Carburation for TM KZ10 144cc Fun 56 Upgrade

Thanks James

Interestingly, a new piston of 55.93 diameter is already a snug fit without the ring

I’ve asked my honer to give me a bore size of 56.08mm dia and as Mikko said in a previous post it’s better to be sloppy.

I am grateful to many of my race colleagues here in Western Australia, however there are many who claim to have 40 plus years experience. I soon found out their knowledge is limited

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I’m no expert on KZ’s but that looks very similar to a cold seize. What’s your water temp. Do you run a radiator blind?

Fundamentally, isn’t a cold seizure the result of a tight clearance issue (owed to inadequate cylinder expansion)?

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I agree if you look at the scuffed piston sides that is a clearance seizure on 125 kz I never run under .003 clearance on a almost 56mm .004 wouldnt be out of the question. You also have to watch the pistons if forged will need more clearance I have built a few of these an they have run well. Good luck.

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Alan, that was a consideration, so I fitted a thermostat which opens at 46 degrees


So, latest info from my original post.

  • Engine rebuilt with a little more clearance 56.08 bore to 55.93 piston
  • Engine run in as per normal
  • 14 laps completed on new piston
  • SmartCarb 38mm adjusted 2 clicks right to richen fuel mixture

Thoughts please on burn pattern on piston

Cant see from the photo but check carefully the left side of the piston edges - arrow side. The right side is sooty so that side should be ok. After wiping off fuel residue, soot etc - piston machining marks should be visible and in perfect condition at the edges. If machining marks are impaired by pitting or erosion - even slightly - its too lean. Or theres too much squish gap.

When I have been too lean, the first sign is that the edge gets a frosted look, like the edge had been hot galvanized. Patchy crystal look. In the next phase that edge starts eroding, machining marks disappear, edge gets a sand blasted look

Search is there any tiny pitting around the edges. If found - adjust the carb richer and do few laps

Keep checking condition of the edges until you are sure you are not running it lean. I just take the spark plug off, rotate engine so that piston is at the bdc and use small light to see the edges. No need to take the cyl head off

That is a bit of a weird pattern to my eyes, very offset to the transfer ports but usually that’s more of a porting thing vs jetting. The center indicates it was rich, so that’s good. But that carbon build up on the right is puzzling.

All that aside, how did it run and how does the pipe header look?

That’s a lot of clearance, are you sure? KZs usually run 0.07mm, you are running more than double I don’t think being too tight is an issue. Here is another idea if all troubles happen at the end of the straight and everything else points at jetting too rich, it could be because you are starving the engine in relation to rpms in the breaking zone at the end of the straight. Did you have a chance to look into somebody else’s telemetry to see your braking point and rpms at the end of the straight? If not, to help things you can go CD1/72 for the idle jet. That will ensure some extra fuel/oil when you lift too soon/for too long.

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It ran very well. Header looks fine.

I was thinking that carbon build up is a consequence of the over rich settings of the SmartCarb SC2

Would like to hear more about the SC2 once you get things dialed.

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Mikko, the feedback from my go-to engine (but not anymore) builder said 56.08mm is to much clearance and will promote piston slap and piston will need replacing after every meet.
I must admit, after my last test of 14 laps, the piston did seem “sloppy” in the bore, but then again, it didn’t seize. I might have my spare barrel honed to 56.07 and test that.

Do we know if we’re talking about for cast or forged pistons. Reason I ask is that the clearance is dependent on that, so when the discussion comes up it should be confirmed.

Maybe all the pistons are one or the other, but in any case a cautionary tale.

They’re cast pistons from Meteor

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Do meteor have a recommended clearance?

They just say clearances should be in accordance with engine manufacturer’s tolerances