How do I know if I need to Rejet my carb?

A clean spark plug means you can skip the rejet. A black, sooty-looking spark plug means you’re running rich with too much fuel in your mix. If you see white residue, that means you’re running lean with too much air. In both cases, you need a rejet.

How do I know what jets to put in my carb?

The old jets will have a number on the side of them to tell you how large the hole for the gas is. If you’re trying to give your engine more gas, install jets with a larger number which means it has a bigger hole. To make your engine run leaner, install jets with a smaller number and therefore with smaller holes.

Where is the main jet on a Mikuni carb?

In this power range, Mikuni carbs pull fuel from from the high speed circuit. As the throttle slide is pulled up almost all the way, the jet needle protruding from the bottom of the slide clears the needle jet and opens up a direct passage from the float bowl, through the main jet, directly into the mixing chamber.

How do I know if my main jet is too rich?

If the pilot circuit is rich, the engine will have a rough idle or may not return to idle without “blipping” the throttle. You may also notice a distinct smell of raw fuel and some eye irritation from the exhaust.

How do I know what size carb I need?

The formula for calculating how much CFM (cubic feet per minute) your engine requires is: CFM = Cubic Inches x RPM x Volumetric Efficiency ÷ 3456. Any ordinary stock engine will have a volumetric efficiency of about 80%.

Are Mikuni carbs good?

There are plenty of Mikuni and Keihin carburetors in the market are not authentic. They’re not safe to use and will cause many problems to your bike. If you think you’ve made a purchase and you’re not sure if it’s a genuine carburetor, here are some tips that will help you to identify them.

Does backfire mean rich or lean?

Lean Air/Fuel Mixture Not only can a rich air/fuel ratio cause a backfire, a mixture that doesn’t have enough gasoline can cause a backfire, too. When a lean mixture combusts, it burns more slowly, meaning there will still be some air and fuel that isn’t used up when the exhaust valves open — leading to a backfire.

When to change main jet in Mikuni carburetor?

If it’s white, the air/fuel mixture is too lean and a bigger main jet will have to be installed. If it’s black or dark brown, the air/fuel mixture is too rich and a smaller main jet will have to be installed. While changing jets, change them one size at a time, test run after each change, and look at the plug color after each run.

What happens to the needle jet in a Mikuni engine?

If the engine needs to run leaner, the clip would be moved higher. This will drop the needle farther down into the needle jet and cause less fuel to flow past it. If the clip is lowered, the jet needle is raised and the mixture will be richer. The needle jet is where the jet needle slides into.

What makes a Mikuni flatslide HSR carburetor so good?

However it is the dramatic improvement in throttle response that makes the Mikuni flatslide HSR carburetors such outstanding performers. No carburetor made today can match the Mikuni in this regard.

When did the Mikuni round slide carburetor come out?

Mikuni was founded in Japan in the early 1920s. During the 1930’s they were reproducing carburetors under license by Amal. The new Mikuni round-slide carburetor started hitting US and European shores in the mid 1960’s mounted on two-stroke, Japanese motorcycles.