Short story;
The engine won't idle in gear. When shifting into gear, there is a big slam and the engine either stalls or it starts moving. If you hold the brake, the engine will stall.

Long story;
We replaced the engine with a low mile engine from a reputable seller after the air filter came apart and dusted a cylinder causing a loss of compression.
The new engine didn't come with any of the CVT components, so those were swapped over from the old engine.

Upon first start it was clear the clutch was not slipping at idle and there was a problem.  I did take it out and drive it hoping it would free up. It drove slow and high speed perfectly, but after 20mins of driving it still wouldn't idle in gear.

I spoke to a local CFMoto mechanic and he suggested that the wet clutch may be locked up causing it to not idle in gear. This sounded plausible to me so I swapped the wet clutch and case from the original engine.

After initial reassembly, it seemed to work. When shifting into gear, it was very draggy and the engine was pulling, but it idled in gear.
I fully reassembled everything, and now it's doing the same thing as before. Big slam and stall.

I'm a pro mechanic, and I'm absolutely stumped.  This seems like a pretty simple system, but I'm at a total loss.

Please help!
Thanks

[2015 Z Force 800 53/Trail EPS]

Either the oneway is in backwards (which I suspect is the problem) or the wet clutch isn't releasing...and that can be because the shoes are not retracting because of a spring issue or in the bind with the bell because of a shaft bearing problem...either one.. or both. There is also the possibility that there was an assembly problem where the wet clutch shoes and bell and in contact with each other other then an alignment problem but one caused because something wasn't assembled correctly like..the clutch wasn't installed correctly on the crankshaft or a bearing wasn't seated fully..a gasket wasn't used on the cover..something that caused then to come too much together and make internal contact. Some kind of marks will show this problem as well as a bearing alignment issue.

If it is the oneway being in backwards, it may now be damaged and may have also damaged the area it contacts on the bell shaft from digging-in under load. Some have been known to break all together.

Just FYI, if it is in and working correctly, with the belt off or in neutral and engine NOT running, the primary will spin counter-clockwise and lock if you try and spin it clockwise. If in backward and there are no other issues..including a damaged oneway, it will be the reverse of that.

Either the oneway is in backwards (which I suspect is the problem) or the wet clutch isn't releasing...and that can be because the shoes are not retracting because of a spring issue or in the bind with the bell because of a shaft bearing problem...either one.. or both. There is also the possibility that there was an assembly problem where the wet clutch shoes and bell and in contact with each other other then an alignment problem but one caused because something wasn't assembled correctly like..the clutch wasn't installed correctly on the crankshaft or a bearing wasn't seated fully..a gasket wasn't used on the cover..something that caused then to come too much together and make internal contact. Some kind of marks will show this problem as well as a bearing alignment issue.

If it is the oneway being in backwards, it may now be damaged and may have also damaged the area it contacts on the bell shaft from digging-in under load. Some have been known to break all together.

Just FYI, if it is in and working correctly, with the belt off or in neutral and engine NOT running, the primary will spin counter-clockwise and lock if you try and spin it clockwise. If in backward and there are no other issues..including a damaged oneway, it will be the reverse of that.

Thank you for the reply.
I mirrored what I took out of the new engine with the replacement parts. If the previous owner assembled it wrong, then I simply repeated his mistake....

I'll let you know what I find!

Thanks again.

I think NMK nailed it. Try this test: on a good hard level surface leave the motor shut off and put it in high range brakes off. Try to push the vehicle forward, then try to push it backward. If you can push it forward but you CAN'T push it backward then this suggests that the one way clutch is installed backward. Try it in reverse to confirm.  If in reverse you can push it backward but cannot push it forward then as above, sounds like a backward installed one way (Sprague) clutch.

This is all predicated on the machine being an 800EX or Trail or other 800 with the constant tension CVT.

(also, if the doner ATV had a constant tension CVT and the receiving ATV has a belt slammer or the reverse, that would complicate things too.)