Yamaha Blaster Jetting Guide – Mikuni VM26 Carburetor
Correct Yamaha Blaster jetting is critical to performance, throttle response, engine temperature and engine life. The factory Mikuni VM26 carburetor is capable of working extremely well on a stock Yamaha YFS200 Blaster and can also support many common performance modifications when it is properly jetted.
The problem is that there is no single main jet that is correct for every Yamaha Blaster.
Exhaust systems, airbox modifications, air filters, reeds, cylinder porting, temperature and elevation can all change the amount of fuel the engine requires. A main jet that works well on one Blaster may be too rich—or dangerously lean—on another.
This Yamaha Blaster jetting guide explains how to establish a safe starting point for the factory Mikuni VM26 carburetor, how each carburetor circuit affects the engine, and how common Blaster modifications influence jetting.
Important: Jetting specifications are starting points. Begin safely rich and verify the final setting on your own engine.
Yamaha Blaster Stock Mikuni VM26 Jetting
The Yamaha Blaster uses a 26mm Mikuni carburetor from the factory.
A common stock baseline is:
- Main jet: 230
- Pilot jet: 32.5
- Needle: Stock needle
- Needle clip: Middle position
- Air screw: Approximately 1.5 turns out as an initial setting
A completely stock Blaster with the factory exhaust, airbox lid installed and an engine in good mechanical condition may operate near the factory 230 main jet.
Once airflow is increased, however, the engine generally requires additional fuel.
Removing the airbox lid, installing an aftermarket pipe, changing the filter, modifying the reeds or porting the cylinder can change the required main jet considerably.
Never assume the stock 230 main jet is safe after modifying the engine.
Use the KOR Yamaha Blaster VM26 Jetting Calculator
Rather than relying on a single jetting chart, Ken OConnor Racing developed a Yamaha Blaster VM26 Jetting Calculator specifically for the factory 26mm Mikuni carburetor.
The calculator accounts for several variables that affect Yamaha Blaster jetting, including:
- Exhaust system
- Airbox configuration
- Air filter
- Reed setup
- Engine modifications
- Outside temperature
- Elevation
USE THE YAMAHA BLASTER VM26 JETTING CALCULATOR
Use the calculated result as a conservative starting point. Final jetting must still be verified on the individual engine.
Yamaha Blaster VM26 Main Jet Starting Points
The following specifications are intended as approximate starting points for a Yamaha Blaster using the factory Mikuni VM26 carburetor.
Stock Yamaha Blaster
Stock exhaust, airbox lid installed
Start around:
230 main jet
This is the factory baseline and assumes the engine and intake system remain essentially stock.
Stock Exhaust With Airbox Lid Removed
Increasing airflow through the airbox generally requires a richer main jet.
A reasonable starting range is:
240–250 main jet
Start with the richer jet and work downward only after confirming that the engine is rich.
Pro Circuit Exhaust
For a Yamaha Blaster equipped with a Pro Circuit exhaust system, a useful conservative starting point is approximately:
260 main / 32.5 pilot
Additional intake modifications, porting, temperature and elevation may require a different main jet.
Toomey Exhaust
The Toomey exhaust typically requires considerably more fuel than a completely stock Blaster.
A common starting point is approximately:
290 main / 32.5 pilot
An open airbox, aftermarket filter, reeds or porting may require additional fuel.
Bills Exhaust
For a Yamaha Blaster using a Bills exhaust system, begin around:
260 main / 32.5 pilot
Treat this only as an initial setting and verify the mixture under actual operating conditions.
CT / LRD and Other High-Flow Right-Bend Pipes
High-flow right-bend exhaust systems such as CT and LRD can require substantially larger main jets than a stock Blaster.
Depending on the intake system, cylinder work and environmental conditions, combinations using these pipes can require main jets into approximately the:
320–360 range
Do not install a specific jet simply because another Blaster with the same pipe uses it. Start conservatively rich for your complete combination and tune from there.
Why Yamaha Blaster Jetting Changes
A carburetor meters fuel according to the amount and velocity of air passing through it.
Anything that changes airflow through the engine can change the required fuel mixture.
