This Shifter Engine document for the Honda CR125 engine is going to cover multiple builds and multiple popular years of the Honda CR125 engine that was adapted for use on the shifter kart, ICC, KZ, or 125cc Shifter Kart Chassis.
A quick background on the Honda CR125 engine in the karting motorsports space. In the 1990's, enthusiasts wanted to have karts with gearboxes. Motorcycle engines were easily accessible and parts were very readily available at many motorcycle dealerships throughout the United States. Yamaha, Kawasaki, Suziki, and primarily the Honda engines were used to create the shifterkart movement. The karting industrys De Facto engine of choice became the Honda CR125 for the conversion from MX to shifter kart racing.
Below you will find multiple sections and differeing tuning tips for the most common Honda Shifter Kart engine modifications and uses for karting. Most of the content provided on this page was sourced from SwedeTech Racing Engines. If you are unsure of the engine or build that you have, you can schedle an appointment with Extra Kart Parts and we can help determine what type of package or build you may have. Phone and Live Video appointments are also available.
Please Read the Below Information Carefully - There are mulitple sections and classifications of Honda Shifter Kart engines.
This is only a guide based on information supplied from SwedeTech Racing Engines. It is the readers responsibility to varify their equipment and how their engine is set up. If you are running pump fuel in your shifter kart engine, then we know you didn't thoroughly read this page.
Modified or Blueprinted Honda CR125 Shifter Kart Engine Info
** Note - This section is for Ported or Modified Honda CR125 Engines built by SwedeTech Racing Engines.
This form of the Honda CR125 engine build was most popular in karting from the mid 1990's to the early 2000's. This class allowed porting, machining, and other engine modifications to improve the power over a stock engine.
YOU MUST ALWAYS WARM THE ENGINE UP TO A MINIMUM OF 110 DEGREES FAHRENHEIT ON WATER TEMP BEFORE APPLYING A LOAD TO THE ENGINE. The water temperature sensor should be placed in the radiator hose between the head outlet and the upper radiator inlet. If you do not have an operating data gauge, STOP NOW, contact us for options or view our Data Loggers page.
The Blueprinted or Modified Moto Engine for karting applications will not run properly on pump fuel (Octane ratings of 87-94). SwedeTech recommends the use of VP Racing Fuel’s C12 product or an equivalent from other reputable brands such as Sunoco.
Prior to driving on the track, always warm the engine up to a minimum of 110°. While you are warming the engine, do a visual inspection on the engine and chassis for any loose, broken, or misaligned components.
Do Not Run the engine through multiple heat cycles while on a stand. This will not properly break in the piston and ring. The piston and ring will need a load to properly seat. Only run the engine on the stand to bring to proper warm-up temperature (min. 110°), then start your break-in cycle on the track.
On a 30 second track slow the pace 4-5 seconds per lap. Allow the engine to go through the RPM range from 6,500 – 10,500 for the first five laps. At 5 lap intervals, increase rpm’s by 750 – 1000. Continue until maximum rpm’s are achieved. Run the engine at full throttle on the track, keeping rpm’s low by short shifting.
Spark Plug
Standard Size - NGK BR10EG or R7376-10
Shorty Style - NGK R7282-10
Must use the NGK TRS1225 Plug cap with short style plugs
** You must use a resistor type spark plug **
Spark Plug Gap – 0.024” – 0.028”
Transmission Oil
Motul Trans Oil 10w30 or Motul Trans Oil Expert 10w40
New engine-650ml (21.9 oz)
Use 50ml less for routine oil changes ( 20.0 oz )
Two-Stroke Engine Oil
Motul Kart Grand Prix or Maxima 927
Mix 32:1 for break-in and regular use (4 oz. per gallon )
** DO NOT USE PUMP FUEL ** Recommend VP C12
Jetting
The Stock Moto Engine for karting application will not run properly run on pump fuel. SwedeTech recommends the use of VP Racing Fuel’s C12 product.
Keihin 38mm PWM - recommended by SwedeTech.
There are multiple options available for Carburetor set ups. SwedeTech has had the best experience with the Keihin PWM 38mm carburetor, but you can also use the Keihin PWK 38mm carburetor, this will affect your jetting so you will need to change your baselines. Some engine builders also used the Mikuni TM 38mm carburetor and you may find information online for baseline jetting ranges.
