This post will cover the many potential reasons for why your ATV won't go into gear.
You will need to locate your ATV's service manual for machine specific and detailed troubleshooting information. You can find this online if you don't have one.
Working on your transmission is tricky and if you do something wrong, you don't just have a still not working ATV, you could have a more messed up ATV. So, if you aren't confident you can do the repair, you might be safer taking it to a dealer to have it worked on.
You may also notice that your ATV creeps forward when it's in gear and idling.
When you can shift gears normally while the engine is off, which means you only have an issue while the engine is running, your shifting mechanism and the inside of the gearbox should be fine.
The drive belt should move freely through the clutch assembly. If it rubs against any of the sheaves of the primary clutch, you'll have tension on the belt that interferes with shifting.
You can adjust the alignment between the clutches by adding or removing the shims between the sheaves, particularly the ones on the secondary clutch.
Most ATVs should idle around the range of 1100-1200 RPM. If your ATV idles higher than it’s supposed to, the clutch will always be slightly engaged and create tension that makes it difficult to shift gears.
You can probably fix this fairly easily on a carbureted ATV. All you have to do is work an adjustment screw until it matches the settings in your manual.
If you have a fuel-injected ATV, you'll probably have to take it to a mechanic.
To determine what's going on in there, you'll have to remove the belt cover and watch the clutches while the engine is running at idle, and during revving while the ATV is in neutral. You'll need to be very careful if you do this because the clutches spin at high speeds that can be extremely dangerous if your hands or any foreign objects get caught in them. Basically, keep yourself and any objects well clear of the clutches when you're checking this.
You may find that the clutches are dirty. If so, you can turn off the ATV, pull the clutches, and clean and service them.
The clutch spring may be weak. This can just happen over time. You can replace them, but you'll need to have specialized tools and you'll have to disassemble the clutch. This job should probably go to a mechanic.
If the motor mount isn't functioning properly, it will put the clutch out of alignment. You can tell if this is happening by taking a pry bar and trying to lift the engine. It should barely move, if it moves at all. If you can lift it, you need to fix or replace your motor mount.
The belt has to be the correct size to interact effectively with the clutch sheaves. Aftermarket belts could be too big or small from the start. Belts can also be worn so thin they don't fit.
Be very careful purchasing aftermarket belts or just stick to OEM belts. You should also check your belt regularly to make sure it isn't damaged or worn.
The belt may be shredded and wrapped on the drive shaft. Remove the belt cover, remove the belt, and replace.
The washers in the engine brake system can be worn and interfere with the movement of the belt. You'll see these on the primary clutch. If they're worn, replace them.
Your EBS may have a one-way bearing in it. This too can be worn.
If your ATV has adjustable gear linkage, this can be adjusted incorrectly and interfere with shifting. You'll need to put the ATV in neutral, remove the gear linkage, and inspect the bell cranks and ball-ends for damage and wear.
On ATVs with manual clutch transmissions, you should check the handlebar-mounted lever and clutch cable.
The internal gearbox can be bad. It is difficult to check this without damaging the case. You can avoid that by turning off the engine, removing the belt cover and belt, shifting into gear, and then trying to manually rotate the primary clutch. If the ATV doesn't move, you should take it to a mechanic.
Some clutches use oil to function. You will need to make sure that the oil is not low, old, contaminated, or blocked.
The good news is that this isn't really a malfunction. You can keep it from happening from applying the parking brake before you put the transmission in park to keep the gears from binding up and having to unbind to shift.
