This is the most ‘Frequently Asked Question’ for us here at Stromberg Carburetor. It’s about the hardest one to answer. And the only person who can truly answer it is you. So please read this before you email us your full engine spec, zip code and cam timing sheet.

The Stromberg 97 – old and new – comes with 0.045 inch main jets and a Number 65 power valve for sea level use. But gasoline is not what it used to be, and even single Stromberg carb applications can benefit from a change of main jet and and/or power valve for optimum running. Most multiple carb applications will certainly benefit, too (we have covered that in a different article).

Yet the selection of main jet and power valve sizes has never been trickier. Your local elevation, engine tune, ambient temperature and humidity, your performance and economy expectations, and more, can all have a bearing on what jets are best, but the most important nowadays is your local gasoline (petrol) formulation. So it is all but impossible for us to answer jetting questions accurately.

So where does that leave you? To be honest, our best recommendation is to get the car running on what you have got (hopefully near to the standard factory jetting), then tune your jets by reading the plugs or get the car on a rolling road (preferably one who knows your engine and Strombergs carbs and is near to where you live so you get the same gas, elevation etc etc). They can analyze the exhaust, read the computer and optimize it electronically. Ask other local 97 users what they use, too. Either way, there may well be some jet swapping along the way. We can provide some guidelines, but in the end, each application will be different.

However you analyze your mixture, remember that the Stromberg 97 main jets come in almost immediately you touch the throttle off-idle and, in partnership with the fixed idle and transition ports in the base casting, they control the mixture pretty much on their own up to about 50% throttle when the power valve comes in, adding further fuel enrichment at the top end. So work on the main jets first. If you are reading the spark plugs, start with the main jet tuning by cruising at less than 50% throttle. Or just remove the carburetor accelerator pump links (flicking the spring off the pump fulcrum lever first) so the pumps cannot open the power valves. Ride around a while with a clean stop and then read the plugs. If you are rich you can go for smaller jets. Then when you have the main jets dialed in, you can reconnect the springs and accelerator pump rods and give it a little more freeway speed – ie over 50% throttle – ideally without sudden heavy acceleration so you don’t engage the accelerator pump discharge jets (ie squirt raw gas into the engine) if possible. That will then allow you to see what is happening when the power valve add enrichment at the top end. Most multiple carburetor systems end up with smaller power valves.

Local Gasoline
There are apparently at least 100 different gas formulations on sale in the US at any one time and it varies with altitude, the typical local temperature/humidity range (summer, winter etc), state laws, ethanol content (growing all the time) and other factors. In California, for example, there are many different formulas mandated by the local air quality authorities, and these are changed throughout the year. It is not a problem for modern engines as fuel injection automatically compensates. Carburetors don’t have electronic brains, but that is why we like them, right?
Blended fuel (high ethanol content) is another problem. Although the pump may not indicate that the fuel is blended, it is always advisable to verify the type of fuel the station carries (or stick to a station you know). Larger jets (a richer mixture) may help when running on fuels with high ethanol content. But, remember too that blended fuels are more volatile than gasoline, so difficult hot starting and poor hot weather driveability may result.
Stromberg 97 expert Jere Jobe once told us that his late model Ford Explorer V8, equipped with Montana smog devices averaged 23.5 miles per gallon on the highway on Montana gasoline. Under nearly identical conditions the same vehicle would average 17.5 miles per gallon on California gasoline. That fact alone says a lot about the efficiency of the various fuels. And why your hot rod feels like it has lost a cylinder when you drive out of State!
Altitude. As a rule, if you live at altitude, you need to look towards smaller jets and power valves because the air is thinner with less oxygen, so you need less fuel to maintain a good air/fuel mixture. Original Ford specification called for 0.043″ main jets for 5000-10,000ft, 0.041″ for 10,000 to 15,000 and …wait for it…0.039 for ‘15,000ft and over’!

Engine tune
Higher capacity engines and those with bigger cams/higher volumetric efficiency will generally flow more air or flow the same air faster. So bigger jets may be required. Just remember though, that on the road you need low end tractability and that does not necessarily mean jetting for max top end power at the expense of everything else. Less power. More torque!

S or W?
Stromberg 97 and 81 carburetors (not 48) have Summer and Winter settings for the accelerator pump rod link (at the bottom end). This adjusts the volume of fuel squirted into the intake manifold if you accelerate quickly – W gives you more gas – but it should not change the idle setting nor the point at which the power valve is opened. Just be aware though that it can change the way your engine behaves on the road.

