missing lynx:
"my understanding is the gt kit is not mildly tuned but a quite a high reving kit were as a standardish big block won't rev much over 6000rpm keeping it within the operating range of a std thin taper crank even with the heavy electronic flywheel"
speedy
"Firstly ditch the li crank, I tried to use one on my Jet 200 with a stage 4, Ancillotti and 30 Delly, whilst usuing the fantastic Motoplat electronic ignition [ I'm presuming that's what you have and bot the points version ], and I sheared my crank taper after 1600 miles. That was taking it easy without caning it or sudden quick down changes putting excessive revs on the crank, so get rid straight away. I replaced with the black web mec crank, which are reasonably priced and said to be fine for motors upto 25 bhp, so ideal for the gt 186.I have just built another bike using the big block gt 200 and used a similar crank in that and a bgm 120w stator, midweight flywheel and have static timing of 17 deg. The gt 186 kit has been tried and tested for many years now and are brilliant imo. A friend of mine has just bought a 198 muggello to use in a motor for touring 2 up, instead of my suggestion of the gt 186. I think he has made the wrong choice for his intentions.cheers,Speedy."
Thank you for clarifying your beliefs & you may be right. I know this will go against commonly held views, but do you mind if I explain my own take on crankshaft tapers?
I’m not completely daft, so don’t think I wouldn’t choose a GP 200 taper crank over any other if I was to build an engine from a ‘blank template’ but there are a lot of riders that use the Servetta/Li taper as the consequence of it being in the machine when they bought it, or that they prefer the Motoplat electronic flywheel.
The Eibar crankshaft has a smaller taper, but any reputation of shearing is probably not justified simply as the result of being subject to more power, or torque, of a tuned engine. Increased revs alone aren’t likely to massively increase that risk either, IMO.
With any crankshaft, it might be subject to higher loads in the majority of UK Lambretta engines as there must be very few unmodified, standard engines left!
As to what may shear the taper on a crankshaft, I believe that the reputation for LI type, smaller taper crankshafts having a tendency to shear was started as the result of 60/70’s tuning & racing & ‘running in’ meant that seizing was almost mandatory!
Badly fitted, badly lightened flywheels, unbalanced flywheels, crankshafts out of true are all likely to be factors in crankshaft shearing. Not power/torque alone.The dynamic loading on the magneto side of the optimum, balanced (not out of true) crankshaft should only be that of a flywheel accelerating & decelerating with no adverse ‘shear loads’ as in the case of a piston locking up.
For the record, the GT186 produced about 16.6 hp & 13.1 lb/ft as tested by ScooterCentre on YouTube. The maximum RPM was 9100.
That’s quite impressive but I’ve known motors with not quite the same revs but more power & torque in tuned Jet200’s that flogged up & down the UK for years ridden by enthusiast owners with standard crankshafts that didn’t & never will break as the result of the crankshaft taper.
Like everything else, set up a Lambretta/Eibar engine to the optimum with no ‘bodgery’ & it will be as reliable as any V*spa.
The problem is that with such an iconic machine as, say, the Lambretta GT200 that looks fast when it’s standing still, there was/is the temptation for amateurs to increase the performance & not do it properly. Then, when something fails, they’re not likely to blame themselves, are they?
Us ‘Brits’ are a picky, weird lot that tend to herd. We have hang-ups that we hold on to for decades! Consider why most members on this Forum would advocate electronic ignition until their dying breath.
“They are more reliable than points” I don’t think all of the problems I read about on this Forum would back up that myth…..
Posted: Thu Jun 09, 2016 7:04 am
