I have an inverted mounted ST v60 with MACS Flow-Thru muffler running a Xoar 13x6 prop. I'm using a 6 oz. metal tank with uniflow muffler pressure. The problem is that I have to launch the plane with the needle valve set extremely rich, almost to the point of dying. Once the plane gets airborne it leans out, too lean, but it's not over-lean. It will slightly "crack" to a 4-cycle for a couple of strokes (maybe an 1/8 of a lap). It remains that way for the entire flight. The only good thing is that it's very consistent. This same engine and setup was used in another, very similar plane, and worked fine.

I've checked the tank for leaks - none. Checked for something restricting the pick-up line - nothing. I did have to adjust the tank height, lowering it, as it went slightly rich when inverted. The engine now runs the same both upright and inverted. This adjustment also made some improvement on the needle setting. My previous flying session I thought I had it resolved as the engine would run a nice 2-4-2 break. But today, with no adjustments, it went back to the same issue I had previously. I'm not sure what to try or where to look. Any suggestions?

Dennis

I finally got it to work! 

I had to switch the location of the supply line and the vent line location exiting the front of the tank. The uniflow line is now attached just below the supply pickup. It now works beautifully.

But I'm not sure why it's working. Can someone explain why the my first setup failed and the revised one works?

Thanks,
Dennis

Dennis,
The drawing is great and gives a lot of information. Since you re-plumbed either the connection tubing did have a way of moving and pinching down once in the air or there was a split in the connecting tubing. The ST 60 is a vibrator and even with cross-grain filler between the mounts can cause some significant shaking which can get to the flex tubing. I have found that the blue silicon tubing can develop a very clean shear through the surface at the connection to the tank hard tube. It may not be visible just looking at it but if you pull on the ends you can see the failure point.

Best,   DennisT

 I have used The ST/46 & 60 for years and never found it made any difference where the pickup exited the tank. One thing that does effect the motor run is the location of the uniflow air pickup on the outside of the body. For years I never had any problems with regular stunt models. But when I wen to wide bodies ,Bearcat, I had a problem like yours. The tube on the body was over one inch offset from the tank. When I moved the uniflow inside the body the motor ran normal.
  What model are you flying?
Ed

Dennis      I see you have your needle valve on the outside of the circle so as to clear the muffler. Many people do it but it has caused me problems in the past.   I see why you moved the pickup tube to that side at the front of the tank. If it works that is great.
Ed

If the vent tube is too close to the pick up the air bubbles will miss the pick up but when you get airborne the higher rpm and/or different G forces will let the air bubbles mix with the fuel and lean the mixture.

I recommend you to buy some helium balloons and put them into your car.
Then go for a little drive and see how the balloons behave in turns.
What I try to say, is that bubbles from vent tube (if too close to pickup) usually cause problems when adjusting the needle, not during flight. L

If the bubbles are not being sucked into the fuel pickup line from the uniflow line being to close what are the other things that could cause it to go lean right after take-off?

What comes to mind would be an air leak around the needle valve, air leak around the spraybar to case hole as the engine heats up, small hole in the fuel tube that move after take-off and allows air leakage, backplate gasket air leak. Anything else that could do it?

Best,    DennisT

Dennis,
Oh that plug thing!!! Glad you got it to work. I think this will help a lot of fliers. Plugs are funny things, they seem fine then puff, there not, but still start the engine just fine. It seems that for almost any thing that changes kinda sudden we need to start by changing the plug to one of the know good brands. Thanks for sharing the info.

Best,    DennisT