Took the bike out for it's first test run to the top of Doi Suthep mountain. Plenty of steep corners, engine felt fine, no problems. The ascent was mostly first and second gear, around 8-10,000+ rpm. Descent was in mostly second & third gear. On returning home, as the bike came to a halt the engine revs stayed at 4,000 rpm.

I tried the bike around the car park. If I applied the brakes the engine revs were brought down to the normal idle speed, 1,400 rpm. If I applied the clutch, and came to a halt, the revs stayed at 4,000 rpm. This, with the throttle grip fulled rotated "home" i.e. no throttle applied. I repeated the test. As the brakes were applied the engine would slow to normal idle. If I slowed with the clutch in, the engine rev'ed by itself, and then settle to 4,000rpm. The only solution was to switch the engine off. The question is, does this sound like a problem with the ECU, bearing in mind the engine has just worked hard over c.15km, a vertical +/- 900m., or does it sound like something inside the carb has come loose/needs adjustment?

With the bike stationary but the engine at 4,000 rpm the clutch would not engage. It "slipped" and made a clattering noise. Perhaps this is related to the problem above, perhaps not. The clutch bite point is at the beginning of the lever travel. I'm not familiar with the NC30's clutch, so any advice on whether this sounds like the clutch needs adjustment, new plates, or sounds fine (surely, if the clutch was worn out the bite point would be towards the end of the lever travel ie nearer the bars). Anyway, this problem might not be related to the engine "over-revving" issue. And, once the idle speed was normal, the bike went into gear, no problem.

really really stupid suggestion, you didn't leave the choke on did you? I forgot the other day and had it on full and it was stuck at 4k. The only reason I thought it could be that is you say the journey was about 15km, and you may have had the choke on to start it

The choke was my first thought, but after driving to the petrol station to fill up with the choke out, I put it in before heading up the mountain.

From the way that the revs settle at either 4,000 or 1,400 rpm I'm not sure if a mechanical fault is responsible. That's why I question the ECU, although I don't know if that's a likely problem given (a) it's solid state, and (b) can the ECU influence the fueling to the extent the bike over-revs on idle? I would have thought it's impossible if the carbs are mechanically governed by the throttle cable.

To check: the throttle cable, the cable adjusters on the side of the carbs, the carbs themselves (sigh, off again!), and the wiring loom to the ECU.

Does anyone have any other suggestions?

The CDI won't affect the fuelling in any way at all, as you said, the carbs are mechinally operated and have no electrics in them at all.

I'd suggest getting the airbox off and checking that the choke mechanism is free, both the lever that the cable goes to and the actual plungers on the carb bodies.

Sounds like the carbs aren't seated properly (likely) or the pilot screws are way out (unlikely) to me.

Normally to run at idle, the butterfly's are closed and air is restricted through the idle jet.

If your bike is idling high, then the it must have extra air getting in somewhere. Whether this is from the choke, or carbs not seated correctly, or your vacuum hose is not sealing correctly will be for you to determine.

The ecu box will control the spark ignition depending on engine rev's, so it will be controlled by the speed of the engine rather than having any control over the engine speed (in normal operation).

Sorry that's just a basic overview, hope it's helpful and not condescending.

Thanks, lads. That's really useful. Now I know where to look without worrying about the ECU.

As you said, it's most likely something simple like the carbs aren't seated right, or a problem with the choke mechanism or vacuum tube. There wasn't time today to get the carbs out but I got the fairings off. The rest I'll do tomorrow, and if it's a simple fix, another run up the mountain is in the offing. What are weekends for, if not for the bike?

Cammo wrote: Sounds like the carbs aren't seated properly (likely) or the pilot screws are way out (unlikely) to me.

80sman wrote:Normally to run at idle, the butterfly's are closed and air is restricted through the idle jet.

If your bike is idling high, then the it must have extra air getting in somewhere. Whether this is from the choke, or carbs not seated correctly, or your vacuum hose is not sealing correctly will be for you to determine.

After careful disassembly, air getting in via the carb rubbers is the prime suspect. The pilot screws, the choke, and vacuum hose were all to spec. Once the carbs were off, I found two of the lower carb rubber fasteners were not fastened tightly. Re-assembled, and the engine idles at 1,400 rpm, and does not hang at 4,000 rpm. Perhaps the revs are not dropping as fast as they ought. Rain stopped a test run, but there might be a chance for a run tomorrow. I'll report back if the problem isn't cured. Thanks again for the help.

Did it make it misfire aswell mate

Josh312 wrote:Did it make it misfire aswell mate

No misfire, but the engine did make a few "pops" that sounded different to normal. I need to take it out again to check as the original symptoms got worse towards the end of the ride.