KT100 too lean?

[Joseph Bath]

Hello,

I’m relatively new to running a KT-100.  This past weekend, I decided to try some 110 octane race fuel.  I ran the same 8oz/gal mix I usually do.  I noticed that the kart seemed happy with my high speed needle at about 3/16 and low speed at about 1.5.  With 93 octane and the same mix, I’m usually at 3/16H and 3L.

The kart ran great, but when I pulled the exhaust off, the flex/header looked like this:

To me, that looks pretty lean, but I’m new at this..

Best laptimes of the year, though…

Thoughts?

[Will McDonough]

They say lean is fast but that is really lean if the pipe has that in it.  when I run lean there is still a little oil look to the pipe.  What kind of temp were you running.  You might want to pull the plug and look at the piston.

[Joseph Bath]

My CHT in the last race was about 450 at its hottest.  I get that hot on 93 also.

I should also mention that my clutch seized during the last race, so it was basically direct drive.  I wonder if it leaned way out when I was coasting down.  I doubt that’d be enough to make it look like that  I’ll check the piston tomorrow and snap a pic of the spark plug.

[Will McDonough]

You ran 450 on the temp with out sticking the eginge.  You might want to go ahead and pull the jug off too and check the skirts of the piston.  To see if it has any scorch  marks.

[Brad Alan]

Lean is fast. In my opinion, that’s too lean. Did you a EGT reading? 1100 is good for EGT. 450 CHT is too hot. I woulnd’t go over 420 CHT.

[TJ Koyen]

I’d fatten the high to 1/4 and let ‘er buck. That header looks a tick too lean, but it’s close.

Driver/Coach/Wrench : Innovative Performance/Exprit
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[Daniel Justice]

450? Haha I’ve pulled 490 before, that’s how one of my engines got the nickname “hot head”. It’s sitting out in the garage with a hole burned through it’s piston, not from that same race though. But ideally you want a dark ring in the center of your flex, your picture looks like it has a shadow there. My header always ends up looking like your picture no matter what. I think my flex looked like that when I lost the aforementioned engine. So be careful.

Daniel

[Walt Gifford]

Now you know how lean to make it for the start of the race. Next you need to learn how much to open it up on what lap to control temp.

Gif

FAA certified jet engine and aircraft technician,
Nicholson Speedway class champion 2001,
Yamaha KT100 Service Center,
41 years karting experience

[Jim White]

I roadraced Yamahas for 20 years. The last few using the carb trigger to lean the hi needle down the straightaway. I ran 110 with 8oz of oil like you did. Not sure how it compares to sprint racing but many times I ran over 450 head temp. Looking back at some data I see one race where I ran 14 consecutive 3 mile laps between 470 and 500 and with egt in the high 1200’s with no damage. I also only used CHT as a reference and actually tuned using EGT. I could always tell when it was running it’s best. Not so much from the gauge but at top RPM the fins just gave off a distinctive ring that told me it was happy.
That being said, in my opinion your flex color is pretty close to ideal. In the top picture of the header on the engine you can see what’s known as the wet line. Some people tune off the length of it. Once again it’s another reference to look at and learn from. I would be happy when mine looked like yours does. You also said you were turning some of your best lap times. That also leads me to believe you were in a pretty good state of tune. Now when you are running that close to the edge you have to be careful. Depending on weather those settings may be too lean some days and even still too rich on others. This all also relates only to the fuel you ran that day. Go back to pump gas or whatever and it all changes.
As you sound to be new to this I would probably be safe and follow others advice and richen it just a tad as you learn, but now you do know the feel and sound of having a pretty good tune. As you learn more you will eventually get a little greedy and stick one. My engine builder told me along the way that I would (and did) eventually stick one. But he also said that’s not a bad thing. At some point you have to find the limit so you know how far you can go.
I would be highly surprised if you damaged anything although running near the edge also lessens piston and bearing life as you are making more power. I would probably at least pop the head and check out the bore and piston crown (the color of which is another tuning aid). I would always do that after every race regardless.
Now that just my opinion…you’re mileage may vary :-)

[Greg Wright]

my experience matches what Jim just said. Road raced Yammies for years, won lots and lots of races and championships. My headers and flex were nearly snow white.

Greg Wright
Rapid Racing Inc.
Vintage B-Stock Pilot
"When in doubt Gas it, It won't help but it ends the suspense."

[Joseph Bath]

Thanks for all of this great advice!!  I think the next step is to move to a EGT sensor and keep a close eye on that flex.

Thanks again!

[Walt Gifford]

Joe, a sprint engine has to run richer than enduro engines.

FAA certified jet engine and aircraft technician,
Nicholson Speedway class champion 2001,
Yamaha KT100 Service Center,
41 years karting experience

[Jim White]

As I haven’t sprint raced in years I’m curious why you say that Walt. And I’m not bashing or arguing here :-) Just wondering the reasoning behind that thought. Is it because you go through the rpm range so much quicker and the load is different?

[Greg Wright]

Not arguing either but would like to hear Gif’s response. Having road raced lots of Yamahas I can’t make a direct comparison but I’ve also road raced and sprint raced with 125 shifters and for road racing they had to be much, much richer.

My reasoning? The heavier load on a road race engine should require more fuel.

 

Greg Wright
Rapid Racing Inc.
Vintage B-Stock Pilot
"When in doubt Gas it, It won't help but it ends the suspense."

[Walt Gifford]

You guys want him to make it leaner? An enduro engine is set richer but it runs leaner, a sprint engine is set leaner but it runs richer. There’s more to it than just jets.

Gif

FAA certified jet engine and aircraft technician,
Nicholson Speedway class champion 2001,
Yamaha KT100 Service Center,
41 years karting experience

[Greg Wright]

Nobody said that he should run leaner.

Greg Wright
Rapid Racing Inc.
Vintage B-Stock Pilot
"When in doubt Gas it, It won't help but it ends the suspense."

[Jim White]

I wasn’t sayin to run leaner at all. I just said it looked pretty close in my opinion. I just wondered why a sprint engine needs to run richer. And as I said I’m not arguing. I’m just trying to get more educated and exploring why that is.
My view would be that optimum power is optimum power. Regardless of what kind of track you are on. The only difference I see is going quickly through the power band many times a lap on a sprint track vs long extended heat building pulls when road racing.

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