Here's a question for the Gurus...

[Jerry]

Just curious...which igignition module are you running? If you kept the 10M stock XJ, the ignition / advance curve is set for the milder cams. I think the XJ also had a bit more static timing, 15 degrees instead of the earlier 10 degrees. Max advance also comes in a bit higher, 5,500 RPM instead of 5,200 RPM.

Good thread on differences in modules and timing curves can be found at: http://www.xs11.com/forum/showthread...t=timing+curve

I suspect the timing at idle is off a few degrees because of the 10M ignition box is coupled with the early cams.

If this is the case, couple possibilities. Switch to a 4RO box, which will work on the XJ, but is still designed for the milder cams of the '81 model year. Or, you can switch back to the mechanical advance system of the '78 - '80 11s. You would need a doner engine to get all the parts and pieces, including the extended left-hand pickup cover. (The XJ's electronic system is shorter, so the cover is much shorter as well.) There is, I understand, a wiring differnce between the 2H7 original module and the 10M box, same colors but plugs are in a different order. Randy may be able to help with this.

Or, you could just live with it. Bike seems to run fine at anything over idle!

[dbeardslee]

Stop me if I'm wrong here, but you say it pulled strong to 9500 rpm's. It wouldn't do that if it didn't get adequate vacuum to the advance, right?

[Jerry]

It wouldn't do that if it didn't get adequate vacuum to the advance, right?

On the XS/XJ engines, maximum advance comes in at either 5,200 or 5,500 RPM. Gasoline engines develope max vacuum at idle, and it drops as you go up in RPM. That is why the mechanical advance, or the programmed advance in the 4RO and 10M boxes is needed; at low speed / partial throttle operations, the vacuum can advance the spark for better combustion. (This is based on load on the engine.) Open the throttle, vacuum drops, and the mechanical or programmed curve provides the spark timing. At the 5K values listed above, vacuum no longer has any effect; you are running strictly on the mechanical / programmed timing values.

So, to answer your question, the ability to rev past 5,500 RPM on an XJ is not related to vacuum. Bike is running strictly on the timing curve built into the 10M TCI box.

[trbig]

I've got a couple 4RO boxes... though unknown if they are good or not. Are they a direct plug and play? Is there a way to check them first?


Tod