I have a ST-90 side mounted (and its about 25 years old)

Tuned high end no prob; set low end; it runs fine on the stand at full throttle and hums along at low throttle; good transition back up again.

At full throttle, lift and point plane up in the air, and everything is fine. Start to point it down and it cuts out right away;maybe 2 secs even before it is fully down (maybe 45 deg incline down). The engine just dies like you hit the kill switch; it does not race up in rmp; just straight died.

Tank was probably at 3/4 full at this stage.

What is wrong; What do I change/adjust?

From your explaination, it's to rich on the low end, Lean it about 1/8 turn.

but I was at full throttle at the time...does low end still affect things then?

Yes, the low end reflects a lean or rich mixture. Take a tach, and run the engine to full throttle per the rpm recommended. then back the high end down around 200 to 300 RPM. If the transition is smooth then point the engine up. If it dies, the low end is to lean, if it dies pointing down, it's to rich.

My favorite method of testing the low-speed needle setting is to pinch the fuel line.

Check this article out:

http://www.*********.org/cars_eng-tuning.htm

Sounds like the clunk is stuck or fuel line in tank has stiffened so does not follow fuel in tank. Of course could also be you have wrong supply line to carb.

Lines are correct. I did change to a new tubing layout inside the tank. I added a straight piece of rigid plastic in the middle of the tubing between the exit and the clunk. I was told this would stop the clunk from "falling backwards" to the front and getting stuck. (I had this happen on another plane after a hard "landing"). I would have thought that this type of set-up would not change the fact that when the plane points down, it is hard to pick up fuel....but maybe the ST-90 wants a lot more fuel than a 40-50 size engine which is all I have run b4...or maybe less flex means it falls back less. However, I was about 3/4 full at the time....

At 3/4 full a 35 degree nose down incline is enough to uncover the clunk and kill the engine.

By 45 degrees the clunk is probably already uncovered and sucking air.

This will happen in the air too and contrary to the popular old timer "myth", the fuel DOES NOT stick to the back of the tank in flight.

Sometimes a longer length of fuel tubing will help smooth things out during downlines.

Try this:

Fill your tank fully, then try the same test.

If the engine does not behave the same way when the tank is full then you know for a fact that the problem is not with the engine or it's needle settings... and the problem lies with the clunk being exposed, etc.

This males sense in that I always wondered how a clunk could address that issue...however, how is it that an almost empty tank (in a 40 sized engine) can still run at any angle?

Is this just magnified due to the larger engine? (ie sucking much more fuel so tubing empties)? I just would be surprised that it would empty so fast (2 seconds)

If that is the problem, could I add a second clunk (off a TEE) on very short tube so it is always at the bottom-front of the tank? Adding more tubing does not sound like a good fix as it might be feet worse to be able to run given the 2 second issue...

Agree that your test will prove one way or the other. I guess I should also try at lower throttle level too. More troubleshooting.

Normally air does get into the lines when the engine is pointed down. However there is usually enough fuel remaining in the tubing to keep the motor somewhat spinning.

As long as the bubble is not too big, it will instantly be dissipated when it hits the carb, and the engine is none the wise with nary a hiccup. This is due to the incompressiblity of liquids. Air literally blows through the nipple while fuel takes a while.

A second clunk and T can also be a problem as air can still be drawn through the exposed line.

However all of this is speculation.

Try the test first... it will help tell you what is going on.


It may also be that this could be related to the rigid plastic tube too.

Normally some bending is advisable, as this lets the clunk move towards the available fuel.

Your clunk may be staying out of the lowered fuel level when the engine is pointed down.


I've also had a situation where I did not leave enough of a gap between the clunk and the back of the tank.

I didn't know this at the time, but my engine tended to cut out on accelerations but only in the air.

It turned out that the tubing was stretching, letting the clunk's opening contact the back of the tank helping to somewhat seal it off from the fuel... duh.... was I red faced on that one!

Doubtful it's running out of fuel. It's easy enough to watch the fuel line and see though.

2 stroke engines can run for several seconds just on the fuel in the crankcase at any given time. Yank the line and watch if you don't believe me.

