Diagnosing Misfire in a Honda CRF250L - Solved

I recently bought a 2017 Honda CRF250L with less than 500 miles on it. It has a full Yoshimura RS-4 exhaust system, aftermarket fuel controller, and Acerbis 3.1 gallon tank. The bike is in like-new condition. It starts up quickly, idles fine, does not leak anywhere, does not smoke, no rattles, and revs up smoothly at idle. The only problem is that it misfires randomly at all RPMs when the engine is under load (e.g., when riding it). No error codes on dash. Decel pops are noticeable, but no more than my XR650L. It feels choked up despite the high-flow pipe/slip-on combo. Here's my progress in diagnosing the cause of the misfire.

Battery & Grounds

Tested above 12V. Terminals are clean and secure. Ground connections to bolt to frame are clean and bright. No issues.

New Gas

Emptied tank and replaced with a fresh tank of 91 octane. No change in performance.

Fuel Injector Cleaner

Ran 1/2 bottle of Techron through bike. No change.

Spark Plug

Old plug was sparking fine. Engine starts right up. Plug was not wet. Color was medium brown (not black or white), indicating the engine is neither running too rich or too lean. Replaced with new NGK Iridium SIMR8A9 (same as old plug). No change.

Fuel Controller

A JD Jetting JDHX03 fuel controller came with the bike. I reset settings to neutral. Controller seems to be working fine. Unplugged the fuel controller. No improvement.

Evaporative Canister / PAIR System

Removed evaporative emissions canister/PAIR assembly. Pinned out the 2 circuits. Capped ports with rubber vacuum caps. Purchased aftermarket block-out plate for reed valve from 550 Performance (PD_250_300L). No change.

Air Filter

Inspected air filter. Removed metal mesh plate from back of OEM paper air filter. Air box was clean. Filter looks fine.

Vacuum Lines

Inspected vacuum lines for leaks or loose connections. All look fine. No leaks.

Fuel Pump

The fuel pump is located the inside of the gas tank. Removed tank and fuel pump-filter assembly. Checked wiring connections and pump function. Engine is getting fuel, idles nicely, and revs out in neutral (no load). No issues with pump.

Fuel Filter

The fuel filter is part of the fuel pump assembly. Removed, inspected, and thoroughly cleaned the fabric "pillow" filter with carb cleaner. Reinstalled pump-filter assembly in gas tank. No change in performance.

Fuel Injector

Removed and inspected fuel injector. No visible blockages. Looks fine. O-ring was broken, possibly damaged upon removal. Added new o-ring and reinstalled injector. No leaks. Appears to work fine. Ordered a new injector and installed it. No change.

Air Intake Test

With engine running, I sprayed starting fluid around various parts of engine and air box. Idle RPMs did not change. Engine does not appear to be drawing in air from anywhere it shouldn't.

Exhaust Pipe and Slip-on

Inspected Yoshimura pipe and muffler. Pipe-engine connection, pipe-pipe joints, and pipe-muffler joint all looked fine. Nothing is loose, out of place, or leaking.

Reinstalled OEM Snorkel

I replaced the rubber snorkel in the air box. No change. Snorkel is not the problem. Ordered an aftermarket 'high flow air box velocity stack' from 550 Performance. Its a fancy rubber snorkel thingy. Will install later.

Ignition Coil

OEM coil looked fine. I inspected the boot, trimmed back the wire, and screwed it back together. Seems to work. I purchased used OEM ignition coil w/ boot off of Ebay and installed. I doubt the coil was causing a problem. Engine is clearly making spark. The bike starts up no problem.

Valve Clearance Check

Removed head cover, cam chain tensioner, rocker arm shafts, cam chain, cam chain guide plate, cam bearing caps, and cams. Set the flywheel to top dead center at the "T". Using feeler gauges, I measured the clearances on all four rocker arms. Intake valve clearances measured 0.15mm (spec is 0.16mm +/- 0.03mm). Exhaust measured 0.25mm (spec is 0.27mm +/-0.03).

Shims

Faint stamped markings on all four shims appear to read "222", but parts of numbers are worn away. All appear to be the same thickness. Partzilla lists tappet shims available for the 2017 Honda CRF250L with thicknesses ranging from 1.200 - 2.900mm for about $10/each. I don't think the shims are the issue.

Cam Chain

Cam chain tight. Cams and rocker arms look great. No strange wear patterns. But upon closer inspection, I noticed that the exhaust-side cam was off one tooth. Alignment marks were just barely tilted. I removed hold-downs, tensioner, and adjusted cam chain to correct setting.

Solved

Problem solved! Cam chain adjusted correctly and now bike runs perfectly. Was this a factory error or was the former owner messing around with the cams for some reason and screwed it up?