Events

This is the 3rd in our series of Club Workshops specifically for owners of older geared 2-stroke Vespas. It responds to a request by a couple of members at Vespa House Open Day. The session will revisit the diagnostic processes used and the solutions found for two breakdowns that occurred on the recent Classic Blue Smoke ride to Warburton which you can read about here. Specifically we will be dealing with fuel and spark.

 

From Piaggio’s innovation of the rotary valve intake which replaced piston ported induction around 1962, the humble Dellorto SI Carburettor has sat beneath the airbox cover across all geared models ever since. Tens of millions of them have been made. It is a very squat carburettor offering a short intake. Being mounted directly on top of the crankshaft housing, the fuel load is dropped straight onto the big end bearing. Along with a needle top end bearing replacing the previous bush bearing, this change enabled the oil/fuel mixture to be reduced from 5 or 6% down to 2%.

 

The other reason for SI carb’s squat low-profile design is that since its mounted on top of the engine case, the float bowl has to be lower than the bottom of the fuel tank otherwise, being a gravity feed system, you won’t be able to use the fuel at the bottom of the tank. Boy, that must have been a design challenge as its only about a centimetre below it! Hence the SI has a flat side sliding throttle valve rather than the usual cylindrical vertical lifting valve with its varying needle jet. All in all a very elegant compact and robust piece of engineering to fit a tight space. Dellorto SI Carburettors are often derided in circles where owners attempt to get 30 hp out of a motor designed to deliver around 9 hp. But as is, SI carbs are reliable and they have a surprising number of options for tuning. 

 

We will completely disassemble a carburettor and explain the workings of each part. We’ll discuss how to clean it’s 3 jets and the interior holes that run through the carburettor body. We’ll examine the float valve and float bowl, talk about gaskets, the role of the mixture screw, fuel filters, grommets and air filter maintenance and what torque settings to use on reassembly.

 

We will examine and explain all the components within the ignition chain and discuss how to analyse which part may have failed when you have no spark. There will be two stator plates on hand to look at, the older points based system and the fixed sensor Capacitor Discharge method. We will talk about coils, hall effect sensors, the CDI unit, the ignition lead and spark plug.

 

To be announced closer to the date. If the numbers are small (around a dozen) I’ll host the event at my home near Box Hill. Otherwise if anybody has access to an inner city undercover space, please let me know. It doesn’t have to be a workshop. We can also use Pete’s workshop in Doncaster East