K-Star 70cc Kit Compendium
A lot of people buy the 70cc K-Star a la Treats. For many, it is a first kit. There is a ton of info on getting more speed out of it, but no one has bothered to collect it in one place. I'll do the legwork for others and post what I did, with links to more extreme threads on MA. They have port pics n’ such. One is from a German, so it must be good stuff, ja? Some of this will be redundant for gurus, sorry. But for the less knowledgeable this should be good.
To those less knowledgeable - read up before even looking funny at your engine. Search the site. Minimal reading is How to Rebuild a Puch E50 Engine. The kit installation article gives some nice pictures (of the K-Star kit) and info as well. Some of this assumes that you have basic tools and know how. Deal with it.
Rebuild your bottom end.
The stock crank’s bushing WILL NOT like going over 9500 rpm with a larger piston displacement than stock. The K-Star apparently likes higher crank case volume, so a new unstuffed crank will work just fine. (Others' words, not mine. I used a stuffed crank and was shallow in my matching to keep low rpm air velocity for my SHA and bottom end.) Also, that means the transfer ports can be brought back pretty deep. Case match! And clean up the oil port for bearing lubrication while you’re at it. There is a chance that you’ll need to add some JB or epoxy in the upper right hand corner of the case to avoid blowing your base gasket. Up to you. Pictures below. For crazy modders, here is a link to a DIY reed valve setup.
Get a pipe and carb.
Maybe work with clutches. Pipes are variable – if you want top end buy a top end pipe. Biturbo perhaps? For low-mid end, I like the Tecno Estoril. A 19mm+ Dellorto PHBG is recommended for carburetion. Hi flow air filters rock stock sucks. Angled intakes are way better than the 90 degree units. They have better flow characteristics, look badass sticking off the front of your ped, and allow easy access for jetting or carburetor work.
If you're feeling like an artistic welder, you can make your pipe a side bleed for a bit of extra kick.
Gap your ring.
You'll want to gap your ring to make sure that it doesn't expand into a port and cause great ugliness.
Okay – porting.
We’ll start with the cylinder. I only chamfered so my terrible dremel skills and lack of mathemagical knowledge wouldn’t ruin loop scavenging or make timings worse. I’ve also heard of people bridging the boost ports, but I haven’t been able to substantiate anything. Gurus and daredevils may want to reference the threads below for better porting (piston & cylinder) information and other stuff. A nice story too. Take your time and read through all of the threads. Some good stuff can be found if you dig. Plus a wiki article!
- Puch Cylinder Kit Summary
- Long E50 Story
- Porting a 70cc K-Star
- K-Star Porting
- 70cc K-Star
- Piston Port Intake Timing - K-Star 70
- 13 Port K-Star/DMP
Next – the piston.
I bridged the boost ports and angled them, making them slightly larger in the process. The angling is something I hadn't found on anywhere before. (If you bridge the cylinder ports, I would try connecting both ports on the piston as well. The pistons are cheapish.) On top of that I ground the top edge of the piston skirt back to increase intake duration. The cutout is as wide as the intake, not wider, and rectangular. I cleaned up the sharp edges with a few strips of high grit sandpaper. Consider balancing the piston to minimize slap wear. More can be found in the links above. I sketched a cross section approximation – look below.
If you buy the kit from Treatland, they give you their 70cc hi compression head. It’s not so good for high rpms. You’ll want to read this thread if you’re planning on creating a new squish. Crazy Wayne isn't "compromising" per se, but he knows his stuff. I went the lazy way and did a bit of dremeling/sanding to make it more of a hemi. Using bouncy balls and golf balls wrapped in sandpaper is the other route. Or using machine shop tools. But not all of us are that awesome.
You can change the gear ratio of your moped at the cost of low or high end. When you use a kit, you can usually raise it from stock. Linked is a spreadsheet that gives ratios between gearing, rpm, and speed. Be warned - you may need to get a longer chain.
Wrap it up.
Time it right and put a good spark plug in (BR8HIX, BU8H). Get anything else that needs to happen done and check for air leaks! The intake and exhaust gaskets that come with the kit are terrible (the exhaust gasket is the wrong size, for example). Cut or buy new ones if you're having problems.
At that point, it’s just a matter of tuning. Check out the category link at the bottom of the page for more information.
Don't forget luck. Good luck. Pray ye to the moped gods.