A owners about this with mixed feedback. The 97 works best with around 2.5psi pressure max, but this could be pump or gravity pressure. I don’t know what the pressure is from a Model A tank by gravity and I’m too dim to work out the math, but you can bear this mind. When you put a 97 on a Model A, the carb sits very high up compared with the original carb so it is closer (higher) compared to the tank so there is less ‘head’ of gravity pressure. Also, a stock Model didn’t rev that high so the fuel bowl didn’t empty so quickly. When the tank is low will it provide enough fuel pressure to keep it full at higher revs? My own Model A has a flathead V8 with the fuel pump on the engine so I cannot answer that question! What I will say is that we have guys running 97s on Harley D’s with no pump and the carb almost level with the tank . So I guess the answer is give it a try!
To Mike Stark, Clive again. The market for used 97s depends on a bunch of things. Condition is everything as new parts cost money. Stromberg Carb does make everything available though, if they need anything. ebay seems to be the place where most stuff ends up and because its an auction the prices are real – not some inflated expectation. If you ‘watch’ a few auctions you’ll see what they finally sell at and you can make judgements off that. Max Musgrove in Utah has a good name in the old Ford and hot rod scenes for rebuilding 97s and he buys cores so you could email him to see what he typically expects to pay. – madmax1946(at)comcast.net