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503 EGT Problem


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I have a 503 SCDI mounted inverted in a pusher aircraft.


Last Saturday I throttled back from 5400 to 4000 RPM to descend after an hour of cruising. I also reduce airspeed to keep from leaning out the engine.


On this occasion the PTO cyl leaned out while the MAG cyl remained normal (ie. slight drop in EGT). If I opened the throttle to 5000 or above the PTO cyl EGT would settle back to its normal position. Likewise for low throttle settings the temp was in its normal range.


Having a single carby I thought if it was a needle/jet problem it would affect both cylinders and if it was an air leak it would show up at other throttle settings. Have I made some wrong assuptions here?


The EGT did not exceed 1150, but as it hasn't done this before I would like to find the source of the problem. Any clues??







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I had a similar problem with mine, the CHT went up to 800 C, with no increase in EGT, pulling the throttle back to descend on base and the CHT fell in line with the throttle, when throttle was added when flap was lowered the CHT increased as well.


sorry i couldnt help with your problem, but after that short flight i have now pulled the engine down for a 150 hr inspection, and noticed the CHT sender looked pretty ratty and i have since replaced the sender, though haven't flown the aircraft yet so i cant tell if it was the sender causing the issue,


though pulling the engine apart showed No evidence whatsoever of running to 800 deg C.


Nb my engine is a 503 DCDI.



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When running a single carb, it is important to make sure that the carbi is installed square to the engine, not vertical to the ground as the airplane sits.


The fuel from the carbi is not delivered in the centre of the throat, but at the bottom and then split to the two inlets.


If the carbi is not square to the inlets, more fuel will go to one than the other.


Another possiblity could be a crack in the side of the carbi rubber mount, allowing more air to one side.





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  • 2 weeks later...

Uneven fuel distribution-503.


Just backing up Arthur . It is almost too much to hope that ONE carburettor will supply fuel evenly to both cylinders in this situation. Manufacturing variables in the Y branch can affect it or the varying pumping efficiency of each crankcase,which could be changed by blowby, piston fit, port height (port timing) & pressure waves in the exhaust system. The high power situation is the most critical. The location of the sensor may have a bit to do with the reading.( EGT.) Are the plugs even? N...



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Thanks for the info and the replies. I have used a slight rotation of the carby to even out the EGT readings. Is this OK to do or am I just camoflaging symptoms of problems I should be dealing with? The maintenance log was a bit sparce when I acquired the aircraft so I am assuming the engine hasn't been decoked for a while (+250 hours). I intend to have the engine checked / overhauled this winter so that I can have some sort of baseline for future engine maintenance.


cheers Alan



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503 Temps.


I think you have corrected it. What do you think Arthur?


Carefully check compressions by hand . If they feel good & the plugs show an even colour,& it idles smoothly, keep using it if you want to.The decision is yours,of course. N...



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the 250 hrs without de-coking sounds a little worrying, i have just done my 503, it had 147 hrs when i took the tops off and de-coked everything, the piston was about 0.5 mm thick and the cylinder head was about 1mm thick.



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503 Inspection.


Yeah, I'm also inclined to think it's too long, but I wouldn't just remove the heads. The most serious situation is where the carbon builds up under the rings, in the ring land.The removal of this, is very difficult to do well, unless you are experienced & carefull,& requires the removal of the cyls, & the pistons from the con rods.


An assessment of whether this is a problem can be done with an inspection through the exhaust ports, by carefully pushing the ring away from contact with the cylinder wall with something soft (like a carpenters pencil ) to see that it has room to move in the land.


Carbon , in this situation, can force the ring against the bore when the piston gets hot,& expands and causes the rings to jam or break into small pieces. Sometimes this, ( the carbon build-up) does not occur,& you are therefore fortunate. It is caused by high temps& poor oils & some of the rotten gums in the shandy we get for fuel, maybe stale fuel also. N...



