Modifying a Stromberg 175 Carb

[142 Guy]

Originally Posted by Burdekin

The dyno guy also said it was spot on, a little rich at max revs was all.

I had a look at your dyno curve when you first posted it and find that comment a little odd. At 5500 RPM your AFR has dropped to about 13.4. That is richer than stoichiometric; but, running a little rich at high engine load typically results in slightly higher horsepower. Most engines are volumetric efficiency limited and adding a little extra fuel helps ensure that all the O2 is used up. Of course, too much is not a good thing because you lose heat in evaporating the excess fuel which starts to reduce power output.

Your dyno guy may be suffering from 'digital EFI think' where it would be easy to tweak specific cells in a fuel map to push that AFR from 13.4 up to 13.7 around 5000 RPM. I suspect the Stromberg doesn't have that ability to adjust fuel delivery at 5000 RPM wide open throttle without having some knock on effects at lower RPM. Your AFR does seem to be excessive at around 2200 RPM; but, if part throttle AFR at 2200 RPM is good then wide open throttle AFR at 2200 RPM may not be an issue.

On the matter of potential modifications to the Stromberg to improve flow, you might want to consider mapping the manifold air pressure during a dyno run. New MPX 4250AP MAP sensors are fairly inexpensive (about $35 Cdn); but, a salvaged MAP sensor from just about any car will likely fit the bill (our local pick and pull salvage yard had a flat $5 charge for any sensor pulled from a wreck). The sensors almost universally have a 0-5 volt linear output and you can usually find the slope calibrations on line. With a 5 volt power supply, an analog input data logger and the sensor you can record your actual static manifold pressure during engine operation. If your manifold pressure is running just 3 or 4 kPa below atmospheric pressure during peak RPM wide open throttle conditions there is probably nothing to be had by beating your self up fiddling around with the Strombergs.