Motorcycle Repair: 1985 Honda Rebel 250, honda rebel 250, 1985 honda rebel 250

Question
Mr. Johnston,

I just bought a 1985 Honda Rebel that had been sitting out on a farm for at least two years.  The mileage reads 9,000, not sure thats totally true but it doesn't seem to have many more miles than that.  I bought it as a project bike and things have been going well but I hit a wall.  The bike runs good.  However, since the first time I got it on the road it has made a sound in 3rd gear.  The noise is like a low pitch scrape.  It makes no sound in 1st or 2nd for sure.  It might make the same noise in 4th and 5th but its not as loud.  I adjusted the chain (it was very loose) but it continues to make the noise.  I adjusted the clutch freeplay.  Any ideas?  What's my next step?

David

Answer
Well, David, I'm not sure what a low pitch scrape sounds like so I'll describe what I'd do and we can take it from there.  First you need to know how that transmission is designed.  All of the gears are engaged at all times.  The shifter slides different fixed gears back and forth on the main and countershafts to lock various free wheeling gears to the shafts which provided the different ratios. So the torque may be going through different gear sets but the transmission is loaded all the time.  I'm having a little trouble thinking how that load path can cause the noise to change with the different gears.   

I'm guessing it may be one of the four bearings supporting the two transmission shafts. If you prop the rear wheel off the ground, can you make the sound without load on the driveline?  I know those bike didn't have center stands so you may have to figure out a way to get the wheel off the ground do you can diagnose the noise.  I made a stand for my wife's so I could work on it. If you can get it off the ground, try using a long screwdriver to listen to various points on the crankcases.  If you press the handle up to your ear and put the blade on various points, it will act just like a stethoscope and you can clearly hear where the noise is coming from.

Having typed all of this, I just thought of something else.  You may just have a worn out drive chain.  Often times, chains do not wear evenly.  You will get a section of the chain with a lot more wear than another part.  We usually attribute it to stretch but I don't think that's the cause. I can't believe a 250 rebel has enough torque to stretch a chain. If you adjust the chain and then check the freeplay at various points as you rotate the rear wheel, I think you will find points where there is more or less tension. You need to check the free play while you sit on the bike because the chain will tighten up when the bike has weight on it.  Those uneven points will cause a periodic tone that sounds like a low pitch beat.

I'd be curious what you find.

Good Luck
Rich