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Guest micgrace



Quite correct there facthunter. Certainly pays to have a look at the rings on a 2 stroke as they have a nasty habit of jamming in the ports (due to carbon) on upwards travel, breaking and scoring the barrel.


Needless to say, the rings have a rather short life. Changing them isn't terribly difficult, but do need to be careful with the carbon removal. And check to make sure side gap/ring gap (new rings) is within specs, as well as bore/piston size , taper and condition.


Certainly wouldn't hurt to remove the pistons to check the gudgeon pin fit. Incredible stress on that part and it's bearing.





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Ultralights. I am surprised at the carbon build up on your engine. I ran a free air cooled 503 for over 150 hours and there was only a very thin film of carbon on heads and pistons. What oil do you use? I was using green slime which I believe is unavailable now.



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Carbon build-up


Some of that carbon could be dust. Generally with modern high-performance two strokes & oils I find carbon build-up practically non existent,often little more than a patchy discolouration on the piston & head . Certainly a major contrast to what used to happen 40 plus years ago, when the exhaust port would completely block up at times. Nev...



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  • 6 months later...

Hi Gang.


Being a relatively new member to this site. i've noticed, after reading some of the posts here, that there is a lot of mis-conceptions going on about two-stroke engines. The old adage still applies today look after me and I will look ater you.


Before we start do yourselves a favour and purchase the manufacturers engine workshop manual and handbook. All the settings and procedures are usually described within.


O.K. Two-stroke engines need basic requirements namely. Air, Fuel and Spark. Preferably in the the right order and mix. Lets take Air in abundance clean filtered air mixed with the right quantity of fuel, sucked in by the piston and exploded by the spark at the right moment.


Lets look at the fuel. The fuel used should be fresh, as stagnant fuel, can and will create problems. Being a two-stroke it will need oil mixed in the fuel lo lubricate the internals.


Oil selection is by your choice. But, remember, what you put in you will get out. Caution, with synthetic oils as it will seperate from fuel after standing for a period of time, some synthetic oils do not act kindly on seals and fuel lines. Just be careful.


Sparkplugs, always use manufacurers recommendations. The carb's when adjusted should be level and in line with the crankshaft to maintian an even fuel mix distribution between both cyl. Carburetor adjustments are very critical, more so when you have 2 carbs feeding the the engine. Spark, (electricity, can be nasty stuff when it belts you across the hangar), is generated by the stator and flywheel and generates the power to fire the sparkplugs and provide power to the battery via a rectifier/regulator.


The generator/stator flywheel generate AC alternating power up to a 110 volts . That needs to be converted to DC or 12 (usually up to 14.6) volt current we can use in our battery and run our cosumables like starter motor, insruments, lights, radio, transponder and the like. Now the stator, flywheel will suppy power to the engine and keep it running (without the use of battery power). Very clever.


But all the other gadgeds will require some power, if your power gernerator or rectifier/regulator fails, for whatever reason than the rest of your gadgeds will draw their power from the battery and eventually runs the battery down.


As mentioned in one of the responses above, the "battery is one of the most neglected items on the airframe". It is therefore imperative that the earth return goes back to where the power is generated from. (Engine).


As for the tuning and servicing of the two-stroke engine read the manufacturers specs and whatever you do check and check again. There are many books written and many a web-sites that deal with the problems associated with operating, maintaining and servicing the "dreaded" two-stroke in their many forms and disguises. The bottom line is "Look after me, I'll look after you" If you wish more information please ask. My 582 Rotax perfomed well for 1158 hrs before no 1 big-end bearing decided to give up. A non event landing saw us back into a friendly farmers paddock (no hassles) and we were back home in the hangar within 2 hrs







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Good advice AVU, it is a lot easier now to get a reliable 2 stroke, basically due to more reliable ignition systems and also better lubricant. I toyed with the idea of running a little 2 stroke oil in my 4 stroke before i went away for 5 weeks, just to inhibit it a bit, but did nothing in the end. Time will tell if any corrosion occurred.



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