Won't go into reverse when running

[sportster64]

2008 nitro, try putting it into reverse while its idling, won't go, shut off engine, then it slides into reverse with no problem, same when trying to shift from reverse to forward.
But, if I take the weight off the track by raising the rear of the track off the ground, it will then shift into reverse while engine is running, I did put a new drive belt on, I'm thinking that is the cause, but not sure why..
Also changed. Chaincase oil, but used same Mobil 1 that was in it, 7 oz, .
Very strange..

 

[SnoNinja]

Check your belt deflection. If belt is riding high in the secondary clutch, it's loading the jack shaft and preventing the sled from shifting. I have the same problem with my phazer. I have to pull the side panel off with the sled running, and roll the secondary back a touch and it will drop right into gear. My deflection is too tight, but I already pulled the washers from the back. I need to put longer screws in to open the secondary a bit so that the belt sits a bit lower.

 

[Dr. FeeLGooD]

Yes those bolts can wear out.

 

[Grimm]

Check your chain tension. Tighten the tensioner bolt finger tight and back off a 1/4 turn.

Another thing to check is the reverse linkage. The bolts can loosen over time and there is an adjustment screw if the reverse is acting finicky.