The Care & Feeding of 2-Stroke Motors

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N523SK

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Apr 7, 2020
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Hi, Everyone:
  1. I'm a student pilot.
  2. I bought an airplane with one of the famous/infamous Rotax 670 motors (with the HO exhaust and the Ducati ignition) last year.
  3. The motor was installed and configured by an A&P in late 2018.
  4. The previous owner only put approximately 15 hours on the motor before he lost interest in aviation.*
  5. The airplane had been sitting, idle for most of 2019.
  6. The only recent use that the airplane has gotten has been the 2-hour flight from Antelope, OK to Wichita, KS (STOL Creek Avaition) and the use that it got when Michael Busenitz performed the annual, made some tuning changes to the motor, and re-pitched the propeller.
  7. The airplane has been in the hangar since I bought it.
* Some pilots that I spoke to during the inspection/purchase process expressed their opinion that maybe, that motor, in a pusher configuration, may have been too much for an inexperienced pilot such as the previous owner - But that's neither Here nor There. But it certainly indicates to me that I'll need to be very careful with this aircraft until I have more experience - much more experience!

Anyhow; I have questions about the care & feeding of this motor and 2-stroke motors in general:
  • How to maintain it.
  • How not to maintain it.
  • How to operate it.
  • How not to operate it.
  • How to store it.
  • How to take it out of storage and get it ready for the Season.
This might seem a little silly. But after reading the posts on this forum and others it's become clear to me that 2-stroke motors have their own unique set of needs. e.g.,
Because this is all new to me and the airplane is going to be sitting for a while longer while I finish my training: I'd like to ask what steps should be taken...
  1. to preserve the motor while I'm unable to operate it and then
  2. to prepare the motor for use once I find a pilot friend to exercise that motor for me. Or once I complete my training and I'm ready to fly it myself! (Very few of my pilot friends fly LSA's. None of my pilot friends fly pushers. And none of them fly LSA pushers!)
TIA,
Eric P.
N523SK
 
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N523SK

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Apr 7, 2020
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This might seem a little silly. But after reading the posts on this forum and others it's become clear to me that 2-stroke motors have their own unique set of needs...

For example: In a post on this forum, Armilite wrote...

For Almost, All Rotax/Skidoo 2 Strokes, Detonation is the #1 Problem account they use High Compression Ratio's in most of their 2 Strokes. Minimum Fresh 91 Octane. Pump Gas can lose 2 Points in Octane in as little as 3 weeks...

Atmospheric Pressure is 14.7 x 11.5cr = 169.05psi at Sea Level, and Good Fresh 91 Octane is Good for Max 175psi. So your Rotax Engine is on the Edge from suffering from Detonation. 100LL has a 5 Year Shelf Life. A 50/50 Blend of 100LL and 91 makes about 96 Octane. For a 670 92-95hp@6500rpm, a High HP Engine Pulling/Pushing a Heavier Airplane, I would use a minimum of 96+ Octane myself.
So, naturally, Armilite's recommendation makes me curious about the possibility of using 100LL in any 2-stroke!

Suggestions? Recommendations?

TIA,
Eric P.
N523SK
 

proppastie

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Might look at this I am not familiar with that engine. If there is a manual for your engine you might check it or get something from Rotax.....Try to get written instruction from the factory if they exist. You could easily get different answers from each person you ask but hopefully the factory has the best answer.

I would not modify a good running engine I believe "stock is the best compromise".....Many here are highly experienced and are capable of performance modifications, but one really has to know what they are doing.

I have never run an engine full bore 100% (that is for racing) ....your training is an example, full power to altitude (or manufacturer's max time) and then cruse power.

Most 2 stroke work better on unleaded fuel, but again your factory manual will give you guidance.
 
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n3puppy

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Jun 25, 2019
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There is a lot of useful information on Rotax engines on the "Fly Rotax" site

Rotax Aircraft Engines - Rotax Aircaft Engines

Unfortunately, as time goes on more and more of the two stroke information is being left behind since most the motors are no longer in production.

But some of the manuals (operator and service) are still available for download in the tech section. Operators manuals give a lot of information on the day to day operation of engine, including the flight parameters you are talking about

 
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delta

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I found a few sites that have some good information, particularly in the last one.
 

Will Aldridge

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Rotax Rick who sells quite a few of them would be a good place to start:

 

N523SK

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Apr 7, 2020
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The lead will cause more problems than it solves.....

Yes - Except for the issue with pump gas that Armilite had pointed out...

For Almost, All Rotax/Skidoo 2 Strokes, Detonation is the #1 Problem account they use High Compression Ratio's in most of their 2 Strokes. Minimum Fresh 91 Octane. Pump Gas can lose 2 Points in Octane in as little as 3 weeks... 100LL has a 5 Year Shelf Life.

