Why is my Yamaha XL800 idling excessively high?
I have a 2002 Yamaha XL800. It says MR-1 2-stroke on some paperwork, but it is idling excessively high when I start it up. What is causing this and how do I fix it?
Answer
A high idle on your Yamaha XL800 is almost always caused by a dangerous air leak leaning out the fuel-air mixture, or a misadjusted throttle cable keeping the butterfly valves open. You must stop running the engine immediately and systematically check the throttle linkages, carburettor base gaskets, and crank seals to prevent a catastrophic lean seizure.
Initial Mechanical Checks
Your 2002 XL800 uses the 784cc 2-stroke twin engine with power valves. The MR-1 designation belongs to Yamaha's 1052cc 4-stroke engine found in later models like the FX140. Rule out simple mechanical faults before stripping the 2-stroke 800 twin down.
Check the throttle cable route and tension. A binding cable pulls the carburettor butterfly valves open. You need about 2mm of free play at the throttle lever before it engages the cable. Next, check the mechanical idle speed screw. This T-handle adjuster sits on the side of the carburettors. Back it off anti-clockwise to see if the RPM drops. A 2-stroke out of the water on a flush hose naturally idles high at around 2,800 to 3,000 RPM because there is no jet pump drag. In the water, a healthy XL800 idles smoothly between 1,300 and 1,500 RPM.
Air Leaks and Lean Hazards
If the butterfly valves sit fully closed against their stops and the engine still screams, you have an air leak. Unmetered air entering a 2-stroke marine engine creates a lean fuel-to-air mixture. This condition causes the engine RPM to surge uncontrollably and raises combustion chamber temperatures. Riding a ski in this state melts a piston within minutes.
Check these common XL800 air leak points:
- Carburettor base gaskets: These fail if reused or if the mounting nuts vibrate loose. Torque the nuts down to 16 Nm (12 ft-lb). Always use genuine Yamaha OEM base gaskets at around £15 each. Cheap aftermarket paper gaskets fail regularly.
- Intake manifold gaskets: The joint where the manifold meets the reed block and crankcase draws air if the bolts loosen.
- Vacuum lines: Inspect the pulse line running from the crankcase to the carburettor fuel pump. A perished or split rubber hose draws air directly into the crankcase.