Rotax Max Engine Idle

2 Stroke Kart Engine Forums
1

Hello,
On my rotax max the engine doesn’t hold up at idle!
My question is, should i tune through the throttle cable?

Thanks :smile:

1 Like
2

Hold max throttle turned off. And press the small thing throttle cabel goes in and if it is a gap on the one on the carb you need to regulate. If not check the carburator jetting.

3

Idk so mutch about rotax jetting but you count the screw as an clock 1 turn is 1 hr a half turn is 30min there is i good ap you can download. Search: rotax jetting. In a appstore

4

The Rotax carb should have an Idle screw that raises the slide to hold idle. It will be on the side of the carb and have a spring between the head of the screw and the carb. You can adjust idle up by screwing in and down by screwing out. It is not meant to hold an idle at zero throttle, but more to keep it from stalling out off throttle. When sitting stationary, you my have to blip the throttle to keep it running. Holding at idle with no throttle input for too long will load up the spark plug causing the engine to bog or even stall out when you try to accelerate.

5

Thanks people :wink:

I’ve already tried adjusting the idle screw but I don’t feel any difference in idle speed.
Is it normal for the engine to stall when not accelerating?

I probably have to adjust the throttle cable a bit then?

6

Has it been laying up for a while? Since it seems non responsive. It’s possible it’s got an air leak. Check the rubber boot where the carb is mounted to the engine.

7

It’s been out for a few years stoped, but now I’ve been using it often.

The rubber boot actually has some cracks on the outside, but on the inside it doesn’t appear to have anything.

8

Hi, just thought I’d add my experience as I found this thread obviously trying to solve the same Rotax Max idle problem.
I just bought a kart that has been sitting for a while, had a non evo Rotax that I thought I’d take for a few practice runs before buying a new engine for it.

Problem is, appeared to need blurps of 1/3 throttle to keep alive, seemed very rich all round, and many comments about people saying “some idle, some don’t”
Which seems strange to me. So I did this.

  1. Download the Rotax Jetting app
  2. I pulled the Carby apart to find quite dirty, so cleaned and blasted out all jets/ports.
  3. fit the recommended jet from the app (app said 126, there was a 160 in it ??? That seemed way off, even if I change all the input data on the app I couldn’t ever make it recommend a 160)
  4. reset float level to 3.5mm
  5. check the needle clip is on setting 3 (middle groove) recommended by you tube video (power republic)
  6. adjust air screw to 1 1/2 turns out
  7. sneak the idle screw almost all the way in

This Rotax purrs like a kitten now :smiley:

It may be a different story on the track throughout the rev range under load, I may well find out why it had a 160 jet in it :grimacing:

9

A 126 seems tiny for a pre EVO. 160 at sea level is what I remember.

10

If it’s a pre-evo definitely do not run a 126. If it is an evo that’s fine

11

Ok, thanks guys,… I’ll go back but it seems to barely run and certainly won’t idle. I was worried that there was something wrong with it.
I’ll give to old girl the 160, thank you for the info :pray:

12

Can you check what carb it is actually. You may have a Rotax Evo carburettor on a non-evo in which case 126 is fine. Stamped on the carburettor you’ll see VSHB34 and then a 2 letter combination. QD and QS are non-evo carbs. XS is the evo carb.

13

Yep, thanks
It got QS stamped on it. So non evo
I’ll run the 160 and see how it goes.

14

I believe that Apps are reasonably accurate. So why did it recommend a 126 jet? The App that I used had to be calibrated but I wouldn’t think that calibrating would make that much of a difference. Is the App assuming an EVO carb/motor?

Idling is not important and I don’t recall any of my Rotax motors idling that well but they kept running at idle with a small blip of throttle.

15

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That should sort out your main jet.

For setting your idle air and idle set screws (Uranus Software. Idle air screw adjustment):

ADJUST PROCEDURE

The default position of the air screw is two turns (anticlockwise) from closed position. This works for most of the occasions but we are going to describe our adjustment procedure to get better results.

First of all, engine can’t be cool for this adjustment; it’s necessary to adjust the carburettor with engine at operational temperature, between 45ºC/113ºF (minimum) and 65ºC/149ºF (best)

With the engine stopped, close the air screw turning it full in (not to tightly) and open it two turns. This is the default position. This is the position we need in order to adjust throttle stop screw.

Now, start the engine and adjust throttle screw to get the engine stable but as low of revs as possible.

Once adjusted throttle screw, stop the engine, close air screw and open it 1.5 turns. Now it’s time to listen to your engine.

Start your engine again and very carefully open air screw till you get a smooth sound of your engine. It’s easy to get the point. Open 0.25 turns each time till the engine runs smooth, stable and with revs slightly higher that with the standard position.

Remember that the mixture is going to be leaner as you open the air. Be careful not to open too much (around 4 turns it’s my limit, but in most of the occasions my point is around 1.75).