Vibration and knocking sound when warmed up

EternalArianne

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So I found myself on the receiving end of a FREE ECHO PB-403T backpack blower from 2005 that would run but it was very rough and hesitating. I did a full tune up (air filter, spark plug, fuel filter) but issues persisted.

I managed to get it running properly after replacing the carburetor with an OEM ZAMA one. It fires up in 2 pulls now and blows great.

But something I've noticed is that once it's warmed up, it makes a slight knocking noise and there is a slight bit of vibration - enough to make my right hand that controls the trigger tingly after use.

When you first crank it up and it's just idling, it sounds like a typical 2-cycle engine. But once it gets warmed up and you've been blowing for like 10 or so minutes, that's when the knocking sound and vibration starts. It's difficult to describe.

You'd think if the blower fan was loose or damaged, it would make noise all the time, correct? And if the engine itself was knocking, it would fail pretty quickly? I'm not sure where to start troubleshooting on this. It runs and blows just fine, it's just more of a concern at this point...
 

EternalArianne

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So you suspect it's an issue with the fuel? I'm using a 50:1 blend that my neighbor uses in all his equipment, but I'll admit he uses the cheapest stuff possible...
 
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