2011 YZ450f engine stumble.. mid-up


We are working on a 2011 yz450f with 90 hours on it.

The valves were checked about 15 hours ago.

The air filter is fresh.

The oil and filter have been changed.

We just replaced the clutch basket and hub.

After all the maintenance listed above the bike ran fine for 10 minutes.

Now it starts and idles fine, but stumbles badly when it hits the mid to upper rpm's.

It never quite hits the rev limiter.. though it feels like it.

It sneezes and will through a small fireball on deceleration occasionally.

Last night we pulled the spark plug and found it was pretty black.. almost like the choke was left on.

When the choke is pulled it does change.. so it doesn't appear to be in the choke circuit.

We replaced it and the same stumble occurs.

We also connected the tuner and didn't see any failures.

The same fuel can was used.. that was also used a few days earlier.. with the same fuel in it.

Any help or direction would be greatly appreciated.


The choke does not enrich the mixture on the EFI YZ's, it opens an air bleed like the hot start on a carb. Enrichment is by the CDI using temperature inputs.

A black, sooty plug is more often an indication of faulty ignition than a rich mixture.

Hello all.. again...

It has been suggested that the fuel injector may be having a problem.

Is there a way to remove it and cycle it to view the spray pattern.. or any spray at all?

We do have a second identical bike if we need to swap parts.. but I'm not a big fan of 'breaking' two bikes to save one..lol

We will be swapping the ignition coil tonight and plug cap assembly.

Thanks again.

You should be able to test it outside the bike. Leave the fuel line and electricals connected and remove the injector form the TB. Crank it with either no throttle or the throttle very slightly opened. Too much throttle will kill the squirt by putting the ECU in the "clear flood" mode. Be keenly aware that the gasoline spray from the injector will be EXTREMELY flammable. Be careful where you do this, and where it's pointed.

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