Home Forums TaG Rotax Max Help! Sr. Engine doesn't rev past 12,500 (upgraded to EVO)


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    • #46340

      I upgraded my current Sr. engine to EVO using the upgrade kit. The engine ran excellent before the upgrade. After the upgrade it’s cutting off exactly at 12,500, right where ignition curve changes, and starts making “ba ba ba…” sounds.

      We initially though it’s running out of fuel on top end, so we gave it more on the floats and the main jet. It didn’t help. Overall, we figured out that the engine responds pretty good to jetting adjustments on the bottom and mid-range, but the issue with the top remains the same irrespective of the jet. We ran all jets from 122 to 135 and the issue at 12,500 was still there. If you look at the Mycron, it never revs past 12,500-12,600 and once it hits that, it immediately drops back to 9,000-10,000. Sometimes it’s choking earlier around 9,000 or 10,000, but then revs up to 12,500 and stumbles there again. It almost feels like an electrical problem now.

      Already spent two days on this, going in circles. Here is what has been already tried (and it didn’t help):

      • Different old-style pipe (with brand new packing material)
      • Different battery
      • New spark plug
      • Different fuel pump
      • Different carb setups on floats, needle, and main. cleaned everything multiple times with carb cleaner
      • Grounded one or both cables for the power valve (just as an experiment)
      • Pulled out crank sensor, cleaned and put it back on
      • Compared EVO kit installation with other karts and all connections visually
      • Different EVO coil taken from another kart

      In terms of the next steps, planning to:

      1. Replace crank sensor
      2. Put one more O-ring spacer to keep it farther away from the crank (people are saying it may help)
      3. Try grounding in a different place
      4. Start looking into replacing all EVO parts one by one (try old-style power valve, replace EVO electronic box, replace EVO wiring harness)

      If you guys have any ideas about what can be wrong, please share.

    • #46351
      marc walsh


    • #46426
      Chad Landers

      I had a problem like this on an old style engine.  It ended up being the ground wire for the coil.  I have pro speed coil re locator.  I had to put the jumper ground wire on the opposite side of where I had it.

    • #46669
      andy graham

      there are several issues that cause the ecu to limit at 12500…see if you can download the manual and it will guide you through a bunch of trouble shooting…I’d almost bet money that you have the gasket between the cylinder and powervalve housing on upside down…theres a hole in the gasket that has to align with the hole in the housing and the cylinder, that’s the pressure feed for the power valve…if its blocked off, the power valve wont work and you’ll limit at 12500…the new power valve has a ground wire thats attached or unattached to change the rpms at where it opens, might want to chase that also…easy to check the PV gasket though and rule that out hopefully


    • #46675
      Steen Carstensen


      I thought the power valve on the EVO engine/upgrade kit was electronically operated, is there still a need for exhaust pressure for it to work???

    • #46678
      andy graham

      Steen — they use the same gasket and PV housing, so the hole is still there..we put one on as an upgrade, so I assumed it still had some pressure activation, but you’re right, the new PV is controled by an electronic solenoid, so not really sure…


    • #46715
      Scottie Melancon


      Are you grounding to the correct location? The EVO requires the ground to be installed on the left rear cylinder head bolt. We had a few folks at our track that had similar issues. They attached the ground to the proper location identified in the installation manual and it fixed the issue.

    • #46722

      Thanks, guys.

      PV gasket seems to be installed correctly, but I will double check. Ground is also correct, installed per EVO upgrade kit manual on the left rear bolt of cylinder head.

      I am going to the track tomorrow to replace EVO parts one by one. Will report progress.

    • #47030

      Problem solved. Crank sensor. The old one didn’t go bad and I’m pretty sure it will work just fine with old-style ignition. But the new EVO stuff seems to be more sensitive to it

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