Geoff on his 208cc Vespa PX125
Geoff on his 208cc Vespa PX125

 

Unless you know better, French scooterboy Guy-Francois Evrard – known to his British friends as Geoff – has become the first to achieve the impossible. He has successfully fitted a large-block Vespa 200 cylinder onto the Vespa PX125 casings.

 

OK, it’s obviously not impossible, but it is tricky enough that it’s taken 40 years for someone to get this working.

 

It is a feat of engineering very much in the spirit of the Italian 1960s ‘duecentino’ (“little 200”) conversions, where a Lambretta 200 barrel was modified to fit onto 125/150/175 casings. However, on a PX it is even trickier as the two casings are substantially different in stud sizes and positions.

 

I’ll let Geoff explain in his own words…

 

Sticky

 

PX200 cylinder on the left and PX125 on the right. Not that similar…
PX200 cylinder on the left and PX125 on the right. Not that similar…

 

Vespa Passion

 

I’ve ridden Vespa since 1984. I love these scooters and their simple engines. I prepare and build engines for people. Very often, people ask me “don’t you have a 200 to sell?” The problem with the 200 is that there are fewer and fewer and they are expensive and often in poor condition. And I don’t understand the interest people have in the 200, they are badly thought out, badly designed and above all, a good 177 well-optimized, it’s much more satisfying.

 

What I like is optimizing the engines, giving more performance to what already exists and making it reliable. I am the only one in France to offer preparation of a 190 on the 125 engine using the 177 BGM kit. These are more powerful and torquey than the 200.

 

Illustrating the difference in capacity – this is a 125 piston inside a 200 cylinder
Illustrating the difference in capacity – this is a 125 piston inside a 200 cylinder

 

But there, I said to myself, Geoff, nobody has ever adapted a 200 cylinder on a 125 block without modifying the casing, moving the studs… If nobody did it, go ahead, try it. It costs nothing and that’s the kind of challenge you like. And yes, we stay on a block 125 and we pass it in 200. That was in February 2019.

 

Reverse engineering

 

I aligned the casings of 125 and the casings of 200, the pistons, the crankshafts. I measured, calculated, drew. I made assemblies just to test combinations of parts. And there I had an idea. In fact, when you can’t solve a problem, the idea is to reverse the data and work backwards.

 

My goal was to be able to assemble an engine fitted with a 200 cylinder but to be able to revert to the standard 125 version if necessary. The 125 and 200 casings are different, but not that much.

 

A modified crankshaft, an old cylinder of 200 adjusted with absolute precision, an adequate gearbox, all judiciously thought-out and there was no reason why it should not work. The main idea is that with casings of 125, we reduce the dead volumes and we can increase the compression by exploiting an unused space. And 125 cc which becomes 208 cc is still 83 cc more!

 

But even if it seems simple, it was not that much! And it took 11 months in my little workshop, between two engines to build, another to prepare and other essential obligations, to arrive at the final result.

 

Yes, everything worked as I wanted. After a few failed attempts, I finally saw the engine in front of me, finished. I installed it on the bench to start it. Two kicks and he was alive. Adjustment of the carburation was necessary; I started on the adjustment of a standard 200 on a carburettor SI 24. But in fact our block 125 with its modified crankshaft and its cylinder of 200 has an exact displacement of 208 cc. I think it’s the largest displacement for a PX 125 engine without modifications.

 

And then after I let the engine run to see how it reacted and pre-run it, with my friend Jay, we installed it on my PX.

 

VIDEO | Geoff rides the 208 cc Vespa

 

The surprise is enormous from the first laps. The answer is immediate, the engine is flexible, there is a huge engine torque and it’s really fun to ride. And the possibilities are much greater! Because here it is only the “basic” plug & play version. We can develop the system and work it much more!

 

I succeeded in what I had been told was impossible.”

Of course, I protected my system; it would be a shame if someone could copy this little wonder. Now I’m going to roll, make the usual small adjustments. I’m really happy that I took up the challenge.

 

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Q&A

 

SLUK: Now you have a working formula, is this a conversion that is easy for you to perform?

Geoff: Quite honestly, it’s a bit of work but if you know how to build an engine, there are not really big complications. So I would say that for me who created this system it’s simple to perform. For everyone who would like to build his own 208 engine, they would not have any difficulties if they are aware of what they are doing and the way they are doing it. 

 

Do you need the customer’s engine in order to perform this conversion or can all the parts be bolted-on? Well, that’s a good question your Honour! For the moment I would say, I will do it for people if they want it to be done, there are still some little technical secrets that I don’t want to reveal. But who knows, if a brand is interested to produce the bits of this system, it would just become as common as installing a usual cylinder kit.