Changing jets
There is no easy way to change the jets in a Stromberg 97 (and its brothers the Stromberg 48, 40, 81 LZ etc). You get at them from underneath, so you are going to have to take the carbs off to do it. The power valves are replaced from the top – they are under the accelerator pump. You’ll find full instructions, with safety notes, under the ‘How To’ tab in our Tech Center for jets (9533k) and power valves (9594k). And here is a time-saving tip. Leave the carb base or bases fixed to the intake (so you do not disturb the linkage set-up), and just remove the three big screws that fix the bowl section to the base. Oh, and buy a Genuine Stromberg Premium Jet wrench (9071K).

A final note
The art to Stromberg jet tuning is to make small changes and most people move just 0.002inch difference in main jets (eg. from 0.045 to 0.043) and one or two ‘numbers’ in power valves (eg from 65 to 64 or 63) at a time. Remember too, that Power Valves use engineer’s number drill sizes, so a smaller valve has a higher number. With a single carburetor, even if you move to Colorado, if you need to change more than say 0.004inch (main jets) and four numbers (power valve) from standard, you may have ignition or carburetor problems that need your attention first. Bigger jets will not solve those air-leaking throttle shafts, for example.

And remember, while neither is good, too rich is better than too lean. Detonation can wreck your engine in seconds.
As with all our Tech articles, we welcome customer feedback and other input. Email us with your thoughts and if it adds to the debate, we can add it in..tech@stromberg-97.com

The new Genuine Stromberg 97 fits wherever an old Stromberg 97 does. So let’s take a look at the usual applications, and everything else you might need to know for alternative uses – manifold stud pattern/spacing/air cleaners/hood clearance/throttle and choke linkage/fuel lines/distributor vacuum and flow in cfm.

Stock, hot rod and alternative applications

The new Genuine Stromberg 97 is built to original blueprint specification (which some improvements), so it fits wherever an old Stromberg 97 does. Let’s take a look at the stock Ford and hot rod applications, then examine everything else you might need to know for alternative uses. This includes manifold stud pattern and spacing/air cleaners/hood clearance, throttle and choke linkage, fuel lines, distributor vacuum and flow in cfm.

The Genuine Stromberg 97 has been around a long time, so let’s cover the traditional applications first. The 97 was standard on Ford V8 from around 1935 to 1938. They were also offered as an aftermarket replacement for Ford V8 from 1932 right up to 1953, though the ‘49-53 applications used a slightly different model (for distributor vacuum, which we’ll come to later). The 97 can also be used as a replacement for the smaller Stromberg 81, which was standard on the small Ford V8-60 engine from 1937-40. No, they’re not too big! They work great – maybe we’ll explain that later too. There’s a whole bunch of new 97s running around on Harley Davidson V-Twins. Go figure.

Then there’s the hot rod applications. As the 97 became the hot rod carburetor of choice, so all the early multi-carb intakes were set up for 97s right through until the muscle car era. Hot rod multi-carb intakes for 97s are still available new for Ford flathead V8s, early Ford 4-cyl engines (Model A,B,C), early Chevy straight 6, Chevy small block V8 and early Cadillac V8. And intakes for other vintage overhead valve engines can still be found at swap meets.

Alternative applications

We are often asked about conversions – making new intake manifolds or adapter plates to fit 97s on for new engine applications. The 97 is a very simple carburetor which makes it ideal for adapting, so let’s look at what you need to bear in mind: Manifold stud pattern and spacing, carb height/hood clearance, throttle and choke linkage, distributor vacuum and carb flow in cfm. Most of these are covered in more details elsewhere, right now we’ll just cover the main points.

Manifold stud pattern and spacing, air cleaners and hood clearance

Our 97 carburetor has a three bolt intake mount – the same on Ford V8 from 1933 right up to 1953. If you’re making an adapter plate, the key dimensions are:

Bore centers are 1.34 inches apart. The two bores are 1 3/16 inches diameter. The two front fixing bolts are 3.12 inches apart and 0.88 inches from the carb centre (throttle) line. And the third rear bolt is centered on the front two, but 1.88 inches back. Aftermarket intakes tend to vary but if you are making an adapter plate and you set the carbs 4.75 inches apart you’ll match the Offenhauser standard and you’ll get a Stromberg TwoStep linkage to fit too.

The OD of the casting at the top where the air cleaner fits is 2-5/8th inch and the overall height of the 97 is 5-5/8 inch plus the intake gasket, though the air cleaners/scoops sit 5/8 inch down over the top of the carb mouth.

