View Full Version : SU Carb Mix...Air filters on?

05-17-2011, 09:08 AM
I've got the carbs pretty well set up. when I install the air filters do I go one flat lean or one flat rich to compensate for the foam restriction?

05-17-2011, 10:03 AM
I too am setting my carbs (strombergs though) and wondered about this. Since the air flow will be slightly restricted by adding the filters, won't the fuel be picked up at the same restricted amount so no change would be needed? I had planned to use the handy lift pins on my carbs to see if adding the filters changes anything.

05-17-2011, 12:22 PM
My observation with SUs and Strombergs is that air filters slightly richen the mixture. The slight obstruction they provide increases the depression inthe carb, causing more piston lift but without corresponding increase in airflow.
I'm not sure I can 100% reconcile the theory, but I will point out that Keihin, with their CD carbs based on SU-type principles, used to use flap-valve stranglers for starting.

05-17-2011, 01:12 PM
We don't have a lot of diagnostic tools available to us to determine the fuel air mixture that our carbs provide under the combined variety of driving conditions we encounter. Idle, acceleration, deceleration, cruising are all conditions supposedly compensated for basically with an air valve and and a tapered metering needle.
Lift the air valve, works pretty good, if done correctly, but I suspect most hold the air valve up too high or too long while deciding
I've come to regard reading the spark plugs after returning from your average drive is about as good as it gets for all practical purposes.

05-17-2011, 02:57 PM
FWIW I agree with your theory, Roger. Any restriction to air flow acts just like a conventional flap choke (on a fixed venturi carb). It increases the depression 'seen' by the fuel jet, and hence the fuel flow for a given air flow (meaning a richer mixture). One way to compensate would be to vent the float bowl to the inside of the filter (which is exactly what the later 'emissions' carbs do, and partly why they do it).

But my paper filters don't seem to present enough restriction at idle to make a noticeable difference. I get the same results with them on or off. If your filters are offering a significant amount of resistance at idle, then what are they costing you in full throttle, redline power (where the airflow may be 100 times greater) ?

I did some experiments once with the O2 sensor from a newer car mounted in my TR3A exhaust manifold. The results were somewhat surprising, in that the meter would swing from one extreme to the other with the slightest change in throttle. Trying to tune the carbs to be mid-scale (theoretically just enough air to burn the amount of fuel completely) under cruise conditions was both frustrating, and counter-productive. The car ran better with the "lift the piston" method.

But I still hope to pick up a "wide band" exhaust analyzer some day, and see what I can figure out with that. With gas projected to be $5/gallon by summer, it could easily pay for itself in just a few years.