Fitting Lambretta’s best lightweight scooter with Vespa’s finest 2-stroke engine.
This project is going to upset a few purists and I’m happy about that. Purism fits blinkers. It tells you that standard and original are superior when all evidence points to the contrary. If something is perfect in its purest form then nothing you could possibly do to it would improve it, but there are always improvements you can make to any complex machine. The whole scooter tuning industry relies upon this concept. Purism be damned.
Having recently written a book on Scooterboys it refreshed my memory on two important aspects of the cult.
- We didn’t care about monetary value
- We didn’t care what people thought
Both of those concepts are massively liberating.
If your scooter is worth £20,000 then you don’t want to take it out in the rain and don’t feel safe to park it anywhere. Basically, it becomes an expensive ornament; which in practical terms means it has no value at all.
If your scooter is worth £200 then it’s not a problem to take an angle grinder to it to make something new. It’s also a suitable client for experimental tuning, so you learn new skills. You can ride it wherever you like, even off-road. You could actually get more enjoyment from a £200 heap than from a £20,000 mantelpiece project.
Cheap is great. Value is a shackle.
Complete mental unshackling is really not giving a toss what people think. That’s a harder trick to pull off after years of conformist conditioning. Scooterboys of the 80s were perfectly happy to go against the flow, to put noses out of joint and even to deliberately offend if necessary.
We don’t intend to go that far this time but there are still purist Lambretta owners that won’t be happy about us building a Vesparetta. Never mind.
My son and I are going to take something cheap and use left-over parts to make it better than when it came out of the factory. We sincerely do not give a toss whether you like that or not because it will be fun.
Launching Luna to the moon
The Lambretta Luna series (Lui, Vega, Cometa) is a work of Space Age styling deftness by Nuccio Bertone’s design studio. Essentially it’s a lightweight moped aimed at teenagers but to my eyes, it still looks more radical than any 50cc scooter up to the Italjet Formula of the 1990s.
While Bertone’s 60s styling looks great, Innocenti’s 60s engineering let it down massively. On an under-powered 1.5hp moped lightness is everything; if you want it to perform.
The Vespa 50 achieves lightness through engineering genius. After welding, the whole Vespa chassis and body is one stiff unit. The Vespa front fork is one stiff steel tube. As a result the Vespa smallframe handles well enough to accommodate much more powerful engines.
The Lambretta Luna chassis shows that Innocenti were watching and learning, but not entirely. The main frame is also one piece made from pressings and fat welded tubes, but still the legshields are separate. They add weight without adding stiffness.
Instead, Innocenti kept the weight down by cutting corners and making things spindly and weak. I’ve eaten with sturdier forks than are fitted to a Vega. The primitive front suspension is entirely without damping. The stand is a joke and the groovy cast aluminium handlebars feature tidy but poorly-designed controls.
If the chassis lacks engineering brilliance then the engine matches it. The upright 3-speed engine configuration of the Lui 50 is a gutless shambles. The 4-speed 75cc version used in the Vega and Cometa is a masterclass in vibration creation. When the guys at Rimini Lambretta Centre analysed the original crankshafts in order to develop a new version suitable for their Casa 135 kits, they found that the crankshaft balance factor was suitable only for jack-hammers and ladies bedroom entertainment devices.
This is before you get onto the 2-plate clutch and the air filter box that you break when kick-starting the engine. In a school report, the Luna engine project would really be a D+ “please try harder.”
Obviously nowadays the Casa Performance boys are steadily dealing with all of these foibles one-by-one on the Casa 135 project, but on a Lui 50 that means pretty much stripping the engine of all major components and starting again with something up to the job.
You can end up with a cracking 65mph scooter but a Casa 135 won’t be cheap, and importantly it won’t use up all the smallframe engine stuff I’ve got littering my garage from my Scootercross and circuit racing days.
We won’t meet our ‘fun on a budget’ target by pouring money into the project.
I mentioned that the smallframe chassis was genius. Well the engine layout is even better.
The Vespa smallframe was reputedly the last scooter project of Engineer Corradino d’Ascanio – the man who conceived the first Vespa. This aeronautic engineer put more than a decade of learning into improving on all the previous Vespa engine layouts with the following features:
- Clutch position: No longer was the clutch on the crankshaft. It’s now mounted on the gearbox input shaft so it no longer acts like a heavy second flywheel.
- Induction: Earlier piston-port Vespa engines required up to 5% oil mixture; which caused smoky running that would foul plugs. With a clever rotary-valve inlet system on the front of the engine controlled by one crankshaft web the engine can now be run on 2% oil mixture because lubricant passes directly over the crankshaft big-end bearing.
- Cooling: This is the main advantage of the smallframe layout. 2-stroke cylinders are inherently difficult to cool properly because one side – where the exhaust exits – is massively hotter than the other three sides where the incoming cold fuel/air mixture enters above the piston. The objective here is to keep the bore of the barrel perfectly round when at operating temperature so that you can run the tightest piston-to-bore clearance possible. Tight clearances reduce noise, reduce wear and increase efficiency, but they risk piston seizure if the bore of the barrel distorts unevenly when it gets hot.
- On a large-frame Vespa the cooling air from the fan is blown onto the cold side of the cylinder which is the opposite of what is required. In order to compensate there’s more fin area on the bottom of the barrel facing the floor. That works but it’s not ideal.
- On a Lambretta – almost all of them – the situation is even worse. Cooling air arrives on the side of a barrel that is cold at the top and hot at the bottom. Max Quattrini’s M210 is the only Lambretta kit that has uneven cooling fin design to try to counter-act that problem in the same way a large-frame Vespa does.
All engineering results in compromise. For the Vespa smallframe the compromise is that the rear-facing exhaust port aims straight at the rear shock, so the exhaust gas has to take a sharp 90-degree turn as soon as it exits the barrel, or divert around the shock like the exhaust shown above.
The other compromise is that while the forward-facing inlet may allow for use of a conventional carburettor (unlike the down-draft PX design), this carb is located deep inside the monocoque chassis where it is hard to access.
Everything about the superior smallframe Vespa engine residing in the single-piece Vespa chassis makes it a pain to work on. Perhaps that engine might be better used in an open-framed chassis where you can access everything. A chassis something like a Lambretta Lui perhaps…
Words and photos: Sticky
READ PART TWO HERE – Where the lads dig around some unused parts boxes to resurrect an ancient Pinasco reedvalve kit with the help of DRT and Readspeed.
Project SuperLui gallery part 1
New products always in development…