Rotax Rick who sells quite a few of them would be a good place to start...

Yes - Rotax Rick built this one! So I sent Rotax Rick a direct message (DM) with all of the history & all of my questions & my telephone number. And he called me earlier, this morning, to get me up-to-speed with all of the stuff that I need to know about the Rotax 670!

Eric P.
N523SK
 

Dana

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What airplane? And what carburetor?

Random thoughts:

I'm not familiar with the 670 in particular, but the most important thing in any 2-stroke is proper mixture. Too lean and you fry the engine, too rich and you foul the plugs and/or it doesn't run smoothly and make power. The engine must be jetted correctly for the altitude you're flying at and the air temperature. I used to change jets twice in the year (spring and fall) as the weather changed.

If you can get ethanol free fuel, do so, but most engines will tolerate up to 10%. But it will run slightly leaner (so must be jetted for it!) and deliver slightly less power. I ran mogas with ethanol in all my 2-strokes. If I went cross country and needed to refuel with avgas, I was safe because it ran richer so I didn't have to worry about frying the engine, though I had to make sure it didn't load up and quit on final approach from running too rich. The difference was probably a half of a jet increment.

An EGT gauge, properly installed in the correct location, is critical for getting the jetting right (look at the plugs too). CHT will tell you if the engine's overheating due to inadequate cooling on hot days, and when it's warm enough to fly on cold days.

European octane numbers in European engine manuals are different from American numbers, it's measured differently. US 91 octane is sufficient.

Use the manufacturer's recommended amount and type of oil, no more, no less.

If you run mogas, switch to avgas for the last fillup and flight before putting it away for the winter... it stores better. Other than that, I never did anything special to put it away for the winter or take it out again in the spring, if I got a nice warm day in midwinter I'd go flying.

A lot of people talk about using fresh fuel. I never found that to be an issue, within reason, I regularly flew with 3 month old fuel if for some reason I didn't fly for awhile. The important thing is to store it in a sealed container. On my plane I had check valves in the fuel tank vent to limit the amount of air moving in and out.

2-strokes are thirsty. Don't run out of gas.

Most of Armilite's experience seems to be racing engines, and most of his suggestions are things that wouldn't (or shouldn't) be done by the average pilot who just wants a reliable engine without much fuss.

2-strokes like to run at full power, if they're cooled adequately. What they don't like is a long fast descent at partial power with an unloaded prop. Chop it to idle for descent.

Use NGK spark plugs with solid tips, not the screw on tips. Don't use resistor plugs if you have a resistor cap on the high tension lead, the ignition module won't be happy.
 

N523SK

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Another pilot in the Challenger Ultralight Facebook Group warned me that...

...with 100LL, you have Lead to worry about. It plates out on the tops of the pistons, on the rings, and on the spark plugs, requiring more frequent changes and inspections.

Properly maintained mogas, or even auto fuel with ethanol, does not have this issue. (Jim Robinson only has auto gas at his Erie Airpark and never has had an issue. I use mogas year round without issue.)

Ethanol fuel additives can affect the epoxy of composite fuel tanks, (which we do not use.) In service, it does absorb up to 4% water that might be in a system. That is a good thing! It still runs fine (with a bit less power, admittedly,) and there is no engine stoppage if the fuel is dissolved. That is why it is used as an additive in the winter ("Heet") in cars and other engines.

I'm puzzled by the comment about the Expoxy of composite fuel tanks (i.e., "which we do not use") because I believe that the 15-gallon fuel tank in my Challenger II is made of Fiberglass-Epoxy composite. But I'll verify that tomorrow... Or maybe someone here can shed some light on the 15-gallon QC tank?

Another pilot in the same Facebook Group added...

with 100LL you need to run decalin to prevent lead buildups
But I'm not sure what to make of that except to take it as a friendly suggestion that If I do burn 100LL in the Rotax: I should be sure to add Decalin to the fuel. But, IIRC, neither Armilite nor Rotax Rick mentioned Lead plating or Decalin - So it must not be that critical!

Notwithstanding all of that: Running 100LL sounds like the best possible solution...
  1. It's got a much higher octane rating.
  2. It's got a much longer shelf life.
  3. It runs cooler.
  4. It lacks ethanol so it won't absorb moisture; damage the fiberglass fuel tank, or; damage the fuel lines.
  5. It's ubiquitous.
  6. Running one fuel exclusively will reduce the possibility of human error.
What are The Community's thoughts about this strategy?

