I have a bit of an issue with my xr650r, it won't rev out to the limiter. It feels like it hits a "limiter" at about 6800 or so at fulll throttle. With less load (4th gear and 3/4 throttle) it will rev to the limiter but not smoothly. Jetting is good (bigger/smaller main has no effect), coil is new, plug and boot are also new. Checked the exciter coil voltage at the cdi (around 145v at the cdi plug), and at the coil (around 75v, so less than the minimum 100v specified by the manual). The low voltage at the coil could be the issue so i checked the loom thoroughly but cannot find any issues.

Another thing, the manual specifies at least 0.7v from the pulse generator when kicking but i'm getting 6-7 volts, ie. more than 10 times more than the specified minimum. I suspected that there might be voltage coming off one of the lighting coil outputs due to a short between the wired in the loom but the wires all check out fine on the multimeter, no shorts.

I started the bike and with it running, measured the output into the coil (meant to be at least 100v at kicking speed) and found that at idle it's around 110v but as i rev the motor it drops off. What on earth could be causing these issues? Testing with 2 different cdi's has no effect on the results and the motor still stops revving at around the same point. Earths are all good.

All of the peak voltage tests are performed using a peak voltage adapter. Measured resistance of the pulse generator is 250ohms and the exciter coil is about 90ohms (hot values). There are no honda values for resistance as far as i can acertain.

All suggestions welcome.
Thanks :)

I measured the voltage coming off the exciter coil BEFORE the cdi, it drops away as the revs rise too. Surely that is not the usual situation? Any ideas?

My first guess would be too large a gap on the spark plug.

Make sure to try the guys on the Yahoo group too- different set of folks.

Has it got a mechanical advance unit?? if so check springs and weights.

Ok. Have had made some headway. Before I forked out for a new stator etc i decided to remake the adapter between the FCR carb to the airbox suspecting that maybe the main air jet was not getting air. The original design was simple, only allowing for the difference in sizes between the FCR inlet and the air box boot. The new design separates the intake air flow from the air bleed jets by use of a velocity stack.
Pics below.

Before it was similar but minus the ram tube which i think may have been causing some turbulence/vacuum across the air bleeds.

There is now a much greater reponse to main jet changes. Using a wideband O2 sensor during tuning as well as feeling for best response/power i am now down to a 160 main, 40pilot, ncvs needle and next step will be a slightly smaller pilot air jet to correct a lean spot around the needle transition to 1/3 throttle area.

Bike will now rev quite cleanly to around 7800rpm (well over the peak power/torque point) and pulls 100mph where it struggled to make 85mph before.

Air fuel ratio is around 12.8:1 at idle, 13.5 at cruise, 12.8 at 2/3-3/4 throttle and 11.2-11.5 at wide open throttle, could possible benefit from a 155 main. More testing is on the cards.
Between 12.2 and 13 seems to be the sweet spot.

6 volts from the pulser is normal. I don't know why Honda always lists .7 for their pulsers, they are always around 5 at cranking speeds.

You` should have at LEAST 80 peak volts after CDI before coil at all times.

Sparkplug caps have a resistor in them that can fail, but you say it is new.

I've seen kill switches with high resistance that redu`ice but don't eliminate post-cdi peak voltage. check continuity across the keyswitch, thumbswitch, and pushkill if so equipped. Be suspicious of any high-resistance readings.

12.5 - 13.2 is ideal.

What are you running for a pipe? Are you SURE it isn't clogged/restricting at high flow rates?

36MotoMarc said:

The kill switch earths to kill the motor, so i disconnected it when checking the voltages. No change. But the issue doesn't appear to have been spark after all.

I'm sure the pipe is restricting it a high revs, it's the stock one. But it is clear and it was only WOT that it would not rev under. Under 3/4 throttle it would rev better. It's restrictive, but not like hitting a brick wall restrictive and that was how the motor stopped revving, like a limiter.

I'm sure the problem is now fixed, it could probably be further improved by adding vac lines to the air bleed inlets and routing them to a small filter outside the airbox. But, it's good enough. The thing hauls!

I have two XR650R's and believe me when I say that you want to dump the stock exhaust.
I have a Big Gun ceramic coated system on one and a FMF power bomb on the other. Not only will you see a nice gain in performance, they both sound awesome.
I like the sound of the Big Gun better and the ceramic coating makes it a lot cooler on your leg :-)

I have had issues with sagged or improper valve springs with other bikes in the past leading to premature valve float.