Common modifications that require checking Yamaha Blaster jetting include:
- Aftermarket exhaust pipe
- Aftermarket silencer
- Airbox lid removal
- Airbox modification
- High-flow air filter
- Reed-valve changes
- Cylinder porting
- Increased displacement
- Compression changes
- Carburetor changes
- Oil pump delete
- Significant elevation changes
- Major temperature changes
Even if the engine ran perfectly before a modification, do not assume the existing jetting remains correct afterward.
Understanding the Mikuni VM26 Carburetor Circuits
Proper carburetor tuning is much easier when you understand which circuit you are actually adjusting.
Pilot Jet and Air Screw
The pilot circuit primarily controls idle and very small throttle openings.
The stock 32.5 pilot jet works well in many Yamaha Blaster VM26 applications and does not normally require the dramatic changes commonly made to the main jet.
Begin with the air screw approximately 1.5 turns out.
With the engine fully warmed, adjust the air screw to achieve the best idle and cleanest response.
If an engine cannot be adjusted correctly within a reasonable air-screw range, inspect the pilot circuit and mechanical condition before assuming that a larger main jet will solve the problem.
Jet Needle and Clip Position
The needle has its greatest influence through the middle portion of the throttle range.
Moving the needle clip up lowers the needle and makes the mixture leaner.
Moving the clip down raises the needle and makes the mixture richer.
Make one change at a time.
If the Blaster runs correctly at wide-open throttle but hesitates, surges or sputters during part-throttle operation, the needle circuit may require attention rather than the main jet.
Main Jet
The main jet has its greatest influence at large throttle openings, particularly approximately three-quarter throttle through wide-open throttle.
A larger main-jet number provides more fuel.
A smaller main-jet number provides less fuel.
When establishing unknown jetting, begin on the rich side.
It is considerably safer to begin with a main jet that is too large and work toward the correct setting than to begin too lean.
Rich vs. Lean Yamaha Blaster Jetting Symptoms
Learning to recognize rich and lean behavior is an important part of two-stroke tuning.
Symptoms of Rich Jetting
An excessively rich Blaster may:
- Sputter or four-stroke under acceleration
- Refuse to clean out at high RPM
- Feel lazy or flat
- Produce excessive smoke
- Foul spark plugs
- Run better when the throttle is backed off slightly
Rich jetting generally hurts performance, but excessively lean jetting can damage the engine.
Symptoms of Lean Jetting
A lean Blaster may:
- Surge
- Hang at a high idle
- Run unusually hot
- Detonate or ping
- Lose power under load
- Respond abnormally when the choke is applied
- Experience piston or cylinder damage
Do not continue running an engine that shows signs of a dangerously lean condition.
Temperature and Yamaha Blaster Jetting
Air density changes with temperature.
Cold air is denser and contains more oxygen. The engine may therefore require more fuel.
Hot air is less dense and generally requires less fuel.
A Blaster that is correctly jetted during hot summer weather can become lean when ridden in substantially colder conditions.
This is especially important for riders who operate the same ATV throughout the year.
Check jetting when seasonal temperatures change significantly.
Elevation and Yamaha Blaster Jetting
Elevation also affects air density.
As elevation increases, air density generally decreases. Less oxygen enters the engine, so less fuel is normally required.
This means a Yamaha Blaster operated at high elevation will typically require leaner jetting than the same machine operated near sea level.
A Blaster jetted correctly at 5,000 feet should not automatically be assumed safe when brought down to sea level.
This is one reason copying another rider’s jetting specification can be misleading.
Before You Jet the Carburetor
Do not attempt to correct a mechanical problem with carburetor jets.
Before tuning your Yamaha Blaster VM26, verify:
- Engine compression is acceptable.
- The engine passes a leak-down test.
- Crankshaft seals are not leaking.
- Intake boots and gaskets are sealed.
- Reeds are in good condition.
- The carburetor is clean.
- Carburetor vent hoses are clear.
- Fuel flow is unrestricted.
- Float level is correct.
- The air filter is clean and properly oiled.
- The exhaust and silencer are not restricted.
- Ignition and spark are functioning correctly.
- Fresh fuel is being used.
- Premix ratio is consistent.
An air leak can make an engine behave lean regardless of how large a main jet is installed.
Installing larger jets does not repair a leaking crank seal, intake gasket or damaged reed.
