hawk56

My friend has a Cub with a Magnum 91 4 stroke on an inverted mount.
The plane runs for 2 minutes and starts running rough and dies. It does this sitting on the ground or in the air and always runs the 2-minutes from a full tank. I adjusted the low-speed and high-speed needle valves and it running great up to 2-minutes. I have been flying for a few years but have not come across this kind of problem, so any help would be appreciated.

Ross Kean

If you try to start it with a half (or less) tank of fuel will it run? If not, then you likely have a fuel tank/lines/clunk problem.
(hole in line, tank too low compared to carb, clunk not at tank bottom etc). Any bubbles in lines going to carb?

If it shuts down and you refill and restart immediately, will it continue to run? What about if you don't refill.

Have you tried with the plane inverted?

Since it is running OK on the ground or in the air (presumably at different throttle levels) I doubt that it is way too rich and loading up but you never know....

Is the engine hot after a run? (Insufficient cooling air or too lean)

Will it continue to run on the ground if the glo ignitor is left on? (Bad plug, too rich etc)

Crankcase vented properly? (Makes a difference on some engines)

Lots of possibilities.

Ross

hawk56

After it runs for 2-minutes and dies, it will not start until its re-fueled.

I did not see any bubbles in the fuel line and the clunk seems to be free in the tank.

The cowl has a pretty big cut out for the engine and appears to have enough cooling.

The plane has an on-board glow igniter which we tried disconnecting and reconnecting and got the save results.

I did not try running the plane inverting and I did not notice the height of the tank to the carb. This seems like it might be the problem.



Thanks
Tim

waagbuck

My Feedback: (1)

Also check for holes in the fuel line inside the tank.

Live Wire

Sounds like fuel lines are reversed. It happens to the best of us

Edit
2 hawks and 2 sterlings in three posts

SST

My Feedback: (2)

Is your muffler pressure fitting clogged?

touch and go

My Feedback: (11)

Tim, Regarding your "clunk seems to be free in the tank" comment, note that if the clunk has come off the end of the fuel onto which it is supposed to be attached, it will be "free in the tank" but not performing its function which might be the cause of your problem. Good luck.

hawk56

There has been many solutions to the 2-minute flight problem posted here. I will keep trying them until the problem is resolved.

Thank you for the help.

Tim