Rotax Max 125 getting stuck at 13,000 rpm for a couple seconds

G’day all, first a couple of photos to set the scene:

1st image is the section of track in question (main straight with incline. Not my footage/image)
2nd image is my data showing the issue. Laps came from 2 different sessions 1 day apart.

As you can see with the data the kart is revving out fine till about 13,000 - 13,100 then proceeds to stall out. At best the kart keeps the same speed and at worst it loses 1kph. I’d estimate I’m losing around 300-400 rpm by the end of the straight which is correlated by other racers in the class on the same gearing saying they were reaching 14600 on same age tires. Jetting was 122/Clip5 which was the same as what others were running from the same engine builder.

EDIT: Top end on 7.5 hours, bottom on around 37-38 hours

Any ideas on what could cause this?

Check your ignition timing first. If it’s correct, swap it with another one (coil/rotor/pickup) set that is known to work. Followed by jetting lean it out a bit and see if it revs higher. Also check float height. Those tests will give you an indication of the path you should be following

I’m no rotax expert, but they are a total loss ignition. If your battery gets weak it could affect ignition strength.

Rotax is very sensitive to batteries and spark plugs. What float height and air bleed screw setting are you running? Also Rotax is done pulling after 12k rpm and you are going uphill. Do you have someone else’s data to see if theirs is doing the same thing?

Battery is Motobatt less than a year old and was fully charged before both days. For the time being i’ll eliminate it since I don’t have another and the battery holds charge incredibly well.

Spark plug is a NGK GR9DI-8 gapped to 0.45mm.
Air bleed screw 1 & 3/4 turns.
EDIT: Float at 3.5mm

I haven’t seen anybody else’s data but following others I’ve watched them either maintain or slightly pull away which really shouldn’t be happening in the draft and I’ve also been hit up the rear near the top of the hill but other karts pulling better. I can probably ask for data if need be.

All that seems fine. I would suggest opening the spark plug gap to the max 1mm rule limit and go richer on the floats.

Also go make a big swing on jetting. Go up like 3 sizes and down 3 sizes. See if you can see any change.

Is it a EVO or non-EVO?

Other then whats mentioned above, the Powervale could be an issue.

Check wireharness and powervalve.

My engine builder and I discussed it’s probably not a powervalve issue otherwise it wouldn’t start accelerating again and it’s opening fine in other parts of the circuit.

We’re going to go for a full rebuild and check in case one of the seals is on its way out.

I’ll try lean out the floats even more within regs and maybe go leaner on needle. Builder isn’t a fan of going below a 122 main. What’s the advantage in a wider plug gap? I thought max ran best between 0.4-0.6mm plug gap?

It may or may not be any better just something to try. Spark plug gap can vary between motors and fuels.

Back off the throttle just slightly when the engine starts to struggle. If the motor pulls better the jetting might be too lean.

Did this problem occur suddenly or ?

Happened all weekend at that track. It’s not my local so I’m not there very often

What was your water temperature. If it’s not hot enough it can cause issues breaking through this RPM band. I always ran >60 deg C. (140 F).

Great idea. Was so easy to troubleshoot shoot a KA100 or X30 since the air box was right there and you could over an inlet tube to look for lean or rich jetting.

It’s been a while since I ran a Rotax, so things might have changed. If the Rotax pipe still uses packing, pull it out and check it. If it’s burnt away, then replace it. Or better yet, if it’s been in there for a while just replace it. I used to replace every other race but that was back a few years ago.

Also almost 40 hours on a Rotax bottom end is a lot of hours. I would consider rebuilding it soon.

1 Like