If your Blaster is bogging and you are unsure whether the problem is carburetion, ignition or mechanical, use the KOR troubleshooting guide:
YAMAHA BLASTER BOGGING TROUBLESHOOTING GUIDE
How to Tune the Yamaha Blaster VM26
Work through the carburetor systematically rather than changing several components simultaneously.
First, make sure the engine is mechanically sound.
Next, clean the carburetor, verify float level and confirm adequate fuel flow.
Set the pilot circuit and air screw so the engine starts, idles and responds correctly at small throttle openings.
Evaluate the needle and clip through the middle throttle range.
Finally, confirm the main jet under high load and wide-open throttle.
When the correct main jet is unknown, start rich and work leaner in controlled steps.
Changing the main jet, needle position and air screw simultaneously makes it difficult to determine which adjustment actually changed engine behavior.
Yamaha Blaster Mikuni VM26 Jet Kits
To make VM26 tuning easier, Ken OConnor Racing offers Yamaha Blaster Mikuni VM26 Jet Kits in multiple main-jet ranges.
Instead of buying a random assortment of jets, select a kit that brackets the expected main-jet requirement for your engine combination.
Available ranges extend from stock and mildly modified Blasters through combinations requiring substantially richer jetting.
SHOP YAMAHA BLASTER MIKUNI VM26 JET KITS
The kits are designed to work hand-in-hand with the KOR Yamaha Blaster VM26 Jetting Calculator. Use the calculator to establish the approximate requirement, select a kit containing the surrounding jet sizes, begin rich and tune toward the final setting.
Individual Mikuni main jets and pilot jets are also available through The Blaster Store.
Frequently Asked Questions About Yamaha Blaster Jetting
What is the stock Yamaha Blaster main jet?
A stock Yamaha Blaster using the factory Mikuni VM26 carburetor commonly uses a 230 main jet.
What is the stock Yamaha Blaster pilot jet?
The factory baseline is a 32.5 pilot jet.
What main jet should I use with a Pro Circuit pipe?
For a VM26-equipped Yamaha Blaster, approximately a 250 main is a useful starting point for a Pro Circuit setup. Intake modifications, engine work, temperature and elevation can change the final requirement.
What main jet should I use with a Toomey pipe?
Approximately a 280 main is a useful initial starting point for a typical VM26 Toomey combination. The final jet must be verified for the individual engine.
What main jet should I use with a Bills pipe?
Approximately a 250 main can be used as an initial starting point, with final jetting determined by the complete engine and intake combination.
What jet should I use with a CT or LRD pipe?
High-flow right-bend pipes such as CT and LRD can require considerably richer jetting. Some combinations fall into approximately the 320–360 main-jet range. Use the KOR VM26 Jetting Calculator to establish a more appropriate starting point for your specific setup.
Does removing the Yamaha Blaster airbox lid require rejetting?
Usually, yes. Removing the airbox lid increases airflow and can lean the air/fuel mixture. Jetting should be checked any time the intake airflow is increased.
Does porting require larger jets?
Cylinder porting can increase the amount of air the engine moves and may require richer jetting. The amount of change depends on the porting, exhaust, intake and complete engine combination.
Can I copy the jetting from another Yamaha Blaster?
Use another machine’s settings only as a reference. Two Blasters with similar modifications can require different jetting because of elevation, temperature, fuel, engine condition, porting and other differences.
Yamaha Blaster Jetting: Start Rich, Test and Verify
There is no universal Yamaha Blaster main jet that works for every engine.
The factory 230 main and 32.5 pilot provide a useful stock baseline, but exhaust systems, intake changes, reeds, porting, weather and elevation can significantly change the fuel requirement.
The safest approach is simple:
Make sure the engine is mechanically sound, calculate a conservative starting point, begin rich, change one circuit at a time and verify the final jetting on the actual machine.
Ken OConnor Racing provides the tools and parts to make that process easier.
USE THE YAMAHA BLASTER VM26 JETTING CALCULATOR
SHOP YAMAHA BLASTER MIKUNI VM26 JET KITS
SHOP YAMAHA BLASTER PARTS AT THE BLASTER STORE
For additional repair information, specifications, calculators and troubleshooting articles, visit the Ken OConnor Racing Yamaha Blaster Technical Resource Center.