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Do high EGT and High CHT go together?


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Hi to all the experts out there.


I have recently bought a Tyro with a Rotax 447 engine.


I am finding that the EGT and CHT are right at the top of the safe range at full power. They are 1200 EGT and 400 CHT. I have checked that the fan is working OK, belt tension is a little below spec but not bad. The plugs confirm the mixture is lean so I have ordered a larger main jet. The correct one is shown as 165 see chart at: http://www.ultralightnews.com/enginetroublshooting/bingjet.html


but the one in there is a 160. I have ordered a 165.


I hope that this will reduce the EGT, but will it also reduce the CHT?





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newairly, I reckon it will help, especially with am aircooled engine. Lean mixture is the quickest way to kill a 2-stroke engine. The main jet affects full throttle, which is when things are most critical, but it's still worth looking at the temps just below full throttle, where the needle position comes into play. Let us know how it goes. Nev..



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Thanks Facthunter,


The EGT does drop significantly at 75% power which is why I hoped that it is just the main jet that needs changing.


How long does a new set of plugs take before their appearance can be used as a guide?





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Plug indications.


newairly, normally your full throttle EGT should be LESS than when you reduce the throttle to Say 80%, because of the enrichened mixture. The EGT is the most accurate indicator that you have, and provided that you consider its readings reliable, I would just run the recommended plugs, and tune by the EGT.


..... IF you want an indication from the plugs, then you can soot the insulators up a little by a candle flame, or run the motor with the choke on for a little while. One and a half minutes at full throttle (on the ground) should give you a colour indication, but don't idle the engine after the ground run, as it will change the indication. Good luck nev..



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  • 3 weeks later...

Tried new needles???


Hi Newairly.


Two points


(a) The right jet is the one that gives you the correct EGT, even if it's different to one in the Bing table! Because of the way two strokes work, the amount of air/fuel that gets sucked in depends on a lot of things , notably resonances in the exhaust system, and whether you have the correct prop.


(b) Has anyone told you about the needles yet? There is a little circlip on them, you move it up a notch to go richer, down for leaner. You may need to change needle position from summer to winter or if operating at high altitude. Different needles have different tapers, and the taper varies along the length.


From your symptoms though it sounds like you have a needle that is worn in midrange , making the mixture richer in midrange than usual, which gives the appearance of being lean at full throttle, pull the needle out, have a look at it, the worn area is usually visible. So you should replace the needles, always good to keep a spare set in the your kit on the airplane! (you do have a kit? under your seat, with a rag, spare plugs, plug spanner, philips/slotted screwdriver , spare cable ties, a bit of safety wire, spare safety pins)


Normally the mixture is supposed to get richer at full throttle, i.e the EGT should go down at full throttle. The worst case EGT is usually on descent, particularly a forced descent at part throttle. If I have my needles setup for cruising, then I find I need to pull the chokes on in a rapid descent (It's a 582, but the same thing will apply).


Note it's not a 2 stroke thing, CHT and EGT are only vaguely related, the situation on descent being a classic one, the EGT measures the temperature of the combustion gases, (and hence the temperature of the piston) irrespective of the volume of gas. On the other hand the CHT measures the heat transferred from combustion which is proportional to combustion temperature times gas volume, this heat is then removed by cooling air flowing past. So on descent you are windmilling, so the engine sucks in more air than the carby expects (as there is no exhaust back pressure) , so the mixture goes lean, and the piston gets really hot (it's not melting yet, but the crystal structure resembles a slush puppy) , but because the throttle is retarded, the volume of combustion gas is small but extremely hot, so heat flow into the cylinder is modest, but cooling airflow is high, so the CHT is quite low, in fact you can have the situation where the piston expands so much in the cold cylinder it seizes up.


Which is why in real airplanes, they have engine cowls , that can be closed to keep the heads nice and toasty warm, and a mixture control that is always set to rich for descent. It's why real pilots make throttle adjustments slowly, watch the temperatures, and use cowls and mixture controls as required. This is why we have downwind checks, as the engine setting needs to be different during the final approach (setup to give maximum power if a go-around is required) than what it was earlier during descent (setup to preserve the engine).


As I hinted at earlier, having an incorrect propeller / gearbox or incorrect exhaust can do odd things with EGT, check with other Tyro owners to see what they use.