There are also various methods of fuel delivery for the Honda CR125 engines when adapted to shifterkart uses. Stock carburetor with floats ( and GFS ), modified carb with floats but restricted inlet valve and seat, Pump Around System ( two round pumps, 1 1 for fuel supply, 1 for fuel removal from carb )
Jetting range is based on Sprint Kart setup using the SwedeTech GFS system and a 1999 cylinder combo.
165 -185 main ( center of the carb, 6mm hex )
Main jet supplies fuel from ½ throttle to full throttle
62 - 52 pilot ( next to main jet, use 5mm flat screwdriver to remove )
Pilot jet supplies fuel from 0 – ¼ throttle
DGH needle - 3rd from top. Experiment with 4th & 2nd clip from top.
Needle regulates fuel from 1/8 throttle to ¾ throttle
Air Screw ( side of carburetor body, above fuel inlet ) – 1 Turn
Clutch adjustment
You need about 3/8" to 1/2" free play at the end of the lever lifter arm on the engine side. You shouldn’t have more than 1 inch of travel when you engage the clutch lever on the steering column. When the clutch lifter arm is at rest, it should sit at a downward 45 degree angle. As you engage the clutch arm, the clutch should start to have an applied load when the lifter arm is at a 90 degree angle to the case or flywheel cover.
Any more than that and the outer plate could jump out of the basket. Make sure your cables have the nylon inner housing and keep them straight and well lubricated. You should not need more than two fingers to engage the clutch.
SwedeTech Quick Tips
Break-in water temperature should be 150° - 155°
Normal running condition water temperature between 125° - 135°
Take the reading anywhere between the cyl. head outlets and the radiator. Do not place the water temp sensor between the radiator and the water pump
Engine will lose performance after 135° water temp.
Overheating and engine damage can occur with temperatures past 160°
New engine break-in 15-20 laps.
New Top End break-in 12-15 laps
Increase main jet 1-2 sizes for break-in
RPM Range – 7,000 – 12,700
Top End Maintenance
Top End Kit – Piston, ring, circlips, wrist pin, bearing, head gasket or O-rings ( depending on the modifications ), exhaust o-rings, and base gasket(s). Always measure gasket thicknesses are a reference before ordering parts.
For normal racing use – 6 – 8 hours
For optimum performance – 4 – 6 hours
Normal use – for the recreational racer that is looking for longevity and cost effectiveness. Typical normal use includes jetting the engine on the richer side.
Optimum Performance – for the avid racer that is looking for every performance advantage. Optimum performance includes jetting the engine for best performance.
Note that the above schedule of maintenance is based on our experience. You may find that these times may be adjusted based on your needs, driving conditions, and overall engine maintenance. Many factors affect wear and tear on components.
Lower End Maintenance
Lower end maintenance will require that you send the engine to SwedeTech. We recommend that this service is performed between 30 - 40 hours of use.
Note that the above schedule of maintenance is based on our experience. You may find that these times may be adjusted based on your needs, driving conditions, and overall engine maintenance. Many factors affect wear and tear on components.
Regular Maintenance
fuel filter and fuel system for debris (Daily)
Drain transmission oil (4 hours)
Rebuild GFS rectangle fuel pump (4 hours)
Rebuild GFS round fuel pump (16 hours)
Drain GFS pulse lines (Daily)
Frequently Used Parts and Part Numbers
Frequently Used Parts and Part Numbers
Piston, 13110 KSR 730
Ring, 13121-KSR-731
Bearing, 91103 - KZ4 - B01
Circlip(2), 94601 - 15000
Wrist pin, 13111 KV3 000
Gasket, 12251- KZ4- A91 (2001) or O-ringsO-ring, Head - Inner and Outer
Base Gasket, 12191 KZ4 L10 (2001) - Multiple Thicknesses
We use the 2001 base gasket with both '99 and '01 cylindersBase Gasket, Multiple Thickness Options
O-ring, exhaust
******
** Note - This section is for Stock Moto with a 1999 cylinder and head Honda CR125 Engines built by SwedeTech Racing Engines. Stock Moto referred to a SKUSA class that did not allow porting, machining, or altering of the engine to improve performance. This form of the Honda CR125 engine in karting was popular for many years, from 2006 to around 2018.
YOU MUST ALWAYS WARM THE ENGINE UP TO A MINIMUM OF 110 DEGREES FAHRENHEIT ON WATER TEMP BEFORE APPLYING A LOAD TO THE ENGINE. The water temperature sensor should be placed in the radiator hose between the head outlet and the upper radiator inlet. If you do not have an operating data gauge, STOP NOW, contact us for options or view our Data Loggers page.
