Adjusting air/fuel screws on brand New England Stromberg 97

[Digger]

My roadster was running a little rough and sputtering, so I treated myself to a brand new England made Stromberg 97. I put it on the car and like Koates says below it does work great right out of the box. However two things I noticed, the first is upon idle in the driveway it does have a little bit of fender shake, body shake, and when driving feels like I could be getting more smooth power out of it. Therefore when I go to adjust the air/fuel screws, what is the best procedure? Concerned about getting it so whacked that I won’t remember how to get it back to at least where it is now. Any advice appreciated


Koates post:
The Stromberg 97 reproduction carb is made in England by Stromberg. It is an excellent piece of equipment and likely better than the original 97s. I have used three of these on 34 and 35 Fords and they work perfectly right out of the box. Parts are all interchangeable with the original 97s and spare parts are all available including various jet sizes. One draw back is they are not cheap at close to $500. All the other 94s and 97s are made in China or the parts are made in China and many reports of problems with many of these but they are cheaper. Pay the money and get the quality product. Regards. Kevin.

[tubman]

On a fresh installation, I start with both screws 1 1/2 turns out (seat them gently all the way in and then back them out 1 1/2 turns). Start the engine; it should run half way decent at this setting, if not something else is probably wrong.) Set the idle to factory specification; somewhere around 500 RPM. Slowly, screw one idle screw in until the engine starts to misfire. Back that screw out just a bit, and do the same with other. If you find the idle speed has changed, reset it using the screw on the throttle arm and redo the mixture screws. Repeat until you get the smoothest idle at the proper idle speed.

[Fritz]

They’re great carburetors. I follow the instructions that come with it:

https://www.stromberg-97.com/cms/upl...manual_web.pdf

[Ronnieroadster]

To set the air/fuel mixture screws to a setting the engine likes. I always use a vacuum gauge. When you move each screw watch the gauge your looking for the highest reading on the gauge. This I have found to be the best setting for single and multiple carbs as well.
Ronnieroadster

[Terry,OH]

Disconnect the wiper hose and install vacuum gauge to the tube. Adjust one screw at a time slowly for the highest vacuum go back and forth between the two adjustments until engine idles smooth and vacuum is high. If the car starts and idles there is no need to go to a starting point with the adjustments.

[Digger]

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Great ideas, any recommendations on vacuum gauges? Don’t have one but will buy

[tubman]

Didn't know if you had a vacuum gauge or not, which is why I didn't mention it. They are very useful as others have said, and I suggest that you get and use one. I also use mine to set the initial idle timing on modified engines where the factory specs may no longer apply.

[Mart]

Originally Posted by Digger

Great ideas, any recommendations on vacuum gauges? Don’t have one but will buy

No, except to say you want a service vacuum gauge rather than an in dashboard one.

I watched coldwarmotors this morning and Scotts friends are using a vac gauge to set the idle on a 340 mopar.

It might be worth watching. Caution: expect some friendly robust language.

https://youtu.be/T1L3H40jn0Y

[Digger]

Gauge on order will report back

[Bored&Stroked]

One thing to note is that the idle screws are just for idle - they have nothing to do with your 'open road' performance and drivability.

You may need to explore different jets to tune the mixture to meet what the engine needs.

[Digger]

B&S: any suggestions? All stock car….

[Bored&Stroked]

Originally Posted by Digger

B&S: any suggestions? All stock car….

The stock jetting on most 97's is probably pretty close. If you feel the engine is surging a bit - which can be a lean condition, you might try buying two jets that are 2 steps bigger - which should be 47's.

Before I changed jets, I would check my float level - as this is very important in general. Also, what orientation do you have the fuel-pump push rod set too? I tend to use the 'outer' spot on the lower linkage - more squirt has usually worked better for me on most cars (stock or performance).

Lastly, what type of distributor do you have in it and if it is stock and has not been reworked by Bubba, Charlie or Michael, then it probably has the stock advance system. Replacing the stock advance system with the later one (around 1940 or so) can really make these engines run better. The advance curve is better on the later units.

Best of luck!

PS: That chrome fuel pressure regulator with the dial . . . they're almost useless and cause more issues than they solve. If you really want a decent regulator, then you need to buy the low-pressure Holley one, include a gauge with it (so you can accurately set it) and mount it AFTER the mechanical fuel pump.

[Digger]

So circling back on this post, here is the answer of how the distributor fixed the carburetor!

I had Mike Hubbard rebuild my original carb thinking maybe the brand new Stromberg out of the box was slightly off? So this morning at cars and coffee we swap the carburetors and as we were getting ready to fire it up, he looked in there and said hey what’s up with this distributor cap?

The driver side cap was not even connected to the body of distributor! It was just hanging there, so since the perceived “carburetor problem”, I have been driving around on four cylinders! So of course reconnecting the cap and getting all eight cylinders, it runs like a champ again like it did when I bought it. All I can figure is the car wasn’t driven much by the previous owner and since I’ve been driving it a lot maybe the retaining ring was not really secure, it came loose during driving at some point. So now I have a nice brand new back up carburetor sitting on the workbench!

