did Harley carburetors progress from their primitive beginnings to the
state-of-the-art CV design they use now?
know the answer because I played a role in the process.
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.
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
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.
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.
it wasn’t back then.
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
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
Black smoke out the exhaust indicates a
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
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.
tuning advice specific to your Harley engine and model, call the FLO
Headworks customer service hotline at 805-481-6300.