Although if I had money to bet, I'd go for the worn needles!


While you are poking around in the carby, check the float level, the float should be just horizontal when the petrol starts to come out, (use a bucket under the carby, push floats up with your finger,and have an assistant work the priming pump) now take the floats off (without losing the fiddly bits) put them in some water or fuel , they should float high like corks, if they are half submerged - buy new floats! Now look inside the carby bowl, it should all be the same grey colour, if it is white near the bottom, then the aircraft has been left standing in the weather, with water left sitting in the bottom of the bowl, if this is the case then be alert for corrosion damage in the rest of the fuel system and the carby, bits of white powder play havoc with the jets.


You may want to check your ignition too, I think the 447 has points? If so then check the advance angle and gap. (It's faintly possible an earlier owner ran it on high octane fuel, and advanced the ignition, but Rotax's can't handle the higher EGT)


Cheers, BobT


PS , I didn't actually answer your question yet; yes the CHT will drop at any particular throttle setting when EGT is lowered, but not by much ,100 of EGT = 25 of CHT maybe).



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Thanks BobT,


There is quite a lot to contemplate in your reply!


I have looked at the needle. When I pulled it out I found that the clip was on the top (leanest) position. Aha I thought, maybe that is part of the problem. The Bing table says it should be in the middle groove. Also went to one size larger main jet. Hovever no obvious difference on the next flight. CHT still reading 1200. It drops to about 1100 at 5400 RPM. I tried turning on the booster pump in case it was a fuel flow problem. No effect. The EGT probe is a single one at the junction of the 2 ports. I know this is not recommended, but that is what I got. I read that the reading in this position can be well above the actual correct reading. Any experience? Plugs suggest that EGT is not excessive. Light brown.


My next effort will be to tether the plane on the ground so I can do full power tests without actually taking to the air. I also want to check the static RPM at full power. In flight it seems low, about 6400 RPM.


I have checked the fan belt tension and checked for obstructions around the heads.


I have also removed the exhaust and checked for any obstructions, eg mud wasp deposits.


I am not sure what you mean about the float level. What do you mean about "the float should be just horizontal when the petrol starts to come out"


I will also check the timing. Yes, my 447 has the Bosch points ignition. The Rotax manual describes the procedure well.


The prop is a fixed pitch Bolly which is claimed to be matched to this engine.


I will get on top of this eventually! Fortunately the plane is operated from my own strip so I can try things without having to leave home.





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Hi Phil.


Ah having your own strip, what a luxury. My thruster is on a trailer in the shed as I am "between hangars" at the moment. So it's a couple of years since I've poked around with the engines.


Ground tethered tests have their own problems, e.g. (a) great difficulty in untying the rope afterwards (b) getting a ring vortex around the propeller, particularly if you have a tailwind. It's not representative of real flight, but can be useful indicating if your engine is under-performing, (compared to earlier ground runs, but if you haven't got a history of earlier ground runs it's difficult to tell).


Positioning of EGT probes is apparently important (but I have no direct experience), I have two EGT probes at the recommended position, and a switch to look at either one. (The 582 has two carbs , I think the 447 has only one?)


As regards the floats , check this page http://www.ultralightnews.com/enginemaintenance/bingcarbtuning.htm (the page before the one in your first post) and http://www.ultralightnews.ca/bing/bingservice.html


This will give you a quick indication of the actual level. When you have the bowl off, the floats stay in the bowl . There is a little brass wishbone shaped bracket the floats push on. If you push this up gently you should feel when the fuel valve just seats, (if some one else pumps the fuel, it will stop dribbling at this point), at this point


the little arms should be parallel to the carby base. There is a little tang (pushes on the bottom of the fuel valve) that can be bent to change fuel level. Aha the second web page I pointed to says it can be done on a bench by turning carbies upside down! That's a much cleaner method!.


Note that wear in carby parts invariably richens the mixture, perhaps a previous owner just changed circlip position and put in a smaller jet to compensate for wear? Its starting to look a lot that way.


The heirarchy should look like this:


(a) Get the fuel level to the correct value (don't tweak it to change mixture)


(b) Select the main jet according to full throttle mixture


© Select a needle to control midrange mixture


(d) Adjust the circlip to suit altitude and temperature.


