It has a misfire while running. The engine has low hours starts easily and runs well. No dirt in the carburetor, valve clearances are correct and I have replaced the spark plug.

Possibly water in carburetor fuel bowl. Drain and clean bowl.

If your Husqvarna backpack blower starts well but starts to act like it's not getting enough gas when you put it at 3/4 speed, there could be a few potential issues causing this problem. Some possible causes include:

1. A clogged fuel filter: The fuel filter is responsible for removing dirt and debris from the fuel before it reaches the carburetor. If the filter is clogged, it can restrict fuel flow and cause the engine to run poorly.
2. A clogged carburetor: The carburetor is responsible for mixing fuel and air to create the combustion that powers the engine. If the carburetor is clogged, it can restrict fuel flow and cause the engine to run poorly.
3. A dirty air filter: The air filter is responsible for removing dirt and debris from the air before it reaches the carburetor. If the filter is dirty, it can restrict airflow and cause the engine to run poorly.
4. A problem with the fuel line: If the fuel line is kinked, pinched, or has a leak, it can restrict fuel flow and cause the engine to run poorly.
5. A problem with the spark plug: A faulty spark plug can prevent the engine from running properly, it can cause misfire and the engine to run poorly.

It's recommended to check the fuel filter, carburetor, air filter, fuel line, and spark plug. Clean or replace them as necessary. It's important to consult the operator's manual for specific troubleshooting instructions and diagrams for your blower model, as the location and procedure may vary depending on the make and model. It's also important to consult the operator's manual for safety instructions, and to make sure you have the correct parts for your blower.
It's recommended to seek the help of a qualified technician if you are not familiar with this process.

a misfire can be mechanical as well as electrical
code p 0300 is random/ multiple misfires detected
low fuel pressure is a probable cause from a problem fuel pressure regulator or problem EVAC system not venting properly causing a vacuum in the tank and starving the pump
erratic could be from valve problems on heating
loose valve seat insert/s, incorrect valve clearance from faulty hydraulic followers, cam shaft lobe failures are all part of the mechanical reason for random /multiple misfires

Clean nicely carburetor make good adjustment pull the choke and without a gas engine must be lit

Possible causes are:
Insufficient valve tappet clearance. Re-check the tappet clearance on each valve. You can confirm this by checking the compression. Proper compression for your engine is 175~190 psi. Much lower than that indicates insufficient tappet clearance - or internal engine wear.
Fuel mixture too lean. This can easily be caused by clogged carburetor jets, from sitting. It only takes a few months on non-use for the carb to become plugged with fuel varnish. Also make certain there are no air leaks between the carburetof and engine. If you are running an aftermarket air filter, rejetting to correct a lean mixture is required.
Whatever you do - do not operate the engine in this condition or you risk damage to the cylinder head and valves.

If your XR600 has ever sat for more than just a few months, there's a very good chance that the carburetor is plugged with fuel varnish. This means that the carb will have to be disassembled and cleaned.
A big four cycle engine like your's may "feel" like it has good compression, but it may not have what it takes to start and run. Check the compression. With the throttle wide open and a few good kicks of the starter, you've got to see at least 140 psi of compression. Anything below that and it isn't going to start or run reliably. If the compression is low, the first thing to do is re-check the valve tappet clearances. Insufficient tappet clearance will cause low compression. Also make sure the kickstart-actuated compression release is not hanging up and causing compression loss after the kick start lever is returned to its parked position. If the valves are properly adjusted and the compression is still low, then internal engine repairs are needed.

It won't be valve timing or clearance. The fuel/air mix would be the culprit. Clean the carburetor with special attention to the idle circuit. Remount the carb with new manifold gaskets. A manifold air leak will mess up the mixture. Also, get a new spark plug one heat range colder than stock. Hope this helps out!

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