Comer C51 Top End Weirdness

My daughter has been running her Comer Kid Kart for a while, and there’s persistently been a weird top end issue where, when she is on the main straight, it doesn’t pull like it does on shorter straights. In the second red box, you can see that the longitudinal Gs are more than double those in the first box, and the speed curve is much steeper. This is also a slight uphill section. In the first red box, it is at the end of a 590’ straight that is slightly downhill, and there’s almost no acceleration at all after 35mph. Acceleration at the first part of the straight looks solid, just not on the second half of the straight.

Could there be a carb problem that comes up after prolonged full throttle only? Maybe something like a leak or flow restriction that shows up after staying WOT for longer than elsewhere in the lap. She swears that she isn’t lifting at this point, and she is the type to not say that unless she really believes it. It is hard to tell from GoPro video, but she does float her left foot over the brake, so it’s not impossible that she could be dragging the brake slightly, but this has been in every single data trace I can recall looking at. I don’t think it is realistic to think she’s been dragging the brake this consistently for so long, and there’s a little dead travel before the brake engages.

Watching from pit lane does she visually slow down? Does she seem to lose pace compared to others through the same sector?

The speed trace inside the red box doesn’t look particularly troubling, she is still accelerating through that pull. What is happening immediately before the second red box though - is that a chicane in the straight? What does the RPM trace look like - does the rate of change decrease as it nears max rpm?

Have you varied the jetting at all?

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The acceleration traces are at the bottom of the red boxes; accel in the first red box is about a third of the second red box. You can also see that the speed trace is much steeper in the second red trace. It is very much real

Jetting doesn’t make sense. The jetting looks great in the second box, so what would cause jetting to change on the main straight only? My first thought when I saw this trace was something causing it to starve or a leak. Something that builds up gradually when running steady WOT

What would cause it to change is probably that you are comparing two runs with what likes a fairly significant variable. The first pull is through a continuous 10’ decrease in elevation, the long pull is an almost 15’ increase in elevation that starts where the speed dips. Would guess that is the biggest factor when you are dealing with 3 horsepower.

Kinda hard to imagine that jetting isn’t a concern when you are talking about it potentially starving for fuel though. Have you tried jetting without success or are you just assuming it’s not jetting?

Props to you for putting in 100x more effort in kid kart data analysis than i ever will. When stuff like this happened with my kid karter, it was always, “oh, i lifted because i didn’t like the engine being so loud” or, “Oh, I forgot to hold it down, dad”.

Unsolicited advice from a stranger on the internet: Just go have fun and get laps. Then go get ice cream. That’s what she’ll remember the most anyway.

You’ve got me wondering if the elevation is not quite right. That’s the base for all my confusion anyways - if I did think it was uphill when she lost speed, I wouldn’t think twice about it (and that’s why I ignored this for a year). The track is CW, so the data looks like it is downhill when she loses acceleration, so that’s what caught my eye. But if it really is uphill (hard to tell in person for such a low change), I have no problem moving on.

@fatboy1dh this is all for me, not her, haha. Data is my fun, and she said she is keeping it flat, so I haven’t worried her with any of this. She is having a blast and I just want to make sure I don’t ever get her frustrated by, say, ignoring an engine issue.

I recently dropped 1.5s with my 206 simply by having the frame straightened. It had a very slight pull to the right on the straights, nothing crazy, but enough to scrub a little power from a motor that already makes nearly none. It was very frustrating doing everything I could to keep up only to drop back on the straights. As Zoey’s mechanic, my main job is to make sure I don’t give her problematic equipment and tell her to just push through when there’s something I can do about.


This is where I would start. You might be at the upper limit of the engine’s ability to pull at the high RPMs.

No, it still has some more revs to give. It hits higher speed/revs elsewhere on the track just fine. I’m thinking I just need to go through the elevation data with a finer comb. Occam’s razer - the simplest answer is the most likely. If it is uphill, it won’t take much to sap the last bit of power at that speed. Looking at my dyno sheet, it is under 1hp. There are other flat sections where it keeps pulling another 5mph much, much stronger, so I know the kart can push through the air, I think it just can’t do it when also going uphill. My data says it is going downhill and should be able to push, but, looking at other elevation data sources, it is slightly uphill. That would explain the consistency in all her data and make all this moot.

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Other ideas. Are they opposite directions? Wind? Draft?

Wind and elevation are two factors that came to mind for me as well.

It is too consistent (every single lap for a year) to be a draft, and kids are bad at poking around the kart in front rather than drafting.

They are opposite directions and wind is usually against you on the straight (the straight goes SW and most weather comes in from the west moving east), but the wind on this session was SE, giving a slight tail wind down the straight.

The theory I am most comfortable with so far is that there is an elevation gain where I originally thought it was a drop.

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