First TaG race, small issues with the Rotax Evo

Yesterday was my first race running TaG masters at OVRP. I had a blast but was running near the back of the pack for the whole day. Doubtless I need to get a grip on driving a kart with power, but I also had two issues with the motor that are a little puzzling:

  • During the first two practice sessions, the motor was running well except for at the end of the straight. It would hit ~12k RPM and then stumble and not rev any higher. It was almost like the Briggs when it hit the limiter but more abrupt. Every now and then it would rev out to ~13,900, and when it did there was a noticeable change in tone in the motor. I hate to make this comparison but it sounded like the early VTEC motors when they get into VTEC. Assuming this is the power valve opening fully? The issue persisted in the morning sessions but then didn’t happen once in quali, heat or race. The only thing I did was flip the PV solenoid in the bracket because it was upside down (realized this after looking at other karts).
  • At the end of the 2nd practice, I got stuck behind a kart going really slow through the hairpin, which is the slowest corner on the track. The motor would not rev about 3k, then stalled and. I’m assuming I flooded it. I waited about 20 min and it started and reved on the stand with no issues.
  • In the afternoon quali, heat and final sessions, the motor would bog badly coming out of the slow speed hairpin/uphill chicane complex. I would brake down to ~6k rpm, turn in and get back on the throttle and it would rev to ~6500 and just sit there for about 3 sec, then it would kick and run normally. I tried overlapping the throttle and brake to keep the revs up, and feeding throttle extremely slowly coming out of the hairpin but nothing worked. I think this is where I’m losing a big chunk of time, as I could hang with some of the guys up until this point, but when the motor bogged, they just took off.
  • One of the experienced Rotax guys took the exhaust off, and the donut gasket was in 2 pieces with just the top half sitting where it should. Could this have caused such a bad bog?

The motor is used, EVO upgrade and (supposedly) 8 hours from full rebuild. I cleaned the carb and did a carb kit and FP rebuild before this weekend. I also cleaned the powervalve. I was running a 122 main jet and needle clip at pos 3. Any ideas?

I would start by putting new packing in the exhaust. If your jetting was off, those 2 things could be contributing to some of your issues. Invest in a weather station and jetting app/program.

The Rotax bog is a common issue out of slow corners. You can’t mash the gas on a Rotax coming out of slow turns, practice being smooth. If you’re racing against other TaG engines there isn’t much you can do to keep up.


I was jetting based on the “Jetting Max Kart for Rotax Max EVO” iPhone app which seemed to recommend carb setup similar toh what the other karts in class were running. Most of the other karts were running EVOs as well and didn’t seem like there were having any issues at all. Are the Rotax really that sensitive to throttle input on slow corners? I was being extremely patient (or at least I thought so) on throttle application but it still bogged.

I will try new packing as well. Thanks!

Ahh the joys of the EVO when it goes bad…
Main things to check with these symptoms…but not limited too.

-Does you engine have the gasket under the ignition crank sensor? This is common issue with EVO’s upgrades. Crank has another relief for the conrod and the reluctor sensor picks this up and thinks is doing more RPM. So therefore rev limiter hits early and power valve opens early. Max is 2 gaskets. Fixed with new crank cases.

-Over 13500 RPM, you need to be on the lean side or the engine will not “clear”. We pretty much run a 122 jet everywhere now, down to a 120 if we have this symptom at high RPM.

-EVO’s eat coils. Hence their change to the non resistor plug recently. This also makes the over rich jetting worse.

General rule, go rich on needle, lean on jet and tune you progression port jet for low speed throttle response. This is more motor specific, but start at 1-1/2 turns and fiddle.

Hope this helps.

Thanks for the info! I’ll definitely check the crank sensor gasket. When you say “progression port jet”, is that the same thing as the idle air screw?

Yip that’s the one. good luck

Just wanted to add an update here. I tore the kart down again and realized the drive sprocket as 1 tooth higher than where it should be, so that would account for some of the bogging for sure.

I also attempted to replace the exhaust donut gasket. The gasket didn’t really want to slip onto the exhaust socket, so I applied a small amount of pressure to the gasket and it split down the middle and broke fairly easily. Is there a trick to getting the gasket socket onto the socket, or has anyone seen any e gaskets or sockets out of round/too large or small?