TIA,
Eric P.
N523SK
 

Dana

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1. The higher octane rating only matters if your engine requires it.
2. Yup.
3. Not really. It runs cooler if the engine is jetted for mogas with ethanol, because ethanol runs leaner and thus hotter.
4. Yup.
5. Maybe. The airport I kept my Kolb at for awhile didn't have gas, so the gas station a mile down the road was more convenient.
6. Don't think that really makes a difference here.
 

N523SK

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Messages
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I'm puzzled by the comment about the Expoxy of composite fuel tanks (i.e., "which we do not use") because I believe that the 15-gallon fuel tank in my Challenger II is made of Fiberglass-Epoxy composite. But I'll verify that tomorrow... Or maybe someone here can shed some light on the 15-gallon QC tank?
I was able to find the manufacturer's documentation for that 15-gallon Fiberglass tank: It was manufactured by Aviation Turbulence. But unfortunately the documentation does not specify whether an Epoxy resin or a polyester resin was used.

Eric P.
Portland, Oregon
 

N523SK

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Apr 7, 2020
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Another pilot on another forum recommended that I watch Bryan Carpenter's video "Is your Two-stroke Engine about to Fail?":

The EAA website has a 1.5 hour webinar on why 2-stroke engines fail - based on the Rotax 503, and also has numerous short tips on 2-stroke engines. The EAA is the best $40 investment any flier can make.

And, after watching the video, I can safely say that there are many factors that go into the reliable operation of a 2-stroke motor. And I can say that Bryan's video will make pilots aware of what most those factors are. Where Bryan's video could use improvement, IMHO, is in the area of "How to correctly measure & adjust those factors". But the video is definitely worth the Time and the Money - And I would recommend it to anyone that has an airplane outfitted with a 2-stroke motor!

FWIW: There are at least another dozen videos available on 2-stroke operation and maintenance on the EAA site. I don't plan to watch all of them - But I'm going to at least give 'em a look to see what I can learn from them!

HTH
Eric P.
Portland, Oregon
 

bryancobb

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Jul 9, 2019
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I will not use the word "expert" but I do have hundreds of hours flying Rotax 2-strokes with not a single hiccup. I put 200 hours on my Tierra II with a single-ignition 503 Fan-Cooled with ignition points. I have 200 hours in Mini-500's. One was with the factory grayhead 582 and the one I own now has a bluehead 582. Both ran with Ducati Dual Ignition. I have a dozen or so hours in a Flightstar II with bluehead and a Fergy II with a bluehead 582. The only event that could maybe be called a Rotax engine problem was when a Rotax muffler had rusted inside and cracked open like an egg...in flight. It was a Tierra pusher and the engine was right near my ears and it was deafening. The EGT's & CHT's stayed stable so I did a 180 and flew 10 miles back to the airport and landed normally. For the first 30 seconds, It made me nervous until I saw the engine would stay running even with the loss of back-pressure. My next several replies to this will try to remember and spell-out all the details to MY success. Some of the bullet-items on my list are fiercely debated.
 

bryancobb

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This reply will make a few general explanations for my reliability with 2-strokes over the last 20 years.
* The 2-stroke engine is a LOT less forgiving than Lycomings or Continentals and to fly one safely, YOU MUST FLUSH ALL KNOWLEDGE YOU GAINED FLYING CERTIFIED ENGINES.
* The most accurate information on how to operate and maintain a Rotax 2-stroke --or any other brand for that matter-- IS THE MANUFACTURER. Follow their published inspection and maintenance instructions, to the letter.
* Keep the engine in its factory configuration. Do not remove accessories that came on the engine.
* A 2-stroke is "harmonized" to run at a certain RPM under a certain LOAD at full throttle. A bluehead Rotax 582 for instance, is rated at 6800 RPM, continuous. A slobbering-rich 582 can spin a flat-pitch prop and reach 6800 RPM
but isn't running right. A slightly lean 582 won't reach 5800 RPM, even if it has the correct prop load, and it will get hot! You need to understand and KNOW the correct prop load for your setup. Gearbox ratios affect that so there's a lot of variables. There are published guidance on determining the correct prop load for YOUR airplane. The one that comes to mind is Mike Stratman's... https://www.cps-parts.com/cps/pdf/Part6.pdf... Select the correct prop and pitch before starting to adjust jetting (mixture) for correct RPM.
* Mike's link above has SIXTY articles that are like the OPERATING BIBLE for 2-strokes. Learn it, know it, live it. Just paste the link in your browser and change the SIX to any number from 2 to 62, inclusive. Read every article that applies to you, at least once per year.
* A 2-stroke delivers the oil by one of two ways. It does not have a sump of oil that circulates through the engine. Oil is DELIVERED BY THE ENGINE, either by someone MIXING it with the gas, or by an OIL INJECTION PUMP that mixes it with the gas inside the inlet port. Either way will keep your engine running, as long as the oil gets where it needs to be but there's ONE MAJOR MAJOR DIFFERENCE. If you mix your gas in the can before pouring in the aircraft reservoir, YOU KNOW WITH CERTAINTY that it's there, but it's at a fixed ratio, regardless of whether you are at full throttle or idle. If your engine has an injection pump & tank, it varies oil/fuel ratio proportionately to how much power (heat) you are making so at full throttle it mixes at 40 or 50 to ONE. The only way you KNOW WITH CERTAINTY that an oil injected engine is getting enough oil is to have NO LEAKS and record and track your oil consumption accurately on every flight. At Idle, the ratio may decrease to 100:1 or more! An idling Rotax doesn't make much heat and cannot burn the excess oil, so where does it end up? It gums-up the piston ring grooves, it sticks on the underside of the pistons and in the rod bearings, and leaves a lot of goo in your exhaust ports and in your muffler.
 

