I'd first change the fuel line and be sure the filter isn't leaking air.  A pinhole can cause this.  Otherwise it's likely vibration foaming the fuel.  You can check the prop for balance or change props.  You might try rotating the prop 90 degrees and see that it's not compounding the internal mis-balance of the crank.  Most likely he'll just need to stiffen the nose.  I have on one occasion sanded dope mostly off,  glassed the whole fuselage and refinish.  That solved the issue completely on that soft profile.

Dave

 The plane  is a Brodak Hellcat, I don't know how stiff the fuse is on this kit. He doesn't run a filter, we did check the lines and tank for pinholes, didn't see any. He just started balancing props, will try changing its orientation. The engine currently runs rich, very little change  without a major needle adjustment. The end it goes very lean. I didn't mention we noticed airbubbles in the pressure line headed to the tank.

I would say we have our best luck with the .51 running it just like the old .46- pretty rich launching in a solid four stroke.  We liked the APC 11/6 well on it pulling a 69 ounce airplane with little trouble.  I burnt up a couple until I learned ( to my amazement) it doesn't run like a typical Schnerle motor, ie OS FP/LA etc.  I don't remember tach settings now but I would guess 8500-8800 rpm to launch.  I cooked em' on 4 pitch.  Curiously it sounds overloaded on more the 11" diameter unlike the .46 which I preferred to use the Rev Up 11 1/2 - 6 EW.

Dave

I believe the prop is a 12x5, Sig 5% all castor, tongue muffler on pressure. I don't know the brand of tank, 4.5oz all forward tubes. We won't  fly again until midweek, hopefully he tries another tank before then. The needle was turn a good 1/4 turn on the ground -no response. Engine goes WAY lean near the end of the tank.
Does no response from the needle say" venture too big or small?" This was a used engine, somewhat square head, Chinese manufactured? It has a black delrin venture and I would assume a ST nva.
Thanks for all the help.
Steve

 thanks

In my opinion, a major cause of bubbles in fuel tube is the micro vibration generated by the engine. Check out nicely, the balancing of the propeller and spinner. The engine mount must be hard too. Try to fly with mixture less rich  and a warmer glow plug.

I'm going to print this thread on Tuesday so he has a list of possible solutions.
Thanks!!

I had a Brodak uniflow oval tank that had steady a stream of big bubbles going through the fuel line. It turned out the uniflow tube interior opening was too close to the pickup and feeding it air. If you haven't tried it, capping the uni vent(and opening the overflow) would troubleshoot that possibility. But i think it's normal to see a few bubbles from that very thing. That's where the filter helps too.
Rusty

Is this the G.51 (as I think many assumed)? ST did make some earlier .51's, including a sleeve bearing one as well as the ball bearing little brother (sister?) to the famous ST .60bb.  

Good choice on the Hayes tank! Might not be the ultimate tank, but they're simple and reliable. My G.51 needed under 6 oz of 10% with a .285" venturi. I used mostly Powermaster 10-22 GMA.

An APC 12-5 is a good baseline prop. The Thunder Tiger 12.5 x 5 and RSM 12 x 5.5 would be worth trying. I also used the 12.25 x 3.75 APC, and it worked fine. I don't recall what the launch rpms were for each prop.  Steve