Hot Compression Loss After Top-End Rebuild
A 2018 250 cc four-stroke motocross bike arrives after a top-end rebuild performed elsewhere. The piston, rings, valve seals, and cam chain were replaced, and the valve seats were lapped. The bike starts cold, but after an 8-minute moto and 5-minute heat soak it will not restart without wide-open throttle and backfires through the intake. Cold compression is 155 psi (spec 160–190 psi); hot compression is 110 psi. Hot leak-down is 22% with audible air at the intake tract; cold leak-down was 8%. Intake valve clearance is 0.10 mm (spec 0.10–0.15 mm); exhaust is 0.19 mm (spec 0.17–0.22 mm). Cam timing marks align at TDC, spark is good, and fuel pressure is 330 kPa (spec 330 kPa). The owner has a race tomorrow and asks you to shim the intake valve to the loose end and release the bike. You have gaskets and shims but no replacement valves or seats.
1. What do the cold and hot compression and leak-down results tell you, and what is the first check before removing the head?
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The leak-down location is the key. Air heard at the intake tract points to an intake valve that is not sealing; ring leakage would normally be heard at the crankcase breather, and head-gasket leakage would show at the coolant or oil passages. Cold leak-down at 8% is acceptable, but hot leak-down at 22% with the same intake noise means the sealing problem appears or worsens with heat. That pattern is more consistent with a valve seat or valve face that distorts when hot than with worn rings.
Before removing the head, recheck intake valve clearance with the engine hot or immediately after a heat soak. Clearance can close as parts expand, and an intake valve held slightly open will cause exactly this hot-only leakage. If hot clearance is zero or below spec, correct the shim to mid-spec and repeat the hot leak-down. If clearance remains within spec and the hot leak-down is still 22% at the intake, the head must come off for valve and seat inspection.
2. Evaluate the owner’s request to shim the intake valve to the loose end and race the bike tomorrow.
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Shimming to the loose end is only a valid diagnostic step if hot clearance has closed. It is not a repair for a distorted seat or valve face. If the valve is already within clearance cold and still leaks hot, extra clearance may reduce the leak slightly by letting the valve sit down harder, but it does not correct the sealing surface. The engine can lose compression again as soon as it reaches race temperature, and a leaking intake valve can lead to backfire, poor combustion, and eventually valve or seat damage.
The shop also has no replacement valves or seats, so a proper repair cannot be completed tonight. Releasing the bike for a race would be a foreseeable safety and reliability risk. The correct decision is to explain that the hot leak-down result disqualifies the bike from release, document the findings, and quote the head repair.
3. Describe the repair and verification plan if the intake seat is found off-center.
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Remove the head and inspect the intake valve face, seat concentricity, guide wear, and stem runout. An off-center seat requires recutting or replacing the seat and refacing or replacing the valve as needed. Check guide clearance before cutting the seat, because a worn guide will make the new seat inaccurate. Replace the valve seal, set clearance to the middle of the spec rather than the extreme, and confirm cam timing after assembly.
Verification should include a hot leak-down of less than about 10%, hot compression within 10% of specification and close to the cold reading, and a cylinder leak-down with no intake noise. Then run the engine through several heat cycles and a loaded moto-style test before release. Document cold and hot readings on the repair order so the before-and-after condition is clear.