GTS 250 Project Vespa.
As anyone who reads this little forum knows I have a little Budget Project Vespa 250 GTS in for a motor job due to a massive overheat.
I decided to take the cylinder head on myself, decking was required at .35mm warpage. Personally I think a new head was in order but for the life of me could not find a working 250-300cc cylinder kit in the USA for sale to me from Piaggio. he SAME head is used on like a dozen or maybe a couple of dozen different models of 125-300cc Piaggio/Vespa/Aprilia motors.
Here is a good place to post some of these pics. Here the bike is seen as it came in, running, but sure as hell not right! 140 pis compression (with a decompressor, hmmm) is very healthy, but coolant shooting out of the exhaust pipe is not good.
Wth 30 psi of pressure in the coolant expansion tank, the blown head-gasket was pretty obvious.
Here is the piston, crap quality pic, just easy to do in a hurry with the phone.
So, does this not appear to be a typical Aprilia style removable flywheel weight? Looks almost like a dead ringer for what is on the RSVR/Tuono of recent vintage. Imagine the difference a loss of 2lbs rotating mass would make on a 17bhp scooter engine? This is a case where I can see the lighter flywheel being a really exciting concept.
This is a look at the stock combustion chamber and ports to a lesser extent, you can see from the burn marks the massive size of the 72mm piston compared to the 50 or so mm "valve bowl"
The micro sized exhaust valves have a large volume of spent gases to deal with quickly, this is not the right porting shape for that, this is the reason that a pipe on a Vespa makes little difference in power, they work, but 1-2 bhp gain are not impressive due to flow limitations of the stock head. This is why the Malossi V4 head makes so much sense!
Here are the stock valves compared to the older stock 36mm exhaust valve of a Rotax V990 in the background. The Vespa valves are long and stable, I will give them that. I do like how they did the face area of the intakes with the relieved stems in a critical airflow location.
Here is what I decided to do to it...much more detail in my "shop" thread.
This is post porting, I figured really, this motor is so starved for air at higher engine speed just simple traditional porting for big "flow" was the way to go, velocity is not a problem in these little ports with a big ass piston pulling real hard on them.
Once again, velocity is not a big problem on these ports due to massively being undersized in terms of valve area and valve pocket size.
This is the intake, man pictures really show every flaw the naked eye trains your brain to forget!
Directions for the future. Thoughts, ideas, scheming...it keep me up at night...
Exhaust valves open, I love taking pics this way, need to get some red led flexlights for doing the exhaust ports!
Intake valves open at full cam lift, if the piston were not 20mm larger than the area we normally think of as the "combustion chamber" this would look so bitchin:
It will be fun to see how this project ends up, stock pipe and all, the bike is only really modded with removal of stock parts in the airbox (works well if you have one of these) so adding a little compression and hopefully the ability to flow and trap more air and fuel! We shall see, it is just as likely a null gain or even a loss, you only learn these things by trying what you feel is right?
I figure more to come, maybe a thread for each Vespa getting modded coming though the shop with real go fast goodies, they are amazing well done machines with the ability to learn and adapt to mods pretty well it seems. As power get much above 18 brake on my dyno bigger injector and a fuel programmer will be required!
Please feel free to post and add pics or thoughts of your own, trying to have fun again working on these little machines!
Reply With Quote
