You mean tune up kit? So you replaced the engine with the fins? When my MX-8 bogs down, its either the engine is wrapped up with palm tree hairs, filter needs to be cleaned, or there is a clog of leaves in the hose somewhere. The clog is pretty rare but it always happens at a joint, so I break open each hose to see. Water flow is the usual culprit. I have a dedicated port for my cleaner, so as the filter cartridges get closer to needing to be cleaned, I have to divert more suction from the skimmers. Sometimes I need to remove the screws over the engine to see if its wrapped up with these hair-like things my palms drop. If you used the tune up kit I think you did, you already opened this part up.
I've had to replace the upper and lower engine housing, but that was causing it to jam very easily, not move slow. Your problem is likely water flow. There are directions in the manual to count rotations of the wheel with the yellow thing on it to gauge your level of flow. TBH I am tired of messing with this thing as its really touchy and always needs parts. I have a robot that I got for free and I'm trying to bring it back from the dead.
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