Rotax 125 senior Evo issues

Gday team

To cut a long story short, ive been chasing my tail for a long time now, and its burning a hole in my wallet and starting to not enjoy it, i havnt raced yrs because of these issues, but have been out for countless practice days, just to end in frustration…

issue is that my rotax just wont run properly, ive tried almost every jetting combination known to man, replaced batteries, wiring looms, reed petals, needles, plugs etc etc the list goes on.

but the motor never revs out to 14k (no matter how short the gearing)
generally will go to 12k sometimes, and feels like a big power surge at around 10k.
other times wont rev past 8k or even 6k.

and i can seem to put my finger on the issue as to what it could be.

ive checked every wire, every connection etc eetc

anyone have any wisdom they care to share with me?

all help is greatly appreciated!!

I think it was @Liverpool had similar issues, he found a second spring on the power valve bellows, keeping it closed most of the time. Check it.

Also check your throttle cable rigging - make sure when your pedal is full travel the carb guillotine thing is fully open.

Maybe it wasn’t Andy but somebody had that recently

throttle cable is brand new and opening the whole way.
ill take apart the power valve tommorow, in regards t the power valve, besides taking it out and cleaning the valve i have not done much with them, is there some sort of tutorial to have a look at?

i do feel though, it is not power valve related, but i shall eliminate that by checking that spring

Not sure to be honest, I’m not too familiar with Rotax, only drove a non evo a couple of months before switching to X30.

There’s a YT vid on power republic channel about power valve maintenance, I think it’s non evo but I guess the mechanical action is the same? The slider needs to be kept free, clean off carbon deposits and file down any burrs in the metal with wet n dry.

I found the Rotax system incredibly complicated, others will help I’m sure, most in this group are stateside and prolly sleeping.

yeah im in new zealand,
so most of our mates from merica are likely snoozing

Yup it was me and I was in the exact same boat as you a month ago. I was fed up and tried so many things. Richard is correct my issue was this:
Maxed out at 12,000rpm and found it was this:
My evo is a gen 1(2015/2016) and the power valve cables to the case/solenoid/pump are ran different and the spring in the power valve is below the bellow and not above. I came across an article about changing the cabling to the new gen but nothing was said about the PV assembly so the cabling was changed by my local shop but nothing changed rpm wise. Brought it back to them and he rebuild the PV and it had a spring below like gen 1 and a spring above like gen 2. Once they pulled that out it was over 13,000 right away. My suggestion would be

A) take a picture of the cables between the case/solenoid/pump and I’ll tell you which one it is. If it’s older I suggest changing it.

B)disassemble the PV…good video from power republic how to do this on YouTube. Check if the spring is between the bellow(green rubber) and black cover or if it’s between the metal body and bellow. If you have both, change the cables and remove the one between the metal and bellow and your good to go. Just make sure the PV blade is moving freely too. Throw some oil on the blade.

C) you can do a quick check of the power valve function on the stand. Rev it up once it hits 7000 rpm you should see the valve open and once below 7000 should close.

I know your frustration so report back because once it’s going good it’s a ton of fun.

1 Like

Was able to find pictures of the 2.

You can see how one has a spring below the bellow and the other is above.

The new gen is above and was told this works much better than below.
Also you can see the pipe runs into the solenoid in the pull position on the top of the solenoid. The other runs in the push position so the green regulator arrow goes in the other direction.

The springs are different. The one below is small about 10mm and the one above that you want has a 20/25mm spring that’s black with a line of green spray paint

All excellent suggestions above. Has the packing material in the exhaust been replaced since this problem started?

Have you had an engine builder look at it?

One easy way to know if its the evo PV is to swap it out for a non evo PV setup that doesn’t rely on electrics to open/close.

well still stuck between that dreaded rock and a hard place.

no joy with the power valve. Its all functioning as should

could it be something to do with the ecu?

any more ideas team?

last resort is taking to an engine builder, as the nearest one, will require me to freight it. and no doubt start costing $$$$

I’m no expert but these are the things I would check to make sure. Not sure if it was running good before but:
-check if exhaust has junior restricter in it
-change the exhaust matting
-rebuild the fuel pump with a rebuilt kit
-look over the wiring harness to make sure no damage
-rebuild carb
-3mm float level
-inspect reed valve petals

That’s really all I can think of. It’s weird that it’s not the PV. My local shop said most issues with Rotax are the PV. Low end or top end it’s usually the PV function.

Have you swapped out the wire harness? I had a similar problem where ran fine up to around 11000rpm then stopped.
Also be sure your exhaust is completely clear all the way through.

Borrow a wiring harness and ecu from someone that you know is working on another kart. If you send this engine to a shop this is exactly what they’re gonna do, swap things out one by one and see if it solves the issue.

Have you tried someone elses carb? If not do so. Ensure you are not using a mix of pre-evo and evo carb jets/needles…the two CANNOT be interchanged. If you’re using the evo carb, ensure your needle is K57 and set to either P4 or P5…ie second richest or richest position.

My head’s spinning just reading these answers :joy::joy:

My apology for jumping on the power valve band wagon, but that would be my guess as well. Rotax went through 3 iterations of their wiring harness for the Evo upgrade before getting it right – I know 'cause I got to do all three. If I remember the sequence correctly, their big mistake was to have the power valve work off engine vacuum at first and then, realizing their mistake, change to engine pressure. Essentially, this “flipped” everything 180 degrees in the circuit. This means you need to be sure your hoses coming out of the engine to the “T” (which sends impulses to the fuel pump AND to the power) are sorted out per the current diagram. I’m away from my kart typing from home, but my memory is that the fuel line from the “T” goes to an inline 1 way valve, then to a small electrically controlled valve, then to the body of the power valve. Hope this helps.

Andy, Scott here from the US.

I am having same problem as OP after upgrade from Jr to Sr. evo series 1.

Is the Jr. exhaust restrictor between the cylinder and the exhaust? Or somewhere else in the exhaust?

I have the Sr. Cylinder with power valve and connected to Jr. Exhaust.

I don’t remember seeing the restrictor on the Jr. cylinder.


If you have a restrictor the exhaust flange will look like the first pic. If not it will look like the 2nd pic.