Vespa PK 100 update 3

tuning run1

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The little PK smallframe has been a battle at times but I’m getting to like it for what it is. As is always the case, I’ve ended up spending more than the scooter is worth but it has been good to use parts from manufacturers that aren’t so well known and to make something quite slow go a bit faster than it was designed to be.

With a lot of vintage scooters there isn’t a guy at the local garage that you can just go and ask about parts and how you do this and that. I’m lucky to have a few vespa mechanic friends that I can call on if I need help, however like most things, you tend to learn the most from just doing the work and the inevitable mistakes that come with that. Also as you try new aftermarket or OEM parts and their various suppliers you get to learn a lot more about the costs and the time and effort required to do this sort of work. Then there is the tooling and the techniques required to just work on old scooters or just old mechanical things in general.

With the ET2 disc brake fork upgrade the original hub had a 5-stud mounting, which meant you could only use wheels off an ET2 or similar scooter. VMC make a front hub drum that uses the original 5 stud mounting but has the normal 10″ vespa wheel studs around the hub. It’s a good bit of kit and fitted perfectly. While doing the front hub I replaced the front shock with a YSS unit for an ET2 fork. This fitted perfectly and is miles better than the worn out shock that was on there before. While not a top of the line shock, I have used YSS units before on other projects and found them to be good enough.

The original stator was working fine when I got the bike but just died one day. I had looked at some other ignitions online and decided on an Italkast IDM ignition. They were a good price and well made. Sadly, the Italian instructions weren’t the best (like other suppliers ignitions to be fair) but because I had some experience with the Pinasco flytech ignition on a customer’s scooter, it was easy enough to work out. I normally use static timing on my projects for various reasons but decided on this one to go with a variable timing unit. It is reasonably easy to set up with a timing light and it automatically retards the timing by 8 degrees as the revs increase. The only issue I have with this kit is the regulator – it doesn’t seem to work, however it may be my wiring but I will try another regulator at some stage. Its starts and runs very well at the moment, so probably best left alone.

The original 19mm carb that came with the bike was problematic. It wasn’t as easy to tune as other carbs I have used and always seemed to develop an air leak which is quite common on smallframe vespas. The engine will start and rev it’s guts out until you turn it off – which is quite disconcerting if it hasn’t happened before and also not having the kill switch wired up was a bit of a pain. I spent some time cleaning it, replaced some gaskets and the felt ring in the intake, tried a new foam air filter and eventually just gave up on it.

Along this path to carburetor enlightenment, I discovered the fuel tap was dodgy and leaked. Worse than that, someone had put the fuel tap lever on the wrong way so the off position was actually reserve. And just when you think it couldn’t get any worse, the on position didn’t work. Fark. Changing a fuel tap isn’t a simple job but sometimes it’s got to be done. All of my scooters have the SIP fast flow taps and I haven’t had any issues with them. If you are modifying/restoring your vintage scooter these are up there with new cables, wiring loom and tyres.

I replaced the 2 bolt intake with a Malossi one and a CP24 Polini carb. These carbs are physically very small and I wanted one to try on my VS3 engine so it was a no brainer. Setting it up was reasonably straight forward but there was a bit of work to get the throttle and choke cabling working properly. The PK would now start and run fine with new spark and a new carb.

The rear hub had a stud missing and had been bodged with a longer bolt and nut through the hole – pretty standard for some rear Vespa hubs. It wasn’t quite round and the brake wasn’t the best so I ordered a new FA Italia brand PK rear hub. When I installed the new hub it was binding somewhere, and the wheel wouldn’t turn. After measuring the original Piaggio part and the new FA Italia one there was a 2mm difference in the internal length of the part that goes through the oil seal and butts up to the bearing. Also there were additional raised areas in the way the new one had been cast which also caused some issues. I eventually got it to fit after a lot of faffing about but it’s not the first time an OEM part didn’t quite fit and probably won’t be the last.

The original engine was a changed to a 130cc unit with a PK cylinder, piston and another head of some sort. I’m not 100% sure as I haven’t stripped the engine but there was some information available about the engine from a previous owner. It came with a Polini snail exhaust which seemed to work OK. I had looked at some smallframe cylinder kit upgrades for this engine and found quite a lot available from various suppliers so have been mindful of the updates so far being able to work with any future engine upgrade. Like most 2 stroke engines you can generally expect the best performance increase from correct engine timing, carburation and the exhaust on a standard engine. On some of my largeframe scooters a new exhaust has added more torque and, in some cases, a higher top speed and just needed an appropriate adjustment of the carburetor.

I got the VMC Evo 52 exhaust system as it met a few parameters of the existing engine and looked like it would fit. The frame of this scooter had been cut in places for a previous exhaust system however you don’t really know if they will fit until you get one and try it. Not only does it need to fit inside the frame but it needs to clear the rear shock, the rear tyre and the stand. The mounting points on some exhausts can be problematic and need to support the size and weight of it in a way that it won’t vibrate and crack. The header pipe can be awkward if it doesn’t attach to the cylinder in a position to then attach to the rest of the exhaust – a well known issue with smallframe engines and exhausts.

The VMC comes unpainted so it’s recommended to apply a clear coating, if you like the unpainted look, or paint it with a high temperature product. I like the unpainted look so may leave it like that. It comes with the hardware to mount it to the swingarm and this is where most of the work is done to get it in the right position. You can’t remove the rear wheel with the pipe in place but that’s pretty common on scooters with performance exhausts. If you can get the mounting right the exhaust comes off easily with removing 2 bolts and the spring between the header and the exhaust. I like the look of it and it sounds great. The engine performs a lot better than with the Polini snail exhaust but if upgraded with an appropriate cylinder kit and modified to suit the VMC exhaust I think there is more power to be gained.

I’m not sure when I will get back to this, as other projects take priority, but it can wait patiently in the shed for a bit.