Relationship between rich/lean and the fuel/oil mix ratio

Just looking for some guidance on the relationship between running rich/lean and the impact of the fuel/oil mix ratio being used.

For example, if I was running a fuel/oil mix that was richer than recommended (say a problematic mixing jug - long story), would that materially change the usual rich/lean relationship via the jetting selection?

Here’s my issue. As I’ve posted separately, I’m wanting to use a 175 SS (not the SSE) for road racing as a replacement to the CR125 - so in a testing phase. Had some problems with the Tillotson HB-15A, so for the moment now using a 36 mm Dellorto. Seems to work perfectly running on my local sprint circuit. EGTs peaking about 680 C (1250 F), depending on jetting for the day, and using a 17/25 sprocket combo. I am blowing a bit of smoke though and have the feeling I’ve got too much oil in my mix ratio. Spark plug looks a bit wet, but to me not too bad.

So, ready for another road circuit test at Phillip Island. A very tough track on kart engines. Again, a frustrating day. EGT’s all over the place and very high in lower gears, so backing off and coming in. Go much richer with the jetting (like a peak 230 with the main jet, while the app is recommending 205 for road racing near sea level), same problem and bogging in the top end of G5 and from 10,000 rpm in G6 (at say 160 km/h). The free advice in the garages is forget the EGTs, they’re deceptive, you may have contaminates etc, let’s look at the spark plug and piston crown. Okay, so I try that. Both look wet/oily, seem rich, so come down in jetting size. Try 220, then 205, exactly the same symptoms. EGTs super hot in the lower gears (particularly as I approach 14,000 rpm before slightly short shifting), then come down to 640 C in G6 while stuck at circa 10,000 rpm.

So, is it a lean bog (of which there is little sign in G1 - G4, other than very high EGT peaks - like well north of 700 C) and I’m at risk of blowing up? Am I too rich? Or is something else going on? For example, I’ve got a feeling my fuel/oil mix was off (like higher than 5%). Would that explain high lower gear EGTs but a somewhat oily mess on the spark plug and piston crown when post the run?

Any thoughts?

I’ll admit that I’ve only speed read so far……

If the jetting is in ballpark, the amount of oil won’t matter as much as you might expect as far as having a markedly visual impact. Unless you go wayyyy off the scale. I’ve run 10oz/gal on air cooled engines and the plug is still very much dry once the jetting is close.

Remember a plug is self cleaning once it gets into its correct temperature range.

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You can be rich and get high egt temperatures if raw fuel is being burned in the pipe. I’ve heard of just that happening in road racing, not as much in sprint as the WOT time is much shorter.


That’s an interesting comment.

That’s a good point too. If your ignition advance is conservative, you’ll have elevated EGTs.

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Oil mostly doesn’t count for combustion; adding oil leans the mixture. If you have more than 1:32 of a 50-weight JASO FD oil in there - or Elf HTX - you are not going to seize for want of oil. You’ll only fail due to detonation.

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Charles, using Elf HTX 909 at 1:20 (5%) mixed with Elf Race 102. I’ll pull the head off tonight and look for signs of detonation. I’m hoping not and that the high EGTs are for the reason Ricky mentioned, and then I need to think about moving the ignition advance from the base/conservative position (as per James’ comment), which for “stock class” reasons I’d rather not do. I’d prefer to just to go leaner on the jetting without blowing up, but perhaps that’s not doable and I simply need to burn more fuel in the combustion chamber (and not the pipe). This is moving outside my pay grade.

It’s definitely best to work on jetting first and then work on ignition timing. Wish I had more specific advice for you, but for now it looks like you’re going to have to establish your own baseline EGTs.

You are using the 36mm Dellorto, yes? What are all the other internals of the carb? IE needle, clip, tube, inner/outer pilots? All of those will contribute to the EGT/performance. The needle has a STRONG effect on overall jetting.

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Yes, using the 36 mm Dellorto. The last run was main jet 205, needle K28 clip P3, emulsion tube DQ 273, pilot B100/65. The only thing I changed during the course of the day was the main jet - 200 (which was what the app was recommending for long circuit at that location on the day, albeit Phillip Island is as tough as it gets with lots of WOT), 205, 220 and 230. I didn’t stay out long on the 200 - as soon as the EGTs approached 700C, I came in (which in hindsight may have been a mistake). Moved to 205, 220 then 230, then started coming down. All same symptoms - big bog from around 12,000 rpm in G5 and hopeless in G6, don’t think I got past 10,000. Make sense?

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Being engine illiterate, not sure what to make of this.


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 :joy: )

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:

  1. we saw way higher EGT’s than what we did under sprint conditions

  2. 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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