Seeking silly Kstar kit advice

Hey folks,

I boomeranged back to mopeds and have been playing with a cheapo kstar. The silly thing struggles to rev past 7500k unless I have gravity helping me out.

- unported kit, B- case matching job (it's just mild), thick enough base gasket to just let exhaust fully clear

- za50, have tried both 20x40 and 18x40, same RPM ceiling basically

- pretty mostly confident it's buttoned up tight with no leaks

- 15mm bing with a 78 jet, richest needle setting

- MLM peoples sidebleed (troubleshot this by putting it on a TCCD kit I also have, rips right up to 9k)

- high comp O ring head

- some mystery CDI that starts at about 25 degrees and retards down to 15 or so (works lovely on that aforementioned TCCD)

SO, the one thing that is pretty glaring is that the piston crown is 1-2mm short of hitting the top of the cylinder. Certainly no measurable squish using a 1.5mm thick piece of solder (I should probably get a bigger gauge solder). This issue has been covered a little bit before on here - most notably this thread - but it didn't seem like there was any good solution. Kinda feel like probably everyone has this kit so maybe I'm missing something.

Roffman already advised me to just sell it, haha, but I like torturing myself. Is there any reason that I couldn't just manually lap the top of the cylinder down 1-2mm? I'm using an O-ring head, so maybe it would be forgiving? Do I just need to throw a bigger carb on it? That doesn't seem to equate with the expectations I'd had reading other people's results with the kit.

Re: Seeking silly Kstar kit advice

Change your squish by decking the cyl to about a .8mm squish and raising timing a touch (1mm +)

Re: Seeking silly Kstar kit advice

Overpriced Parts /

Sell it off or use it as a doorstop and get a DMP kit which is spaced correctly

Re: Seeking silly Kstar kit advice

Winners never quit roff

Re: Seeking silly Kstar kit advice

thanks for the feedback and comic relief!

I dug up my old special VERY FLAT GLASS™ and got to work. I measured in six different locations of the cylinder skirt to check my work periodically. Then I got impatient (you can only treat ADD so much) and took it to the belt sander, and finished with more lapping.

For the first time ever I got a measurable squish, looks to be right about .95mm on the very thinest end part, and ramps up a bit to 1.05mm or so, I feel so competent now.

Of course I'm expecting absolutely no changes cause its mopeds, but I'll report back since I'm sure you're all dying to know what happens. Also opened up those piston boost ports and a wee bit of exhaust, so hey who knows. (edited)

Re: Seeking silly Kstar kit advice

I think that should give a very noticeable improvement. Think about how tiny the space is for gas to expand and push the piston. You increased the efficiency of the use of that expanding gas exponentially. Because it’s mopeds, it makes that much more of a difference. I’d be surprised if it didn’t.

Re: Seeking silly Kstar kit advice

Jack Rutherford /

You also may be just at the cusp of where that pipe wants to hit (7500+ rpm). 1-2mm is a lot to hand sand on a cylinder. I did it once or twice and then promptly bought a belt sander. Report back with results. (edited)

Re: Seeking silly Kstar kit advice

I didn't notice a damn thing. Yes you are correct, it gets to that cusp, whereas the tccd kit I have on the other bike hits that and shoots up again to 9k+. No difference with a circuit pipe vs that MLM sidebleed. TCCD kit came with a 19phbg on it (I put that on my main bike and that's a joy), guess I'll swap the carbs to see what happens. Wouldn't be satisfied even if that did 'fix' it, cause I'd read how a 15mm bing seemed enough to get people going pretty good (well at least Ken lol) (edited)

Re: Seeking silly Kstar kit advice

raise the exhaust port to match the tccd

Re: Seeking silly Kstar kit advice

Get a DMP with a 19mm phbg and go 50 mph like I do

Re: Seeking silly Kstar kit advice

Jack Rutherford /

You may have raised the exhaust port too much with that thick base gasket. Depending on how much you took off the other end of the cylinder, maybe you can lower the exhaust port with a thinner base gasket.

Re: Seeking silly Kstar kit advice

thanks dudes, I seriously love reading the feedback here that can help me understand the theory more. when I was younger and deep into this stuff, I would just throw more money at a problem and buy the bigger kit/carb/pipe, rather than trying to get actual insight. I'm still just really curious why some seem to have no issues with this kit, and some do; maybe differences in castings from different iterations, of course people are more likely to share positive results that may be ticking time bombs re: air leaks or advanced timing or whatever else.

Once I get some more time I plan to swap the 15 bing/19phbg between the kstar and tccd kits, note the differences, and then maybe look at making port maps to try and sync them up - never did that before and I imagine it would be fun to learn.

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