Rotax Evo Senior Keeps Stalling

Hi guys,

I’m having an issue with a current iteration rotax max Evo senior.

The kart cannot complete one lap, under hard braking it keeps stalling and then will not restart unless left alone for a bit.

Prior on the stand, it starts and revs perfectly…on track without fail, it stalls under heavy braking at the end of the longest straight.

I’m pretty familiar/knowledgeable with rotax. I’ve stripped the carb multiple times, verified everything float height etc, even tried an entirely different carburetor same thing. I opened the fuel pump and cleaned it, it was fine to begin with. The vacuum lines are all plumbed correctly. Changed the spark plugs.

I’m absolutely at odds end with this, any advice/help would be appreciated.


Could it be a clutch issue where the clutch locks up and kills the engine when braking hard?

Good thinking …but then it shouldn’t be a pain to get it restarted if that were the case I’d think.

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Not really. If you think about it, what is the first thing you instinctively do when it feels like the engine is going to stall? You give it more throttle in an attempt to keep it alive.

My last race I was trying to late brake for the hairpin at the end of a straight. I was going down the inside and carried too deep. When I braked as much as I could, I ended up looping it and immediately gave it more gas to prevent stalling the engine. All I did was manage to flood the carb and it took nearly a lap of my competitors to re-fire the engine.

Could the same be happening to you? Check your clutch for any hairline cracks that may be keeping it engaged below the stall speed. Those symptoms will not likely happen on the kart stand, but certainly will happen on track.

Ok will check the clutch out…this package is however brand new…one race old

Does it always happen in the same corner? Did it happen to you any other time? What was the situation when it happened? Hardest braking at the end of a straight? Could be you are braking too hard too quickly that you are locking up the axle before the clutch can disengage.


Could it be electrical? Wiring?

It doesn’t run well even before it completely stalls. The stalling is after any situation of braking moderately hard…no I am 100% sure it’s not from brake “too hard”.

As for it being electrical, I won’t discount it at this point.

Obviously we want to help, but we need to understand the symptoms more. You stated “It doesn’t run well even before it completely stalls.” This means something more than how you first posed your issue. How is it not running well? Is the engine down on power? Does the engine struggle to rev? Does the engine stall easily when running at idle if you don’t keep blipping the throttle? Being a new engine, did you go through the break-in process? I have so many questions, but we need to start somewhere in order to lead to a path of diagnosis.

In the automotive industry and most of the engineering industries, we follow something called Diagnostic Troubleshooting Trees. Its basically a chart of ever branching tests that start with the initial symptoms and will fork off on a different path depending on your test results. Ever leading to a new branch until you isolate the issue.

First symptom…Engine stalls.

First Question…When does it stall?..and so on…and so on…until we get a final answer.

Next question…Can you replicate the same symptom on the karts stand like it happens on track? If you rev the engine on the stand and then apply the brake, does the engine stall?

Inside the fuel tank - is the fuel suction hose still flexible and does it have the weight on it ?

Mikko, the tank is brand new with brand new pick up tube. Yes the weight is attached.

Greg, it’s hesitating and won’t rev cleanly on the track, then when you go on the brake it dies. It’s dying at one point in particular on track because it’s a tight hairpin where you have to slow all the way down.

On the stand prior to going on track it’s perfect, it revs cleanly and idles, even if I press the brake.

Yes the engine was broken in properly.

Can you try someone else’s ECU, and ignition box?

Yes I can try that, good idea

I have a couple of things to check.

  • Drain the float bowl into a cup and see if there is any debris in the fuel.

  • With the kart on the stand, lift the kart from each side (front, back, left & right) and see if the engine still revs cleaning or if it bogs/hesitates.

  • Pull the carb and blow out all of the jets and orifices and retest on track.

It sounds like the carb might be starving for fuel on track. The fuel demand is always higher under a load than on the stand. Each test will help give us direction in what to look for next. Example, draining the float bowl will tell us if debris is blocking the jets. The Tilting test will tell us if the floats are cutting fuel. Blowing out the carb will tell us if there is a blockage in one of the jets. Let us know what the results are.

Fuel pump pumper diaphragm? Clogged fuel filter?

Greg I tried a completely different carburetor that is known to work. Same thing.

I also open the pump, sprayed it out it seemed fine. Engine and pump is 1 race day old.