 

Are you planning to offer this conversion on the open market to customers? For the moment, I know that the system is working really well. It is simple in its conception but it needs some special parts made on order. So yes, if there are producers interested in my conversion, of course I would be very pleased to offer this new product on the open market. This is absolutely the type of kit I would like to find in a shop, something I would buy to install on my own Vespa.

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I can see this conversion being popular for those who want to make a PX125 go faster on a budget, as used standard 200 parts are freely available. Which parts are needed for the conversion (barrel, piston, head and cowling, exhaust)? Faster is one thing but I think that there is something more interesting than being faster here. The point is that there are less and less P200 and this conversion is a new alternative for the Vespa owners. Being faster, it is something quite easy to do, but to have a reliable powerful engine with a lot of torque to do long journey is more interesting I suppose, especially when you can do it with a standard 125 block which can develop more performances.

 

About the parts needed, 200 barrel and piston and cylinder head, cowling and exhaust – the special adaptor I made, of course, and an appropriate transmission. My prototype is built with a 23/65 standard primary. I will have to see if a different transmission, based on the 68 teeth primary driven gear, is as efficient.

 

For your engine to be 208cc you must be using a longer stroke (60mm) crankshaft and oversize piston. Will this conversion still work with a 57mm stroke crank and standard bore barrel? Ah, ah!!! I told you that I would not be too technical… hem, well I am not going to talk about the crankshaft, because there are several possibilities and I need to be in touch with a producer to be able to offer several options to the riders and tuners.

 

About the piston, don’t get me wrong Sticky, I only use a standard P200 piston and a standard P200 barrel, something that I did just grab from one of my workshop shelves. So we develop here 208 cc with a basic 66.5mm bored piston. With 60mm long-stroke, the equation seems to give the good result.

 

Would this conversion also work with kit barrels for the PX200 engine like Malossi or Polini? Oh, you naughty, naughty boy! What a cheeky question… No, at the moment, only the standard P200 barrel can work; this is the maximum size you can use with the PX125 casings. But there are some possibilities to tune the barrel and piston, those who are familiar with O-tuning will directly see this interesting option. And even if it is the basic version of my system that I would call “plug & play” if you want to work on the casing, you only can improve and upgrade.

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What price will this conversion be if the customer supplies all the required parts? This is a difficult question to answer. When I had the idea of this conversion, it was just thinking: nobody did it, do it! And I did it. You know, I am quite old school; I am a scooterboy not really a businessman (sadly!) I build things, I try them, if they are working then that’s great! Then what is working well, I just do it and make people happy with the engines I build for them. About pricing this conversion, I have no idea. This is the question to ask to a producer, with the cost of material and other things that I have not a clue. But, yes if some serious people ask me to build them this type of engine, I will do it: at the right price.

 

How much testing has been done on this conversion so far? Not enough, but you know, my best friends are always taking the piss because I spent my life doing running in. I need to do more road test and long rides, but so far, I am not worried about it.

 

Are there any difficulties of the conversion that you think might make it less reliable than a standard engine? Not at all. An engine is an engine, if the right setting is done, if you use the right oil and don’t ride like a crazy loon, what could possibly go wrong? After all, this is just a standard engine with a bigger displacement.

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You said that you like the 177/190 conversions for the PX engine. Are there any advantages to your 200/208 conversion in your eyes apart from cost? Yes actually, I see several advantages. The piston and cylinder are genuine products, not too expensive, you can find them everywhere. The fact that I use the 125 casings is another advantage, they are quite common. The structure of the 125 crankcase is a plus for the conversion because I use the dead volume as a bonus and take advantage of unused space. Another significant point of my system is the compression we get from the combustion chamber. What was a problem on the P200 cylinder heads becomes an advantage here. The combustion chamber becomes modular and facilitates the adjustment of the squish according to the effect sought by the user. To conclude, at low cost (compared to the purchase of a pair of Malossi, Pinasco type casings and their components), I can say that to date, my system offers the largest displacement possible for an engine block 125.

 

Is there anyone that helped you with this conversion? I’d like to thank my brother Ludovic Loiselet, Sean Brady (Scooterotica) and Al Harpham (Hotrod Scooter USA) for their friendship and advice.

 

Questions and edit: Sticky

Images and video footage: Geoff

How can people get in touch with you if they want to know more about this conversion?

 

You can contact me on my FB page – J&G 2% Vespa tuning  or send me an email at: optimized.engines@gmail.com

 

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