I have an SK90 on a Big Stik 60. The engine runs great except in the following scenario:

I go full throttle into a vertical climb. When in the vertical position, if I throttle back from full throttle in an attempt to hover, the engine will stall.

I know it's not the plumbing in the tank. I checked it. Also, the engine runs fine when wide open throttle during a vertical climb. I also did some trouble shooting on the ground. I went to full throttle and held the plane in a vertical position. Engine runs great. But then if I throttle back from wide open, it dies.

The high speed is already set slightly rich and there's a nice smoke trail from the exhaust. I tried to fatten it up even more but then the engine would burble and not run smoothly at full throttle. I don't think it's the low speed needle either because I can idle for several seconds and then transition to full throttle without any problem.

The interesting thing is this only happens when the plane is in a full vertical position. In any other scenario, I can transition from one throttle position to another without missing a beat.

Any thoughts? One positive aspect is i've become very good at dead stick landings. I can land it on the runway on a dime.

It seems to me that the issue is that, because you can run at full throttle in the vertical and the engine dies when you throttle back that the tank pressure is causing a rich cut when you throttle down. I think you maybe have too rich an idle needle setting or the air-bleed for idle needs opening a bit. Are you using a non-return valve on the pressure feed?

Personally, I'm a big fan of Perry pumps and, although they tell you not to, I use a return to the tank from the needle to avoid fuel over pressure when throttling back from high speed.

I could try to lean out the low speed needle. I'm not sure where the air-bleed for the low speed needle is. The only adjustment I can find for low speed is the screw inside the throttle arm.

When you say Non-return valve, can you elaborate? The engine is a basic setup. Tank has two lines, one going to carb, one going to exhaust.

??? i do not understand , can you show a pic ?

One of my SK .91s is doing that too but in my case the fuel tank is too far back from the engine and it isn't drawing enough fuel on the low side. It's not an air bleed carb so you aren't going to find that hole mentioned. If you fatten up the low end a little it may help but it sounds like a draw/pressure problem? Check and make sure you don't have any air bubbles in your fuel line, if you do it could be a hole in a line or a loose bung. It's just something simple that you haven't located yet. It could even be the glow plug cooling down. The temps are starting to drop now and I'm starting to have to fatten up my engines already. My engines are tuned for 100+ degrees and it has been in the 40s at the field in the morning, that's a pretty big drop so I'm opening up the needles a click at a time until the weather gets stable again.

I've checked the fuel line and there's no bubbles at all and I just replaced the glow plug with a "hot" A3 plug. The fuel line has a clean stream of fuel going to the carb. It's really strange that it only does this while in the vertical climbing position.

This can happen because of a LEAN condition.

The engine is leaned out because of the nose up attitude, then when you pull back the idle speed setting of the needle does not supply enough fuel to keep the engine running.

I would start by richening the idle four clicks and doing a nose up test AT IDLE.

Then richen the HS needle one or two clicks and with the nose up, run the throttle up and down, to see if you've cured the problem.

And just for grins try a new OS #8 and if you can then even a type F.

Opjose - thanks. I'll try that. Stupid question here: the idle needle is a flat head screw within the throttle arm connection. What would you say is the equivalent turn of the screw to equal a "click"?

So for example, if i richened the idle by four "clicks", would that be a 1/4 turn on the idle screw?

I tried a #8 initially when breaking in the engine and couldn't get it to idle reliably. So I've been using an A3 since then.

It doesn't click, you just turn it out very little, the SKs low end is very touchy on mine. If you go to tower and read up on glow plugs you will notice the A3 is a hot plug for .10 to .60 engines, the #8 is used for most the rest of the two strokes. Reason I said try an F is because they retain the heat longer and don't cool down as fast. Just turn out the low end enough to see it move, a 1/16th is a big move on these engines, just a little at a time!! I have a very long screw driver and do it with the engine running so I can hear it, sometimes I use a tach so I can see the rpms.

Start with the lsn about 1/8th turn and go from there. I agree with opjose it sounds like a lean condition.

I'm amazed by the number of flyers who don't set the high speed needle with 1/2 a tank and with the nose of the model straight up. Once the engine is in a fairly clean scream for 1/2 a minute, give the line a quick pinch. The rpms shouldn't drop, they should not go up either, just remain the same. From here you have a solid basis to work on the low end mixture, but the low end adjustments will make you revisit the high speed adjustment as a double check.

Well, the engine might not have an air-bleed screw. Some do, some don't.
It sounds like you don't have a non-return valve, You'd know if you'd fitted one!

Done

DenverJayhawk

He has a pump installed, you do not.

You should not do what he did unless you install a pump.