Introduction: Honda GX25, Part 3: Rebuilding

Torque specs:

Intake 8mm nuts:

Exhaust 8mm nuts:

Lower crankcase: 6. 4nm

Flywheel: 14.7nm

Valve adjusting lock nut: 4.9nm

Spark plug: 11.8nm

Step 1: Tools

This engine is easy to reassemble with basic tools and parts. You will need:


-3mm and 4mm Allen key

-8mm, 10mm, 14mm spanner and sockets, spark plug sockets and wrench

-flat and Phillips head screw driver.

-flywheel holder

- Stanley knife

- feller gauge

- torque wrench

- anti sieze

-gasket sealant (oil stable)

-brake cleaner and paper towel

-any replacement parts as required

- Honda engine oil or equivalent

Step 2: Crank Case Assembly


1. Ensure all parts are clean and free of dirt. Ensure mating surfaces of the crankcase are clean and wiped down using brake cleaner or similar and all gasket material has been removed (use knife). Lightly oil inside of cylinder

2. Apply small dab of oil to stem of each valve and insert valves into head. Big valve: intake, smaller valve: exhaust. Make sure the valve seal is fitted on the intake side. Place a spring over a valve on top of the head and fit the retainer to the valve. The intake can be inserted easily using a magnetic pick up tool as per above pic. This will not work on the exhaust valve as it is titanium (you can balance the valve on the tool or use a bit of blue tac).

Crankcase Assembly

3. Apply a small bead of gasket goo to the bottom part of the crankcase and the seal journals on the main crancase.

NOTE: To assist assembling the crankcase, place the top half in a vice or similar, upside down.

4. Place the timing belt onto the passage in the crank case so the belt stick into the air and the crank can be passed through it.

5. Apply oil to the piston and rings. Ensure ring ends do not line up with one another as per the picture above. This is bad, so rotate the rings around the piston.

6. Fit the rotating assembly (crank, piston, bearings etc) to the crank case. Insert the piston into the cylinder. Gently push all piston rings into the cylinder without damaging or snagging them and scratching piston and cylinder. Flat screw driver may help poke them in or hold the assembly and conrod and use it to wiggle the rings in.

7. Once rings are inside the cylinder and crank bearings in position, bring the two crankcase halves together ensuring no oil has dripped onto them. Replace 2x 4mm screws near pull start and finger tighten. Replace the 4x4mm screw (the slightly longer ones) into place and finger tighten.

8. Tighten the 4x4mm screws in a criss cross fashion to 6.4nm with a torque wrench, then the remaining two.

9. Pull through the timing belt to top of head. Fit the oil tube from lower to upper crankcase.

Step 3: Timing

10. Refit the flywheel aligning the key with the key way (slot on the shaft). Tighten the nut to 14.7nm with a torque wrench using a screw driver to prevent the assembly from spinning. A proper fly wheel wrench works better but not everyone has one.


They will snap. If you do snap one, you need to snap one 180 degrees opposite so the engine does not shudder. You could even weigh the piece to make sure the same weight is removed. Just dont do it!

11. Flip the engine upright. Make sure tappets are resting on top of valves. Line up mark on the flywheel (I coloured it red to help see it, 2nd pic) with bottom bolt hole. Take note of the two marks on the timing gear. Line them up horizontally and inline with the top of the head (4th picture showing timing marks lined up with timing gear fitted). Put the timing gear in position and slide the timing belt onto it. Check the flywheel timing mark again then the timing gear. The timing gear might need to be rotated a tooth or two for correct alignment.

WARNING: Do not install the timing gear or rotate the engine until the timing is correct or significant damage may result.

12. Check clearance of tappets (cold engine: intake- 0.08 +-0.02mm, exhaust 0.11 +-0.02mm) and refit tappet cover and 2x4mm screws.

13. Ensure engine turns over easily and smoothly. If you feel any binding stop and check your timing. Because there is no plug fitted, there's no compression so only the resistance of the piston rings should be felt.

14. Fit the ignition coil and finger tight the screws. You will note the mounting holes are elongated. This is for the clearance adjustment and must be done once fitted. Using a feeler gauge ensure 0.35-0.5 mm clearance between both parts of the coil, as per last photo.

Step 4: Attachments

15. Refit remains parts in reverse of disassembly.

With the exhaust studs, add a dab of anti sieze before inserting into head. Be sure not to touch the clutch material or it may fail due to contamination. Make sure the small hole in the gasket is lined up correctly or the carby will not work. The throttle housing maybe facing the wrong way. Undo the two phillips head screws on top and simply rotate the housing. Use a bit of engine oil to lube the air filter. I needed a new filter so just used the packaging to keep it clean.

16. After sealant has cured, fill engine with 80ml of 10W/30 API SG oil and fill the fuel tank. Ensure air filter is clean and oiled lightly. Start engine. NOTE: if you have completed the mods from my other instructible it will require some tuning but refer to that section.

Step 5: Carby Tuning

The engine will now be sucking more air, so even if you use the stock carby it will need to be adjusted or it will be running really lean and could cause engine damage.

Take note of how the settings are from factory and record what you are doing as you go if you worried you are going to struggle. If it all goes pear shaped, just dial everything back to factory and start again. Its really easy.

Assuming you went with the upgrade carb, start the engine. Leave the filter cover off and just work somewhere fairly clean and dust free. It might take quiet a few pulls to get the fuel to come through, 10-20. If after that its still not going you may need to wind the idle in a few turns. Few pulls and it should go.

High Screw

It will now be revving fairly high but we will fix that up. Take a screw driver and while holder the throttle flat adjust until it sounds good. The engine may want to die if you go straight to Wide Open Throttle WOT. If it does just increase RPMs steadily. The engine before should run rough and cough and splutter at WOT. Adjust to until it is running smoothly. Go past that until it starts running rough again and then return to happy medium.

Low Screw

Adjust the idle back to a fast idle. Using a flat precision screwdriver, remove the plastic grommet on top of the carby throttle screw (the big brass one with a slot in it). If you look into it you will see a very small slottled screw. Tap the throttle. It will most likely bog. Keep adjusting the screw 1/4 turn until it does not bog anymore. Again go past that and find the happy medium.

The first carby I did adjusted very easily. The second one was a bit finicky. Just stick with it. If you think you have completely ballsed it up and are lost, take it back to factory and start again.

Adjust idle.

The end result should be an engine that responds quickly to taps on the throttle without bogging and runs really nice at WOT.

Its a really good feeling when you get the tune right and watch this thing scream!

If i have not explained something with enough info or needs clarification, let me know.

Happy tinkering.