Fit & tune – BGM 177 kit: part 1 | WORKSHOP
Conquering the world on a shopping moped – part 1
The Vespa 125 and 150 have always been the bread-and-butter-versions of the P-range, outselling the P 200 and T5 by far, but they have never really been the ideal first choice of a rally-going scooterist. However, new tuning options on the market might just change things. In this series, we’ll be building and tuning a touring engine based on an LML 150 lump and put it to the test on the way to the Venlo rally in Holland.
Last year our trip from southern Germany to Venlo was a last-minute affair. With my P2-sidecar-combination being in bits and none of my other classic scooters considered up to the job of 600 kms of German motorways, using the GTS 300 was the last resort. However, despite (or rather because of?) needing only half as much fuel as the other guys on two-stroke Vespas and the trip being totally effortless, I found it – hmm – let’s say a somewhat muffled experience compared to riding a traditional scooter. No harm intended, fellow GTS-riders, I don’t want to start another flamewar about automatic scooters, each to their own – it simply didn’t feel right for me.
Fast-forward one year – and the forthcoming summer rallies are imminent. The sidecar combination now has made her way to the paintshop – but still won’t be ready on time. Plus I sold the GTS because I was bored with it. However I recently bought a cheap PX 80 in bits to replace the GTS as a daily ride. Of course the PX 80 standard engine is completely useless for any other application than propelling 16-year-old German kids to school (and being late of course because of the sluggish performance of both teenager and machine). But with a more powerful engine, capable of pulling a longer transmission, it might become a suitable touring bike. Wait a minute – I still have a 150cc LML engine kicking around…
Now there are good reasons why the most widely recommended rally tools have always been the P200 and the T5. The P200 has tractor-like pulling power that, combined with its tall gearing, offers a very relaxed and reliable touring setup. The T5 to the contrary features a high-revving engine, however its alloy barrel means that reliability is not being compromised by this and it can happily be thrashed all day long. The PX 125 or 150 have neither of these features. Their standard engines are simply too slow for mile munching, and the cast-iron tuning cylinder kits traditionally available for these engines have never been the ideal choice for long distance trips on the motorway.
However times are changing, and with a new generation of alloy-barreled tuning kits from the likes of Parmakit, Quattrini etc. all arriving on the market, there are a few new options to help enhance the smaller PX engines’ touring qualities. So the decision is struck – I will build myself a touring engine based on the LML-lump and see what this does when coupled with a modern barrel design. The latest newcomer to the market in this class is the 177cc BGM kit supplied by Scooter Center Cologne, which has had some very positive reviews and therefore will be the kit of my choice to test as a touring tool.
The unusual bit about the LML engine I’m going to use is, that it is one of the latest designs and comes with reedvalve induction and a new cylinder layout with 5 transfers as standard. You could say the engine is a blend of a standard PX 150 and a T5, and this is also how it behaves on the road. Now while you’d assume that this will form an ideal basis for a tuned engine, things are not all rosy.
The original reedvalve used by LML has only two petals, the focus here was clearly for fuel saving (originally using a 20mm carb) rather than performance. To feed my touring engine I want to upgrade to 24mm – we will see how this pans out later. Experience gained by some other people indicate that the LML reedvalve is rather inferior to a well-tuned rotary induction with a gas-flowed crank, especially in terms of torque, but it doesn’t help – as this is what I’ve got.
Secondly, the crank used by LML is a source of potential problems – mine was not the only one to suffer from a premature con-rod-bearing failure. From today’s perspective, this comes as a bonus, as the 57mm Mazzucchelli crank I fitted when this happened about a year ago means that the bottom-end now should be fit for further tuning.
Finally, the port layout of the LML cylinder and casings is slightly odd – similar to most 177cc tuning kits, it has a larger transfer area than a standard PX. But while these tuning kits usually have their additional transfer area above the standard ports, i.e. towards the two upper barrel studs, in the case of the LML engine the additional transfer area is further down, towards the exhaust side. That means most Italian kits will not be a straight fit, as the transfers are in a different area, even if they have roughly the same size.
Getting kitted out
According to the info from SCK (Scooter Center Koln), their BGM 177 kit has been designed both as a straight out of the box bolt-on kit or as the basis for further tuning. Having said that, it already features a heavily increased transfer area and an exhaust port that covers 66% of the bore, which is what you would be aiming at when tuning a barrel (provided that you have good quality piston rings to cope with that port width). Port timings with a 57mm crank are 171 deg exhaust and 118 deg transfers. So for a touring engine, there should be absolutely no need for further modifications in that area.
The same usually applies to the transfer area at the cylinder base. This apparently has been designed with the standard PX transfers in mind, so that the actual transfer area in the cylinder base is comparatively small, with the transfers opening up after a few mil inside the cylinder. With the LML casings, this leads to the strange situation that the transfer ports at the base of the barrel are actually smaller than those in the engine casings. This is something that will need addressing in the further course of things.
Other components I’m going to use are pretty basic: with the Mazzu crank already in place, there is no need to split the engine again. The other things that will need attention are the gearing and clutch. The late LML engine already features the larger 115mm clutch (P2 and T5) as standard, and as I’m building a touring engine rather than race so this should suffice. The PX clutch sits on the crank, so that clutch slip is less of an issue here than it is with Vespa smallframe or Lambretta engines. Problems are created by high revs, the weakest point of the old PX clutch design is the basket cracking due to centrifugal forces. This issue will hopefully be cured for good by using a CNC-machined clutch basket from Austrian supplier Ddog, which is much stronger than the original item, but is still a very cheap offering compared to “proper” high end race clutches.
With the gearbox staying where it is, the only option for changing the transmission ratio is the primary drive. The standard 68 teeth primary gear cog can normally be combined with three different clutch cogs – 20 (PX80), 21 (PX125 and late LML 150) or 22 (PX150) would have been the only available options. The standard 23-teeth clutch cog of the P200 doesn’t fit as the cog wheel is too big. But thanks to the ingenuity of DRT, there now is also a 23-tooth clutch cog available, made specifically to suit the 68-teeth primary. That means that you can have a 2.95 primary drive ratio, which is exactly the same as if you’d been using a 65-tooth P200 primary cog with a 22-tooth clutch (which has always been the recommendation for 177cc cylinder kits). So far, so good.
Exhaust wise, I will be using a SIP Road box – again this has been kicking around for a while and has proven its worth as a highly suitable touring exhaust which offers both a healthy increase in power as well as standard easy access to the rear wheel and engine. Altogether, the potential is there for a capable motor while mainly using parts that I’ve already got at home.
Words and photos: Boris Goldberg