The Yamaha G1 Mikuni BV21-16/BV21-161 carburetor uses a #97.5 main jet and #42.5 pilot jet. These are factory service-manual specifications for the documented G1 carburetor.
G1 • Main & pilot jet sizes
G1 carburetor jet sizes at a glance
Main jet
#97.5 standard.
Pilot jet
#42.5 standard.
Altitude
Use the G1 service specification for the matching BV21-16/BV21-161 carburetor; do not substitute G2 jet sizes.
Before ordering
Identify the engine and carburetor, then read the number stamped on the existing brass jet.
Lean symptoms
Needing choke, surging or load bog can also come from a clogged passage, weak fuel supply or intake leak.
Rich symptoms
Black smoke or plug fouling can also come from a stuck choke, high float level or leaking needle valve.
How to choose the correct Yamaha G1 jet
G1 jetting symptoms
Needs choke to run
Clean the pilot and main circuits and check fuel supply and an intake leak before fitting a larger jet.
Bogs under load
A restricted main jet can cause this, but so can weak pump flow, low bowl fuel level or a blocked filter.
Black smoke / fouled plug
Check choke position, float level and needle sealing before assuming the main jet is too large.
Runs well only at low speed
Main-jet or main-nozzle restriction becomes more likely once the pilot circuit is known to be clear.
Poor idle only
The pilot circuit matters much more than the main jet at idle.
High-altitude operation
Use the model-specific elevation chart where available; do not guess how many jet sizes to reduce.
G1 jet specifications
Applies to: Carbureted gasoline version.
Engine: Yamaha 2-stroke gasoline.
Carburetor: Mikuni BV21-161.
Main jet: #97.5 standard.
Pilot jet: #42.5 standard.
Altitude: Use the G1 service specification for the matching BV21-16/BV21-161 carburetor; do not substitute G2 jet sizes.
Model-specific note
Yamaha G1 service data lists a #97.5 main jet and #42.5 pilot jet for the documented Mikuni BV21-16/BV21-161 carburetor.
Yamaha G1 carburetor jet FAQ
What is the Yamaha G1 main jet size?
#97.5 standard. Yamaha G1 service data lists a #97.5 main jet and #42.5 pilot jet for the documented Mikuni BV21-16/BV21-161 carburetor.
What is the Yamaha G1 pilot jet size?
#42.5 standard.
Does altitude change the Yamaha G1 main jet?
Use the G1 service specification for the matching BV21-16/BV21-161 carburetor; do not substitute G2 jet sizes.
Can I install a larger jet in a Yamaha G1?
Only after confirming the engine, carburetor, altitude and cause of the lean symptom. A clogged passage, intake leak or weak fuel supply should not be corrected with an oversized jet.
How do I identify the correct Yamaha G1 carburetor jet?
Use the stamped jet number together with the engine code, carburetor ID and model year. Cross-check those identifiers against the matching factory parts or service information.
What is the difference between the main and pilot jet on a Yamaha G1?
The pilot circuit controls idle and very low-throttle fueling. The main jet becomes dominant as airflow, throttle opening and engine load increase.
Jet sizes can change with engine, carburetor revision, model year and altitude. This page applies to the carbureted gasoline version. Do not drill jets or assume a jet from another model is equivalent. Confirm the engine, carburetor ID and existing jet markings for your exact Yamaha G1. For manufacturer documentation, visit the official manufacturer website.