Anything less than 11 m.m. does that on mine .

There's no more flow from the muffler to the tank than the volume of the fuel being burned off...relatively little...so little heat. If that was the cause of the bubbles, then all systems with muffler pressure should have the same problem. They do not.  I would suggest that there's a leak someplace in the hoses, filter, or tank that gets worse with muffler pressure. You are hooking the muffler pressure hose to the uniflow vent, right?

For the record, I didn't want to like muffler pressure, until I tried it.    Steve

Within reason, the size of the pressure tap from the muffler will make no difference, because there is such a small amount of flow. A 6-32 or 8-32 tap is fine. There's also no accumulation of oil/gorp in your tank, OBTW.

With a uniflow tank, you can leave the overflow or fill tube open and then have a "conventional" or "old-style" tank, like folks used in the '50's. I don't see an advantage to have the engine speeding up through the run! Less trouble with timing the handle wigglings.

The nice thing about adding muffler pressure is that there is no longer a mixture change as the model heads up/downwind. You can also fix that by pointing your uniflow straight up or down,  or tuck it away inside a shielded area, so there is no air pressure change. Still, some engines just seem to like muffler pressure, but not all. Nobody seems to know exactly why that is.  Steve
 

So many possibilties, Profile or full fuse, where on the muffler is the pressure tap, one piece fuel filter or t piece?

An arf profile.  Hangar 9 PT 19.  The motor is an evo 36 with the small venturi sleeved with 7/32 od aluminum tubing and 2 head gaskets an OS RC needle valve screwed into the venturi with form a gasket.  I am getting a good seal at the venturi.  I run the green 10% cool power.  The motor is very smooth and seems to get smoother as I put more time on it.  Vibration could be a problem but I am experiencing this problem only on pressure.  The muffller is the stock tube muffler. so the pressure tap is about 2/3 away for the exhaust manifold to the exhaust outlet.

The plane flys pretty decent.  I had to put a lot of lead in the backplate to get to balance per specs.  I would say the nose isn't as robust as it should be, but I am wondereing why I have this problem on pressure only.  This happens with all the tanks.  My next thing is to put a sintered bronze filter in the tank.

Now I think that it is a vibration problem.  Ther sintered bronze filter helped but I think the deal is that I was fooled.  Muffler pressure lowered the RPM slightly causing more vibration.  Like 200 rpm which makes a big difference in the vibration.  10,000 RPM no bubbles, 9800 bubbles and getting worse as the RPM drops.  Maybe it gets better after it goes below a certain point but it I haven't tried that.  Maybe a 10/6 would work better and lower RPM?

The motor is very smooth though.  That nose is like built up out of some strange wood.  I think it would explode with a Fox 35.

Next plan is a metal tank with Brodak adjustable tank brackets.  I'm going to put some small faucet washers or O rings between the fuse and the brackets.  Sort of a trick vibration isolation system.  Sort of fun.  Probably been done before.  I drilled some holes in the fuse and found out it was hollow.

If this idea works, you will hear about it.  If it blows up in my face you won't.  Wish me luck!

I had the same problem with my RingMaster. No metal tank would work, not matter how creative I was using rubber and foam to isolate vibration. Armor All fixed the problem.

Later I fixed the problem with a Sullivan RST tank with a piece of foam in between the tank and the fuselage and secured wrapping several layers of Scotch Clear tape. No more need to add Armor All


Martin