A poor glow plug can give similar flame outs too, easy to check with a new plug.

brand new plug was in at the time (OS#8) so that is not it (the old one was changed as pulling the glow ignitor showed a dramatic change in pitch etc)

I pulled the tank and it was fine. I decided to replace the line to the clunk to just regular tubing. If anything, the rigid portion was restricting flow due to smaller ID.

I pressure tested the tank/tubing and it was good.

I ran with full tank and it was fine on any throttle settings.

At 1/2 tank, it was fine at low throttle but cut out on high when pointing down. However, it cut out after a much longer period. Ran maybe 5 seconds, slowed, then got fast, then died. I pulled out a 51 size engine and it did the same thing but ran for 10 seconds. I guess the fuel usage explains that. The 51 runs at any tank level and any orientation in the sky so I think this ST90 should be OK too. Now time to maiden and find out....

(I have been doing much more checking as this was a used engine and I want to make sure)

We often talk about "pointing the nose down" but often no one explains to what degree it is pointed down.

If you point the nose straight down, you may expose the clunk almost instantly if the tank is not absolutely full.

When I test "nose down", I angle the plane about 30 degrees or less, nose down, to try to keep the clunk in fuel.

If you've angled the plane more than that, then things sound pretty normal.

In the air, you'll hardly ever have the engine running full throttle nose down for more than a half second.

This is our standard SuperTigre carb setup. It applies to ALL SuperTigre carburetors. This is the same text we have on the ST website as well as in the ST Direct Support Forum, here, on RCU.
****

To properly set up your carburetor, there are a few steps...

Make sure the spraybar is set so that the fuel slot is pointed straight down the center of the carburetor. You will have to look through the carb from the bottom, with the venturi opened to full. Loosen the two screws next to the high-speed needle valve to rotate the spraybar so that the slot is centered.

The idle needle can be seen in the slot of the spraybar. It looks like a piece of wire that has the end squarely cutoff. It should be less than one-half way across the slot when the venturi is opened to full.

The high-speed needle should be opened about 2-1/2 to 3 turns out from closed.

All of these settings are very rich. You'll have to start the engine at about 1/4 to1/3 throttle. Once the engine starts, advance the throttle to full. You can then lean the high-speed needle until the engine is running just rich of peak RPM.

Once you have the high-speed needle set, you can retard the throttle a bit and use the idle needle to adjust the engine. Retard a bit, and adjust. Keep doing this until you have reached the desired idle RPM and mixture. Remember, since we started out with an intentionally-rich setting, the idle mixture will have to be leaned.

Finally, the midrange can be adjusted by rotating the spraybar a hair. Just loosen the two screws next to the high-speed needle and rotate the spraybar just a tiny bit. Either direction is OK. This will help you get a good transition and midrange.

I wish those sentences were removed.

Doing this does not improve things and may exacerbate transition problems.

The best fix I've ever run accross for ST transition problems is to richen the low speed needle for good transitions and do what is pictured below to adjust the idle... it's worked like a champ on problematic Supertigres.



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However none of this sounds like the original poster's problem... His engine runs just fine until the fuel hits a certain point...

Yes. The tuning seems fine on the stand. Like I said, the original problem was that it cut out in less than 1 second when pointing down at maybe 30-45 deg. Now, I can point down at 60 deg and get a good 5 seconds so I think things are OK to try out.

Are you useing a pressure line? If so make sure it's not restricted
and the line doesn't have holes. On an engine without a pressure
line it's very typical to run rich when the nose is pointed down
and adds to the difficulty to tune it.

yes pressure line; three line system and yes no leaks (pressure tested)

I agree.

At 60 degrees you are almost instantly uncovering the clunk.

In flight, you ( should normally ) throttle back before putting the plane into that attitude.

This permits it to run even longer even if it is sucking air... the prop gets fully unloaded as the plane picks up speed, and fuel consumption drops precipitously.

Once you level back out the clunk is covered again, and the air in the lines can quickly be expelled through the needle because of pressure differences.


Remember that with 3/4 of a tank of fuel, even 30-45 degrees of nose down, can expose the clunk too.

This was a periennial problem on my Funtana s90, which could decend nose down very slowly, particularly during inverted blenders, causing it to cut out when I pulled up....

Now I just do not attempt such manouvers with less than 3/4 of a tank full of fuel....