Why did my engine blow up?

Please help me figure out why my engine blew up! As you can see in below pics, the piston looks like it’s never seen a drop of oil! Link to video of the failure (occurs at 3:25 in this video):


IAME KA100 motor with Tillotson carburetor, approx 4 hours on bottom end, 8 on top end since last rebuilds. 48.22 piston size. I rebuilt the carb with a standard Tillotson carb rebuild kit before raceday. Motor ran great in practice last week and also during this night’s practice and qualifying heat. Failed a few laps into the prefinal and wouldn’t turn over after failure. MyChron 5s 2T data revealed peak EGTs were approx 940 degrees during this session. This is higher than I usually see (I’m usually in the mid 800s). My mechanic says peak should be 1000+ degrees, but I’ve never been able to get temps this high even if I lean out the needle screws a bit. I replaced the EGT sensor not long ago, so I’m fairly certain these temps are accurate.

I have three theories about why the engine might have failed. What do you think?

  1. fuel:oil mix was wrong. I’m using VP Racing MS98L with Red Line 2T Two Stroke Kart Oil. I bought a “5 gallon” drum of MS98L at the track. I planned to mix 5 gallons of fuel with 16 Oz of oil (40:1 mix), but when I poured the contents of the fuel drum into my VP racing gas can, I noticed that the fuel level in the can was a little bit (I’m talking maybe 1-1.5 inches) higher than the “5 gallon” marker on the gas can. I should’ve just used my transfer pump to pump some fuel out of the race gas can and back into the drum, but I didn’t think of this at the time. Instead, to compensate for this, I added a very small amount of extra oil from a second 16-oz bottle of oil (so maybe I had 16.5 or 17 oz of oil instead of 16). I mixed it as well as I could by shaking, rotating, and tipping the can many times before I added this new fuel to the kart’s tank. Maybe I didn’t put enough oil in the fuel? Or too much? Or I didn’t mix it well? Or I should have used Motul?

  2. I didn’t prime the fuel system. When I went to start the kart, there was a lot of air in the fuel line from when I had disconnected it to rebuild the carburetor. I drew fuel into the engine by cranking it (using the onboard battery and starter) with my hand covering the airbox. This sucked fuel into the engine, but only after cranking for at least 10 seconds. Maybe the piston got damaged during this time because there was no fuel (and thus no oil) going in during these 10 seconds? But shouldn’t it have been lubricated already from the last time I ran it (just a week before this raceday)? Should I prime the fuel system before cranking the engine every time I get air in the fuel line, or is this not necessary?

  3. Something was wrong with the carburetor or its needle settings. I ran the low fuel screw at 60 seconds out and the high screw at a bit more than that (maybe 65-70 seconds, not exactly sure since I adjusted it a bit during practice using my hand, not a carb watch, while I was going down the straightaway on track). If I leaned it out too much, wouldn’t my EGTs be higher?

I really appreciate your help! As you can probably tell, I have a lot to learn.

First, for the EGT, what sensor are you using? I have the basic stick type, and typically run the EGT around 1000 degrees at 15k RPM. Also, how far from the header is the EGT?

Also, for fuel mixture, what you are running may be a little lean. I run a quart for 5 gallons of fuel, so 20:1 and have no issues, besides we typically run SKUSA rules, which that’s the required mixture. Any reason why you’re running the mixture so lean?

Your settings for the mixture screw are on with what is normal, so not too lean from a carb point of view.

Who did the rebuild? The only times I have seen a KA fail like this, the piston was put in backwards and the ring got caught in the port causing a failure. That would be my main guess as to what happened, the other could be the oil to fuel mixture, but that would only be a slight reason.

Are you using spark plug color to tune?

2 weeks ago I started my quest to fine-tune our KT100. I swapped our Mychron 5s from the CHT probe, which is affected by outdoor air temps and the 2 driver’s arm/elbow positions, to EGT.

EGT readings were low so I started leaning the carb. I went from 900 to 950 to 1000. I also threw new plugs in at each increment. The plugs told me I was WAY lean, but I believed the Mychron (my pilot training told me to trust my instruments). We stuck the motor on the next pass. Same failure as yours (melty piston aluminum grabs ring). Zero compression.

Use and trust plug readings. Just finished a 140 lap race this weekend and engine was still working at the end of it, lol!

I think you are on the right track thinking your 40:1 ratio is a problem. That is considerably leaner than the recommendation of 20:1 stated in the manual. I run 16:1 using Amsoil Dominator. We have racers at our track that use Redline with success, so I doubt that is an issue.

Your needle setting are about the normal baseline.

As for temps I’ve never had much luck with lean mixtures running very well so trying to chase a temperature never worked for me. Some sensors read differently but again your window is fine. Do you check the tightness of your spark plug and cylinder head nuts? I had a KT100 once I seized because they came loose.

The only other thing that has happened to some KA’s, especially with a lot of time on them is the ring locator pin coming loose. With the amount of time you stated I doubt this was the issue.

There’s a really, really, really simple explanation.

Redline 2-stroke kart oil is horrible. It is too thin to protect the piston from seizing to the wall.

