Dirt Bike Maintenance

This is a collection of all the noteworthy maintenance and fixes I've done over the years to either my dirt bikes or my dad's.

Top End Rebuild on 2017 Honda CRF 250r

Issue

Intake and exhaust valve clearance was going out of spec quicker than normal, leaving the bike hard to start and more inclined to stall when idling.

Resoning

This makes sense because the valves had a high number of hours on them and were due for new ones. The old valves began to "tulip" or bend and cause too much air to escape when they're supposed to be shut, therefore losing compression in the cylinder.

Solution

Put new valves in the cylinder head and then readjust valve clearance with new valves. While I had the top end of the engine open I also took the cylinder to a local machine shop to be honed out for good measure. I also put in a new piston, piston rings, and all new head gaskets.

Images

2017 Honda CRF 250r cylinder and cylinder head taken to machine shop.

Valve Adjustments

Issue

Intake and exhaust valve clearance going out of spec, leaving the bike hard to start and more inclined to stall when idling.

Resoning

Normal for this to happen every once in a while from valves deforming with lots of hours on them.

Solution

Open the cylinder head cover, set engine to Top Dead Center, take measurements of valve clearances with a feeler gauge. After that, remove cam chain, camshaft, and valve lifters. Then, adjust valve shim size based on measured valve clearance and desired valve clearance given in manual. Put valve lifters, camshaft, and cam chain back on and measure clearance again to ensure it is within spec, and repeat the process again if needed.
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Carburetor Rebuild on 2005 Honda CRF 450

Issue

Bike not starting, sounds like it wants to but can't.

Resoning

Fuel was left sitting in the carburetor for too long and clogged the passages in the jets inside the carburetor.

Solution

Pull out the carburetor, tear it apart, and clean out all the jets with carburetor cleaner and compressed air, and then reasemble with new gaskets.

Gallery

2005 Honda CRF 450 carburetor body disassembled

2005 Honda CRF 450 carburetor disassembled pieces

2005 Honda CRF 450 carburetor jets and seals

2005 Honda CRF 450 carburetor back in bike

Clutch Replacement on 2025 Yamaha YZ250F

Issue

Clutch slipping under higher RPM, not transferring power to gearbox correctly. Unusual problem to have on a new bike, at this time with only 20 total hours on the engine.

Resoning

The rider was on the clutch a lot, not being very forgiving. This caused the friction plates inside the clutch pack to wear down quicker than normal and cause them to start slipping.

Solution

Replace friction plates and steel plates with new ones, quick and easy fix.
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