1984-2007 Honda CR80R, CR85R, CR125R, CR250R Carburetor Tuning, Troubleshooting, Removal, Installation, Cleaning & Inspection Guide.

Carburetor Tuning & Overhaul

Always refer to your service manual before attempting to do any work on your motorcycle, important data including procedures, diagrams, specifications cleaning & maintenance information will help you care for and lower the chances of premature parts failurer. Below is a great source for service manuals available for instant download straight to your computers in just seconds.
All manuals are simple to use pdf downloads and are delivered straight to your computer in just seconds. Not sure what a pdf is? Here is a sample of what a  Service Manual looks like (sample is restricted to one page only).


Honda CR125R, CR125-R, CR-125-R, CR125 1984, 1985, 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006 & 2007 models.

Honda CR250R, CR250, CR-250R, CR250-R 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 00, 01, 02, 03, 04, 05, 06 & 07 year CR250R models.

Honda  CR80R, CR85R, CR80, CR85, CR-80-R, CR-85-R 1984, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 00, 01, 02, 03, 04, 05, 06 & 07 year CR-80/85-R models.

This carburetor guide describes the service procedures as general procedures for Honda CR80R / CR85R / CR125R & CR250R motorcycles. It is only recommended to follow the carburetor guide in the service manuals listed above as these they contain the same information mechanics and technicians use to service your bike.

Important information not contained in this page like special notes, tips, proper procedures, complete specifications, torque specs, additional torque specs and recommendations must be followed in the instant download manual to ensure that the vehicle is in peak operating condition. For maintenance information on your motorcycle see 1985-2007 CR80/85/125/250-R maintenance section.


Below are some common problems that could be associated to a malfunction with the carburetor or related parts. When considering if the carburetor should be examined further check the list below to determine if you are experiencing any of these conditions.


Engine cranks but won’t start
Lean mixture
Engine stalls, hard to start, rough idling
Afterburn when engine braking is used
Backfiring or misfiring during acceleration
Poor performance (driveability) and poor fuel economy

Some of the causes of carburetor problems are listed, again we insist on downloading the manual as it helps match the effect with the causes helping you pinpoint and diagnose the trouble spot quickly and efficiently. As you can see there are many possible causes of poor performance with your motorcycle. Stop the guess work, invest in a manual and resolve the issue effortlessly.


No fuel in tank
No fuel to carburetor
Clogged fuel strainer
Clogged fuel line
Clogged fuel tank breather tube
Misadjusted fuel level
Too much fuel getting to the engine
Clogged air cleaner
Flooded carburetor
Intake air leak
Contaminated/deteriorated fuel
Clogged jets
Clogged starting enrichment valve circuit
Improper starting enrichment valve operation
Improper throttle operation
No spark at plug (faulty ignition system)
Clogged fuel jets
Faulty float valve
Float level too low
Restricted fuel line
Clogged carburetor air vent tube
Restricted fuel tank breather tube
Intake air leak
Faulty vacuum piston
Faulty throttle valve
Restricted fuel line
Fuel mixture too lean/rich
Contaminated/deteriorated fuel
Clogged jets
Intake air leak
Misadjusted idle speed
Misadjusted pilot screw
Misadjusted float level
Restricted fuel tank breather tube

Clogged air cleaner
Clogged slow circuit
Clogged starting enrichment valve circuit
Faulty ignition system
Leanmixture in slow circuit
Faulty air cut-off valve
Faulty ignition system
Faulty ignition system
Clogged fuel system
Faulty ignition system



Before removing the carb download the service manual for pictures, illustrations and diagrams for assistance with these procedures. Procedures differ between all years and models and the following should not be used as an exact guide.

To remove the carburetor start by removing the side covers and air cleaner housing. Place and approved fuel container under the drain tube. Then loosen the drain screw and drain the carburetor. Then release the carburetor heater wire from the clip and disconnect its connector if equipped. Disconnect the fuel tube and loosen the starting enrichment (SE) valve nut.

Remove the screw and the throttle drum cover and loosen the insulator band screw to remove the carburetor off the insulator. Disconnect the choke cable by turning the SE
valve nut, being careful not to damage the SE valve. Loosen the throttle cable lock nut and remove the adjuster from the carburetor body, and disconnect the cable from the throttle drum. Now you can remove the carburetor.


Start by inspecting the following:

Check the SE valve face for scores, scratches or wear.
Check the SE valve seat at the tip of the valve for stepped wear.
Check the seal ring for wear or damage.

Start by removing the following from the carburetor body:

air vent tubes
drain tubes
carburetor heater


Remove the attaching screw and the air cut-off valve and then remove the O-rings and joint pipe. Apply vacuum to the vacuum tube and the vacuum should be maintained. Air should not flow through the valve ports when the vacuum is applied, and should flow when the vacuum is not applied.