The Stock Moto Engine for karting applications will not run properly on pump fuel (Octane ratings of 87-94). SwedeTech recommends the use of VP Racing Fuel’s C12 product.
Prior to driving on the track, always warm the engine up to a minimum of 110°. While you are warming the engine, do a visual inspection on the engine and chassis for any loose, broken, or misaligned components.
Do Not Run the engine through multiple heat cycles while on a stand. This will not properly break in the piston and ring. The piston and ring will need a load to properly seat. Only run the engine on the stand to bring to proper warm-up temperature (min. 110°), then start your break-in cycle on the track.
On a 30 second track slow the pace 4-5 seconds per lap. Allow the engine to go through the RPM range from 6,500 – 10,500 for the first five laps. At 5 lap intervals, increase rpm’s by 750 – 1000. Continue until maximum rpm’s are achieved. Run the engine at full throttle on the track, keeping rpm’s low by short shifting.
Spark Plug
Standard Size - NGK BR10EG or R7376-10
Shorty Style - NGK R7282-10
Must use the NGK TRS1225 Plug cap with short style plugs
** You must use a resistor type spark plug **
Spark Plug Gap – 0.024” – 0.028”
Oil
Transmission Oil
Motul Trans Oil 10w30 or Motul Trans Oil Expert 10w40
New engine-650ml (21.9 oz)
Use 50ml less for routine oil changes ( 20.0 oz )
We recommend changing the gearbox oil every 4 hours. Oil is cheap compared to the cost of replacing clutch or gearbox parts. If possible, change the oil while the engine is warm.
Two-Stroke Engine Oil
Motul Kart Grand Prix or Maxima 927
Mix 32:1 for break-in and regular use (4 oz. per gallon )
** DO NOT USE PUMP FUEL ** Recommend VP C12
Jetting
The Stock Moto Engine for karting application will not run properly run on pump fuel. SwedeTech recommends the use of VP Racing Fuel’s C12 product.
Keihin 38mm PWM - recommended by SwedeTech
Jetting range is based on Sprint Kart setup using the SwedeTech GFS system and a 1999 cylinder combo.
160 -180 main ( center of the carb, 6mm hex )
Main jet supplies fuel from ½ throttle to full throttle
50 - 60 pilot ( next to main jet, use 5mm flat screwdriver to remove )
Pilot jet supplies fuel from 0 – ¼ throttle
DGJ needle - 3rd from top. Experiment with 4th & 2nd clip from top.
Needle regulates fuel from 1/8 throttle to ¾ throttle
Air Screw ( side of carburetor body, above fuel inlet ) – 1 Turn
Clutch adjustment
You need about 3/8" to 1/2" free play at the end of the lever lifter arm on the engine. You shouldn’t have more than 1 inch of travel when you engage the clutch lever. When the clutch lifter arm is at rest, it should sit at a downward 45 degree angle. As you engage the clutch arm, the clutch should start to have an applied load when the lifter arm is at a 90 degree angle to the case or flywheel cover.
Any more than that and the outer plate could jump out of the basket. Make sure your cables have the nylon inner housing and keep them straight and well lubricated. You should not need more than two fingers to engage the clutch.
SwedeTech Quick Tips
Break-in water temperature should be 150° - 155°
Normal running condition water temperature between 125° - 135°
Take the reading anywhere between the cyl. head outlets and the radiator. Do not place the water temp sensor between the radiator and the water pump
Engine will lose performance after 135° water temp.
Overheating and engine damage can occur with temperatures past 160°
New engine break-in 15-20 laps.
New Top End break-in 12-15 laps
Increase main jet 1-2 sizes for break-in
RPM Range – 7,000 – 12,700
Top End Maintenance
Top End Kit – Piston, ring, circlips, wrist pin, bearing, head gasket, exhaust o-rings, and base gasket.
For normal use – 6 – 8 hours
For optimum performance – 4 – 6 hours
Normal use – for the recreational racer that is looking for longevity and cost effectiveness. Typical normal use includes jetting the engine on the richer side.
Optimum Performance – for the avid racer that is looking for every performance advantage. Optimum performance includes jetting the engine for best performance.
Note that the above schedule of maintenance is based on our experience. You may find that these times may be adjusted based on your needs, driving conditions, and overall engine maintenance. Many factors affect wear and tear on components.
