Tips to Tuning the Carburetor (2)
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 bestapproach is to set it up on the rich side, then lean it back until you get quick throttle response.
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 bestapproach 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.
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