503 exhaust gas temps

cgmg

Senior Member
Joined
Oct 30, 2003
Messages
1,149
Location
El Paso, IL.
Aircraft
Empty hangar right now, have owned and flown Air Command & RAF
Total Flight Time
375
I've been trying to dial in my carbs to get my 503 exhaust gas temps into spec, which is 43 degrees or less difference between cylinders. The best I've been able to do is get the temps about 20 degrees apart up until 6000 rpm or higher. At wide open throttle, the difference climbs to 80 degrees plus, with the #1 cylinder running hotter. The only way I've been able to overcome this is to set the carbs so cylinder #2 runs hotter up to the 6000 rpm point, but being over the recommended temperature difference. This doesn't seem like the right thing to do, so I'm leaving the carbs set for temps to be close up to the 6000 rpm point.

Anyone out there have any suggestions as to what is going on, or is this typical of 503's. I've checked my clips, needle jets, and jet needles. Everything matches up. The only changes I made over the winter were rebuilding #2 carb, which is older than the #1 carb. I also changed my main jets to 155 from 158.

Any help, or things to look for, would be greatly appreciated.

Mark
 
Re:503 exhaust gas temps

Mark,

I have a similar problem with my dual carb 503... the EGT spread always varies between about 40 to 80 degrees F. I've just learned to ignore it because I've been unable to find any cause for it... other than maybe crappy probes or the dual gauge itself.

I've torn my engine apart twice in the last 150 hours for cleaning, new seals, gaskets, etc., and I've never seen any sign of problems from the apparent excessive EGT spread. I've also carefully rebuilt the carbs twice.

I've just attributed the EGT readings to possible minute differences in the probe manufacture, their locations within the exhaust manifold, or the construction of the exhaust manifold itself. One of the runners on the exhaust manifold is not quite as tall inside as the other... manufacturing tolerance at Rotax I guess.

The CHT is also always cooler in the front cylinder as well... undoubtedly from receiving the bulk of the cool air from the fan.

Relax,

John L.
 
Re:503 exhaust gas temps

You have both probably done this, but it is a good idea to swap the probes in the manifold for a test.

Once you know where the problem is you just have to allow for the difference.

Non aviation un calibrated gauges have convinced people to adjust things and do serious damage to the engine and also to the gyro if the engine stops at the wrong time!!!!!

Aussie Paul.
 
Re:503 exhaust gas temps

It is very hard to have both cylinders on a 503 to have exactly the same compression. Unlike the 582, the 503's do not have proper crankcase pressure seals between the cylinders, so uneven crankcase compression means uneven mixtures and consequently different temperatures.
Two strokes are always set to run a little on the rich side to be on the safe side. Variations in the EGT are not critical as long as they do not do over a certain figure. What that figure is depends on guage accuracy, correct wiring and the probe placement.
 
Re:503 exhaust gas temps

Mceagle and Paul:

Well put!

The stock recommended jetting from Rotax seems to be a little on the rich side on my 503 (EGT's are usually on the low side of the green range) and I've just avoided trying to "optimize" things by jetting leaner. The slightly richer mixture (than optimal) is my insurance policy against the Chinese manufactured dual EGT and CHT gauges I use... not that Westburg gauges are much better.

John L.
 
Re:503 exhaust gas temps

I think this is one of those KISS cases. Check the plugs' color -- if both are chocolate brown, the exhaust temp is correct, no matter what that little plastic guage says.
 
Re:503 exhaust gas temps

Buy a bunch of sets of jets richer in each step. Run them up number at a time until the motor "4 strokes" you will know the sound when you hear it at full throttle then back them off a number or 2 until it does not 4 stroke. The engine will run fat but safe. Anything the egt says after that is engine blueprinting issues, ignition timing, gauge issues, etc. Tuning by ear and plugs is the best route. Egt gauge is just to confirm it. The engine may make a little bit less power rich/fat mix but you will save the engine in the long run. Talk to the guys at lockwood if you want any other technical questions. Also switch carbs to see if its a mfg tolerance between new and old flow rates. No 2 are ever the same !