Lower End Maintenance
Lower end maintenance will require that you send the engine to SwedeTech. We recommend that this service is performed between 30 - 40 hours of use.
Note that the above schedule of maintenance is based on our experience. You may find that these times may be adjusted based on your needs, driving conditions, and overall engine maintenance. Many factors affect wear and tear on components.
Regular Maintenance
fuel filter and fuel system for debris (Daily)
Drain transmission oil (4 hours)
Rebuild GFS rectangle fuel pump (4 hours)
Rebuild GFS round fuel pump (16 hours)
Drain GFS pulse lines (Daily)
Frequently Used Parts and Part Numbers
Piston, 13110 KZ4 506
Ring, 13121 KZ4 A91
Bearing, 91103 KZ4 B01
Circlip(2), 94601 15000
Wrist pin, 13111 KV3 000
Gasket, 12251 KZ4 701 (1999)
Base Gasket, 12191 KZ4 J00 (1999)
O-ring, exhaust
******
** Note - This section is for Stock Moto with a 2001 cylinder and head Honda CR125 Engines built by SwedeTech Racing Engines. Stock Moto referred to a SKUSA class that did not allow porting, machining, or altering of the engine to improve performance. This form of the Honda CR125 engine in karting was popular for many years, from 2006 to around 2018.
YOU MUST ALWAYS WARM THE ENGINE UP TO A MINIMUM OF 110 DEGREES FAHRENHEIT ON WATER TEMP BEFORE APPLYING A LOAD TO THE ENGINE. The water temperature sensor should be placed in the radiator hose between the head outlet and the upper radiator inlet. If you do not have an operating data gauge, STOP NOW, contact us for options or view our Data Loggers page.
The Stock Moto Engine for karting applications will not run properly on pump fuel (Octane ratings of 87-94). SwedeTech recommends the use of VP Racing Fuel’s C12 product.
Prior to driving on the track, always warm the engine up to a minimum of 110°. While you are warming the engine, do a visual inspection on the engine and chassis for any loose, broken, or misaligned components.
Do Not Run the engine through multiple heat cycles while on a stand. This will not properly break in the piston and ring. The piston and ring will need a load to properly seat. Only run the engine on the stand to bring to proper warm-up temperature (min. 110°), then start your break-in cycle on the track. Water temp during break in should be in the 145 - 155 degree range.
On a 30 second track slow the pace 4-5 seconds per lap. Allow the engine to go through the RPM range from 6,500 – 10,500 for the first five laps. At 5 lap intervals, increase rpm’s by 750 – 1000. Continue until maximum rpm’s are achieved. Run the engine at full throttle on the track, keeping rpm’s low by short shifting.
SwedeTech Quick Tips
Break-in water temperature should be 150° - 155°
Normal running condition water temperature between 125° - 135°
Take the reading anywhere between the cyl. head outlets and the radiator. Do not place the water temp sensor between the radiator and the water pump
Engine will lose performance after 135° water temp.
Overheating and engine damage can occur with temperatures past 160°
New engine break-in 15-20 laps.
New Top End break-in 12-15 laps
Increase main jet 1-2 sizes for break-in
Spark Plug
Standard Size - NGK BR10EG or R7376-10
Shorty Style - NGK R7282-10
Must use the NGK TRS1225 Plug cap with short style plugs
** You must use a resistor type spark plug **
Spark Plug Gap – 0.024” – 0.028”
Oil
Transmission Oil
Motul Trans Oil 10w30 or Motul Trans Oil Expert 10w40
New engine-650ml (21.9 oz)
Use 50ml less for routine oil changes ( 20.0 oz )
We recommend changing the gearbox oil every 4 hours. Oil is cheap compared to the cost of replacing clutch or gearbox parts. If possible, change the oil while the engine is warm.
Two-Stroke Engine Oil
Motul Kart Grand Prix or Maxima 927
Mix 32:1 for break-in and regular use (4 oz. per gallon )
** DO NOT USE PUMP FUEL ** Recommend VP C12
Jetting
The Stock Moto Engine for karting application will not run properly run on pump fuel. SwedeTech recommends the use of VP Racing Fuel’s C12 product.
Keihin 38mm PWM - recommended by SwedeTech
Jetting range is based on Sprint Kart setup using the SwedeTech GFS system and a 2001 cylinder combo.
