ETLB Squawk Forums — Archive

My buddy has a 503 spitting fuel out of both carbs at idle and midrange we are thinking bad rings, what do you guys think?

If you’re talking about the throat of the carbs then small amounts of fuel will spit out due to timing of the intake port. If the engine runs fine off of idle then it is probably not a problem. A compression tester will validate the rings. Might also check the fuel level in the carb bowl as the floats have a tendency to get saturated and create a rich mixture.

Monte

65 pounds on compression test.

Should be over 80. Time for re-ring.

Monte

You will find the extra power you get from a re ring on the first take off run. The difference is astonishing.

Thanks guys, I'll let you know how it turns out.

Even 80 to me would be low.  More like 100-120 is what I have seen in the past.

But, automotive type of compression tester or leak down tester?

Automotive

Yep, way low then (as leak-down type can read lower).  Anticipate over 100 once you re-ring.

to re-ring an engine some other checks need to be done. the cylinders need to be checked with a bore micrometer for concentricy at the top and bottom along with a hone to establish a cross-hatch pattern to seat the new rings. pistons need to be de-carbonized and checked for wear on the skirts and dome.

monte

You didn't say how many hours are on the engine.  I'm assuming that if the rings are worn out that the engine is high time.  You might be money ahead by sending the engine to a reliable service center and have a complete "zero time" overhaul done.  The cost is usually about $500 for labor plus parts...maybe $2500 total.

https://dealerlocator.flyrotax.com/

Why? Rings don't do more than 150 hours, if that. The crank will comfortably exceed Long John Rotax's  300 hours. Provided that the bearings are still within tolerance (which is easy to check,) there is nothing wrong with glaze busting the bore and fitting new rings.
The likelihood of engine failure is much reduced because there is less risk of ring breakage, bore wash or plug fouling and the power will increase. Done properly (and there is no reason to expect otherwise,) it will improve the engine greatly.
I would fit a new float lever, needle, main jet and needle at the same time, just to set the engine's calibration back to stock.

has the carburetor been disassembled? sounds like a carburetor issue to me, find a bing carb assembly manual and carefully study it.

I fail to understand hot that could affect both carbs and result in low compression. The OP is almost certainly right.

Quoted from tomshep Why? Rings don't do more than 150 hours, if that.

I did not know that.  I put way over 150 hours on a Rotax with no tear down.  A guy I know ran his 503 over 1000 hours with no tear down ever....until the crank finally failed.

Rings are the hardest worked part of a two stroke. Firstly they get rubbing friction rather than rotation. Secondly they stop and start twice per revolution. Thirdly they get hot exhaust gases on just one part of the ring and lastly they wear at different rates around the circumference. Give a well run two stroke a new set of rings and a glaze bust and you'll see the power come back up nicely both from better sealing and from less tilt of the piston which, even if it isn't contacting the bore still has to be "switched" from one side to the other as the rod goes over centre.
If you reckon on 5000 RPM, in 300 hours, those rings have been dragged sideways for just under 7,000 miles.

Interesting that the Rotax service manual doesn't agree with you about ring replacement at 150 hours. They require inspection and cleaning, but no mandatory parts replacement until 300 hours.

I'm a qualified Rotax tech, have been since '89 (whatever that means!) and I've always replaced 'on condition'. Much depends on how the engine is used, type of oil etc. Bore ovality and ring gap and ring groove condition all play a part in WHEN to replace. If Rotax have a mandatory time for any component, then warranty and liability come in to it but in practice, especially with motors that are no longer supported then good engineering practice should apply.

I have found from experience that the 50 hr de-coke is a GOOD idea. I tried letting a 582 go to 150 hrs once just out of interest but the rings were sticky or stuck. At 50 hours that generally isn't the case.

Invest in a bore micrometer and check the shape and tolerance at each 50hr de-coke as well as measuring ring gaps (you do remove the rings from the pistons at this time don't you?  ). My advice anyway  

Quoted from Max SSDR I'm a qualified Rotax tech, have been since '89 (whatever that means!) and I've always replaced 'on condition'.

Is it quite common for rings to be worn out and needing replacement at 150 hours?

Qualified rotax reps gotta eat

Quoted from radfordc

Is it quite common for rings to be worn out and needing replacement at 150 hours?

No not really. As I say, depends on how they have been treated. I'd never have earned a living from servicing Rotax, it was a hobby really and I charged around 40% of my normal hourly consultancy rates!