I have a 150cc motorcycle and am planning to install an after market air filter(K&N) since the stock one was favoring fuel efficiency and not performance.

The bike has a carburetor (Mikuni) not fuel injection, I was suggested that I should Jet the carburetor, can someone please explain how to do it?

  • by the way. What kind of bike is it? May 6, 2015 at 8:04
  • its a TVS Apache RTR 160
    – Shobin P
    May 7, 2015 at 6:23

1 Answer 1


Jetting a carburettor - short answer

You will require two things more than likely.

  1. A size larger main jet
  2. An adjustable needle jet

Smoothbore Carburettor Image

smoothbore carb

Your main jet sits in the float bowl, it's number 11.

Off idle and higher the main jet contributes more and more to air fuel mixture as you open the throttle to wide open where the main jet is 99% of your fuel delivery.

The tapered needle, number 3, gets pulled up in the main jet pipe and since it's diameter get smaller in the main jet pipe as it is pulled up in the pipe as the throttle opens, it delivers more fuel the higher it is in the main jet pipe.

In order to properly jet your motorcycle you will need to get a kit that provides those two components for our carburetors and install the two new components.

The kit may also have you perform a couple of tasks.

1. raise the float level in your float bowl so the fuel level is higher and thereby the mixture is richer. Increasing the height of the fuel allows the bernoulli effect to pull the liquid from the main jet pipe easier, due to it's proximity to the venturi of the carb.

2. Drill out air jet plugs. There will be plugs that hide your air jets. All Japanese manufacturers have done this since 1980. You need to carefully drill out the air jet plugs and pull them away without drilling into the air adjustment screw. This screw set's rich/lean for the idle circuit. They will probably have add fuel and pull back on the air screw a bit

These measure are to richen up the air fuel mixture. Having better airflow through your intake tract will force you to richen things up for the added air. If you do not, the engine will run hot because it has a higher % of oxygen in the mixture, this lean condition can burn a hole in the top of your piston as an extreme result in hyper lean conditions.

The instructions in most jet kits are very good with photo's. As well, someone has probably done it one youtube. Best of luck to you! Hope you get an extra few HP out of this.

Rich is better than leaner. An engine runs cooler with a rich condition. A very, very lean condition can destroy a motor.

Adjustable Aftermarket Jet Needles

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Non-adjustable OEM Jet Needles

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