170-190 main ( center of the carb, 6mm hex )
Main jet supplies fuel from ½ throttle to full throttle
58-65 pilot ( next to main jet, use 5mm flat screwdriver to remove )
Pilot jet supplies fuel from 0 – ¼ throttle
DGH needle - 3rd from top. Experiment with 4th & 2nd clip from top.
Needle regulates fuel from 1/8 throttle to ¾ throttle
Air Screw ( side of carburetor body, above fuel inlet ) – 1 Turn
Clutch adjustment
You need about 3/8" to 1/2" free play at the end of the lever lifter arm on the engine. You shouldn’t have more than 1 inch of travel when you engage the clutch lever. When the clutch lifter arm is at rest, it should sit at a downward 45 degree angle. As you engage the clutch arm, the clutch should start to have an applied load when the lifter arm is at a 90 degree angle to the case or flywheel cover.
Any more than that and the outer plate could jump out of the basket. Make sure your cables have the nylon inner housing and keep them straight and well lubricated. You should not need more than two fingers to engage the clutch.
Top End Maintenance
Top End Kit – Piston, ring, circlips, wrist pin, bearing, head gasket, exhaust o-rings, and base gasket.
For normal use – 6 – 8 hours
For optimum performance – 4 – 6 hours
Normal use – for the recreational racer that is looking for longevity and cost effectiveness. Typical normal use includes jetting the engine on the richer side.
Optimum Performance – for the avid racer that is looking for every performance advantage. Optimum performance includes jetting the engine for best performance.
Note that the above schedule of maintenance is based on our experience. You may find that these times may be adjusted based on your needs, driving conditions, and overall engine maintenance. Many factors affect wear and tear on components.
Lower End Maintenance
Lower end maintenance will require that you send the engine to SwedeTech. We recommend that this service is performed between 30 - 40 hours of use.
Note that the above schedule of maintenance is based on our experience. You may find that these times may be adjusted based on your needs, driving conditions, and overall engine maintenance. Many factors affect wear and tear on components.
Regular Maintenance
Inspect fuel filter and fuel system for debris (Daily)
Drain transmission oil (4 hours)
Rebuild GFS rectangle fuel pump (4 hours)
Rebuild GFS round fuel pump (16 hours)
Drain GFS pulse lines (Daily)
Replace carburetor float needle valve (with each Top End)
Frequently Used Parts and Part Numbers
Piston, 13110 KZ4 A90
Ring, 13121 KZ4 A91
Bearing, 91103 KZ4 B01
Circlip(2), 94601 15000
Wrist pin, 13111 KV3 000
Gasket, 12251 KZ4 A91 (2001)
Base Gasket, 12191 KZ4 L10 (2001)
O-ring, exhaust
For a short period of time in the US Shifter Kart scene, there was a Honda CR125 Junior class called S5. This Junior shifter class utilized a spec airbox and a spec exhaust restrictor to tame the Honda CR125 for the use with younger kart drivers, aged 11-15. Below is a sample guide to jetting if you happen to have the Junior Shifter setup
Jetting
The Stock Moto Engine for karting application will not run properly run on pump fuel. SwedeTech recommends the use of VP Racing Fuel’s MS98 product.
Keihin 38mm PWK - recommended by SwedeTech
Jetting range is based on Sprint Kart setup using the SwedeTech GFS system and a 1999 cylinder combo with the RLV 23mm Airbox and SKUSA Exhaust restrictor installed.
160 - 140 main ( center of the carb, 6mm hex )
Main jet supplies fuel from ½ throttle to full throttle
58 -48 pilot ( next to main jet, use 5mm flat screwdriver to remove )
Pilot jet supplies fuel from 0 – ¼ throttle
DGJ needle - 3rd from top. Experiment with 4th & 2nd clip from top.
Needle regulates fuel from 1/8 throttle to ¾ throttle
Air Screw ( side of carburetor body, above fuel inlet ) – 1 Turn
Once again, most of this information was collected from SwedeTech Racing Engines.
Torque Specs
| Nut, 8mm cylinder head | 28 nm, 20 ft / lbs |
| Nut, 8mm cylinder base | 28 nm, 20 ft/ lbs |
| Stud, 8mm Cylinder (case/head) | 12 nm, 9 ft / lbs – apply loc-tite |
| Stud, 6mm Cylinder (exhaust) | 10 nm, 7 ft / lbs – apply loc-tite |
| Impeller, water pump | 12 nm, 9 ft / lbs |