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Updated 2008-01-31

Crankcase Leak down Test Apparatus for Honda CR250R

Objective:Click for Larger Image

  1. Seal crankcase openings to determine if there are leaks in gaskets, "O" rings, or other seals
  2. Pressurize the crankcase and measure the pressure over time
  3. Determine condition of crank seals or failure of power valve.
  4. Listen to seams, covers, and bolts with ultrasonic tester (borrowed) to find leaks
  5. Remedy excessive amounts of black leakage from the exhaust pipe. Verify that the wet side crank seal is not leaking gearbox fluid into the crankcase. This would richen up the mixture and leave lots of unburnt black funk around the silencer.

Leak down Test Apparatus Main Body

There are a handful of components that make the leak down test apparatus. Click image at right to enlarge. Each component is readily available from either a hardware store or auto parts store. The freeze plug and pressure gage are from a car parts store. The schrader valve, hose barb, tubing, PVC elbow and PVC cap are from a hardware store. The beer tax was more expensive than many of the components used.

Schrader ValveFitting A
The schrader valve
from plumbing section of hardware store

Fitting B
The hose barb
from plumbing section of hardware store

click to enlargeThe main body of the leak down test apparatus is a PVC elbow with a PVC cap fitted. Click image on left to see a larger image. I used ample amounts of PVC glue just to make sure there are no air leaks. It makes purple and blue marks on the nice white PVC, but who cares. It covers up the greasy fingerprints. Let the glue dry thoroughly. The PVC components are really cheap and have thick walls to fit the valve and barb well.

click to enlargeDrill and tap the threads for the fittings when the PVC glue is dry. Be sure to use the correct drill bit for the tap, and locate the holes on the top and bottom of the tester body. The hole for the barb should point up so the gage and tubing create the most direct route when installed. Click image on right to see the larger picture. The Schrader valve hole should point down where the foot pump can sit on the ground. Tap threads into the thick parts of the PVC cap where the elbow and cap are close and tight. A gap may form if you tap a hole too close to the closed end of the cap because the parts have a taper on them. Use tapered taps sized to the barb and schrader valve body to create threads on the holes you just drilled. Note the diameter of the barb and schrader valve are different and have different threads.

Put a dab of sealer on the threads of the schrader valve and hose barb and screw them into the main body of the tester. I used teflon tape on the threads with good success. These threaded pieces must be airtight. Where the mold that made the elbow parted, there was a ridge created that could lead to an air leak when the tester is installed. It is also a good idea to sand down the parting line on the elbow so it is flat.

This is my new version (click here) of the leak down tester. The PVC body allowed air to leak around the molded o-ring on the inside the carburetor boot. The aluminum base has a groove to accommodate the o-ring and assure a quality seal. The new tester uses a schrader valve to allow the crankcase to be pressurized and a ball valve to isolate the air pump. The PVC tester suffered from pressure loss when the air supply was removed from the schrader valve. The air supply and schrader valve are isolated when the ball valve is closed thus solving the air supply problem. The image on the right will show an enlarged picture.

The pressure gauge is mounted to a hose barb located on a coupler mounted between the base and ball valve.


First clean the motorcycle to get all the dirt off the frame (including the underside of the upper frame) and throw it on a sturdy lift.

Remove the exhaust pipe, springs, and grease/RTV that may be gunked around the exhaust port. Take time to obsess about the black gunk in the pipe. I have tried several solvents and even stove cleaner, but the black gunk will not budge. Argh! Install the freeze plug in the exhaust port and tighten the bolt to compress the rubber that seals off the port. I had to modify the diameter of the freeze plug to make it fit better.

Remove the carburetor. I found that it is easier to just remove the entire subframe in one big chunk. This means that the seat and side plastics come off. Be sure to stick the bolts back into the holes they came from so you don't lose them. Be sure to put slack in the clamp that secures the air box to the inlet of the carb. Lesson: somewhere in the process of removing the carb without removing the subframe I bent the choke lever shaft just slightly. That was a $40 mistake.

Install Leakdown Test Apparatus into the reed valve boot (the side the carb slides into) using an aftermarket hose clamp. The diameter of the PVC elbow is 1.636". The clamp that was on the bike will not collapse the reed valve boot enough to fit the PVC elbow. Make darn sure this connection does not leak, but do not damage the reed valve boot.

Attach the tubing between the leak down test apparatus and a vacuum gage that is sensitive enough to see 0.5 to 1 PSI changes in pressure. Mine will only measure 10 PSI max. I used enough tubing to prop the gage between the throttle and the brush guard at eye level for comfort. The gage is available at most auto parts stores. It is a handy gage to have.

Attach a foot pump to the schrader valve on the bottom of the leak down test apparatus.

Pressurize the leak down test apparatus with the foot pump to 5 PSI and measure the rate air leaks out of the crankcase using a clock with a a second hand. Note: my foot pump leaks thus invalidating the first set of test data. To resolve the problem, pressurize the crankcase and then quickly remove the air pump nozzle from the schrader valve before any leak down can occur. Be sure to compensate for any air that escapes when you remove the pump nozzle.

What rate indicates a problem? I am not sure, but I used 1 PSI/Minute leak down as a threshold. Fortunately my CR250R leaked 1PSI in 5 minutes after I fixed a leak in the CRV cover. It had a ripped gasket. This was enough to make the bike run like crap.

A borrowed ultrasonic tester can be used to hear air leaks around the jug, seams, and bolts. The tester can also be used to verify the soundness of the test rig. I also use a solution of dish soap and water can be sprayed allover the engine to see leaks in seams. You will see a white foamy lump on the leak and the bubbles make a crackling sound with the ultrasonic tester. The ultrasonic tester is not necessary, just cool to mess around with. Too bad I had to give it back.

Stick everything back together. Use factory torque specs, new bolts where important, and dab anti-vibration compound on bolts.