QuestionQUESTION: Hi Bill, Thanks for your time. I recently purchased a 1974 Honda XL-250 in good original condition, although in need of a cam chian tensioner assy. I bought a Clymers manual and carried out repair with no problems. Except, now I can't get bike to run. I've re-adjusted points and timing to no avail. All I can get is a backfire,usually through the carb. At TDC, should the cam be pointing down with all valves closed? Or, should I set timing with intake valve open? Bike ran before I took it apart. Also, there is spark, but it is not a big fat blue spark. Please advise. Thankyou, Alan Thomas, FAllon, NV
ANSWER: Alan, The camshaft is going to be oriented with lobes DOWN or UP at TDC depending on what part of the stroke you are on. For cam installation, putting the crankshaft at the T mark alignment and then rotating the camshaft so that the lobes are DOWN and the sprocket marks are UP should make camshaft timing correct. Check your compression.. should be about 160-175 psi. If way lower than that, the cam timing is probably off a tooth.
Did you verify your ignition timing at F mark on COMPRESSION STROKE? If you pulled the spark advancer apart, you may have installed the points cam in 180 degrees out of phase with the engine.
Make sure that the points are clean and shiny, set to .014" maximum gap and then move the backing plate until they just OPEN at the F mark alignment on the rotor.. at Compression stroke.
Bill Silver
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QUESTION: Hi Bill, Thanks for replying. I'm not clear though with positioning cam lobes down with sprocket marks up. I found that when lobes are down you cannot connect cam to sprocket using the holes provided. Also, what does "skip a tooth" mean ? Any advice greatly appreciated. Thanks again, Alan Thomas.
AnswerAlan, I am looking at the illustrations at http://www.cmsnl.com/honda-xl250-motosport-250-k1-us_model838/partslist/E++06.ht trying to see if they used a scribe mark line or round punch mark on the camsprocket for reference purposes. Not clear to me, at this point. However, the basic concept remains the same.
(cam)
/ \ lobes
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o piston at T mark.
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Back off the camchain tensioner so you don't have interference with that.
Once you have the camshaft oriented to so that the lobes are down (compression stroke) you have to move the camsprocket around on the chain until one of the holes lines up. If the second hole is obscured then just feed the first bolt into the camsprocket/camshaft and then rotate the motor around until you can access the second bolt.
I just found a dingy photo of a used camsprocket on Ebay and it looks like it has the LINES through the sprocket, which will align with the horizontal split in the cylinder head cover, so when installed the lines should be parallel to the horizontal surface of the head/camcover interface.
If you install the camshaft a tooth off from the correct position, the ignition timing will be difficult to set and the compression will be lower, due to the valve timing being out of phase with the piston's motion. Unless the camchain tensioner breaks, it is hard for an engine to skip a tooth in normal operation. In some cases, some engines will jump timing if the camchain is excessively slack, when you shut it off.
Bill Silver
