OK, it’s been a while since the last episode of this engine transplant saga, but we’ve not given up. We’ll get to the gory modification part shortly, but before then, here’s a bit more about setting up the Vespa motor.
When it comes to 2-stroke timing, taken from a simplistic perspective, there are combinations of exhaust and transfer timings that are known to work well.
Longer port open durations (taller ports) will tend to increase peak power but also shift power higher up the rev range. Higher revving engines tend to have narrower operating rpm (narrower ‘power band’) and thus demand more gears. High revving engines also use more fuel and wear out more quickly.
With only a 4-speed gearbox in the Vespa motor and the Lui only having a 5-litre fuel tank, we can’t afford for this engine to be too thirsty. Using different combinations of packing plate and head machining we have the freedom on this engine to put the ports pretty much at whatever duration we liked.
We decided to choose moderate timings as a starting place. Initially we adjusted the height of the barrel by putting washers on the studs before the barrel in order to get the bottom of the transfer ports level with the piston at Bottom Dead Centre before measuring the transfer port timing.
Port timing measurement with a Buzzwangle.
BUZZWANGLE TO THE RESCUE
Many people still don’t understand my Buzzwangle tool kit. Fair enough, it’s difficult to grasp the benefits until you’ve tried one. Steadily more and more scooter dealers are using them daily – including Ben for AF Rayspeed – because they not only produce more accurate results than a degree disc, but mostly because they are faster and easier to use.
With the Buzzwangle tool mounted to the flywheel (the 4-way tool fits Lambretta and most Vespa flywheels) and the Buzzwangle-O-Meter mounted onto it, measuring port timing is as simple as closing the port, zeroing the meter, spinning the flywheel to open and then close the port fully again, and reading the displayed measurement in degrees.
You can buy a Buzzwangle kit here in the SLUK Shop.
With 4.2mm of washers fitted, the main transfer port timing was 108.4 degrees, which is on the dull side of moderate.
With 5.4mm of washers the transfer ports were fully open at BDC and the timing was a much more healthy 119.1 degrees but the exhaust was still only 163.3 degrees. No wonder this kit wasn’t very powerful!
5.4mm seemed a good place to settle because that allowed for a 5mm aluminium packing plate – which was available off-the-shelf from SIP Scootershop – and a couple of 0.2mm gaskets either side.
Once you have picked a transfer port timing by adjusting the cylinder height then it should be easy enough to adjust the exhaust port timing to suit by grinding the port to increase its height and therefore open duration.
GIVE ME HEAD
The one remaining adjustment after setting the height of the barrel is to sort out the cylinder head, in terms of both compression and ‘squish clearance’ (the gap between the outer edge of piston crown and cylinder head at Top Dead Centre).
A quick check saw that the piston and the head could barely shout to each other, let alone come close enough for a kiss. We measured the squish clearance a 4.05mm when ideally it needs to be closer to 1mm on a small capacity motor.
A massive squish gave us two options:
- ‘Top’ the barrel. In other words machine 3mm off the top gasket face of the cylinder. This was common practice converting iron barrels for ‘Jap’ piston conversions, but on a plated barrel it risks chipping off the top part of the Nicasil bore plating. Additionally, if you machine off large amounts of barrel then you start to lose cooling fins.
- Machine-back only the gasket face of the cylinder head by 3mm. This would allow the combustion chamber and squish to actually sit inside the bore of the barrel. This is a harder trick to pull-off since it is not so easy to hold the head for machining, but it offers an advantage in that the head self-centralises inside the bore.
In the end I left the parts with Jerome from Readspeed and asked him to do one or the other. In the end he chose to machine the head because it was less risky and offered more advantages. Jerome also calculated and re-machined the head for a compression ratio to suit modern fuel.
Despite being only a small shop Readspeed is remarkably well equipped and Jerome is skilled at this sort of engineering as you’ll see shortly when his new kart-barrelled smallframe is revealed to the world.
REEDVALVES TO BE CHEERFUL
The standard induction layout for the Pinasco kit is a bit basic. It comes as standard with a 4-petal reedvalve of early-80s variety with a really oddball stud pattern. Bolted on to this is a bizarre alloy manifold with a long rubber section aimed at locating a 24mm carb inside the frame of a Vespa.
For our project, none of this was any good. This crude manifold would put the carb exactly where the frame sits on a Lui so that had to go. I also wondered if we could find a better reedvalve. After Mark Broadhurst told me about a tiny 6-petal one he’d had good results from, I decided to find one of those on eBay. When it arrived Sam took a Dremel to the cylinder casting to allow enough room for it.
With the square stud layout of the smallframe barrel, you do always have the option of reversing the cylinder so that the inlet is near the shock and the exhaust port exits down and forwards so you can have an expansion chamber under the floor.
Reversing the barrel offers some advantages in exhaust design but it also removes the one about the fan blowing on the hot side of the cylinder. As such we decided to stick to a conventional layout with a curly exhaust on the right hand side and the carb facing forwards.
We’ll show you how we resolved the inlet manifold issue and how the engine fares when leak-down tested in the next episode.
New products always in development…