Razor MX650 Chain Tensioner: Fix, Replace, or Upgrade?
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The stock Razor MX650 chain tensioner is fine, right up until it isn't. When it starts giving you problems, most riders aren't sure whether to adjust it, replace it, upgrade to an aftermarket option, or just skip the whole chain thing and convert to belt drive.
Here's how to figure out which one makes sense for your situation.
Key takeaways
- Most tensioner problems come down to the spring losing tension, the mount shifting, or the chain being worn past what the tensioner can compensate for.
- Adjusting first is always worth trying before spending money on replacements.
- An aftermarket rigid tensioner solves the flex and alignment issues the stock setup has.
- If you're going to do a belt drive conversion anyway, skip the tensioner replacement and go straight to the kit.
How the stock tensioner works and why it fails
The stock Razor chain tensioner is a spring-loaded arm that presses against the chain from the inside, keeping it under tension as the chain stretches and the motor mount flexes. It's a simple design that works well enough on a stock bike ridden gently.
The problems start to compound when you ride harder, mod the bike, or just accumulate enough hours that the tensioner spring weakens and the chain stretches beyond what the spring can pull out. The mount itself can also shift or bend over time, especially on rough terrain, and once the tensioner isn't pushing at the right angle the chain can slip off the sprocket.
Signs your tensioner needs attention
- The chain keeps jumping off, especially under hard acceleration or on rough ground
- You can see visible slack in the chain even with the tensioner fully extended
- The tensioner arm is bent or obviously out of position
- The chain is making more noise than usual, a scraping or slapping sound
- The tensioner spring has visibly weakened or broken
If you're seeing any of these, the tensioner needs attention. The question is what kind.
Option 1: Adjust or repair the stock tensioner
Before replacing anything, try adjusting. If the chain has stretch but the tensioner spring is still healthy, you may have adjustment range left. Check the tensioner adjustment bolt and back the wheel position out slightly to take up slack.
If the tensioner arm has just shifted position, reset it by hand and check that the mounting hardware is tight. Sometimes a tensioner that seems broken is just loose on its mount.
If the spring itself is visibly weak or broken, there's no adjusting your way out of it. Move to option 2.
Best for: New problems on a well-maintained bike where the underlying cause is just looseness or minor adjustment drift.
Option 2: Replace with OEM
Razor replacement tensioners are available through their parts program and through third-party parts retailers. If your stock tensioner is bent, broken, or the spring is shot, a direct OEM replacement gets you back to stock performance quickly.
The replacement is straightforward: remove the old unit, bolt on the new one, adjust chain tension. It's a 15 to 20 minute job.
Best for: Stock or near-stock bikes where the chain system is otherwise in good shape and you just need the tensioner functional again.
Limitation: You'll have the same spring-loaded design with the same flex and alignment limitations. If you've been fighting tensioner issues repeatedly, OEM replacement probably just buys you another season before the same problem comes back.
Option 3: Upgrade to a rigid aftermarket tensioner
This is where things get meaningfully better. A rigid adjustable motor tensioner replaces the spring-loaded stock setup with a solid mount that lets you set belt or chain tension precisely and holds it there without spring fatigue or flex.
The MxMods Motor Tensioner is cut from 3.2mm cold-rolled steel, mounts to the stock motor bracket without modification, and includes a precision-machined spacer and M6 mounting bolt. Once you set tension with a solid mount like this, it stays where you put it.
This is also the tensioner you'll need if you're running a belt drive kit, since the stock tensioner isn't compatible with belt drive systems.
Best for: Riders who want to stop fighting the tensioner for good, anyone running an MY1020 motor swap, and anyone who is considering belt drive conversion now or in the future.
Which option makes sense for your situation
| Your situation | Best option |
|---|---|
| Tensioner just needs adjustment, chain is in good shape | Adjust first (free) |
| Spring broken, rest of drivetrain is fine | OEM replacement |
| Repeated tensioner issues, stock motor | Rigid aftermarket tensioner |
| Running an MY1020 or planning to upgrade soon | Rigid aftermarket tensioner |
| Planning a belt drive conversion | Skip to the belt drive kit (tensioner is included in the plan) |
| Tired of ongoing chain maintenance | Belt drive conversion |
FAQ
Can I run the MxMods motor tensioner with my stock chain?
Yes. The motor tensioner works with both chain and belt drive setups. It mounts to the stock motor bracket and gives you a solid, adjustable tension point regardless of what's running on the drivetrain.
How do I know if my tensioner spring is worn out?
Push the tensioner arm by hand with the chain off. A healthy spring should push back firmly and return the arm smoothly to its resting position. If it feels weak, sloppy, or the arm just falls back without resistance, the spring is done.
The chain tensioner looks fine but my chain keeps falling off. What's actually wrong?
Check the motor mount bolts. A motor that has shifted even slightly in its bracket changes the chain alignment enough to cause derailment regardless of tensioner condition. Tighten all the motor mount bolts and see if that resolves it before replacing the tensioner.
Does the MxMods motor tensioner work on the RSF650?
Yes. The motor tensioner is compatible with the MX650, MX500, SX500, and RSF650.
If you're leaning toward a full drivetrain upgrade, read our complete belt drive conversion guide to see what the full setup looks like.