The Bings are extremely reliable carbies (if you don't neglect them). Though you should expect to replace the needles every 200Hrs, and fuel valve, floats,(maybe float pin, maybe jets) every 400hrs.


Also just check whether you have the ancient needle with three rings? And whether you have a tiny o-ring on top of the cir-clip (without the O-ring, the needle rattles a lot more and wears out quicker) aha see here http://www.ultralightnews.ca/bing/needleclipupdate.html (they reckon on 150hrs for the needles).


I've never really been a big fan of reading spark-plugs or tea-leaves. With two strokes the type of oil and mix ratio affects plug colour, and whether you are burning unleaded (automotive) or high-lead (aircraft) fuel.


Cheers, BobT


PS It's winter now , expect to use a richer mixture, try the bottom groove, if it runs smooth over the entire range then all's good



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Ah this might be interesting:




look particularly in the area where the posting date changes from 11sep to 12sep.


two interesting tips (a) use a shim under circlip (b) use a thinner needle


see also http://www.thirdshift.com/jack/firefly/firefly58.html


cheers, bobt



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  • 3 weeks later...

Not out of the woods yet


Firstly, thanks for the many really useful replies.006_laugh.gif.0f7b82c13a0ec29502c5fb56c616f069.gif


After looking through the incredible amount of information available about setting up Bing carburettors I found a description of how to check the fuel level in the float bowl




The level was about 3-4mm low. wonderful I thought, maybe this is the problem, so I adjusted the Float Arm to get it to the specified 1/2 inch below the rim with the floats removed. So far so good.


I then tied the aircraft down with a longish rope to allow full power runs on the ground.


Result. EGT at full power is still about the same, 1200+, but at lower power levels the EGT drops to about 1100 which is fine. But I still find that is is impossible to get a steady RPM around 5400. It wants to go up to 5800 or down to 5000. I have heard that this may be caused by too lean a mixture and I guess that the main jet is still the main controller for this region.


I now have the needle clip on the middle position and a 165 main jet, up from 160 originally. Both as per the Bing chart. It seems that I need an even bigger main jet. At full throttle the fuel flow is probably not influenced much by the fuel level because of the high suction in the venturi.


What really puzzles me is that the previous owner claims that he did not have these problems, and in fact the plugs seemed to confirm this when I first removed them.


Can anyone shed more light please:help:





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I don't know about the 2 strokes, but "hunting", which is rpm going up and down at a set throttle position in 4 strokes is usually caused by a rich mixture,



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Hi Phil.


Since my last notes, I found my Californian Powers Systems catalogue, several hundred page gold mine of info about Rotax engines. And lo and behold Mike Stratman has scanned his excellent articles onto a web page. http://www.800-airwolf.com/articles.htm


Page two of this part shows the different needle shapes http://www.800-airwolf.com/pdffiles/ARTICLES/part11.pdf


Page two of part10 shows the expected mixture vs rpm




He also makes a throwaway comment about exhaust systems being responsible for the "not holding a midrange rpm syndrome". Exhaust system tuning was to be one of the things I would have suggested to you also, but presumably the exhaust system hasn't changed, and a leak at the ball joint is normally noticable. I had an exhaust spring fall off during climbout once, and it was a real struggle to clear the terrain at reduced full power, (but power was reasonable at 75% throttle where the tuning is less critical)!


Mikes part12 talks about CHT and EGT which was your original post anyway.


Did you get a chance to look at the weblink I posted earlier at matronics.com , the guys there had similar problems with EGT and RPM hunting, and found switching to smaller needle solved their problems. I'm thinking a needle with a steeper slope will help you out (15K2?) certainly a new needle has a steeper slope than a worn one so there's a first choice


I'm a bit surprised you still have high EGT's on the ground, the expectation is that max rpm will be lower, and EGT will be lower when on the ground.


Here's a summary list of High EGT causes:


(a) New Needle most obvious - yet to be tried


(b) Low fuel level - fixed that


© Main jet- tried a 10% bigger one maybe try a 170?


(d) Your altitude is lower or its colder than before


(e) Exhaust system detuned


(f) Air leak on inlet side, check rubber carby mount


(g) your aircraft is much slipperier than before, check your ailerons: do they both point up?


(h) your propeller pitch is finer than before.


