Vibration.

[Boppin Bobby]

My mate's 750 ss ( same motor) is sick. He's getting abnormal vibration which increases with more throttle. Could this be ignition related or, as he's fearing the worst, the crank giving up the ghost due to only running on 1 pot.
No visible or audible signs of running on 1 but it's eating plugs.
:confused:

[YourOldNemesis]

Surely if you were running on 1 pot you'd know about it as you'd only have 1/2 the number of bangs and probably less that 1/2 the amount of power you should have?

Besides, why would running on 1 cylinder hurt the crank? There are plenty of singles around that don't need rebuilding every 500 miles, also, the BMW X5 doesn't run on all cylinders when cruising to improve economy and I don't think it causes any crank issues... A 90deg V twin is not a balanced engine so I can't see how running on 1 pot would cause vibrations any worse than normal, AND running on 1 pot would still have the other one going up and down inside the cylinder, so it would not cause an abnormal imbalance.

Given the above I can't think what the abnormal vibration would be apart from some really nasty timing/fuelling issue so that your timing is way out...

[paul620sie]

"its eating plugs" - maybe a silly question but is he using the right plugs?

Virbraion is most likely due to for ignition or fueling than a mechanicial problem like the crank. Get them checked first.

[Zimbo]

As above, the eating of plugs is likely to be a big clue, and an unbalanced crank wouldn't cause that. Likely fueling i suspect. When you say "eating plugs" can you be more specific, are they melting the plug, fouling it with oil? What colour are they when they come out? Is it the carb or injected model?

Lastly, has he had the valves shimmed? Could be that too, in fact thinking about it that's probably the most likely cause ....

[superlight3]

Shove some new plugs in and see what happens. If it's fouling the plugs (with oil) within 50 miles or so then most likely piston ring or valve guide failure.

Bottom end bearings will be a 'rumbling' at tick-over (stick some rags over the exhuast exits to better hear the engine).

Else it could be the dreaded alternator rotor come loose (you'll know that if it happens!). Which would mean a new crank/rotor/bearings/re-mortgage ete etc.

Cheers.

Lee.