Hone the cylinder, put in a new piston, switch from whatever nasty racing gas you’re using to fresh 93 octane premium pump gas. If you can get it, use Elf HTX 909. If you can’t, use Motul 800. If you’re pinching pennies use Mystik Sea and Snow.

Here is a pic with the exhaust bung plug missing. You can see what type of sensor I’m using and how close to the header it is.

I run 40:1 just because that’s what the previous owner advised. I am not opposed to running 20:1. I didn’t know that was SKUSA spec. I have only ever raced at club level, and they don’t test my fuel because I’m too slow for them to care.

My mechanic did the rebuild. I would like to think that he wouldn’t make an elementary mistake like this because he does a lot of KA rebuilds, but we’re all human…

No. How do I know what color is correct? I’m reading this other thread below, and most of it is going over my head: Reading spark plug color

The ring appears to be intact, so I don’t think it’s gotten caught in the bore, just seized from metal-to-metal contact of the piston in the bore.

20:1 is IAME’s standard recommendation, while the Australian importer suggests you can run down to 16:1 to be on the safe side of things.

Definitely not a locator pin failure. I’m with the others that have pointed to oil type and oil mixture as the culprit. Elf HTX 909 seems to be the best, and Motul 2T/800 is a good substitute if needed.

I would also come back to your carb. Ensure that all passages are clear, and double check that you have the high and low needles installed in their correct locations. Be sure that you’re running a fuel filter as well.

Check for air leaks as well. Make sure all parts are tight, gaskets are in good health, and reeds are not broken/chipped.

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I agree with the above, don’t run that oil. I don’t think it is the sole cause here, but a good oil can help when you are tight on other factors whereas a bad one will work against you as soon as you are running a tad too lean, at the limit with oil %, high EGT etc.

By looking at the video, engine seems to be in trouble already at 3:10 exiting the corner, if it happens again in the future pull over, sudden loss of power like that exiting corners very rarely fixes itself, most often is a sign of impending doom.

I would look into 2 areas: sudden ingestion of air and carburetor. Sudden air ingestion could explain why you were running low EGTs then all of a sudden everything goes wrong. Most likely a crank seal failure if that’s the case. On the carburetor, you said you rebuilt it just before race day, before that change everything else was fine. So check the arm height setting/popoff pressure that could completely throw you off if not correct. Keep in mind all of the jetting baselines (minutes out) refer to a common popoff pressure window…if that is off, everything else will be off too

What is the “arm height?” Is the arm the part labeled “25” in this schematic? What is the ideal popoff pressure? Does the bend (or lack of bend) in part 25 controll popoff pressure?

Ultimately I think it was the oil mixture that was your issue. I’ve run KA’s lean enough to where they lean bog off the turns and lay down on the straights with obscene EGT’s without eating a piston…stuff that would have for sure stuck a piston on a Yamaha but the KA is pretty resistant to the piston damage you saw unless the locator pin failed or something of the like.

900 is far too low on EGT, but part of your problem is that you have the jam nut still on your EGT sensor. Pull that second nut off of the sensor and run it all the way down in the pipe and your EGT’s will read around 100 degrees hotter than they do now and be much more in line with your builders recommendation.

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Maybe a bit too simplistic, but take a look at the ceramic a little ways into the plug. You want the color of a cardboard box, light medium brown, if you run gasoline. Much darker or black is rich, much lighter or gray or white is lean.

If your ring is stuck, or there’s aluminum smeared on the cylinders or along the piston skirt, it got too hot - either due to mixture or friction (lack of oil).

Disclaimer- my prior experience is limited to aviation piston engines, 1970s big blocks & turbo v6, and now starting on KT100s. I’m new to karting.

This is great stuff! I would like to try this. Do i need to check the plug when it is still hot fresh off the track? When do you check?

Wow, I never thought about this.

You guys are great. I’m gonna have to start posting here more often. It seems the consensus is that my fuel mix is too lean. I will try again with 20:1 ratio. I have ordered some of the SKUSA spec Elf two stroke oil. I will lower my EGT further down into the pipe. I’ll learn to read spark plug color, at least a little bit.

Thumbs up to the oil ratio and jam nut on the EJT probe answers. Ran with two 16 oz bottles of Redline mixed with 5 gallons of gas in a KA without problems.

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I use cheap plugs that I can toss (no iridiums here). They need to be fresh out of the box. Run the kart on the track. Don’t idle too long coming into the pits. Kill the engine as you roll to a stop. Check the plug. Make your adjustments. Run a new plug. Repeat. I number my plugs with a sharpie after the runs and make notes, and refer back if needed at home. But they are one & done for me.

Inspecting the piston is a good way to check the mixture. The IAME Selettra ignition rarely allows a clean “chop” to read the spark plug. Other guys use borescopes, I have a pair of leather gloves and a 13mm socket!

What do you mean by “clean chop”? Are you saying that checking by removing the plug immediately after a sessions isn’t accurate?

When we did it in gearbox karts it was full throttle on the last straight before the pit in then clutch in shut engine off and coast to the pit.