[Mytmerc83]

I picked up a Aeroworks pro x 260 with said YS Fz70s four stroke on the nose. This YS is a beast... it throws this profile around like a chew toy. I do have an issue on start up however. I prime the motor and start it with no issues, it's once I start to raise the throttle to clear it out and set the high side. It seem to start to knock and most times once it does this it stops dead and loosens the prop. It also feels (when it does the knocking) to be out of balance? But if I feather and play with the throttle I can get it to rev up and clean/clear up and the rest of the flight its spot on. An thoughts?

[Cougar429]

For some reason you are leaning out on accel, causing it to slam to a stop.

Note that YS is using normal fuel draw at idle, (misture set with the regulator screw and should start with it even with the end of the housing) and transitions to pressure feed once above a certain RPM, (can't remember the specifics). It sounds like it may be happening at when it switches over so you may have a stuck regulator or even a stiffened up diaphragm.

Are you having any trouble setting the HS mixture?

[Mytmerc83]

Hey Gary, idle is perfect, high sets no trouble. The FZ70s is the new version that does not have a screw to adjust the regulator as far as I've seen and researched. It does smarten up but on the next start up it's the same till I get it to wot again then it's fine the complete flight. It makes a ton on power and runs good otherwise.

[floyd]

I would clean the complete engines fuel system first before trying to trouble shoot. The pump has a small diaphragm that can be contaminated very easily. Even running Castor oil can contaminate the pump unit, as it gums up in storage. When I ran my YS engines I changed the diaphragm every spring before I flew. The diaphragm only fits one way so be careful if you remove it. Cheers Floyd.

[darkith]

Maybe a bad check valve?

[Mytmerc83]

Went through everything and cleaned. Nothing stuck out diaphragm seem fine everything looked pretty good. The check valve is good, flew the plane then let it sit for about an hour as I flew other birds, called it a day and when I removed the line to release the pressure to drain the tank there was lots of pressure still there.

[Cougar429]

What fuel are you running?

[Mytmerc83]

I'm running 20% nitro 20% oil as YS recommends.

[Yser]

I would replace the check valve like the other poster said. These engines rely on fuel pressure to the fuel regulator from the tank. The tank is pressurized from the pumping action of the piston. So also make sure you are not losing pressure to the fuel tank. Check all the fuel line connections and fuel line for leaks. This sounds definitely like a lean problem.

[Cougar429]

Rene, that's true to a point, but as I posted originally YS allows for natural fuel draw to the carb from idle up to a certain RPM. This is between the regulator and the carb so is independent of the pressurization system. It acts basically like a Cline or Iron Bay external regulator below that threshold.

Once above then it switches to pressure feed to the carburetor, which is where a lack of pressurization would immediately begin to show up. This behavior does not sound like what the OP is experiencing and he states the system holds pressure for a considerable time after shut down.

I would be interested if Ryan could post if the problem was identified and solved. That is the original purpose of this type of thread anyway.