Posted by Michael Frank on July 11, 1997 at 00:33:35:
In Reply to: SU Carburetors on Flathead Sixes? posted by Alan Bowes on July 09, 1997 at 18:36:49:
Having experience both with SU's and various American carbs, I feel compelled to reply.
I run triple SU's on my '69 Jaguar. The carbs have an undeserved reputation for being finicky. In fact, they commonly develop air leaks around the throttle body, making tuning difficult. This aside, a good SU is a joy. Properly set up, it will run for thousands of miles without attention. The constant-depression design minimizes flat spots and hesitation. And when it is time to rebuild, 30 minutes a carb is all you need.
You will need a manifold or adapter to convert. I have no idea where to get one for a 251. You will also need to invest in about $70 worth of special tools, since American mechanics have no clue how to set these carbs up. You will be on your own: but it is a simple job, once learned. All SU's are single barrel sidedraft carbs, which is why so many British cars have distinctive hoodlines.
Part of the initial set up will be selection of metering needles. Initial needle selection will be a big, big deal. SU makes thousands of needles. The proper way to select needles is to run the car on a dynamometer, and measure HP output at various RPM's the needles are then resized to provide smooth, optimum power output. Since you likely don't hava a dynamometer, you will need lots of needles, patience, and a good ear.
As to emissions, most PW's were built prior to major pollution restrictions. This being the case, a well set up SU will provide cleaner running and more power than, say, a Carter or a Holley. My Jaguar runs cleaner than most new cars.
My favorite reference on the subject is SU Carburetors Tuning Tips & Techniques by GR Wade. It is published by Brooklands Books, ISBN # 1 85520 2557.
Mike Frank