coughing 503 dual

[Greg]

a friends 503 dual keeps coughing on full power. On idle and 90% power its runs perfectly. But on take off at full power it will run for about 20 seconds and then die! Not right off but as if somebody sut off half the fuel. Once back on the ground the motor will reve again. (the motor will never die completely)
We have taken off the carbs and worked through them (all settings are right). We have replaced The fuel pump. Changed spark plugs. Changed fuel lines. Changed all the filters. Changed all the pertol in the tank. It is not oil injected so it cant be anything to do with oil mix. We also changed all the gaskets. There was no change to the prop, gear box, or any component before this problem. I dont think the problem could be electric as it does not backfire or miss fire. Please help me I dont know where else to look.

[Morph]

Sounds like it's starving fuel. One of our guys had a problem with his 503 in his trike, running near to full throttle it would start coughing and switching the petrol pump on temporarily solved the problem. He found the air venting hole in the tank had a small plastic burr, from when the hole was drilled. When he was running at higher revs the air flow would suck the burr into the hole and restrict the airflow. As soon as the plane was at idle the airflow into the tank was sufficient to maintain idle without any problems. Took him weeks to find.

[Greg]

Thank you, we did try that one too
Looks like this is going to be quite a thing!

[vincent]

I had a similar hassle on my 503, replaced the vacuum pipe from the engine to the fuel pump. It had a pinhole in the pipe about 1cm from the engine. No more hassles after that.

[PMarshall]

Check mag switch wiring for a short to earth. Sometimes the loom can get damaged by vibration and can short causing some strange faults.

good luck

[ZU-ANE]

Have you checked your float level. Maybe starving fuel when in need of a lot.

[Greg]

Thanks Guys

PMarshall, The engine is dual ignition, both work on a mag check, if 1 fails shouldnt the engine keep going? or should i look for a main short else where? I think we might be onto something here, as we havent checked electrics

[Microwave]

Hi Greg,

I had a mag problem some time ago where one mag failed at high revs under hot conditions. The engine carries on running, albeit a little bit rougher, however, its not a nice feeling knowing that you only have one mag.

From the evidence presented, I reckon the problem is either fuel, the vacuum pipe to the fuel pump or a dirty air filter. It is very possible that the air vent to the fuel tank is partially blocked and the fuel supply to the carbs is restricted or the fuel filter is dirty. Possibly even some gunk in the main jet. I have seen all of the abovementioned at some time or another. In most cases its the smallest and most unexpected that gives you the biggest problem.

Have fun, but please let us know what it was when you do eventually solve it.

[PMarshall]

Mag switch shuts off the engine by shorting to ground. One way to test them is 2 disconnect the mag wires at the plug on the engine and run the engine and see what happens. Be ware as soon as the mags wires are disconnected the engine is "live" and can be started by rotating the prop. you will have to use choke to stop the engine. And if this does not stop it you might have to reconect the wires to stop the engine. Reconect wires asap to avoid injury to others and yourself.

[The Agent]

Allthough 2 strokes are not as full of sh!t with the new fuels be careful if there is fuel starvation at high revs you could burn a hole in the piston.
Now that will not be pleasant and we saw it this weekend on a single ignition 503.

[Greg]

Tried everything you guys said!
Still no change
Sent the trike to a well known agent,
who couldnt find a thing, he sent it on to his technical guy,
and yes he couldnt find a thing.
we will be test flying again this afternoon!
I think there must be nothing worse than a problem that shows
up when it wants to.
if i find anything i will let you guys know.
Thanks for all the ideas

[Bennie Vorster]

Greg

So what happened

[Greg]

In total desperation we took the plane to glen at rhino park.
He took a few days and went through it bit by bit. We collected it fri, test flew her on sat and yes she died again! We phoned glen who went out of his way to come through on sunday to have a look again. After the engine began to cough we checked the carbs again. 1 of them was almost empty and one full! glen then reset the carb again and we tried again. yes she died again! So glen then took a carb off a second trike, checked jet sizes and settings, put this carb on the trike and off she went!!! Looks like the problem was in that 1 carb but every component and setting is correct, so perhaps a small crack? we will not know untle glen has finished with it. But for now we are back in the air again. Thanks glen

[Bennie Vorster]

[Aviation Engines]

I have to agree with Bennie!

HAve you check the pick-up line in the tank, it may be getting sucked up against the wall of the tank.

Also check the breather on the fuel tank, it may be blocked or too small.

These are typical signs of fuel starvation.

Please advise the following:
Total operating time
Last maintenance prior to this problem and how long ago (hours?)
aircraft type

It is also possible that the cylinder is so badly worn out that when the piston is on the down stroke it's unable to pressurise the crankcase enought to force the required fuel mixture up through the transfer ports.