i've ran almost 9 tanks thru the Magnum XLS 46 (breaking in). I adjusted the High Needl first and not much with the Low needle. I get a smooth transition from idle to Full Power. I can point the prop straight up with full power with a noticeable increase in RPM. I can point it straight and back up and the engine still runs.

I then decided to simulate what would happen in the air. At full power I I pointed the prop straight up and then cut the fuel to quarter stick. engine running fine. i then pointed it down 45 degrees less than quarter stick and then increased throttle at horizontal. the engine stuttered and then died. I repeated this a couple times and noticed it happens only when I point the prop down and then straight again.

Could it be the clunk not reaching the front of the fuel tank while nose down? If so, what do I do? or this doesn't happen in flight?

First, the clunk doesn't need to reach the front of the tank, nor do you want it to as it will sometimes stick there. When you are in a dive, unless it is a dead stick dive, you are going to be dropping faster than the pull of gravity. IE 16 ft per second per second. The fuel is going to be like anything else, and floating or pinned to the apparent top of the tank. Static test, IE holding the nose down should result in a dead engine if held long enough. That is not going to happen in the air though.

Don

In that case, off tomorrow for the first maiden flight.

Your running the engine a little lean, give it a click or two to the rich side.

rrragmanliam

I agree. if you are in a dive you should be throttled back and holding it for eight or ten seconds tops. The reason to point the nose up in a ground test is to see if it leans out too much. Don't worry about a nose down test.

The others are correct in that you don't want the clunk going to the front of tank. If the clunk goes forward many times it will get stuck there and then your engine is going to die as soon as the clunk will no longer be under the fuel. Agood way of preventing this is to simply put a piece of brass tubing in the clunk line so that the clunk will no longer be able to move to the front of the tank.

Ken

I assume I'm going to richen it with the High needle, correct?

Yes, richen it with the high needle.

this is incorrect, the fuel doesnt stick to the back of the tank. the only thing that keeps the engine going is the fuel in the fuel line. this isnt a problem (unless your a mile in the air) because that fuel in the line will supply the engine for around 15 seconds. that will put your plane in the ground if it takes that long for you to get down, so it isnt a issue.
dont believe me? next time you are running a engine take the line off the carby, it will run for about 5 seconds before dieing thats with no fuel line. now add 6in of fuel line and it will run for a while, long enough for the plane to level out and get fuel again

Thats true - I won't be pointing down for that long.

How long is it gonig to run on the 6" of air in the line??

Actually surprizingly long!

What happens is that the fuel moves through the line at a relatively slow speed as it is vaporized at the carb.

Once the bubble ( or in this case the several inches of air ) hits the carb, if there is still pressure in the tank the air gets pushed out relatively quickly as it can flow out of the carb need much more quickly than fuel.

The engine stumbles and is helped by the inertia and airflow, but it may just resume operation.

6" is extreme, but I've seen engines dealing with 3" of air in the line without shutting off because of the above.


That's not to say having air in the lines is ever a good idea...


But his point is absolutely correct, that fuel - DOES NOT - "stick to the back of the tank" as often espoused by even veteran RC'ers.

It's the remaining fuel in the lines that keeps the engine going, and the clunks DO become uncovered and start sucking air.