A BIT MORE FOR THE ET4
Piaggio¹s Vespa ET4 is a superb town scooter, as the many sales in the UK attest, but it¹s hardly been welcomed with open arms by the traditional scooter scene. I don¹t think this has as much to do with the fact that it is a four-stroke or an automatic, as the fact that it¹s simply not fast enough for rallying. I¹m sure that if the ET4 was a 75mph Vespa with whatever engine, you¹d be tripping over them at various events nation-wide like you are with Runners and Dragsters, but you aren¹t going to get that kind of performance from a two-valve 125cc four-stroke.

The reason the ET4 is such a stunner in town is because Piaggio sacrificed some of the machine¹s top speed potential to maximise on acceleration from the lights. When we tested one back to back with a Vespa T5, they were almost identical on acceleration up to 50 mph (the ET4 actually gets the jump off the lights) after which the two-stroke slowly romps away beyond the ET¹s 60 mph top speed.

If you look at the blue line on the dyno graph below, you¹ll see why it accelerates so well. Right from the word go maximum power is delivered to the wheel. The machine tested was not new either. It had already done 15,000 miles, although it was recently serviced. While peak power of just under 9hp might not sound like a lot, it¹s not far behind many standard Vespa PX200s. The problem for the ET4 is that the power output trails off as the speed of the machine rises. By 55 mph the engine has lost 33% if its power, which will tend to make it lose speed with a heavy load or in adverse conditions, such as up hills. In favourable conditions the engine won¹t go much faster than 60 mph due to a rev limiter in the ignition system. Without altering the gearing in some way you aren¹t going to get a much higher top speed, so the best you can hope for from a simple performance modification is to increase peak power slightly and to get the engine to hold on to the power for longer.

PM Tuning appear to have done just that with their stainless steel PM56 exhaust for the Sfera/ET4 125 (pre-Leader engine versions) as can be seen from the green line on the graph. Power is increased throughout the speed range, with a rise in peak power of almost 0.5 hp. That¹s not the full story however, because the improvements multiply as the speed of the machine increases. At 55 mph, where the standard pipe has dropped off to 6 hp, the PM56 allows the engine to carry on making 8 hp: a 33% improvement. This makes a real difference on the road, particularly when climbing hills, since the modified scooter now produces as much power at 55mph as it used to have at 45 mph. The little Vespa will now romp up a long local hill with two people on board at an indicated 50 mph. No way would it do that before.

PIPE (& SLIPPERS)
For such an efficient conversion, you might imagine that there must be a downside, but so far there doesn¹t seem to be any beyond PM¹s suggestion that the scooter should now be run on 97-98 octane fuel (Super Plus Unleaded or LRP) rather than standard unleaded. In terms of fitting, it couldn¹t be much simpler provided you read the instructions, and make sure that the exhaust isn¹t under tension when everything is bolted up tight. It may be necessary to slightly bend your stand leg to give a little extra clearance, but otherwise there is nothing at all to modify. Carburation, ignition and transmission settings remain totally standard.

Four stroke exhausts have a tendency to rust from the inside outwards due to a build up of condensation, particularly if the machine is only used for short journeys. Since the PM 56 is made from rust-resistant stainless steel it should last and look better for longer than the original system. Depending on your outlook, one benefit is that the scooter no longer sounds like an emaciated sewing machine. The new exhaust gives it a totally unfamiliar throaty burble, which is more reminiscent of an old British single-cylinder bike than anything. Early versions of the PM 56 did not have the same small diameter tailpipe, giving them a marginally greater power output, but overstepping the mark as far as noise was concerned. For £205 + VAT, the PM 56 has got to be regarded as a simple, good value way improving the performance of an ET4.

C¹MERE, THERE¹S MORE
If you wish to go further with the ET4, then the next step should be to raise the gearing somehow. If you combined the PM 56 with an upgear kit and/or performance variator it should be possible to get an extra 5-10 mph top speed without loosing too much in terms of acceleration. Malossi make a 137cc alloy conversion kit which will boost power and torque slightly. If you combine all these modifications then you should end up with a machine with a genuine 70 mph potential, which isn¹t bad for a basic 125cc four-stroke.

As ever with an ET4, the main consideration is to keep an eye on your engine oil level and to use high quality synthetic oil wherever possible. Early ET4s had a habit of drinking a little oil. With only a one litre sump and a discoloured sight glass it is easy to think the engine is full even when it is empty, and the results of neglect are disastrous. If you keep on top of your servicing then an ET4 ­ and particularly a tuned one ­ should be a more rewarding town tool than most traditional scooter riders will ever suspect.

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