Hi,
you mean the "jerky thump effect", that sometimes used to cause old XT's to stall at low revs, the reason for the dual cv carbs. ...?
Two different things here: chain lashing is one thing - if that is what you mean and loss of manifold vacuum is something very other. The CV carbs of the Raptor ( and KTMs, too) and the dual register carb of the Skorpion/SZR/SRX etc combat this by not being able to open without manifold vacuum regardless of how hard one twists the grip. Chainlashing -drivetrain backlash - is a problem of all big bore engines, whether singles or twins, have. Modern injection mapping helps a lot towards relief. Some like the current Ducati 4 valve twins are almost free of chain lashing. The brand new Morini is pretty good, too. The press made a big thing about the supposed undrivability of the MZ1000S below 3000rpm - chainlashing again. You don't read diddlysquat about the same bad habits in the Aprilia Mille which is in reality
much worse than the MZ, even the every first one. And the 1200 4 valve BMW twin is no exception either, despite shaft drive. Their new 800 twin has, like the single, a cog belt!
Of course big singles all have the problem, more or less. Classic thumpers - the MZ and SZR are not really thumpers - with their long stroke (undersquare), low compression, low performance and extremely heavy flywheel are soft on the throttle and easy to drive. A short stroke (oversquare) engine like ours with much higher compression even in OEM state must chop at low RPM when the trottle is opened too agressively. Especially since they also have, compared to a real thumper, a light crank and flywheel assembly. The flywheel of the dated SRX is
much heavier, needs to be, too, since the thing has a kickstarter. Even so, it is a notoriously bad starter. This is more due to the CV carb and automatic deco than the flywheel, however.
Removing the counter balancer: I do
NOT recommend this unless you are ready to take a beating and ride very actively, sporty, even agressively. With correct riding position and good physical condition, the vibes are not a problem - for the rider. They tend to loosen and drop almost anything on the bike until one has learned where and what and what to do about it. In the old days of real thumpers, this was commonplace. With any 50ies British bike, even parallel twins, you spent as much time looking after it as riding it, sometimes even more until you had learned the ropes. Been there-done it. Remember I am over 60. And I have a BSA A7 running now as well.
Until the newest models, KTMs had no counter balancer and the early 610 and 640 models had no oil pump, either. A pure racing engine, meant to broken after every outing.
enuf: the counter balancer needs up to 5hp to rev up and down. A lighter one needs less of course and OVER used to offer three (I think it was) different sizes. Of course none at all needs even less, namely nothing. There is a lot more to it, and this getting long.
Acceleration and braking is quicker. The motor responds like a twostroke - almost at least. Reducing the flywheel mass does the same thing to a lesser degree. But also makes the bike less drivable if one is prone to low RPM riding and an uncontrolled right wrist. The reduced mass increases the chainlash. THe CV carb is per se uncontrollable, so that augments the problems which Wonky alluded to a while back. A finely tune flatslide and good riding habits get around this. In fact, I can coast thru town at 2200RPM in top gear as long as I don't need too quick response. Can even accelerate (slowly) out thru the city limits without changing down. No chainlash. No chopping: no counterbalancer, lightened flywheel and freewheel, quickturn grip, flat slide, 60hp at the rear wheel but also 43 chainwheel on a much lighter Brembo wheel and 155kg with gas in the tank.
C3 is the rating of "space" between the balls or rollers and the races, tolerance, but not in terms of manufacturing quality. One reason to use C3 bearings is because the casings, when they get hot, tend to cinch up the outer races and the tolerance is reduced. Another is alignment. Another is changing axial tolerance at different temps.
One of the cardinal points in gettng performance
and longevity is to get everthing to spin as freely as possible: reduce friction!. In racing two-strokes (125, 250, marine engines) C4 bearings colloquially know as outboard bearings are used. Not a good idea for 4 strokes with their much higher loads per cc, but where lubrication is only supplied by the fresh gas and the engine gets dismantled after every race...
Magneto problems: this was the PVL magneto used by many. In my case the circuitry was finicky. The beast started up alright - out onto the track and after one round at best dead. Turned out, that the circuitboard must have had a thermic problem; as soon as it reached a certian temperature caused by the circuittry itself (not from the bike), it quit. PVL flatly states that they are finicky and never ever should be turned without a plug attached and grounded. I assume the main transistor (thyristor?) was damaged. This happened without apparent cause from one day to the next. The magneto worked fine for a year or so. We looked for days for a cause, thinking it was fuel starvation since that was how the bike acted. nada. Found lots of little things, too, but not the cause.
Such a small magneto has potential advantages: very low rotating mass, very small and light, no battery, thus selfcontained.
It also has the usual magneto disadvantages of being dependent on RPM to deliver any noteworthy spark. Below 500 is nothing, so starting a very high compression big single can get to be a problem. Without a starting machine you can forget it. My bike, without deco, could not be jumpstarted regardless of how many were shoving.
A battery igniton like the SilentHektik delivers max spark at any RPM, the SilentHektik delivers 3 sparks in milles. series. The ignition is even lighter than the magneto and the rotating mass is nill. Disadvantage is needing a battery and of being dependent on its charge to run. Or you modify a small light ca 100W generator to run the system, necessary for fuel injection if you have it, but then you have the higher rotating mass you were trying to get rid of. Drivability of the SilentHektik ignition is much better, not to mention starting, which is now child's play.
All three bikes I use on the track - the 1978 1200cc Laverda triple, the 1986 Bimota DB1 and the Skorpion SOS - have constant loss battery ignitons now. The Bimota never had anything else and was raced in the BOT that way. Just a little 4,5ah battery like the little one now in the Skorpion. The big triple needs, due to its three current-devouring coils, something a bit bigger.
Merry Christmas.
in the flesh, sounds wicked. Wonder if he sold it? I'll get the Martin Sweetmeister on the case...