I have exhausted my brain and the brains of several other engine experts with an issue im having with my Rotax 277UL (free air) so I thought I would ask here.
I acquired this engine from a board member a decade ago, its hours and condition largely unknown. I cleaned it up, rebuilt its carb and mounted it up last spring with a 2.58:1 gear drive swinging a 3 blade ultraprop. static runs with stock carn jetting were fine so I attempted a short flight, 5 minutes in while running at 6000rpm with a cht of 430 and an EGT of 1050 the piston burned through on the intake side and the engine seized. (neither me or the plane were harmed thankfully).
I rebuilt the engine last October, replaced crank seals and installed a piston and cylinder from my fan cooled 277 ( a known good motor with about 80hrs run time). Now the real fun has began.
on the first break in run I had to abort early as at anything over 5k rpm the CHT quickly went pas 475deg. I built a simple cooling shroud around the head and increased the main jet size from the stock 148 to a 150 then a 152. No matter what combination of jets, needle clip positions or shroud designs I try, In an attempt to keep the CHT below 450 im jetted so rich on the main at 152 that the EGT never exceeds 900 deg at any power setting and im starting to foul plugs.
here are the particulars for my engine:
tractor configuration, free air cooled all temps recorded in flight at 65deg OAT and 40mph airspeed premix at 40:1 Bing 54 carb and stock exhaust.
max static rpm: 6340, in flight 6450
EGT at WOT: 850deg CHT at WOT:400deg
EGT at 6000rpm: 900 CHT at 6000rpm: 450
EGT at 5500rpm: 950deg CHT at 5500rpm: 440deg
EGT at 5000rpm: 900deg CHT at 5000rpm: 425deg
EGT at 4500rpm: 800deg CHT at 4500rpm: 375deg
I have checked and verified timing multiple times I have pressure checked the crankcase, no leaks I have checked piston and cylinder clearance I have checked, checked, and rechecked gauges for accuracy
most people cant get their free air 277s to reach 375deg on a hot day and I cant keep mine at 450 or under unless I jet it so rich it doesn't want to run well, at full power the carb is aspirating fuel out the intake but she still wont stay cool. if I go to a 146 main jet to get the EGTs up to 1100deg then the CHT will skyrocket past 500deg at anything over 5000rpm.
what gives, this plane is going to miss another flying season if I cant sort this out. so please lay any and all advice on me, im going to get a priest and some holy water.
Chris
Remember, in aviation death sentences are administered by the laws of physics, not the FAA.
Hi Chris, I am very much interested on this case and following. What if, you install the standard carb set-up and do the break-in using rotax's program but running half rpm as your first stage then the standard program's recommended rpm as final. Just a thought. Regards, Phil
The head hasn't been painted has it, other wise maybe vacuum test the crankcase, perhaps a seal "seals" under pressure but leaks under vacuum?
Might be impossible with the prop blast, but might be worth a shot, spray starting fluid around everything while it's idling, carb flange, case gaskets etc. and listen for a change in the engine.
I am kind of wondering about a possible exhaust obstruction, mouse nest, etc.... does anyone know a good way of cleaning debris out of a Rotax muffler can? It seems like the straight forward method is blowing compressed air through it but I know there are a lot of nooks and crannies that cant be cleaned that way.
Chris
Remember, in aviation death sentences are administered by the laws of physics, not the FAA.
>>I have checked, checked, and rechecked gauges for accuracy<<
I don't doubt that you have - but how did you do that?
Because of reading of your woes this morning, when I went to re-install my rebore re-pistoned 503 with its new temp senders this afternoon, I checked the gauges themselves.
There are two dual gauges; one for CHT one for EGT. Both guages showed about a 5% difference in the current needed for them to show their full scale readings. Makes a mockery of Rotax's requirement of a "40 degrees maximum' difference between the two pots. Maybe Rotax require atomic clocks to be involved in the cal of their two-stroke engines' gauges.
BlueMax, your problem is hugely bigger than mine, but how DID you calibrate your CHT indication?
I would have asked the same question, how accurate are the gauges Bluemax is using? I've seen some quite marked differences between different gauges on the same cylinder, so it's not out of the question that the gauges are both inaccurate.
Any possibility of when you replaced the piston and cylinder that it modified the compression ration? (Wrist pin to piston crown length difference, connecting rod length difference, piston crown profile etc)
How did you check timing? Try regarding a bit and see if temperatures go down. Low EGT and high Head temp means that the burn is mostly done in the cylinder and escaping gases don't burn anymore. This leads me to believe that your timing is too early. If I'm correct, you should also feel a bit of power boost when you retard the timing. Basically, set your idle high (2000-2500 RPM) and if you retard the timing at idle and the RPM increases, you're on the right path.