Updating this thread again, as it seems the issue has returned. See below for a video of this past weekend’s final that I think shows the issue:

First, the motor stalls at low RPM coming off the pre-grid. They you can see (and hear) during the race, the motor seems to trundle along and bog coming out of the hairpin complex, then finally reach a certain RPM where the power just seems to hit (you can hear this in the video, it’s usually at the little bump in the uphill section). The other karts either blow right by me or pass me in the uphill.

I was running the same main jet and gearing as the other Rotax. I fiddled around with the idle air screw but that didn’t seem to make a difference. At this point I’m thinking the Powervalve is not closing at low RPMs.

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Starting with the exhaust, removed the pipe and check the color on the exhaust header. You’re looking for a medium brown. If it’s black or white, check the carb. If you haven’t replaced the packing in the exhaust, do so. Pop the cover off the pop-off valve. If you can push the valve in freely and it returns on its own, that’s a good start. There is an optimum length for the power valve. Sorry, I don’t have that measurement handy.
On the carb, check to see if you have the same floats as everyone else and float height. Check the inlet strainer for debris. Look at the needle to see if the rubber has any deformities. Caution, do not spray the inside of the carb with brake cleaner with the needle valve installed.
I’m not familiar with the mobile jetting apps. Does your app have suggestions for spark plug? And one last thing for now, what’s your water temperature?

Does the Power Valve ‘push’ in from neutral, or ‘pull’ out from neutral when testing function?

The reason I ask is that my kart did very similar type of low rev behaviour and when I checked the PV I pulled it up and it slowly springs back down.

My kart is non-Evo and didn’t rev out past 12500. It was literally my first ever session and the track was cold and damp so I wasn’t really brave enough to push it too far as it was.

There is a spring under the cap that pushes the valve towards the cylinder. As the crankcase pressure builds at higher rpm, the valve is pushed out.


Thanks. Makes sense.

Okay, so now to add to the ongoing saga of this engine…
I decided to skip the race this weekend and practice yesterday instead. I had installed a new PV solenoid, new exhaust socket gasket and socket, and re aligned the exhaust. Went out for the first session, and the bog was much improved. Went out for the second session, and it was running like crap - bogging, sometimes completely losing throttle coming out of corners. at this point I’m thinking it HAS to be a loose pin or wire in the harness, as the issue is so intermittent. I swapped in my spare harness and the kart ran well the rest of the day.

BUT, when attempting to start the engine sitting on the grid, the clutch started to grab. You can hear it in the first 15 seconds of this video: . It would also stall eventually if you put the kart on the ground. It would run fine out in the track and would idle with no issues after the session was done and the engine was warm.

I took it home, removed the clutch drum and didn’t find anything suspicious. I also removed the clutch itself and it looks fine. I cleaned the drum and clutch with brake cleaner and put it all back together and the grabbing is worse :

The clutch is so simple that I am kind of at a loss to explain the grabbing. I am running the 11t driver so maybe the plastic bushing is worn, or the clutch is fatigued and is engaging at idle (this seems unlikely as it is a pretty solid piece) or the clutch drum somehow is out of round.

Has anyone seen this issue before? Thoughts?

What are the conditions of the bearing and O-rings? Does the clutch drum wobble on the shaft? The old clutches, as I remember them, had a brass bushing for the 10t and a needle bearing for the 11t.

A couple other things to check.
The nut that holds the gear to the drum.
The screws that affix the clutch.
The nut that hold the flywheel

Assuming that when you inspected the clutch, all the air gaps were the same and when you turn the motor over by hand, that there is zero runout for the clutch. Check the drum for any unusual wear marks.

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The 11t gear uses the bushing, and it looked fine. Same with all the washers and o-ring, no signs of any wear. The inside of the clutch drum was a little scuffed up but nothing crazy. However I did notice that the bushing was completely dry and not greased at all. I put a bit of high temp grease in there when I assembled it. Maybe it just needs some lubrication?

Surprised that the 11t driver uses a plastic bushing. I looked at a couple of web sites that show a needle bearing for the 11t on an EVO engine. With either a bushing or bearing, I’d grease it at least once a day.

How much slack to you have in your chain as it looks loose? The chain also has shinny spots as if its hit the ground.

In the 11t the bushing doesn’t hold the grease and they run dry. Then they wear and try and cock over, hence the grabbing and noise.
Just apply a little grease, and all will be good again. I do it after each day on the track. And they do need to be replaced periodically.

Hope this helps

Thanks guys. The more I think about it the more I agree its got be be a squeaky bushing. Hopefully the grease will be all it needs.

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