Remove the four screws with the tube clamp and the vacuum chamber cover while holding it. Remove the compression spring and diaphragm/vacuum piston from the carburetor body and turn the needle holder counterclockwise by using a screwdriver while pressing it in and release the holder flange from the vacuum piston. Then remove the needle holder, spring, jet needle and washer.

Inspect the following:

Check the jet needle for stepped wear.
Check the vacuum piston for wear or damage.
Check the diaphragm for pin hole, deterioration or damage.
Check the vacuum piston for smooth operation up and down in the carburetor body.

Air will leak out of the vacuum chamber if the diaphragm is damaged in any way, even with just a pin hole.


Remove the two screws while holding the primer knob body and then remove the primer knob assembly and spring. Inspect the diaphragm for pin holes, deterioration or damage.


Remove the four screws and the float chamber and then remove the following:
baffle plate
float pin
float valve

Inspect the following components:

Check the float for damage or fuel in the float.
Check the float valve and valve seat for scoring, scratches, clogging or damage.
Check the tip of the float valve, where it contacts the valve seat, for stepped wear or contamination.
Check the operation of the float valve.

Remove the following:

main jet
needle jet holder
needle jet
slow jet
starter jet
rubber plug

Turn the pilot screw in and carefully count the number of turns until it seats lightly. Make a note of this to use as a reference when reinstalling the pilot screw and then remove the pilot screw, spring, washer and O-ring.

Check each jet for wear or damage.
Check the pilot screw for wear or damage.
Clean the jets with cleaning solvent and blow open with compressed air.


Start by removing the following parts:

air cut-off valve
diaphragm/vacuum piston.
main jet, needle jet holder and needle jet
slow jet
starter jet
pilot screw

Blow open all air and fuel passages in the carburetor body with compressed air.


Install the pilot screw with the spring, washer and a new O-ring and return it to its original position as noted during removal. Perform the pilot screw adjustment if a new pilot screw is installed.

Install the following:

needle jet
needle jet holder
main jet
slow jet
starter jet
rubber plug

Hang the float valve onto the float arm lip and the install the float valve and float. The next step is to install the float pin as shown in the service manual picture illustration. Lastly install the baffle plate.


NOTE: Check the float level after checking the float valve, valve seat and float.

With the float valve seated and the float arm just touching the valve, measure the float level with the float level gauge. The float cannot be adjusted. Replace the float assembly if the float level is out of specification (see service manual for spec's). Install the baffle plate by aligning its groove with the lug on the carburetor body as shown in your service repair manual. Install a new O-ring into the float chamber groove securely. Install the float chamber and tighten the four screws.


Install the primer knob with the spring and tighten the two screws, being careful not to pinch the diaphragm.


Install the needle clip onto the jet needle and the washer onto the jet needle and insert the jet needle into the vacuum piston. Install the spring into the needle holder and set the needle holder into the vacuum piston. Turn the needle holder clockwise while pressing it until it locks. Holder flange should be fitted the vacuum piston after turning.

Install the diaphragm/vacuum piston into the carburetor body by aligning the tab of the diaphragm with the air passage, then insert the jet needle into the needle jet. Lift the bottom of the piston with your finger to set the diaphragm rib into the groove in the carburetor body.

Install the spring and vacuum chamber cover while the piston remains held in place. Align the concave of the cover with the air passage in the carburetor and secure the cover with at least two screws before releasing the vacuum piston. Install the screws with the tube clamp and tighten them.


Install new O-rings onto the air cut-off valve and joint pipe. Install the joint pipe into the air cut-off valve with the stepped side facing the air cut-off valve. Install the air cut-off valve and secure it with the screw. Turn the throttle stop screw to align the butterfly throttle valve with the edge of the outside by-pass hole in the carburetor body, If the throttle stop screw was removed. Install the collar and carburetor heater with the stepped side of the collar facing the carburetor and tighten it. Install the following tubes to the carburetor and secure the air vent tube with the clamp as shown: air cut-off valve vacuum tube air vent tubes drain tube.


Connect the throttle cable to the throttle drum and install the cable adjuster into the carburetor body. Connect the choke cable by screwing the SE valve nut, being careful not to damage the SE valve. Install the carburetor into the insulator by aligning the lug with the groove, and tighten the band screw. Tighten the SE valve nut and connect the fuel tube.Now install the throttle drum cover by aligning its tab with the slit in the carburetor and secure it with the screw. Route the tubes and wire properly and connect the carburetor heater connector (See diagram in your service manual).

Continue by removing the following parts:

air cleaner housing
side covers

Perform the following inspections and adjustments. Engine idle speed (refer to your service manual) and throttle operation (refer to your service manual). Adjust the pilot screw if it was replaced.



NOTE: The pilot screw is factory pre-set and no adjustment is necessary unless the pilot screw is replaced. Use a tachometer with graduations of 50 rpm or smaller that will accurately indicate a 50 rpm change. Turn the pilot screw clockwise until it seats lightly, then back it out to the specification given. This is an initial setting prior to the final pilot screw adjustment. Warm up the engine to operating temperature.

