My LA-46 "surges" (speeds up) after coming out of a manuever or at inappropriate times during maneuvers.  The surge doesn't last long and seems unassociated with its situation (position in the manuever.)  The motor ran very steady and consistently when mounted on a profile, but now sitting inverted in a full-bodied stunt-ship is acting up.  Similar uni-flow tank and plumbing in both situations.  Didn't surge then, but does now.  What might be causing this?

Whatever it turns out to be PLEASE make sure it makes it here.  I am designing my PA ship for an LA46 now and I am strongly considering keeping the engine side mounted.  Although the effects are probably small, gravity and centrifugal force are different between a side mount and an inverted.  I have also dropped uniflow on clunk tanks and I can run a fast 4 with no breaks in normal conditions and a 4-2 in wind using muffler pressure only.  Your random problem sounds a bit like air bubbles or a soon to be clogged filter and don't exclude fuel.  Bad fuel can be used to kill fire ants.

Ken

Just a random thought - is your tank removable?  Could you try it with the tank from the profile?  Personally, I have all but given up on metal tanks.  The last two I bought were both bad and the last one I made was just too much work.  My soldering skills aren't what they used to be.  About one of the bad ones purchase recently from a well know source that I will not name - whomever built it totally roasted the flux on the inside of the tank and didn't bother to flush it.  I was having problems similar to yours and eventually it would just quit without warning.  What I suspect is that I was getting very small flakes of the corrosion breaking loose at first causing erratic runs followed by larger flakes as more fuel went through it which stopped the engine (no room for a filter, even one of those skinny ones).  The other one had the uniflow soldered to the bottom of the tank about an inch from the pickup.  It's plastic from here on.

So, here is what I did: built a new tank and replaced the old one, changed engine, changed fuel (same mix, different jug), changed prop (went from a 12-4 to a 12-4 cut-down to 11 5/8 - 4), changed venturi.  All with no solution.

Replaced the OS needle valve assembly with an ST-46 assembly (to account for the larger diameter of the ST, I went from a .297 to a .305 venturi.)  Ta-da!  Solution.

What is interesting is that the pattern (speed) I want is at about 9200 RPM.  For some reason, the engine doesn't like being there.  With the OS needle valve assembly, adjusting the engine speed for that target was really difficult.  The ST is much easier.

So, here is what I did: built a new tank and replaced the old one, changed engine, changed fuel (same mix, different jug), changed prop (went from a 12-4 to a 12-4 cut-down to 11 5/8 - 4), changed venturi.  All with no solution.

Replaced the OS needle valve assembly with an ST-46 assembly (to account for the larger diameter of the ST, I went from a .297 to a .305 venturi.)  Ta-da!  Solution.

What is interesting is that the pattern (speed) I want is at about 9200 RPM.  For some reason, the engine doesn't like being there.  With the OS needle valve assembly, adjusting the engine speed for that target was really difficult.  The ST is much easier.

Great news!  I already have the ST NVA on mine!  Don't know what venturi size is but it is larger than a .297.  Maybe I won't have the problem too.

Ken

Scott,
The change to the ST NVA was a good move. Some of the OS NVA's I have would leak air on the spring end, air leaks around any part of the NVA will cause crazy engine runs. To get to the flight speed you want seems like you need to adjust the prop pitch to where the engine gives the most reliable run. I would go to the 9400 and then add line length to get to the lap time you like (probably only adding a foot or so in length). Other option is to reduce pitch by like 0.1 at station 8 on the pitch gauge or try a different brand prop with the same pitch (some brands pitch the tips lower and will do what you need). What lap time and line length are you flying? 9200 on a 4 pitch seems a bit slow for common lap times on the LA46.

Best,    DennisT

Nope!  The correct way to go about this is to change one thing and then fly it to see what affect that one change had.  Takes time, but ferrets out the problem.  I checked out tank issues weeks ago.

Last change was the needle valve.  And that was the cure.

But I am still trimming.  Today I used 20% (15% syn/5% castor) instead of 18% (13% syn/5% castor).  That got rid of the end-of-flight speed-up (I think because of the cowled engine - engine heat build-up.  My profile flew with 18% no problem.)

Also added nose weight with a heavier prop nut.  That helped flight-wise.

I still don't like one thing it is doing: I need 9200 RPM for my most comfortable flight envelope.  The engine likes to run at 8800 or 9400 and is hard to needle to 9200 (however, it's easier with the ST then the OS needle.)  I will continue to play with it.

   OK, I mis-read reply #5. I thought you just changed up a bunch of stuff at one time. Yes, one change at atime is the way to do it. Might have just been the needle. I have noticed that sometimes the needles are a bit different. As for the 9200 RPM, you might want to play with prop diameter just bit. or maybe shim the ventui down a few thousandths with some shim stock and see if that lets you needle to 9200 easier, then have a venturi made to that diameter.
    Good luck with the rest of the trimming,
  Dan McEntee

I have done the same to my OS H60F powered Sakitumi 60+ for the needle valve assembly.
Switched from Enya NVA to the Tigre NVA. Now, just need to get it in the air to see if that works.
Engine was hard to needle to 8600rpm then it took off after the plane was in the air.

On further thought, I'm wondering if it isn't heat related. Can you try leaving the cowl off for a flight or two? Or, add more castor oil to cool that puppy down?  Steve