Microlighters

My 582 has now decided to start spluttering for a few seconds (sometimes 10 to 15) after I have switched both mag switches off. If I switch the mags back on during this period the engine springs back to life and carrys on as if nothing had happened! Could this be caused by worn parts in the carbs or perhaps a timing probem? Can anyone throw any light on this as I would like to get it sorted out by this weekend to be able to fly to the Parys do.

Most common cause (that I know of) is carbon build-up. Get this fixed ASAP - anything that can cause running on can also cause pre-ignition (detonation), and is probably doing all sorts of evil things to your engine...

-justin

Piepie die plugdrade nat, of slaat hom met n dropperpaaltjie dood. Last time a 2 stroke engine did that to me, da bugger quit when I least expected it, and I had to survive on instinct. Fix it, either you have carbon build up, or a timing or mixture problem, or not enough oil in the petrol...... YOU HAVE A PROBLEM. DO NOT FLY UNTILL YOU HAVE FIXED IT. :D

justin.schoeman wrote:Most common cause (that I know of) is carbon build-up. Get this fixed ASAP - anything that can cause running on can also cause pre-ignition (detonation), and is probably doing all sorts of evil things to your engine...

-justin

Spot on right.....in all probability

Boet wrote:.. YOU HAVE A PROBLEM. DO NOT FLY UNTILL YOU HAVE FIXED IT. :D

My sentiments exactly :D

Thanks for the advice guys, have arranged to have it looked at. Will post an entry for those interested as to the cause.

That will be nice. Then we can all learn from it. Thank you. :D

Had the same hassle when I was still flying trikes. My 503 had a faulty pickup behnd the flywheel...changed it and all was okay!

If it keeps on running with mags off ,the one wire that should earth the mag to kill the motor is broken off ,check that one, I had the same on my 503,had to kill the motor with the choke because i didn,t have my shotgun with me.

Got the trike back on thursday. Everything was checked out and apparantly the problem was with the carbs. Had them rebuilt and everything is running smoothly again!

"justin.schoeman wrote:
Most common cause (that I know of) is carbon build-up"

As a total novice in this field, please elaborate? Carbon buildup can foul the valves I guess and what else? Where does the fuel/air mixture come from if the fuel pumps are off, the throttle is closed? I can still understand a loose earth providing a curcuit but unless there is a lot of surplus fuel (running rich?) why would the cylinder keep firing?

Would using a higher octane fuel help? I run my Rotax 914f on 95 unleaded at the coast and have once experienced a 'run-on' for 2 seconds which worried me. I mixed 100 octane with the unleaded and it has never done it again. I might add that someone mentioned to me that running 100 octane from time to time was good for it. Dont quite understand why and would appreciate your advice.

Len

JetRanger wrote:"justin.schoeman wrote:
Most common cause (that I know of) is carbon build-up"

As a total novice in this field, please elaborate? Carbon buildup can foul the valves I guess and what else? Where does the fuel/air mixture come from if the fuel pumps are off, the throttle is closed? I can still understand a loose earth providing a curcuit but unless there is a lot of surplus fuel (running rich?) why would the cylinder keep firing?

Would using a higher octane fuel help? I run my Rotax 914f on 95 unleaded at the coast and have once experienced a 'run-on' for 2 seconds which worried me. I mixed 100 octane with the unleaded and it has never done it again. I might add that someone mentioned to me that running 100 octane from time to time was good for it. Dont quite understand why and would appreciate your advice.

Len

Solid carbon deposits in the cylinder head can get hot enough that it provides an ignition source. In the rotax engines, they have mechanical fuel pumps (the electrical ones are backup), and automatic carburettors. You do not kill the motor by leaning it - you set the throttle to idle, and kill the mags. This should stop the motor, but if the carbon deposits inside are hot enough, they provide an ignition source, and the motor will keep on running.

The same effect can also be achieved by 'dieseling', where residual heat + high compression can provide enough heat to ignite the fuel-air mixture, and keep the engine running.

In both cases, ignition timing is not controlled by the spark, but by the secondary ignition source, this means that ignition timing can be completely out, resulting in very serious shock loads to the engine (and associated damage).

Higher octane fuel is harder to ignite, and can solve some self ignition problems.

-justin

Yeah as far as I am aware it's the burning of the 2-stroke oil that was added to the fuel that causes the carbon buildup. In 4 strokes i.e. all 91X motors you do not add oil to the fuel so carbon buildup is greatly reduced or even never a problem.

I am no expert on the 914 but I do recall in cars that if the engine is not tuned for a lower octane it will cause rough running and pinging. Whenever we Vaalies went down to the coast on holiday you would notice this. The way to solve it was change the ignition timing slightly.

Maybe the 914 needs to be tuned slightly to get it running properly on the 95 unleaded.

Thanks guys. Its due for its 100 hour service soon and I will ask the AP to be sure its tuned correctly. In the meantime, why would 100 octane be 'good' for it? does it burn colder? cleaner?

Hi Jetranger

You can burn your valves using AVGAS in a 914.Attached is an article from sagpa.co.za regarding fuel.

A Number of engines that have been brought into the various aircraft and engine service centres recently, have showed signs of premature engine wear or damage that may have been caused by low grade or contaminated fuel.

Although it is sometimes difficult to pinpoint the exact cause, burnt valves, spark plugs and other internal damage seems typical of the type of damage caused by detonation. As too low octane or contaminated fuel is one of the known causes of that sort of damage, I would again like to urge everyone to be careful in selecting the fuel used for your aircraft.

Unleaded fuels typically have a lower octane rating, and some of the modern environmentally friendly additives used to boost octane, also degrade quicker than those used in the past. It is impossible to ascertain the octane levels of every tank of fuel, therefore it is important to use the highest octane fuel available in your area and refuel from a reputable supplier that has a high turnover in fuel. Also use the fuel within a couple of weeks, do not let is stand unused for months and then use it to refuel your aircraft.

Bigger service stations have 95 octane (inland) and 97 octane (coast) unleaded fuel available at about a 2% price premium. It should make sense that the little extra paid for the fuel, is easily offset for the peace of mind and reduced wear on the engine.

The colour of the fuel, engine temperatures, spark plugs condition and general behaviour of the engine can also be early indicators of an engine that may potentially be affected. If unsure, contact your nearest service agent or approved person for another opinion.

Have a look at Reading Spark Plugs for a rough guide to spark plug colours on an engine.