How-to: Cylinder Leak Down Test on Honda
XR650R Motorcycle Engine!
In this engine diagnostics and troubleshooting how-to HD video, Moses Ludel discusses cylinder leak
down testing. The magazine's Honda XR650R motorcycle is the candidate. Learn how to perform a cylinder leak
down test on any internal combustion engine and why this is a superior tool for pinpoint engine
The Honda XR650R cylinder leak down test begins with setting the piston at top-dead-center (TDC) of
the compression stroke. TDC can be found with the timing mark or piston height reading, using a dial
indicator and fixtures. A "two-stroke timing kit" with piston TDC finder will work here. This motorcycle engine
is a four-stroke, single cylinder "thumper".
Caution: Make sure the
piston is at TDC on the compression stroke with all valves closed. If the automatic
decompression start/release mechanism is holding the valves open at TDC on the compression
stroke, loosen the rocker arm adjusters enough to establish valve
clearance and seat the valves completely...A leak down test requires all valves to be
This HD video describes how the leak down tester works. At left, Moses Ludel demonstrates how the gauge
tester registers leakage. The gauge reads in percentage of leak. Percentage of leak is the amount of air
leaking past the valves, piston rings, head gasket or a casting crack.
At left is an adapter made for the Snap-On MT324 leak down tester. Honda XR200R, XR350R and XR500R
four-valve, single cylinder motorcycle engines have a small, difficult to reach spark plug. This Snap-On MT324
gauge has been a shop mainstay since the early 1980s. Maintained properly, this equipment will serve for a
long time. At right, the actual leak down test with compressed air reveals considerable air loss at
the upper cylinder of this XR650R Honda motorcycle engine.
At left, the air leak past the closed exhaust valves is considerable. At right, sealing the exhaust pipe tip
causes air to rush out of the air intake box! This engine's extreme 95% cylinder leakage is largely
due to leaking intake and exhaust valves.
At left, any piston ring "blowby" can be read as air seeping into the crankcase. In this engine, the
majority of leakage is intake and exhaust valve issues. There is also air leakage into the crankcase. At
right, the air box shows abrasive debris at its edge. Dirt seeping into the air intake stream can cause
intake and exhaust valve pitting or score the cylinder's Nikasil liner. Nikasil wear at the top of
the cylinder is evident when looking through the spark plug hole.
This engine requires an upper cylinder tear down to confirm the
extent and cause of damage. It is clear that the damage is severe enough to cause substantial compression
loss. We will now rebuild the top end of this Honda XR650R motorcycle engine. Follow the step-by-step "how-to" coverage right here at "The Off-Road Motorcycle
Curious how this iconic and "bulletproof" Honda XR650R motorcycle engine
got to this wear point in less than 1000 original miles? Get the whole story in Moses
Ludel's comments on carburetor jetting and air filtration—exclusively at this
4WD Mechanix 'Tech & Travel Forums topic—click