I set timing with a dial gauge on the piston and an ohmmeter across the points per the manual and mike strattman's notes on timing. I currently have it set at the retarded limit per Rotax.
Chris
Remember, in aviation death sentences are administered by the laws of physics, not the FAA.
Do you have a different exhaust pipe? Where is the EGT bung located on the exhaust? Can you share a picture? Can you measure the exhaust temperature difference between cylinder side and output side? Is it hard to start the engine? How low does it idle?
All those questions to figure out how the exhaust behaves and if there really is a rats nest in there.
In my experience with 2 stroke engines I've always found that if you retard the ignition the engine runs hotter. Try advancing the ignition to either the centre setting or slightly advanced and see if that helps to reduce the temps at high revs.
The reason for this is that 2 strokes ideally need to have an advance retard like 4 strokes, but as the range is so much less than that needed to stop a 4 stroke from pinking, most manufacturers don't bother and instead just specify a set range of only a couple of degrees. However, within that short range, if you retard the ignition it will help with starting because the mixture is ignited almost at TDC (so it won't try to kick back), whereas at high revs the mixture needs to be ignited earlier, or it'll still be burning as it goes out of the exhaust port. This will cause an EGT to show a higher reading than if the ignition had been ignited earlier at high revs, thus giving it more time to burn before the exhaust port is uncovered.
One other thing is the carb needle. I notice that you've tried changing the jets, but if the needle has the wrong profile it could affect the mixture at a particular rev range, and I notice that the temps tend to be correct up till about 4000 or so, and only get really hot above 5000.
If those don't make a difference then like Valy said, how far from the exhaust port is the EGT probe? Even 1/4 inch can make a significant difference to the reading you'll get. Don't forget you have to measure from the actual inside edge of the port where it meets the cylinder wall and go outwards from there, not forgetting to add in the thickness of the exhaust gasket in order to get the correct distance to drill the hole for the probe in the manifold.
Best of luck with it. I hope you get it sorted, and please let us all know what you had to do to get it reading correctly when you've succeeded.
I've just reread your original post on this thread and noticed you said the fuel/oil ratio you're using is 40:1. That's fine for running in, but once run in the engine should be run on 50:1 mixture. Running it with more oil in the mix means less petrol, so it's running weak. Weak running engines run hotter, so try adjusting the mixture and see if that helps.
The high CHT could indicate a high loading on the engine, maybe due to propeller size/blades/pitch. I highly recommend the following video with Brian Carpenter from Rainbow Aviation ("Is your 2-stroke engine about to fail"). Do also listen to the questions in the end. You might find the answer to your problem here.
I pulled the engine yesterday and mounted up a replacement. After getting the engine on the bench I proceeded to disassemble and inspect it. What I found was discolored cylinder walls in the upper combustion chamber and ALOT of scoring around the bottom of the cylinder sleeve on the exhaust side along with the piston showing lots of evidence of ring blowby. with the exception of the scored areas, everything checks good dimensionally the ring gap is still good and piston/ cylinder clearance in spec. rings had no carbon in the grooves and floated freely on the piston.
on an engine with a fresh piston / rings / cylinder, broken in per the book, what would cause the rings to fail to seat, all damage in the below photos occurred 4.2hrs run time and as previously mentioned, EGTs never even got within 200deg of redline.
In your photos above, the almost total lack of a visible cross hatch finish on the cylinder walls after only 4 hours of running indicates that whoever last honed the cylinder didn't break through the work hardened surface with the hone. So although there is a cross hatch finish, it's so shallow that the first time a piston ring scraped along it, the honed surface will have been smoothed over and thereafter the rings slid along a hardened polished surface.
I used to work as a full time engine reborer and honer, and whenever I rebored an engine I always, and I do mean ALWAYS had to leave the cylinder at least .001" smaller than the finished size, so that I could hone it out to size. This allowed the honing stones to cut through the work hardened surface of the newly bored cylinder (the boring cutter hardens the surface as it cuts it), and reach the slightly softer metal beneath. So that when the honing is complete and the cylinder is the correct size, the surface is not only cross hatched, but is softer than the piston ring. This allows the rings to cut through the metal and bed themselves in correctly.
Below is a photo showing a rotax 462 cylinder. After only 4 hours of running, the honing marks in your cylinder should still be very visible like they are in the newly honed cylinder in the picture.
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