Jonathan
 
Re:503 exhaust gas temps

Everybody,

Thanks for the suggestions. Actually, last year I had to lean my carbs out, as I kept having trouble with carbon buildup on my plugs every 10 hours or so. It also got to where the engine would bog down in midrange rpm's on takeoff. According to my probes, though, the temps were not more than 60 degrees out.

I do check the condition of the plugs, and since re-jetting for a little leaner, my plugs look about the right color, and I'm getting 25 hours out of them without oil or carbon deposits on them.

I took the exhaust off this winter to paint it, and my inspection of the rings, pistons, and cylinder walls showed good conditions in all 3.

Paul's idea of switching probes is good, but my engine is the older style, and I have to bend the end of one probe to get it into the exhaust manifold. Based on what I'm hearing here, I don't believe, at least at this point, that my problem(if it really is one), justifies buying a new probe, and tearing my harness apart to get to it.

I'll pass on my experience if anything comes of this situation as I fly this year, though.

Mark
 
Re:503 exhaust gas temps

Mark, just swap the wires at the instrument. Aussie Paul.
 
Re:503 exhaust gas temps

Paul,

I can probably try that. My connections for the temp probes are in a short piece of wire harness cover that I could pretty easily open up and switch the leads.

Thanks,

Mark
 
Re:503 exhaust gas temps

Mark, I am waiting to see what happens when you switch leads. I had the same problem years ago, I switched leads & it was the instument or sender. Now I just read accordingly.

Look at the right CHT. It runs 100 degrees hotter than the left. uh oh, now you got me thinking,was it the gauge or the sender or was the rear cylinder hotter? Damn , now I gotta recheck!

I think the gauge just reads 100 hotter. Damn memory! Now I can't even remember which is for my front & which is for my rear cylinders. This year I am gonna mark them! I think the Left CHT/EGT is for the rear.
 

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Re:503 exhaust gas temps

Nope but I've been thinking about that piece of yarn. I always head straight for my target anyway! Could someone explain it to me again please .
 
Re:503 exhaust gas temps

"I always head straight for my target anyway!"

Chris,
That is exactly the reason that you need a drift flag - a very important instrument, especially in a low speed aircraft.
 
Re:503 exhaust gas temps

Chris: The yaw string keeps you pointed straight into the relative wind...making you fly more efficiently and smoothly.

Treat the knot end of the string as a snakes head. Step on the snakes head with the rudder that it is pointed at. For example..if the tail of the snake is pointing to the right with its head at the left...step on that snakes head with left rudder as needed to straighten the tail so its coming right back at you.

Here is a picture of my Air Command landing on runway 18 at Mentone International last summer. You can see that the head of the snake needs to be stepped on with right rudder.....but NOT WHILE landing of course... ;D
 

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Re:503 exhaust gas temps

test the EGT by heating water and use a cooking thermmomater put the prop in the water and see what you get.
 
Re:503 exhaust gas temps

Stan,

So if you were going to do a fly by straight down that runway you would step on its head (right rudder) and fly down the runway somewhat sideways?
 
Re:503 exhaust gas temps

Chris: If I wasnt intending on landing but just wanted to fly the runway..then I would add right rudder..and bring the string straight back. This has the gyro flying straight into the relative wind which in the case of the Mentone picture...would have me flying down the runway with the nose to the right.
 
Re:503 exhaust gas temps

Isn't it weird that all of Tim C.'s pictures taken from his 447 powered Air command... Compaired to Chris' machine...... Chris is tracking straight and his rudder pedals are centered and Tim is going straight and yet his pedals are really offset.
 
Re:503 exhaust gas temps

Mentone looks so................................ Green! can't wait to go up this summer for a few days.
 
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