Bike Evaluation

EVALUATING USED DIRT BIKES

Whether you’re figuring out what to do to modify your own machine or buying a used bike, the best way to start is to use the following simple tests to evaluate the bike. Here is a list of the major components of a data bike, and some tips on evaluating the repairs.

Spark Plug

A spark plug is a record of the engine’s condition. Large globules of aluminium melted to the plug denote that the piston is disintegrating because of a crankcase air leak. A glazed finish on the plug denotes sand that passed through the air filter and has probably ruined the engine. Heavy carbon build-up denotes a leaking crankshaft seal on the clutch side. A bike with this program will have white smoke billowing out of the exhaust pipe and oil oozing from the exhaust manifold.

Compression Test

The correct way to perform a compression test is to thread the gauge into the spark plug hole, hold the throttle wide open, hold the kill button on, and kick the engine over until the gauge needle peaks. An 80, 125, 200, or 500 cc bike should have 150-190 psi at sea level. A 250 cc bike should have 170-230 psi. If the compression gauge reading is far below these numbers, the top end will need to be rebuilt.

Air Filter

You can tell a lot about how a guy maintains his bike just by looking at the air filter. If he neglects the filter, he probably never works on the rest of the bike. Check the filter for tears that could have allowed dirt to enter the engine.

Frame

Lay the bike on its side and inspect the underside of the frame. Look for smashed frame tubes, a cracked shock clevis [common on YZs], and bent link bars. Smashed frame tubes and broken motor mounts look expensive to fix, but a good fabricator can splice in a new tube or weld a mounts if you strip the entire frame down.

Suspension Components

Check the rear shock for oil leeks. If the seal is leaking, then the gas bladder may be punctured. Look closely at the shock. If the chrome has peeled, has deep scratches, or is blue from overheating, then the shaft will need to be replaced or re-plated.

Hold the front break and compress the forks. Does oil ooze out from the seals? Are the forks difficult to compress? Check the fork tubes and the aluminium sliders for rock dents. Rock dents in the tubes can be incredibly expensive to repair.

Coolant System

Remove the radiator cap and check the fluid level. A low level means the system has a leak. If the colour of the coolant is brown, the head gasket may be leaking internally. That means combustion pressure has leaked into the coolant system. The worst-case scenario is when the leak has caused erosion on the top edge of the cylinder. This is expensive to repair.

If the colour of the coolant is grey and foaming, that indicates that the water pump seal is blown. The blown seal allows transmission oil to mix with the coolant. Check the radiator for crash damage. Radiators can be welded or sealed with epoxy when minor damage occurs. Check the side of the cylinder head whether head-stay is mounted. Look for white or green residue of leaking coolant. This is common on older CR250’s and all KX’s. This indicates that the head gasket is leaking externally. The common course for external leaks is excessive forces transferring from the top shock mount through the head stay brackets. Check the rear suspension linkage on a bike with an external leak, chances are the bearings are seized or worn.

Linkage and Swing arm

Sit on the bike and bounce up-and-down to compress and rebound the rear suspension. Do you hear a screeching noise? Is the rear suspension seized in an extended or compressed position? This would indicate that the bearings, bushings, and seals need to be replaced because they weren’t greased.

Grasp the linkage with your hand and try to move it back and forth to feel for free play. If the linkage is bent or the bearings have disintegrated and elongated the mounting holes, then the linkage will need to be replaced.

Wheels and Brakes

Put the bike on a stand and grasp each wheel with your hands. Try to move the wheel from side to side with the axle held stationery. If you feel any movement, then the wheel bearings are worn out. This could require new bearings which is relatively inexpensive, or the hub could be damaged from riding with bad bearings. Check the surface of the break disks for deep scratches or signs of overheating. Disks can be resurfaced, or replaced. Bent front disks on endure bikes are common. All break disks have wear indicator lines scribed into the sides of the pad for quick visual checking. If you can’t see the lines, then the pads need to be replaced.

Drivetrain

The drivetrain consists of the clutch, transmission, chain, and sprockets. The trans oil can give clues to mechanical problems. Dip a strip of white paper into the Trans oil. If the oil sample is grey and bubbly, then the water pump seal is blown, and the coolant has been polluted the Trans oil. You can confirm this problem by test riding the bike. The clutch action will be prone to slipping when loaded and dragging when engaged. The chain and sprockets can be visually checked for wear. The chain can be checked by putting the bike on a stand so that you can rotate the wheel and check the chain slack at several different points. If the chain slack varies greatly, the chain is worn out.

TIPS ON TEST RIDING

Clutch

Pull in the clutch lever and put the bike in first gear. Rev the engine. Does the bike creep forward? If so, then the clutch basket may need to be replaced. When you rev the engine does the clutch slip? If so, then the plates and springs may need to be replaced.

Throttle Response

Ride the bike in second gear at 1/4 throttle. Snap open the throttle quickly. Does the bike bog or die completely? If so, then the engine may have a problem as simple as a clogged pilot jet or a major problem such as a crankcase air leak.

Transmission

Ride the bike in third gear. Accelerate while gently applying the rear break. Does the clutch slip? Does the transmission pop out of gear? Third gear is the most abused gear in the transmission. If the transmission pops out of gear, the shift forks may be bent or the engagement dogs on the gears may be worn.

Brakes

Apply the brakes separately. Does the lever or petal pulsate as you apply the brakes? This means that the disk is bent or warped and needs to be replaced. Does the front break have a spongy feeling? If so, then the problem could be as simple as trapped air in the system or is major as a worn master cylinder. Check the break line for leakage. Even if the guy cleaned the leak, brakes fluid will damage the plastic cover on the break hose.

Crankshaft

Put the transmission in neutral. Rev the engine and left off the throttle. Does the bike make a loud shuttering vibration? Do you feel the vibration transfer through the foot pegs and handle bars? If so, then the engines lower end may need to be rebuilt.

Once you’ve been through the bike, you can calculate how much it will cost to bring the bike back to tip top condition. At this point, you can decide whether or not it’s worth fixing a bike or begin to budget.

Click here for a chart to make notes as you evaluate a bike (in pdf format).
Bike evaluation chart