Stop and go riding for 10 minutes is sufficient. Stop the engine and connect a tachometer according to the tachometer manufacturer’s instructions. Start the engine and adjust the idle speed with the throttle stop screw. Turn the pilot screw in or out slowly to obtain the highest engine speed.

Lightly open the throttle 2 3 times, then adjust the idle speed with the throttle stop screw. Turn the pilot screw in gradually until the engine speed drops by 100 rpm. Turn the pilot screw out to the final opening. Readjust the idle speed with the throttle stop screw.


The carburetor must be adjusted for high altitude riding (between 3,000 8,000 ft/1,000 2,500 m).

STANDARD SETTING: Below 5,000 ft (1,500 m)
HIGH ALTITUDE SETTING: Between 3,000 8,000 ft (1,000 2,500 m)

The high altitude carburetor adjustment is performed as follows:

Remove the carburetor (see diagram in your manual) and the float chamber. Replace the standard main jet with the high altitude


Check that the O-ring on the float chamber is in good condition and replace it with a new one if necessary. Install the float chamber and the carburetor. Turn-in the pilot screw the specified number of turns from the initial setting.

3/4 turn in from initial opening

Start the engine and adjust the idle speed at high altitude to ensure proper high altitude operation. Sustained operation below 5,000 ft (1,500 m) with the high altitude settings may cause engine overheating and engine damage. Install the standard main jet and screw out the pilot screw the specified number of turns, when riding below 5,000 ft (1,500 m.

Pilot screw change for low altitude: 3/4 turn out.

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1. Carburetor Assembly

2. Screw

3. Throttle Valve

4. Cover, Diaphragm

5. Jet Block Assembly

6. Spring

7. Diaphragm Assembly

8. Ring

9. “E” Ring

10. Ring

11. Needle Jet

12. Cover

13. O-Ring

14. Throttle Shaft Assembly

15. Ring

16. Seal

17. Spring

18. Packing

19. “E” Ring

20. Cap

21. Screw

22. Drain Screw

23. O-Ring

24. Washer

25. Adjuster

26. Spring

27. Pilot Jet

28. Main Jet

29. Washer

30. Jet Needle

31. Float Assembly

32. Float Body Assembly

33. Float Pin

34. Needle Valve

35. O-Ring

36. O-Ring

37. Plug

38. Screw

39. Screw

40. Guide Holder

41. Spring

42. Plunger Assembly

43. Spring Washer

44. Screw

45. Air Jet

46. Cable Guide

47. Spring

48. Ring

49. Adjust Screw

50. Screw and Washer Assy.

51 Plate

52 Screw

1. Fuel Pump Assembly

2. Diaphragm, Gasket Set

3. Screw and Washer Assembly

4. Screw and Washer Assembly

5. Screw and Washer Assembly

6. Pressure Regulator

7. Fuel Inlet

8. Fuel Outlet





Mity Vac

Pressure Test Tool


Adjustment Tool

Carburetor Float


Gasoline is extremely flammable and explosive under

certain conditions.

Always stop the engine and refuel

outdoors or in a well ventilated area.

Do not overfill the tank. The tank is at

full capacity when the fuel reaches the

bottom of the filler neck. Leave room

for expansion of fuel.

Never start the engine or let it run in an

enclosed area. Gasoline powered engine

exhaust fumes are poisonous and can

cause loss of consciousness and death in

a short time.

Never drain the float bowl when the engine

is hot. Severe burns may result.

Do not smoke or allow open flames or

sparks in or near the area where refueling

is performed or where gasoline is stored.

If you get gasoline in your eyes or if you

should swallow gasoline, seek medical

attention immediately.

If you spill gasoline on your skin or clothing,

immediately wash with soap and water and

change clothing.


Changes in altitude and temperature affect air density,

which is essentially theamount of oxygen available for

combustion. In low elevations and cold temperatures,

the air ismore dense and hasmore oxygen. In higher

elevations and higher temperatures, the air is less

dense with reduced oxygen.

Carburetors are calibrated for an altitude

of 0-6000 ft. (0-1800 meters) and ambient

temperatures between +40 and +80

C). Carburetors must be re-calibrated if operated

outside this temperature and/or altitude range. The

jetting installed in production is not intended for all

altitudes and/or temperatures. In addition, air screw

/ pilot screw adjustments and PVT adjustments may

be required to suit operating conditions.

F (+5 to +26



A main jet that is too small will cause a lean

operating condition resulting in serious engine

damage. Select the correct main jet carefully for

elevation and temperature according to the

charts in the Specifications section or in the

Owner’s Safety andMaintenanceManual for each

particular model.


followed when establishing a main jet setting:

1. Select the lowest anticipated temperature at

which the machine will be operated.

2. Determine the lowest approximate altitude at

which the machine will be operated.

3. Select the correct main jet from the chart on page

on the Specifications page.

4. Clutching changes may also be required for

changes in elevation. Refer to clutching chart in

the Specifications section for recommendations.

The following guidelines must be