Throttle and choke

The 97 throttle and choke levers were designed to work with solid rods to the pedal/choke and have a 0.275 inch OD ball end fitting. Aftermarket 97 models designed originally to fit the 49-53 Fords had a different choke lever for a cable choke (with a bracket to hold the cable outer) and we offer this conversion both as a kit (9552K-C) or a complete carburetor with the cable choke parts installed (9510A-C).

Something else. The standard 97 throttle lever is designed to be pulled from the firewall end. If your throttle pedal pushes (like the 1949-51 Mercury for example) you need our 9581K-LZ kit or 9510A-LZ carb with it installed.

The fuel inlet is usually the easiest part of any conversion. We offer brass hose end fittings (9080K) and stainless steel banjo fittings with one of two outlets (9082K and 9083k). If you want to build hard lines, we offer ‘Ford Nut’ compression fittings (9081K) to install 1/4inch hard lines into the S-jet inlet fitting. The thread into the 97 inlet is half inch by 20 teeth per inch.

Distributor vacuum

There is no ported vacuum take-off on the regular Genuine Stromberg 97 carb, so if your distributor can use ported vacuum (and please check the spec) you will need to specify your new 97 with the -VP (vacuum port) option. All Stromberg BIG97 Primary carbs have this port as standard. There’s a separate Tech article, “Stromberg 97 and Vacuum Advance. Your Questions Answered” on this Tech Center.

What we don’t recommend is drilling the base casting to install a vacuum take-off. A few rare Stromberg 97 models (called the ‘1-1’) did have a vacuum port built in to the base, but this was specifically for the 49-53 distributor which had no base-level mechanical advance built in. So the carb provides vacuum from at least two different points through special base and bowl castings.

In case you’re wondering, the early Ford 32-48 crab-type distributors use a vacuum brake, not advance, which takes vacuum off the intake manifold. Completely different deal.

Flow in cfm

Whatever conversion you are planning, you need to ensure you have enough flow through the carburetor to feed the engine underneath. There are plenty of places on the internet that will help you work out your requirements based on cubic capacity, maximum RPM and volumetric efficiency, so let’s just say that the Genuine Stromberg 97 has been independently measured at 162cfm per carburetor measured at the 1.5inch Hg pressure drop measure that is usually quoted for 4bbl carbs. Traditionally, 2bbl carbs were measured at 3inch Hg pressure drop, but in reality carbs never see that much vacuum at full throttle. The 3inch measure comes out higher so sounds good, but we believe it’s misleading.

Just remember, as a rule you can run a big carb on a little engine, but a small carb on a big engine will run out of cfm and literally choke off the revs.

Stromberg 97: 0.045in main jets and a Number 65 power valve. All new Genuine Stromberg 97s are supplied with this as standard.

Stromberg 40 and 48: 0.048in main jets and a Number 63 power valve (a 63 PV is a bigger jet than the Number 65).

Stromberg 81: 0.035in main jets and a Number 71 power valve.

Remember though, that this was the standard jetting on 1930’s gasoline at sea level. Ford supplied smaller jets for cars supplied at high altitude. And modern gasoline is nothing like it was back then. Nowadays, you need to consider the best jetting for your own application, and the local gas and local elevation, engine tune, the season (ambient temperature and humidity), your performance and economy expectations, and more, can all have an effect on the jetting you should use.

The short answer is no. But here’s the long one…

Adjustable main jets sound like a great idea. No need to buy extra jets or spend hours swapping them out. But we don’t make them and here’s why (quite apart from the fact that they look weird).

Call us old fashioned, but we want to know what jet sizes we’re running so we can balance the carburetor set-up so each is doing equal work. With adjustable jets you won’t even know whether each barrel of the same carb is jetted the same, let alone each carb. So your intake could be rich at some cylinders and lean at others. And we all know what lean cylinders can do for your engine. And what if they slip with engine vibration? In our opinion, it’s not the best way to fuel your engine.

Ok, so swapping jets the old fashioned way can be lengthy and drive you nuts. All we can say is, try to think of it as ‘all part of the fun’ – tinkering with your engine. That’s what hot rodding is all about…

Original Stromberg drawings don’t call for a seal here, even though it’s below the fuel line. There’s just a small seat and a fine thread. Try tightening the screw (but remember that it’s not meant to be flush with the casting), or add a super-light smear of sealant/instant gasket on the thread. Just don’t get any inside the fuel bowl.