(i) you are much lighter than the original owner


(j) Different (thicker) two stroke oil, or higher oil:fuel mix





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Responses to your suggestions,


Altitude is higher than before. Previously about 500ft, now 2500ft. This should give a richer mixture.


The exhaust system is untouched and is a standard Rotax. I have checked it for any blockages, eg. from mud wasps, by removing and soaking with water for a day and vigourously shaking. Nothing came out.


I have carefully checked for possible carby leaks and can not find any places. The O ring is in place on the top cover.


Prop is a fixed pitch Bolly, so no issue there. Static RPM on ground is approv 6350


I am probably 20kg lighter than previous owner, but ground run is similar to climb in flight for EGT.


Oil is the same. Still using some given by previous owner. Pennzoil and ULP at 50:1 carefully measured.


I don't think the rigging is different, but yes, both ailerons point very slightly up compared to the bottom surface of the wing. I asked Dafydd Llewellyn about this and he did not think it was an issue. Do you have any other comments?


I have looked at http://www.800-airwolf.com/articles.htm


What an excellent resource! Pity the scanning is a bit lght in density. In part 11, page 2 he again talks about problems holding RPM and comes to the conclusion that as well as exhaust systems it can also be caused by mid range mixture. He suggests using the choke as a diagnostic. I will try that.


I also looked at matronics.com which is where I got the idea that lean mixture might be a cause for hunting in RPM


I guess the next thing is to order another main jet, 170 this time and see what happens. Given that the EGT is fine at lower throttle settings it seems that the needle may be OK. At full throttle the needle is out of the needle jet according to the descriptions. I wonder if the needle jet may be partially blocked?


The engine runs smoothly from idle at approx 2000RPM to full power, 6350 static and does not complain about rapid throttle openings. My next thing to try is the effect of applying choke while at full power. Apparently it will still enrichen the mixture and give a guide as to what is the problem. I can also try turning on the electric priming pump.


Thank you again for giving me so much advice.





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you haven't got the jet support sitting above the plastic slide instead of below it have you. Just a thought? This will cause the needle to come out of the jet and run exceedingly rich.



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Where should the sieve sleeve sit?


I am puzzled about the location of the sieve sleeve in the carby.


The one that was in when I got the aircraft was located at the top of the mixing tube, and fitted quite tightly because it appears that the plastic rings top and bottom had shrunk amking the sleeve barrel shaped. In this position I can not see what it would do as no fuel seems to pass through it to the main jet.


The new one I got is a loose fit and sits in the well at the bottom of the float chamber where it would appear to be in the fuel flow. One reference says that the purpose of the sieve sleeve is to stop bubbles entering the main jet so this location makes sense to me. However none of the guides to reconditioning Bing Carburettors mention where the sleeve is supposed to be.







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While looking for the correct location for the sieve sleeve in my book, I came across a snippet of info different from what I have posted. It stated that failure to hold rpm is often caused by the "tuned exhaust" and you just have to live with it. So much for my rich mixture!


It appears that the sieve goes in the bottom.



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I saw that one also. In a another article he says that later information from people who read the exhaust suggestion leads him to believe that mixture is often the problem. I would think that if the mixture is wrong in that intermediate region where both the needle jet and main jet control the mixture it may not be possible to hold an intermediate RPM because as the RPM rises the incorrect mixture may cause the power to drop. I suppose this could be either too rich or too lean. In my case I think too lean because this the full power condition.


My exhaust is Rotax standard.


I have also seen a reassembly guide for the Bing which says that the sieve sleeve should be positioned with the bottom level with the main jet, and that it will "assume the correct position" when the float bowl is fitted. This suggests that it is meant to sit in the recess at the bottom of the bowl.





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Guest babs1aus

Before wasting endless amounts of time and money Like I did once.


Borrow some one elses carby if you can and try. As gauges are just that a gauge of performance. They do go wrong. We discovered our problems by putting in someone elses egt gauge for a run on the ground it was reading better by 100 we then put the other carbs on with standard jetting lost another fifty so we had an over reading gauge and a slightly incorrect carby setup. Which was returned completely to standard and a new mark made on the gauge and confirmed by visaul inspection of the plugs. So we now have a perfectly tuned motor and a gauge that has a new mark for the range. It has now done 170 hrs since then and Yes I do have a new gauge nowdays. So if you can beg borrow and try interchanging a few components you can quickly isolate the problem.



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