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WHAT JETS SHOULD I USE?
This is one of the most ‘Frequently Asked Question’ for us here at Stromberg. 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. On the one hand, your local elevation, multiple carb applications, local gasoline (petrol) formulation, engine tune, ambient temperature and humidity, your performance and economy expectations, and more, can all have a bearing on what jets are best. But on the other, the biggest influencing factor on what jetting you need is actually the size of your carburetor’s own venturi. So always bear in mind, certainly with Strombergs, that moving more than three or four jets sizes from stock is unusual, and if required, might indicate a problem elsewhere. One example which springs to mind is restrictive air cleaners. Another is excess fuel pressure. Bigger jets will not solve those air-leaking throttle shafts, either.
So where does that leave you? To be honest, we usually recommend that you get the car running on what you’ve got (hopefully near to the standard factory jetting but always check!), to set a base level by reading the plugs or better still, using an exhaust lambda sensor to tell you what the AFR (Air Fuel Ratio) is under certain running conditions. These might include idle, cruise, Wide Open Throttle etc. You can also ask other local 97 users, of course.
However you assess the rich/lean condition, 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) too much. 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.
Multiple Carburetors
In our own experience, multiple carbs usually end up with smaller jets than stock for regular street use. For example, we see a lot of flathead Fords with 0.043 main jets and number 67 or 69 power valves (the same in all carburetors) – at regular elevations. There are various reasons for this, including the lower vacuum seen at each carburetor.
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. 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. More importantly if you do choose the S setting, double check that the throttle linkage does not go over-centre. While the original Ford applications were designed to prevent this, your own set-up might not incorporate this fail-safe.
Changing jets
Stromberg jets are not the easiest to change. You get at them from underneath the float bowl. The power valves are replaced from the top – they are under the accelerator pump, or in the float bowl if you have a Stromberg BIG97. 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. 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.
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
