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How did Harley carburetors progress from their primitive beginnings to the state-of-the-art CV design they use now?

I know the answer because I played a role in the process.

Back in the early 1970s, I was working in Southern California and getting pretty burned out tuning stock Harley carbs. So were other hard-core H-D engine builders in the area.

Some of us decided to look for a cheap replacement. A couple of guys went out to a foreign car boneyard and picked up two SU carbs from an old British sedan.

The SU carb, made in England, was an advanced design for its day. In the 70s they were readily available in sizes from about 1¾” to 2”, depending on what kind of British rust-bucket you scavenged them from.

The tricky part was tuning the things. This carburetor was unique in that it was designed to meter the fuel and the air simultaneously by using a piston and needle with the standard manually operated butterfly. This design is now known as the constant velocity (CV) carburetor, and it’s used on motorcycles almost universally.

But it wasn’t back then.

It took a couple of years of trial and error testing but we finally got the tuning of these SUs right for Harley-Davidsons. Gradually the word spread from the tuners to the dealers and manufacturers, who talked to their connections, until . . . SU in England announced a CV carburetor especially for the Harley-Davidson motorcycle!

Their CV carb looked a hell of a lot like the ones we developed, and when properly tuned gave more horsepower and better gas mileage with no transitional flat spots. Just like ours!

About 20 years later the H-D factory figured it was about time to put a CV on their new models. When I see one I think to myself, “Thanks, pals. Nice work, Perry.”

Later came electronic fuel injection (EFI) and another new tuning challenge.

The solution to H-D’s early EFI is laid out in the May 1997 issue of Hot Bike Magazine. I am proud to say that FLO Headworks played the leading role in that project, too.

And since that date we have managed to pick up another seven horsepower with an otherwise stock displacement 80 C. I. engine with 8:1 compression, using my blueprinted CV Keihin carbs and 700cfm air filter kit.

So far SU haven’t sent me any checks, and I haven’t got any thank-you letters from the engineers at H-D either. But my customers are happy, and that’s been the most important thing for me over the past 25 years.

Setting up your carbs

A general rule about setting up carbs is to expect your fuel consumption to rise proportional to newly-found horsepower. When setting up your carb, the best approach is to set it up on the rich side, then lean it back until you get quick throttle response. Tuning too lean is dangerous, because if can burn out your engine.

Idle adjustment should not be a problem on your Harley’s carb. If idle isn’t smooth and steady, look for a vacuum leak in the intake manifold/cylinder head seal area. An old mechanic’s trick to check for such a leak is to use a spray can of carb cleaner and a straw. With the engine idling, direct the spray strongly around each clamp. If the engine RPM changes suddenly, you’ve found the problem. Vacuum leaks at manifold joints are common and should be checked for frequently.

Another common problem is “spitting back” through the carburetor. This indicates that the mixture is too lean.

Black smoke out the exhaust indicates a too-rich mixture.

Checking your plugs

You can get an idea of your carb’s mid-range mixture quality by checking spark plug color.

Here’s how to do a reliable spark plug test: Grab some heavy gloves, an extra pair of plugs and a plug wrench, and head out to a flat, empty stretch of highway. Maintain 60 – 70 mph for at least five miles. Shut off the engine at cruising speed and pull in somewhere safe. Before the engine cools, pull your plugs and have a close look at them in good light.

Observe the round, flat surface perpendicular to the threads. This area should be mid-brown/gray. Black indicates you’re running too rich in the mid range, white means too lean.

Take into consideration your altitude (higher means a richer-running engine), barometric pressure, air temperature and the conditions under which you normally ride. It’s best to tune in the same conditions, and preferably on a warm, clear day when the barometer is high.

Some other carb adjusting tips

Oil consumption can have an effect on your plug color.

The main jet is easy to adjust, but requires more speed. You can get a feel for the main jet in lower gears, but the ultimate test is flat out.

And again, plug color is the final indication of mixture.

There are quicker ways to get you to the ballpark. When your motor is revving over 4000, is it heavy-feeling, sluggish, and unwilling to pull strongly? This probably means the mixture is too rich.

Rev up to redline, then back off the throttle slightly. If the motor wants to run faster with the throttle backed off, the mixture is probably too lean.

As always, check your plug color – it’s the most reliable indicator of carb tune.

For tuning advice specific to your Harley engine and model, call the FLO Headworks customer service hotline at 805-481-6300.

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