Could be that there is not enough airflow, or improper airflow around the cylindar of the engine. Could also be that you need a bit more castor . Does it start this at a particular part of the pattern consistantly? Have you changed fuel jugs lately, though I doubt that is the issue its not a bad idea to be aware of it. What are you turning for a prop on the engine.
It would be a good idea to Know what the venturi size is, and which needle assembly your using, and perhaps a photo of the cowling around the motor area regarding cooling to get a better idea. But my first guess would be to up the castor level some.

     Mike, I have my LA .40 in a Brodak Oriental arf. I have 3 of these and was warned against buying all of them. The concensus said that the .40 doesn't run well for stunt. I disagree as I like all of them. I use a Randy Smith tongue muffler on mine opening the holes up a bit. I didn't add any additional holes. I also removed the remote needle as I found that it needles far superior to the stock one. Having my engine fully cowled, I don't find any overheating or runaway problems. I would think that your engine is retaining too much heat either through the stock muffler your venturi could be slightly larger. I haven't used Sig fuel as I still have an ample supply of Powermaster 5/22. I recently purchased the Thunder Tigre prop your using but, have yet to use it. I use the APC 10.5x4.5. I will say that the Thunder Tigre is an extremely heavy prop over the APC. I don't think that is the problem as I know several people using that prop on the .46 and also .40. Is your engine getting leaner as the flight progresses? If so, could this be a tank issue in which your starting out rich and then it goes overlean. I found the best obtainable runs for me was using a uniflow setup but I run muffler pressure to the uniflow pipe. I would think the things that would make a engine run hot would be too much prop load, not enough fuel, restricted muffler. I have successfully run the stock muffler though. I used it for 2 seasons , its just its weight is a big penalty especially on the Oriental which was designed around the Fox. The La weighs almost 2.5oz more as is. I know Brett Buck has mentioned more than once that the stock set up is usually the best bet. I just went with the tongue due to weight and I found I could set the needle better due to the engines note change when I hit the inbetween 2-4 cycle. The stock set up was more of a monotone sound. I would make sure the engine is tight in its mounting. I would then make sure your not suffering from the case of the bad vibes which could be foaming the fuel causing a lean run. If so and its minor, a squirt of Armor All in the fuel may surely help matters there. One squirt will do in a gallon jug. It would be imperative to check all fuel tubing connections. I've had small splits in tubing and also chaffed fuel lines which went undected which can give you fits beyond belief even though they don't appear they're leaking fuel. This can lead to harder starting and surely leaner runs. All my engine have the stock plastic backplate. I was told they're typically not a problem until you take them apart. Well, I did and reassembled without any gasket or sealers and never had a problem to date. This is going on 3 years now. I still keep an eye on them though and you should do the same checking for leaks and also cracks. Especially keep them snug and not to overtighten them. With the glow plug removed, is there any kind of bind in the engine? I'm just shooting in the dark here so try anything. Ken

Could castor gum baked on the head fins be a contributing factor?  Or varnished piston/cylinder?  Or carbon deposits inside the muffler (I've seen some pretty heavy deposits inside OS35-S mufflers).  Sometimes just changing the plug or jug of fuel solves a pesky problem for no readily oblivious reason.  My 2 cents FWIW.    

What Brett says should take care of the problem.  A little cooler plug could also be helpful.

Jim Pollock

Thanks Guys -

Looks like its time to buy my "summer" 15% fuel and a less hot plug. Normally I run the hottest Thunderbolt plug I can get, but maybe in summer the timing is thrown off by the heat retention and detonation results. It may also be over propped with a 4.5 pitch, so maybe an 11 x 4 APC would help out. I do have a tongue muffler that I can try. I am using an old-style OS muffler, much smaller than the current boat-anchor stock muffler. I have had great luck with 3 LA46s and two LA40s and one LA25, so no complaints here about this engine series. The 46 is a plug-n-play gem.

      I would be surprised if the glow plug was an issue. I think it's likely to just be a matter of thinner air and leaning on the needle to compensate.

     Brett

Brett,

Is there a formula to determine the increase in fuel load required to maintain the same run time when increasing nitro (assuming no prop change), or is each engine different?

With 25% nitro I get about the same performance from an LA40 as from the 46 on 10%.  If the motor overheats you are trying to get too much power out of it.  Either more horsepower(nitro) or less prop. 

The Oriental is not a huge plane.  You might try running a 10/4 APC and let the motor rev up into its comfort zone.

Nobody has brought up a possible fuel tank issue.

Maybe it's going lean later in run because of a tank-based failure.

Nobody has brought up a possible fuel tank issue.

Maybe it's going lean in later in run because of a tank-based failure.

   Not if it only does it when it's hot outside or the air is thin. I think that's what I got from the parent post.

    Brett

Mike , i don't know if you have your problem sorted out yet or not but i just fought the same thing with two engines on the same plane. To make a really long story shorter, the fix was getting rid of the fuel foaming on my profile. I tried everything i could think of, could get kind of decent runs but not consistent and every time the engine was very hot. Rubber banded a plastic clunk tank with the bronze sintered filter pickup just for giggles and got perfect runs everytime and no more hotter than hot engine. Hope you get yours figured out.

If the motor is overloaded more nitro makes it less sensitive to needle valve adjustments. More power, more fuel, more open needle valve, less critical setting..  For the most part, adding more nitro is the same as adding more displacement.  Like most folks, I was trying to get the 40 to do the work of a 46.  That just doesn't work.  It seems to like running over 11,000 rpm with the stock muffler, no more than a 10/4 prop.  I switched to an LA 46 on the same plane and it gets a very nice run on a 10/4 APC 4 cycling at 12,200 at launch with 10% fuel.  Thankfully, now with .015 lines allowed you can put a 46 in a reasonably sized 48 oz. plane and get very good performance.

Phil, Thanks for your reply. Very interesting.  4 cycling at 12,200!  The most popular prop around here for the the LA46 is the APC 12.25 x 3.75, either full size or cut down a bit.  Seems like a highly versatile engine that will accept a variety of props, venturi sizes, head clearances. 

Mike, Sorry if I hijacked your topic.  I owe you a coffee next time we're at Peets. 

Kim, if you can drag yourself to Alameda I will buy YOU a Peets just as a  reward.

Brett, is there some problem with having molten aluminum sprayed all over you? I have to laugh when hearing about the "old world craftsmanship" versus modern technology. A guy standing at a lathe with a micrometer cannot duplicate CNC machining even remotely. Which is why those beautiful old machine tools can be had for peanuts these days. But I still like Alfa Romeos.

Too many good tips given, but one left out in my mind (while reading all the others).  I know that there have been instances where engines got hot because of the set up the Smoothie has.  The air flow is not directed around the head, because it is simply *out in the breeze*.  While this might be fine on *normal days*,  it could pose a problem on hot days.  Many stunt gurus have spoken to having the inside of the cowling shaped to direct air flow *around* the head, actually shaping the inside of cowls to insure it, instead of the air just blowing *across* the head.  Profiles have exhibited this problem when it is really hot.

maybe, maybe not, but the consensus of higher nitro would be my first pick, too.

Big Bear