Motorcycle Repair: 79 Honda xl100

Question
I have a 79 Honda 100xl. I need to know how to set the timing. It trys to start but it backfires threw the exhaust.

Answer
John, unlike the earlier XL100 models which began production in 1970, the 1979 version is based off of the XL/XR75-80 engine designs, which used a magneto ignition instead of the battery powered points/condenser of the early days.

First I hope you are asking about ignition timing and not valve timing. If you have removed the cam for any reason and not timed it back up properly, there is no getting the engine to run until you get the cam set properly. Check your compression readings to be sure that your valves are seating and you have at least 150psi compression in the cylinder. Verify that the valve clearances are .002" at TDC compression phase of the engine.

Ignition timing is dependent upon several components and adjustments. The ignition points are under the flywheel. If they need adjustment, you can do it through the slots in the flywheel. The points need to just OPEN at the F mark alignment on the flywheel and engine case marker. The gap should wind up in the .012-.016" range. IF the points are not excessively worn on the contact faces and/or the rubbing block, then you should be able to just use a screwdriver to loosen the lockscrew, adjust the point gap by the slot in the point base and then tighten it all back down again. Verify that the points are opening at the F mark and then you can check to see what else might be happening. If the points can't be adjusted, then the rubbing block is worn down and the points need to be replaced. You MUST buy/borrow the correct flywheel puller tool to remove the flywheel, so you can access the point set. They are commonly available at many bike shops or buy online. The point contacts must be CLEAN and the rubbing block surface on the inside of the flywheel lubricated with point grease.

The condenser and coil are up under the fuel tank. The condensers can fail and cause random firing of the coil. If the condenser fails, you will see deep pitting on the point contact faces. Check the ignition coil for loose connections and test the spark plug cap for 5k ohm resistance. Plug caps can fail causing misfiring.

http://www.cmsnl.com/honda-xl100s-1979-usa_model1139/partslist/  gives illustrations of the parts involved.

For the engine to run you must have:
Compression, spark (correctly timed), clean/fresh fuel metered through the idle jet, idle mixture adjustment set correctly, and air (clean air filter). If the bike has been sitting, you will have to clean out the carburetor and ensure that the idle jet is open.

Bill Silver