Extra vibration at 3100rpm?

XSNate

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Is this normal? It's really only at 3100rpm. Goes away above and below. Hope this is not a main bearing?
 
I dont want to advise you one way or the other.I can only say if you have real high miles or ran it out of oil I might be concerned.If not I wouldn't be to worried about it,I have some increase of vibes at mid throttle.I like to believe one reason the XS is so popular it is very reliable and the main components hold up over time.:bike:
 
Mine has almost 18k on it and I run the piss out of it with no worries.The vibes are typical and unless you see smoke from your exhaust our hear unusual noise you should be good to go.:bike:
 
I believe you have a 79? so it would have the mechanical advance and 3100 is about where it's hitting full advance might want to check the Auto Advance Unit to make sure it in good shape and working right
 
If you haven't done so, check that all the motor mounts are tight. These bikes vibrate and things come loose. When I got mine, not one but ALL the mounts were loose, lol.
 
On a traditional connecting rod reciprocating engine, the 2700-3000 rpm zone is a magical area where internal dynamic forces go thru a transition.

At low rpms, the engine internal forces make sense in term of what we've been taught. Starting at bottom dead center initiating the compression stroke, the piston rises, compressing the mixture, and the connecting rod is experiencing compression force, increasing all the way up to top dead center, and increasing more from the combustion pressure pushing on the piston, delivering this push force thru the connecting rod to the crankpin, all the way down to bottom dead center and the end of the power stroke.

At high rpms, the connecting rod experiences reversed forces. Starting at BDC of the compression stroke, the piston is starting at zero speed, and must be accelerated to its maximum speed, which occurs around 80° BTDC. This acceleration plus compression of the mixture puts a very large compression force on the connecting rod. However, after this point, the piston speed rapidly reduces, and its inertia (plus the wristpin) induces a tension force on the connecting rod. This inertial slowing-down force is greater than the force of gas compression, producing the tension force on the rod. After TDC, even with combustion pressures acting on the piston, the accelerative forces of the piston being pulled down by the connecting rod exceed the combustion pressures, and the connecting rod is still in tension, until the piston max speed at about 80° ATDC, where the piston begins to slow, and the connecting rod now experiences compression.

Now, at mid rpms, around 2700-3000 rpm, (what GM calls the 'detonation prone zone'), these competing forces, compression/combustion pressures versus piston/wristpin inertial forces, enter an interesting limbo state, where the rod can experience either compressive or tensive force, depending on throttle position and ignition timing.

Since the connecting rod angles to-and-fro relative to the cylinder wall, the slipper piston will experience scuffing forces either on the frontside or backside of the cylinder, depending on whether it's pushing down on the rod, or being pulled down by it.

So, there's three zones of force dynamics. Low rpm, predictive conrod compression. High rpm, predictive conrod tension. Mid rpm, the 'limbo' zone', where rattling of a worn engine can occur.

Experiencing a change of vibration while moving thru these rpm zones is normal, for a 360° vertical twin, but can be mitigated by being in a good state of tune, proper carb adjustment, and ignition timing.

However, rattling and knocking in this mid-zone is a sign of problems...
 
If you're cruising around at that low of an RPM, you really shouldn't be anyway. That's lugging the motor.
 
Really? The bike seems to want to be in the 3k rpm when I'm just riding around? 3rd gear 30-35mph = 3100rpm this is bad? Seems harder on the engine to me to keep the rpm up. Is this not true?
 
2nd gear would probably be better for 30 mph, 3rd maybe for 35 to 40. I like to cruise mine in the 3500 to 4K range. This puts it at the beginning of the power curve and rolling the throttle open gives good, near instant acceleration. Yes, running at very high RPMs all the time, like near red line, is not good, but an engine works best when kept in the power band. Typically for bikes red lining around 7 or 8K like ours, that's around 4K.

Another problem you may incur from running the motor too slow like this all the time is excessive carbon build-up in the combustion chambers. This was a problem in the '70s on the BMW R-bikes because many owners just putted around on them and didn't run them hard enough.

If you can't seem to get the right speeds at the proper RPMs and haven't changed from the stock 34T rear sprocket, I suggest you do. I don't feel that sprocket size was a very good choice for this motor. Dropping one or two teeth is a much better set-up. It only drops the RPMs maybe 400 to 800, but that makes a real difference. The European Standard model came with a 33T sprocket which I feel is ideal for this motor running an 18" rear wheel. A 32 is nice on a 16" rear wheel but robs a bit too much lower gear "lunge" on an 18" for my tastes.
 
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