My FP.40 powered Twister kept getting shorter fuel runs on a full tank. Finally my pit man called me back from walking to the handle to show me fuel dancing around the base of the venturi and spilling off the side. I took it home and removed the venturi and Supertigre NVA, checked for cracks in the case and venturi, cleaned it all with acetone and brake parts cleaner, and replaced the NVA with a brand new one out of the Supertigre package.

I sealed the venturi base and behind the hex nuts with RTV. I also replaced all the fuel lines and the filter. I pressure checked the tank with it submerged under water and it's tight as can be with no bubbles.

When I test ran it at the field Sunday, I could see it getting wet around the top of the collet as if fuel was creeping past the threads and coming out the top. But not nearly as bad as I'd seen the previous week before replacing it.

I went ahead and set the engine to run like I want to launch it. After shutting it off, I tightened the collet enough that I could not turn the needle. Cranked it for a launch and it all looked dry, and I flew a full pattern with 12 laps after the clover. A perfect run, even a good 2-4 break.

But this is a problem. If I'm about to fly an official flight and need to tweak the needle, I'd like to have that option without worrying about it leaking. What  I do not understand is how it's pushing fuel past the threads out the top while running on straight vacuum. No muffler pressure or bladder pressure, just a forward pointing open-air uniflow vent on a metal tank.

Any ideas before I rely on this plane in Huntersville this coming weekend?
Thanks,
Rusty

HI  Most likely   it is  not leaking there,  the venturi needle vale  has suction in that area, no positive pressure, the odds are that you have fuel from the front bushing being pushed out and it goes directly to the venturi/NVA  and collects there, the only other place it cone from is from the top of the venturi, most happens when you are running very rich.  I would look very close at the  drive washer  to see this, and it is hard to see  as the fuel will move backwards very quickly

Randy

Randy is right, there should at most be negligible positive pressure there and more likely, negative.

  While I can't immediately explain what you are seeing, ST needle/spraybars are probably the least good of the commonly-used types. they are also different from copy-to-copy and the fit of a particular needle in a particular spraybar matters sometimes (although usually from a vibration standpoint rather than leakage). If it was me I would replace it with whatever PA or RO-Jett assembly fits it. The original OS system is also just fine but you have already drilled out the hole and have a proper venturi for the larger spraybars.

    Brett

If I remember correctly, these spraybars are the same diameter as the OS one I measured in my drawer.

No they're not. They're 5/32 and 9/64.

I strongly recommend you get hold of one of Randys needle valve assemblies.

They have a built in seal and work superbly, Randy also sells packing sealing washers so that you can get them to fit right. I wish I had "discovered" them earlier in my life.

If Anyone hasnt said so , on the S T NVA ;

the ' tightening down immovable ' on a NEW ONE will likely seat things ,
so after with it set ' firm ' , it'll be fine .

Though they do like to be set fairly firm , or just backed off 1/8 turn on the collet from frooze .

if theres naff all resistance on turning the needle , likely as not theyre not fully sealed .