Year, Make, Model

The bike used in this video is a 1999 Honda XR70R. However, if you engine looks anything like this horizontal OHV engine, then this procedure is the same, whether you own a Z50, CRF50, CT70, etc.

Symptoms

While this motorcycle had no symptoms of tight or loose valves, lets discuss some symptoms of either case.

Tight Valves

• Loss of power due to valve not closing properly, engine will lose compression.
• Might be hard to start when HOT.
• A valve that does not close properly will build up heat. This is especially true with exhaust valves. Heat is dissipated when valve makes contact with seat on cylinder head.

Loose Valves

• Loose valves may make engine noisy.
• Loss of power due to less valve lift.

NOTE: If for some reason you are experiencing low compression even after a valve adjustment, you may have to perform a leak down test. I have a full engine rebuild video on the Z50 here.

Valve Clearance Specification

The valve clearance on this model is .002″ ±.001″. Verify with your service manual as specs may differ.

Valve Adjustment First

If performing a tune up, the valve adjustment should always be done first, then ignition timing if applicable, followed by carb synching/tuning. (NOTE: no carb synching needed on a single carb engine). If your 50/70cc needs a carb clean, then check this video out.

Tools

•  9mm box wrench or socket for adjuster locknut
• Pliers to turn adjustment screw
• Feeler gauges: .001″ thru .004″. It helps to put a 90° bend in them to reach under adjustment screw.
• Phillips/JIS #2 or 8mm for LH cover bolts
• 10mm for shifter
• 17mm box wrench or socket for tappet cover

Finding Top Dead Center (TDC)

The easiest way to find top dead center is to remove the intake (on topside) tappet cover, spark plug, and LH engine cover. Rotate flywheel CCW while looking at the intake valve. Once the intake valve gets pushed down, the engine is on the intake stroke and will soon be coming up on compression stroke, or TDC. Now focus your attention on the flywheel & engine case mark. Bring the 2 marks together. Your engine is now at TDC. At this point I like to go wiggle the valve rockers to see if they are both loose, another indication that you are at TDC. If they are tight, you may just have a tight valve. Just make sure they are not pushed down, meaning you are not at TDC.

Get The “Feel” Right – GO/NO-GO

It is very subjective on how much “drag” a feeler gauge should have when adjusting valves. I typically keep increasing thickness until one size will not fit. For example, a .003″ gauge “GO’s” while a .004″ will “NO-GO”. In this case the clearance is .003″. So go by feel and check the next size up to verify it is a “NO-GO”.

Comments

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