I was wondering, does going to large diameter props with low pitch (3D props) cause an engine to die when I throttle down to idle on a steep down line? I changed from an APC 12-6 to a 13-4W and 12.25-3.75 on my Irvine 53 and ever since the engine quits when I'm flying. It seems to be more on down lines while throttling down to idle. I could be wrong, the problem might be something else but I wonder if braking can actually stop the engine at idle.

Most clunk tanks I have seen set up with too stiff a fuel line to let the clunk fall foward. Most of the time if you throttle down when going straight down the fuel in the line will be sufficient till you level out. But a low pitch prop will rev the engine more on the down line which will pump more fuel out of the fuel line. So you probably need to set the tank up for a more flexible line to the clunk, longer fuel line, or a header tank.

The correct way is for the clunk to never fall forward, in my opinion.
Following that, you should never have a flexible enough clunk line to allow that to happen

(Because invariably, if it does go forward, it will stick there, causing you to scratch
your head for an hour trying to figure out why you're dying on uplines, afterwards.)

I was referring more to the air hitting the front of the prop on the down line stopping the engine when it's throttled down. Engine braking.

Sorry about that. I guess I paid more attention to the first reply, than the original post

I'd have to answer "no" to your question... it shouldn't make the engine die. (In my opinion)
If anything, it should still speed the engine up, just as any sort of engine braking would do.
You might try switching props back. temporarily, just to make sure... but I'd doubt it.
I guess it might be "possible" that the new prop isn't revving the engine as much on your
downlines. Perhaps someone with a good handle on a formula for that might chime in.

I would've thought a larger prop would idle even better... due to greater "flywheel effect", if nothing else.

I agree with your logic. I think there is another reason for my engine problem (a leak in the head/cyl). I'm always learning something in this forum.

If anything the idle revs should increase because all load is taken off the prop..the ultimate unloading. The airspeed is actually trying to spin the prop faster which causes drag which slows the model. The two things I'd check first is that the idle mix isn't a bit too rich and then maybe go to a slightly hotter plug.

Well, in my ipinion it should definitely fall forward... especially with slow moving aerobatic planes. I have gone as far as making my own (much heavier) clunks.
For smaller models, I use the smallest, most thin walled clunk line I can find.
The clunk should ALWAYS follow the fuel IMO...

Of course, if the tank is too small, the clunk will never go forward... and you just have to pray...

Felt clunks can solve some problems too in the respect.

Hi!
I agree with watkins....
The clunk should never fall forward in a fuel tank!!! If it does it will get stuck there. Thats why Sullivan and other fuel-tank manufacturers recommends soldering a piece of brass tube to the clunk...so that the fuelline can't bend forward....
I haven't seen a tank that has space enough up-front for the fuelline and clunk bending forward...and then retake its position again.
The clunk and fuel in a fuel tank isn't constantly swooshing around inside the tank... they are both more or less in a fixed position only moving slightly depending on the G-forces.
When you dive down with an airplane you are most of the time accelerating down so the fuel will stay at the back of the tank ...not falling forward!

Regards!
Jan K
Sweden

Make sure you have at least 1/4 inch clearence from the back of the tank to the clunk.

ZB

Not so! That just depends on how long you dive and how much drag your plane has. And if the plane stops accelerating faster than 9.81 m/s² the fuel WILL fall forward. That happens faster than you think... unless you have a really fast plane, and keep the power on during the dive... and then still...
The only things keeping your engine from running out of fuel are the facts that it uses very little fuel at idle and that most dives don't take very long.

Any square, rectangular or cylindrical fuel tank has enough space up front for the clunk...

Hi!
Don't agree with you!
Take a Du-Bro or Sullivan 4-15 oz tank...where is the space for the fuel line and clunk up front and for it to reposition to it's original place just 1 cm from the rear wall of the tank ??
No! Do as the tank manufacturer says: on every thank solder a piece of brass tubing to the clunk so that the clunk can't bend forward.This set-up is proven to work in practice.
Or better yeat.... get a Tettra "bubbleless" fultank that doesn't use a clunk at all.

Regards!
Jan K
Sweden