[koates]

You should fit that new Stromberg carb that is sitting on your shelf back on your engine. It is high quality made and brand new with no wear in anything and would most likely run better than the one you have on the engine now. Funny how all fuel problems are electrical and all electrical problems are fuel. Those new Strombergs are better than the originals in quite a few areas so use it. Regards, Kevin.

[tubman]

An old adage from my racing days : "Always run your best parts".

[Flathead Fever]

There is no such thing as a carb out of the box that is adjusted and ready to go. Even a factory carburetor coming off an assembly line would need to have the final idle speed and idle air fuel ratio adjusted for each individual engine. And its jetting might need to be altered once it gets to its final destination due to altitude differences. Then if were talking about an engine that has been rebuilt and the bore has been changed or it's an older engine that is worn and it doesn't pull the same vacuum that it once did, that would require carb adjustments. Every carburetor will at a minimum need to have its air fuel ratio and final idle rpm adjusted.

The very first adjustment on a carburetor is the float level. If it is too low the engine will run lean and if it's too high, it will run rich. If the float level is off all the other circuits will not work correctly. We're going to assume that since its new carburetor the float level was set correctly at the factory.

The idle air fuel mixture and idle speed are always the last adjustments made on an engine. First the point gap (dwell) is adjusted. That is because changing the point gap also changes the timing. Next is to set the timing. Changing the timing obviously does not affect the point gap but changing the timing will affect the air fuel mixture. So, before you adjust the carb you always check the dwell and timing first.

A modern automobile's computer makes precise idle air/fuel adjustments to compensate for altitude, air temperature, engine temperature, load and rpm. It takes input signals from various sensors and compares them to an algorithmic chart that tells the computer the most likely amount of time to pulse on the fuel injector for the correct mixture, but it's not done yet. The computer will then look at the exhaust's oxygen sensor and check what the exact air fuel ratio was and then further fine tune the air fuel. Your carburetor cannot do all of of that. The carburetor's idle air fuel setting is the best single compromise for a bunch of constantly changing factors. Once those screws are adjusted its stuck with that amount of fuel delivery at idle.

The majority of the time it should idle smooth but at times it might idle slightly rough. So when we adjust the idle air fuel mixture on a carburetor, we want to compensate for those environmental differences by adjusting the idle mixture screws slightly to the rich side.

Many people use a tachometer and adjust for A/F ratio for highest rpm at idle or a vacuum gauge for highest vacuum. That is how I did it until I became
a smog shop test and repair mechanic in 1982. The State of CA mandated that we use the propane enrichment method. They wanted every vehicle tested to have its carburetor adjusted to the lean side. We leaned out the carbs until the rpm dropped 50 rpm. That produced the least amount of emissions at idle. It also meant the engines ran rough or died with the air conditioning was tuned on. If not used to CA specifications it is an excellent way to adjust a carb to the optimum idle A/F ratio.

Set the idle rpm to your desired rpm. and turn off the engine Turn the air fuel mixture screws in while counting the number of turns, gently bottoming them out. CAUTION, over tightening them can distort the needles and seats. Mark the top of the screws with a dap of white paint so you can turn them back out the identical amount of turns as they were. Usually, 1 1/2 turns are the normal starting point on most carbs. Both screws must always be turned the same amount of turns so the fuel delivery is balanced. Start the engine. Pass your slightly propane or map gas bottle over the carb and if the idle goes up this indicates the mixture is too lean, the engine liked that extra fuel you gave it. The extra fuel made more power which created more pressure on the piston which forced it down faster (the rpm went up). If the rpm went down that means the engine was already getting too much fuel and you made it worse. When you have them adjusted properly the propane will have no effect on the rpm. Now if you want you can adjust them out another 1/8 to a 1/4 turn to the rich side to help compensate with those environmental changes we discussed. You will need to adjust your final idle rpm.

[Digger]

Yes there is such a thing as a carburetor right out of the box it’s ready to go, because I put a brand new Stromberg on my three window last year so the boxes worked perfectly made zero adjustments and runs like a champ, self everybody has their own opinion about that in their own experiences about that

[Steve/IL]

Digger,
I'm sure it ran like a champ, but with a little tuning it may run better.

[glennpm]

As B&S said, "PS: That chrome fuel pressure regulator with the dial . . . they're almost useless and cause more issues than they solve. If you really want a decent regulator, then you need to buy the low-pressure Holley one, include a gauge with it (so you can accurately set it) and mount it AFTER the mechanical fuel pump."


Holley 12-804
Low-Pressure Fuel Regulator (std. finish)
Low-Pressure Fuel Regulator (chrome finish)
1 to 4 PSI 3/8” NPT Ports