Thursday, April 19, 2012

DR Evolution 70cc upgrade for ET2

Here is a brief introduction to "Vespa" - my 2003 ET2. It's a 50cc 2-stroke CVT scooter I have owned since 2005. I conducted a series of upgrades on the scooter recently (See my Leo Vince SP3 exhaust upgrade thread on Modern Vespa and my struggles to get a 17.5mm Dell'orto PHVA carburetor working).

The SP3 upgrade increased the acceleration of the scooter a fair bit, and the top speed from ~37mph to ~42mph. This was after upjetting the stock 12mm Weber carb from a 76 to and 80 main jet, and lightening the rollers to 5.7 gram (included with the exhaust).

Adding the 17.5mm PHVA carb didn't really help the scooter. Overall, my conclusion has been that the 17.5mm carb is unnecessary on the 50cc stock engine, even after a modest exhaust upgrade. The engine doesn't breathe enough, in my opinion, to keep up with the more open throttle positions on the 17.5mm carb.

Following these experiences, and having become used to the power of my 200cc P200E Vespa, I decided to upgrade my ET2 from a 50cc cylinder to a 70cc. I mulled over kits for a while, but ultimately settled on the DR Evolution kit. It is a cast iron cylinder with 2 ring piston. I chose it over the Malossi and Polini cast iron options because it was cheaper, but has a reputation for being very durable. It also is tuned for more modest rpm levels according the SIP scootershop from where it was ordered - importantly, it's optimum RPM (~9000) is roughly that of the SP3 exhaust I have, so my thinking was that their intended power-bands were well matched.

Here is a shot of the DR Evolution 70cc kit (arrived incredibly fast from SIP in Germany!):

The instructions are charmingly sparse and poorly translated from the Italian manufacturer, but I read extensively online prior to the modification, and felt pretty comfortable in installing the cylinder.

The first step was to chamfer the ports in the cylinder. This means rounding the edges of the ports to reduce the likelihood of catching a ring in higher stress situations and trashing your top-end. The edges of the ports typically come from the factory VERY sharp and abrupt, and if you run your finger along them you can imagine how an expanding ring passing over the port could snag.

Here is an image of the ports before chamfering. The goal is to take a file or dremel bit and GENTLY soften the edges. People often recommend ~45 degree angle cut into the edge, no more than 1mm along the length of the bore, to create a ramp which will help the rings ride back up beyond the port without catching:

Here is an image of the ports following my dremel work:
Hopefully you can see the softer edges on the ports. I discovered, however, that getting the edges just right is very difficult, and I would not say I achieved a perfect 45 degree angle on each edge of the ports. I think the most important part is to be as consistent as possible, and avoid highs and lows within the same opening which might facilitate snagging of a ring. I recommend the slowest speed on the dremel and be careful not to nick the inside of the cylinder! Note that the edges of the ports parallel to the front and back of the cylinder are the most important, since the rings come against these edges, while they travel along the edges of the ports which run from front to back (perpendicular to the rings). Remember, the key thing here is you are NOT trying to open the ports up - you don't want to take of a ton of metal and change the port timing! All you are trying to do is soften the edges to help the rings cross the openings without incident.

The next step was to gently clear out grinding dust from the bore, then remove the pipe and old cylinder from the scooter.

Prior to installing the new kit, I prepped the piston by inserting the circlip on the left side. This is because there is very little space to work on the left side of the scooter with the engine installed (yes, I did this project without dropping the engine!). So by installing the circlip on the left side first, all I had to do was insert the piston pin through the right side and attach the right hand circlip (which is easily enough done on the right side of the scooter).

Here is the piston prepped for installation - note the green arrow indicating the circlip installed on the left side of the piston in advance (the bottom/exhaust side of the piston is marked with an etched arrow on the crown). Also note that the openings of the circlips should be toward the front or back of the piston, not up or down.

Finally, after installing the piston I lightly oiled the bore of the cylinder with 2-stroke oil, installed the base gasket for the cylinder-casing interface (also oiled), and gently worked the new cylinder into place! It was fairly easy to compress the rings and slip the cylinder on - much easier than on the P200. After installing the oiled head gasket, followed by the cylinder head, and reinstalling the exhaust pipe, the upgrade was complete!

I began with the following jetting in the 17.5mm PHVA carb for the DR70 cylinder with SP3 exhaust: 74 main jet, needle clip at 2nd notch from top, and a 34 pilot jet. So far, this jetting seems to be working beautifully - at break-in speeds (under ~42mph) the spark plug has a very good color and the scooter is responsive.

My first impressions are very favorable. This machine is FAST. I suspect it will hit 50mph on a flat pretty easily - while the top speed of the stock machine was ~37mph, it now hits 37mph almost effortless and will happily accelerate past this speed on a moderate incline. I don't know the top speed yet because I plan on gently breaking the cylinder in for about 100 miles first, but if nothing else I am very pleased with the drastic increase in low-speed power over the stock machine. It will be much better for my frequent two-up riding.

I will post back with updates on top speed and fuel economy (likely in the low 40mpg range), as well as refined jetting (if any) in the future. But for now, I will say that the DR Evo appears to be a great kit.