help with engine problem
 

I am having some issues getting my VQ models AT6 up and running. I have a magnum XLS46A installed sideways with a Jtec pitts muffler. I have tried several different new glow plugs. Here is what's happening. The engine starts fine and seems to run okay. I have run about 3 tanks through it and have leaned it out. . Well, I took the plane to the field the other day and for some reason it starts running goofy. When I hold the plane up in the air to do a run-up. It immediately dies as soon as the plane is held upright. It runs fine on the ground in the horizontal position, but as soon as I hold it up in the air it dies. I would assume this is a fuel problem. I checked and my clunk is not stuck. The fuel lines appear to not have any leaks. The tank itself seems sealed without leaks. I also checked and all my lines are correctly identified. So what gives?
At this point, I have pulled the tank out and I may replace with a better, great planes tank. I hate to do all this though, if its something else causing the problems.

RE: help with engine problem
 

Could be too lean now.

RE: help with engine problem
 

It's too lean. Period.

After only 3 tanks of fuel, it's not time to completely lean out your new engine. The "pinch test" is a much more reliable indicator of fuel mixture. With the engine at full throttle, briefly pinch the fuel line. If the engine sags and dies immediately, it's too lean. If it increases speed slightly then begins to sag, the mixture is about right.

Dr.1

RE: help with engine problem
 

Those are good thoughts, but I don't think being too lean is the problem.
I should have mentioned above, but after that started happening, I backed the needle off and it still did the same thing. I actually backed it off like 5 turns until it was slobbery wet. It still will run on the ground, but as soon as I held it upright, it quits immediately. I noticed some fuel was spitting out from the cylinder head, so I tightened those screws up. I thought that would have been my problem, but that didn't help either. I am still thinking something is goofy with the tank or the clunk myself.

RE: help with engine problem
 

What size prop are you running? Which glow plug? Which fuel? Are you sure that the pressure fitting on the muffler is clear and open?

Some fellows flying models with radial engines (scale) over compensate by running a larger than normal prop. This lowers the rpm and the cooling air over a new engine (you do have an exit area for the heated air - right?). This extra heat also advances the timing, which will make your mixture too lean, especially on a new engine. Three tanks isn't really enough to break-in an even ABC engine properly.

If you are running a larger than normal prop, but need the diameter to get out past the cowl on the AT-6, consider dropping down in pitch an inch or so, unless you are already down to 6" of pitch. I wouldn't go lower than that for a scale type of model. They are not 3D floaters.

Also, make sure there is plenty of exit area for the cooling air once it hits the hot engine and expands.

Frankly, a few more tanks of fuel (up off the ground so you don't wear out the engine by sucking dirt through it) just might make your problems go away.

Get back to us and let us know what's up. Good luck.

RE: help with engine problem
 

Let me give you some more info:

VQ texan with cowl off- so cooling is definately not the issue. I purposely left cowl off for break in and my anticipated first flight, although I haven't been able to make that first flight yet due to the problems listed above.

The prop is a 10x6 three blade. Yes, the exhaust fitting on muffler is free of debris.

I have tried two plugs: one brand new Tower plug and the other a brand new Thunderpro with idler bar.

Fuel is omega 10%.

I have pretty much ruled out all the common things that could be wrong, that is why I post here. I am pretty experienced with nitro and gas engines, but this one has me stumped. I still think something is wrong with fuel lines or tank so I plan on replacing those and seeing what happens.

RE: help with engine problem
 

Check the clunk line inside the tank.

RE: help with engine problem
 

If it is running with five turns out on the high speed needle, you have an air leak somewhere. Check the carb throat/o-ring for tightness. Make sure the o-rings on the needle valves are tight. Make sure the back plate is on tight. Get rid of the idle bar plug, you do not need it and it does not do you any good, maybe some lack of performance. Check your fuel lines for cracks/holes in and out of the tank. Check for air bubbles in your lines, etc - that is a give away for an air leak in the line somewhere. Make sure everything is tight, if the head was loose, what else is loose? Just because it is a new engine does not mean that it is all snugged up tight as you found out with the loose head.

Cheers,

Chip

RE: help with engine problem
 

thanks chip. that explanation seems the most logical (and the easiest to deal with) solution. I will go ahead a change out the tank and all fuel lines and tighten everything up and see if that works before i dive into anything else.

RE: help with engine problem
 

Also since the tank is installed when the fuselage is inverted check that it is right side up when the airplane is sitting on its gear or the fuel tubing reversed to the engine. Those items can give that problem.

RE: help with engine problem
 

One other thing for you to try...block off one of the outlets
on your Pitts style muffler. Some engines have problems
with Pitts mufflers due to low tank pressure. Closing off
one outlet will increase the pressure somewhat.

You will need to readjust the needles after doing this.
Might help...might not. But its worth a shot.

Mike Hammer