I’ve never seen a piston top like that before. Is that carbon deposition?
Here’s a random question: what direction do you have the airbox inlets facing? Have you tried flipping the top section so they are facing the rear of the kart?
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That wash looks pretty good, so you were close on jetting at one point.
What gearing and head volume do you have? I wonder if it’s just got too much compression and that’s holding back overrev. Although 10,000 RPM is pretty low.
(I feel like we had the conversation before 
)
As in I’m forcing too much air in through the air box at higher speeds? No, I haven’t tried a flip, but I’m interested to understand your thinking.
On gearing, it’s stock internal, with a 20T on the engine and a 21T on the rear axle. In sprint I’ve been running 17 engine and 25 rear.
I have no issues with G5 and G6 running sprint - they run out fine to the limit (14,200 or so). That wash pattern includes running sprint, so presumably I haven’t been too far from correct jetting when running short/sprint track. I’ve got no idea on the head volume.
5th gear at 10,000RPM with at 20/21 ratio works out at under 80MPH, how does that compare with KZ and other 125s at the track?
Could too much air being pushed into the air box (due to running speed say over 150 km/h, 93 mph) cause some form of cavitation/pressure wave/interference that actually reduces/retards air flow into the carburettor (thereby making a richer outcome rather than leaner, which all seems a bit counter intuitive but perhaps that’s what happens at speed if the inlets are facing forward)?
James, I have used that spreadsheet (thank you) and my experience pretty well matches the calculated outcome. The other KZs (TMs) and 125s (DEAs) run in open class and are using a long circuit internal gear set. So I’m not sure I’ve got a benchmark to compare other than to say my top end speed in G5/G6 is hopeless. My Stock Honda/CR125 will hit 180 km/h, 111 mph at about 12,700 rpm. I should be easily above 185 km/h with this engine and gear set (remembering I have no restrictor on the SS, unlike the SSE, and will peak at just over 14,000). The TMs and DEAs (running the internal long circuit gear set) hit well over 190 km/h, 118 mph coming into T1 depending on the wind direction.
I’m thinking it’s an easy thing to test. I know some very credible folks that insist on running with the inlets turned around.
I have experience with the 175 SSE on a hybrid long circuit where we hit 95-99 MPH over a fairly long period of WOT (~17 seconds), and observed a few things during that time:
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we saw way higher EGT’s than what we did under sprint conditions
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we had to run MUCH richer jetting. With the Tillotson carb this was anywhere from 1/4-1/2 turn richer on the high.
Have you tried the 30mm Dellorto, or only the 36mm?
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Noted. Just the 36 mm so far (which is stock for the engine), not the 30 mm (as used in stock KZ).
A bit more information.
Image 1 below is me coming out of Siberia at Phillip Island heading north. It’s late in the afternoon, the sun’s in my eyes and thus the SmartyCam 3 lens (so the image is even worse than usual) and there was a brisk southerly, so my engine performance seems to be better with a northern run (goes to support Evan’s thinking on the air-box inlet orientation). Remember this is the Southern Hemisphere, hence the sun orientation. Ignore the indicated gear selection in the tacho - for some reason the MyChron has got itself confused. This image is me in G4 just shy of 14,000 rpm (and a ground speed that is close to James’ spreadsheet). The EGT is looking good. I’d say all happy at this point, no complaints. Yes, I know the water is colder than ideal, but I just couldn’t get the temperature up - a massive stock radiator which I may need to change
Image 2 below is in G5, starting to bog at just under 13,000 rpm and the EGT heading into an uncomfortable zone (assuming there was no fuel being burned off in the pipe and giving off a deceptive EGT reading - but thanks to Ricky’s suggestion, I reckon there’s a good chance that’s happening). Again my ground speed against the RPM for the 20/21T gear set is looking close to James’ spreadsheet.
Now image 3 is being in G6 having short shifted from G5, hopelessly bogged and EGT worse (goes a bit higher then but immediately starts to fall despite my foot planted, and speed starts to fall). Not happy.
Evan, a very logical suggestion given my circumstances. However, I have now tried the top flip but unfortunately the inlets foul with my Dellorto (probably okay with the lower level Tillotson). As an alternative, I wonder if I could use my rain cover? I’ve never used it before and I’m not sure wherther it will do the opposite and restrict needed airflow too much at lower speeds. Any experience out there? I suppose just need to test and see what happens.
Images below
I’ve used the rain cover before, but only on a sprint track in wet conditions. I suppose it couldn’t hurt to try, but you may be better off buying a Righetti Active airbox, as it won’t interfere with the carb when flipped.
Have you tried covering one inlet when at speed and in 5th/6th gear? Another easy test to see if it helps/hurts the mixture.
Well, if I had half a karting brain, the answer would be yes. Regrettably, that is not the case. Clueless.
Anyway, I’m going to try out a different airbox as suggested. I have noticed that all of the KZ drivers at my local sprint track are now using the Righetti Active airbox. So that would seem like a good suggestion. However, our local homologation for the 175 SS does not provide for it and for the moment I really don’t want to move outside our “stock” framework. Instead, the KG Nitro Airbox (30 mm) is available in the homologation, which I know has been around for a while. The literature for the box talks to a static mode as well as dynamic, with images of the inlet half flipped. That’s positive. It also seems taller in design than my current airbox (being a NOX 30 mm) , so presumably won’t be fouled by the Dellorto.
Picture below.
So, unless there are any counter thoughts, I’ll try that and report back in.
That all sounds good. Technically the one you have pictured is the KG Power, which is an updated version of the Nitro, and is substantially more expensive.
Dez’s latest video education below - this time on superkarting/road circuit running.
What I’d like is a discussion about ignition advance (or not) for those that have no idea what they are doing (i.e. me). Is that really the issue Dez had at the end with a stuck piston, despite working with a stock KZ setup using a 30 mm Dellorto? Or was it “just” a matter of jetting choice. Is it the case that for stock set-ups on road circuits, we do need to change the ignition advance/retard to deal with heat/lean issues, or rather it should be best dealt with in the first instance with carb settings (main jet, tube/emulsifier, pilot, needle, clip)? Thoughts?
I guess it’s hard to know what happened in their case. But it sounds like they have a maximum advance rule? If that’s the case, you could consider converting that to mm BTDC for the 175 and use that as your timing baseline
You’ll need to note the stroke and rod length difference to compensate for the 175 (assuming there is a difference).
/edit looks like stroke is the same as KZ, but the rod is 5mm longer. You’ll have to run a calculation to verify what the mm BTDC should be in the 175.
I don’t think advance limit per se was the problem (particularly as he wasn’t racing in an official event, just an open practice session). More advice from others to bring the advance back (that is retard) to no more than 1 mm or else “you’ll cook the piston”. That’s the part I’m confused about. Is that really an issue (sounds very conservative), or it it much more to do with the carb settings? That is, if Dez hadn’t touched the ignition advance in the first place, it would have had no bearing on the piston seize at day end - it was always going to happen because of the carb setting choices on the day; that’s a question not a statement.
You generally do have to back off (retard) the advance when running long course. I don’t have any personal experience, but know guys who burned holes in their pistons from running sprint advance on the long course.
Okay, noted. Are there early symptoms (warnings) before disaster strikes? Water/EGT temps changes/unusual peaks etc, as distinct from just running too lean?