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I have a 2016 750 crew (it is a Cub Cadet but same machine). It has 108 hours, 49 miles. Basically new, but I got it used with 22 miles on it. The guy said it was too big, and that he wanted a RZR. It ran great for a little over a year. It then developed a rough idle and won't wind up to get past about 20 mph in high range. I have tried countless things, and I keep coming back to overfueling. It doesn' seem to be throwing a code.  I have done the following...

Spark plug (multiple times)

Check, clean air filter 

Sprayed brake clean around intake while running (to be sure it isn' sucking air).

Checked valve adjustment

Replaced fuel pump assy., after breaking it trying to remove to check in tank filter.

Checked wiring and connections

Checked/cleaned spark arrestor screen

Replaced fuel, trying different octane. 

Checked compression.

Compression should be 1200 kpa,  mine was between 700 and 800 kpa.  Also noticed oil had been increasing and is now milky looking. I figured gas in oil due to overfueling. Dad said milky or foamy meant coolant in oil so we figured blown head gasket. So, we put a new head gasket on it. Old one didn't look blown. Valves were tight like new, could still see cross hatch marks in cylinder wall (like new as it IS still new!) Put it all back together, torqued bolts properly etc... no change. Still runs poorly, compression still low! I don't think it is a compression issue. We pulled the rocker cover back off to check out the automatic compression release that we had noticed on the timing chain  sproket on the end of the camshaft. It seems to be working properly. Then a lightbulb went off. "Automatic compression release" appears to slightly open an exhaust valve... when?... while cranking engine to start. There is no way that we're going to get 1200 kpa compression (unless I were to back that exhaust valve way off)! I may try that when I re-install the rocker cover.  

Anyway, I'm pretty convinced that it is fuel in the oil, and that it is over fueling like I originally suspected. But how do I know the cause?? Book says over fueling could be caused by Air Pressure sensor, o2 sensor, Solar Term door position fault, or ECU program error or damage. Those are some pricey parts.  Any input??

 

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Disregard compression for now.  What about injector?  Remove it then reconnect it and hold in a glass container. Wind the motor over and observe spray pattern. If  the fuel "squirt"a  fine mist all is OK. If a "stream" you have a faulty or clogged injector. 

Fault codes can be difficult to follow but will only guide us to an electrical fault. Test the injector then we can dig further. PS higher octane fuel should not be necessary.

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Update... I tried to clean the fuel injector after watching some how to videos on YouTube. The spray pattern never changed, still shooting a "v" pattern. So I ordered a new injector for about $150.00. It arrived today and I installed it. It sprayed the same pattern as the old injector and it still runs like crap! 

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I tried unplugging the 02 sensor while it was running. The idle immediately sounded healthier so I took it down the road. It still wouldn't quite let me give it full throttle without it loading up, but it had much more power, and I was able to get it up to 43 mph. With the 02 sensor plugged in it will only get up to about 35 mph. Top speed on this thing when it ran right was about 54 or 55 mph. Would this indicate a bad 02 sensor?

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