Honda XR650R Motorcycle Upper Engine Rebuild Part 1: Tear Down
In this two-part HD video series, Moses Ludel rebuilds the top end of the magazine's Honda XR650R
motorcycle engine. Low compression led to this tear down. (See the
HD video how-to on the cylinder leak down test.) Here, "Part 1" covers the step-by-step tear
down, including cylinder head and cylinder barrel removal. This tear down will lead to
subletting the cylinder head and cylinder for machining and reconditioning.
Top engine tear down begins with removal of the complete exhaust system. At right, the stream of
draining coolant increases with removal of the radiator cap. Drain coolant into a clean pan and prevent
contamination if you intend to reuse the coolant. Strain used coolant through a paint filter when refilling the
At left, two triangle support brackets connect the upper engine and the frame. Removal of the three
bolts and brackets will gain access to the rocker box, cylinder head and cylinder barrel. At right the removed
rocker box shows rocker arms and shafts in good condition.
Note: Poor air filtration and an ultra-lean 125 size main jet on this "uncorked"
XR650R Honda engine overheated the valves and cylinder but did not damage the oil-cooled parts—a testimonial to
the use of high quality synthetic motor oil. Uncorking calls for a 175 main jet. We purchased the
motorcycle in this condition.
At left, camshaft sprocket comes loose with the removal of two allen head bolts. Prevent parts
from falling into the crankcase, or you will be recovering these parts. At right, the cause of the low
compression is evident: The valves are badly "coked up" with carbon and were not seating or sealing
properly. A wrong main jet and lean air/fuel mixture contributed to this problem and heat damage to the
cylinder wall and piston. Seepage of dirt around an aftermarket air filter added to the issue.
Warning: Any time you "open up"
the exhaust flow and intake air stream on a motorcycle engine, the carburetor jetting (or EFI flow) must be
adjusted. For this iconic Honda XR650R engine, the North
American main jet is excessively lean for even a stock engine. When "uncorked", these engines demand
a richer main jet. (At sea level, the 125 stock main jet increases to a 175 main jet in this XR650R application. The pilot jet should also increase in size.)
This engine allegedly has less than 1000 original miles on it, and the lean mixture, plus debris leaking
past the air cleaner, created the damage shown. The 1000 miles might well have been the Baja
At left, the cylinder wall is tough Nikasil plated yet shows scoring and heat damage. A lean fuel
mixture on this size engine raised havoc with the valves and overheated this cylinder's wall. At right, the
cylinder barrel is carefully lifted away from the crankcase, the piston and rings during an upper cylinder
rebuild. A four-stroke engine requires considerably more work during major service than a
The cylinder shows a mix of factory cross-hatch and glazing from severe heat. Fortunately, this is the
only XR Honda engine built with liquid cooling. An air cooled XR would have seized a piston from
this lean fuel mix. The piston does show considerable skirt wear, another result of heat damage. Quality
oil limited the damage to the valves, cylinder wall surface, piston skirts and rings. This damage would
likely have been prevented with the use of a 175 main jet and proper air filtration. This lower-mileage
motorcycle illustrates the importance of proper air filtration and correct air-fuel mixtures.
The cylinder head and barrel have now gone to L.A. Sleeve Company for reconditioning. We will keep track of the work
needed and share details during the engine reassembly in Part 2
of this HD video how-to series!
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