Avistar .46LA engine trouble. Please help
 

So Not a total newbie with rc planes but first time with these types of problems. So I have the engine running great while grounded but I've flown it three times and all three times the engine shuts down about 3-6 seconds after takeoff. I've been able to glide it down each time and land but have gone thru a prop and a set of nose gear in the process. I'm trying to tweak the elevator so the take off isn't so steep thinking that may be an issue but not sure if this is the root cause. Could there be another problem with the carb or tuning that I'm missing? Any help is greatly appreciated!!!

Avistar .46LA engine trouble...Please Help
 

Just sounds like you have the engine set too lean. Once a plane is released and taking off the engine is no longer under a load and tends to rev up a few hundred rpm and dies. It can be a combo of both the high and low end being slightly out of tune. There can be other little problems but it just sounds like the high end needs to be fattened up a little.

If the engine is new run it a tad rich for a dozen flights before you try to tune it for max performance

Are you losing power and hearing a gradual slowing for a couple of seconds before it dies, or does it go from running strong to dead in an instant? The former probably is a tuning problem, too lean on the top end. The latter is probably a plumbing problem, likely a damaged or slipped off clunk line inside your tank. Of course, it could also be an air leak at the carb o ring or backplate, so do check to be sure that bolts are tight and o rings are in good shape.

Also, what happens when you raise the nose of the plane with it running full throttle? You should hear the RPM increase a little and stay that way for as long as you care to hold the plane. That's an easy test for whether or not you have a plumbing problem, and also will tell you if you are tuned too lean.

Thanks for all the tips guys I really appreciate it! @ jester- I've checked the tank and seems okay. I'll check the bolts and the glow plug for air leaks today. It's shut down the last two times right after getting airborn but the first flight I was able to fly a lil bit before it shutdown. It isn't a new engine I bought the whole rig from a guy on eBay so it's run in id say. Again thanks for the help guys .

Also it's not a rev up when it dies its more of an instant stop

Fuel lines can look ok but still have a small split that causes them to leak air. The nose up test should tell you the story on that one though.

(EDIT: multiple post, I have merged your other post with this one. )
THAT being said, are you at all familiar with tuning a glow engine?
try this for starters, as it sound most likely that the engine is tuned slightly too lean.
do this with a helper to hold the airplane!!
after the engine is started, with the helper having a firm hold on the airframe,
have him nose the airplane UP. if the engine quits immediately (or nearly so) turn the high end OUT (counter clock wise) half a turn and try again.
turning the needle valve OUT allows more fuel, making the mixture more rich.
another good idea is to turn the needle valve in all the way COUNTING how many turns it takes, so you can turn the needle valve out a specific number of turns.
one other thing that MAY be causing your issue is the 'clunk' AKA the fuel pickup line inside the fuel tank.
if that becomes clogged, or kinked forward, that will certainly cause problems.

This type of problem can occur if the vent and carb lines are accidentally reversed. The engine may start and run on the ground while it's full enough to draw fuel through the vent outlet, but will die once it's in the air and the throttle is punched.

gg3606 said he checked the tank, so hopefully he confirmed that the carb line with the clunk is attached to the carburetor and the vent line is attached to the muffler. Other problems that could cause this type of sudden engine stop include:

-a plugged exhaust nipple on the muffler cutting off air pressure back into the tank
-a tear in the clunk line will allow the engine to run fine but cut out and die when punched to full throttle

Check these things also. If you believe you simply need to get a good tune, remember it's easier to lean an engine that is too rich. If you try to tune an engine that already starts out as too lean, it can be frustrating trying to get it dialed in. With airbleed carburetors, turning the idle screw in richens the engine rather than leaning it out; this can be confusing to pilots who normally only run twin needle carbs.

Good luck, and keep posting questions and results.

I have an idea: I once had the same thing happen to me - with the same engine as it happens. I'd switched the fuel lines, i.e. fuel was being taken from the tube without the clunk. This meant it ran on the ground normally (not lean) as it was still able to get fuel from the top of the full tank, but when the nose went up for takeoff the fuel ran back and let it go lean until dead, several seconds later.

It happened twice before I left the field and thought of it driving home. Quickly verified at home.

Worth checking. That engine has modest power, but runs very well.

PD.

some engines get tuned to lean when they have a bad glow plug. if the glow plug is getting tired the engine does not run to well unless it is very lean but as soon as you take off the engine unloads and goes too lean and dies

You also hint in your first post that the takeoff is really steep. Are you holding up elevator all the time you are rolling out? If so, the plane may be trying to come off the ground before it attains enough speed to actually fly. LA engines are really good, reliable engines but are not stump pullers. You might try leaving that elevator centered and letting the plane taxi until it has way more than enough speed to take off, then just a gentle touch of the elevator and the plane will lift off in a very gentle climb out. I personally think those long, shallow climb outs are beautiful. A hot .46 engine will jerk an Avistar off the ground after a very short roll out and go nearly vertical, but a .46LA may not.