K-Star 70cc Kit Compendium

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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.

This guide can be used for DMP 70cc kits as well, I think. The K-Star's evil twin - or is it the other way around? It may even be useful for other 70cc kits... If you see similarities to the K-Star or DMP, add links to this page. People will thank you.

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. You must re-gear to 17x40 or above if you want to run a stock crank. I recommend buying a performance crankshaft. 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 16 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.

The stock crank dislikes going over 9500 rpm. A lot.
It was recommended by some to add case material. I didn't. Your call. Pardon the unfinished porting job.


Clutch, carb, and pipe.

Maybe work with clutches. I used a primitive clutch tuning method that tied one of my two shoes to a shelf and the other to a coat hanger grip. From there, I pulled and eyeballed the evenness of the shoes as they extended. The method may adapt to a 3 shoe, but I'm not certain. Pipes are variable – if you want top end buy a top end pipe... Biturbo perhaps? For low-mid end, I like the Tecno Estoril. Note that those are budget pipes. If you want a Simonomi on this kit... Have fun. A 19mm Dellorto PHBG is max size recommended for carburetion. A 14mm Bing will also work wonders. Hi flow air filters rock stock sucks. Angled intakes are way better than the 90 degree units. They have superior 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. A proper gap is .007in or .180mm.

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. Wayne added ports. 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
K-Star Full Potential

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.

Rough cross section of the piston porting scheme I used. NOT to scale.


Re-squish.

If you buy the kit from Treatland, they give you their 70cc hi compression head. It’s not so good for high rpms or cool engines. 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.

Re-gear.

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.

Deal with wiring.

At full RPM a K-Star kit can dump out 25 volts of shit into your lights, causing them to burn out every few seconds. Get a rectifier, regulator, new coils, HPI/CDI, or whatever else you want to get it working properly. Be aware of the electric power generated by a kit, and act accordingly. Do your own research on this part. Your wiring varies too much for me to play omnipotent and try to make a guide. However, I will recommend grounding wires inside the stator instead of having a rat's nest on your frame.

Wrap it up.

Time it right and put a good spark plug in (BR8HIX, BU8H). Roffman informed me that 17 degrees is the timing to shoot for, which may require a dremeled stator plate. 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 and keeping your kit cool during break in. Break in tactics vary so much and are so fiercely debated that I dare not expound on them. But if you're feeling a bit OCD, the Kit break in guide on the wiki could be a good place to start. Once you're finished breaking it in, you should hit 45 mph (with respectable acceleration) easily.

Check out the category link at the bottom of the page for more information. And don't forget luck. Good luck.

Pray ye to the moped gods.