Ah yes, that old wives tale (can I even say that now?), or is it?
I’ve had the Pearl from new, and have to say that fresh from the showroom there was no noticeable flat spot at all. Certainly when you look at the electronic ignition curve you can see that it does have a sudden and steep mountain to climb. It should be noted those particular ign boxes were fitted across a number of Italian bikes back in the day so maybe it was kind of a one size (doesn’t really) fit all. That does mean that they are at least readily available as replacements on the second hand market – though they seem very reliable.
At some point down the track, maybe c. 48k miles I did a top end refresh (new valves to replace the supposedly super stretch and snap OE version), and also pulled the timing chest and replaced the chain in there. Foolishly, and I can’t think why I did it now, I removed the ign sensors from the case whilst I did this. Well resetting the timing on these is really quite a pain – there is no provision for fine adjustment at all. There is a factory tool listed for this, but no living person has ever seen one. Initial attempts were only partially acceptable and eventually I had to settle down to an entire morning of restarts and adjustments. What I read at this time was that not just the timing was important, but also the air gaps between the trigger and the sensors must be identical – this affects the point at which they go up that ‘curve’.
So then I had a good running a bike again.
But then some miles later I had that flat spot becoming detectable. Well there is no way those sensors are going to move in there. Further reading suggested that at higher mileages impurities in the fuel will have opened out the jets. So a replaced the full set on both sides. All good once again. I’m running the std air box, all std jets and with perhaps a slightly more open (but not much) Keihin stainless exhaust.
Next up, just a couple of years back the bike threw up a couple of glitches on the way to the Dragon. Had a cylinder drop a couple of times, but we got there and back OK. Not long after that I lost a cylinder entirely. Turned out to be one of the ign boxes had failed – they’re easy enough to swap over to make the check. And as I said, they are easy enough to get used replacements quite cheaply these days. With another fitted I had things running again but with a fairly vicious flat spot. I checked all the numbers on the replacement box and it seemed to all match. I bought another couple, from a V35 in fact, but plugging in one of these made no difference.
I’ve barely used the bike this last couple of years so looked no further until I had the engine out earlier this year – I was getting the gearbox refurbed. So whilst it was out I checked the gaps on the ign sensors again. Still spot on. Ho hum. So I put a new set of spark plugs in. I’ve always run BP7ES in this bike. I don’t tend to treat plugs as a service item these days, they just stay in. But what do you know, the flat spot has gone again.
Meanwhile, my pal has a V50 II owned from new and lower mileage – maybe around 27k miles. Runs real peachy, no sign of a flat spot and has never exhibited one.
So no requests for advice here, just some observations on various possible causes that help perpetuate (perhaps arguably) the myth.