Tomos vacuum/air leak

What is an vacuum/air leak?

Having an air leak means that unwanted extra air enters the engine, not through the air filter but trough leaking gaskets or openings. Such a leak makes the air/fuel ratio too lean. In practice, you often recognize air leak because the idle speed is unnaturally high, it no longer drops back to idle, the engine needs larger jets to run, it's hard to start, or your Tomos won’t run at all.

Consequences of air leak: This makes the mixture too lean (too much air, too little fuel). With a lean mixture your engine doesn’t cool or lubricate sufficiently. Your moped may then stutter, stall, overheat or seize. It is therefore important to address this as soon as possible to prevent damage.

Testing for air leaks

This method is a super easy way to see if you’re drawing air leak. You simply use a flammable spray (for example brake cleaner) to spray along all the gaskets and connections of your cylinder and intake.

How does it work?

The spray vapor is flammable. If there’s a leak somewhere, that vapor is sucked in and alters the mixture in the engine. You immediately notice a brief change in RPM: the engine may drop or suddenly surge, idle briefly, or stall. As soon as the engine runs differently when you spray at a particular connection, you know that’s where the air leak is.

Tools and materials needed

• A flammable spray like brake cleaner
• Basic tools
• Possibly new gaskets

Spray test step‑by‑step

1. Let it idle: Start your Tomos and let it idle. (It may refuse to start, in that case, spraying the gaskets can actually help it start)
2. Spray along all connections: Briefly spray the brake cleaner anywhere two parts connect like between the intake manifold and the cylinder or between the . Be careful not to spray toward your air filter.
3. Watch for RPM changes: If the engine speeds up briefly or drops in RPM when you spray, that’s where the leak is.
4. Replace leaking gasket: Replace the leaking gasket with a new one. You can apply a thin layer of liquid gasket over your gasket for a better seal. Make sure to let it dry for 24 hours before starting your Tomos.
5. Tune the carburetor again: After fixing the air leak, you'll need to readjust your carburator. In this video and blog we explain exactly how.
6. Final check: Perform the spray test again to ensure you've sealed the air leak.