bryancobb

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By far, the most important tools, second only to knowledge, to operate a 2-stroke safely is to have sufficient, accurate ENGINE HEALTH MONITORING INSTRUMENTS in your panel and in plain view in flight. That means each cylinder needs Exhaust Gas Temperature and Cylinder Head Temperature digital readouts with a monitoring/scan feature that alerts the pilot if the high limit is exceeded. Important but not as much is a coolant temperature gauge if your engine has liquid coolant. Example: My 582 bluehead in my current Mini-500 helicopter gives me the most power between 1050 and 1125 DegF on my EGT's. Between 1125 and 1150, there's no benefit and above 1150, I risk seizing my engine. The Rotax Factory Redline is 1200, and aluminum MELTS around 1350. As you can see, it is very close to melting when it's running right. There's no way to know you are in the sweet/safe spot in flight unless you have EGT/CHT readouts. On Lycomings and Continentals, if you accidentally get a little too lean, the engine makes less power and less heat, bit keeps running because it has an oil sump and pump and lubrication is still sufficient. Conversely, on 2-strokes, If you fly it a tiny bit too lean for a short period of time, the engine starves for oil and your pistons get smeared on the cylinder walls like butter. It WILL seize or at least damage it so badly that you have to rebuild it. Learn how a 2-stroke operates and know with certainty that your engine is getting enough oil at all times... NOT TOO MUCH OIL, that's bad too, as I mentioned earlier.
 

bryancobb

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I flew RC helicopters in the early years of RC helicopters before battery technology made glow-engines almost obsolete. Much of my 2-stroke education happened during those years. I learned that helicopters, with their CONSTANT RPM requirement was the perfect-storm for seizing your expensive 2-stroke glow engines. The oil was delivered by pre-mixed gas and a constant RPM meant the piston was sliding up & down the cylinder wall at the same times per minute at full throttle as it was at reduced power during descent. The oil supply was varying with throttle setting but need for lubrication remains high.

As I learned, it became obvious that the 2-needle-valve carburetors had to be set very rich at high throttle so that there would be enough oil in mid-range throttle settings to prevent seizure. Later in this era, engine manufacturers came out with 3-needle-valve carburetors, specifically for RC helicopters. The purpose was to allow us to lean-out the high (Main) needle valve to get max power at high throttle settings while allowing us to set the mid-range slobbery-rich to get enough oil to the engine in the midrange like when you are coming in to land.

This was a period when my level of understanding of two stroke aircraft engines increased TEN-FOLD !!!!! This contributed more to my hundreds of hours of success in the air, than anything. The three-needle-carbs that became state-of-the-art in the most expensive RC helicopter engines IS EXACTLY WHAT THE BING 54 CARBURETORS ON ROTAX AVIATION 2-STROKES ARE !!! My education in the 1980's in RC transfers 1:1 to Rotax 2-strokes.

Learn the BING 54's. Learn all about "DENSITY ALTITUDE" and how it dictates what jets you need to install in the BING 54's. Learn all about the difference between a BING "Jet Needle" and a "Needle Jet." These are very different and have a totally different effect on mixture (EGT's) with variations in throttle opening. Learn what they do and how. These two items are collectively, the same thing in function as the 3rd needle (midrange) that O.S. Max and Enya added to my RC carbs in the 1980's.

Gain an understanding of the MAIN JETS in the BING 54's and what they do. Learn how to change them and always have a change tool and several sets of jets in your aircraft at all times. Density altitude can change significantly from sunrise to sunset and may require jet changes. Learn what throttle setting on your aircraft results in "crossing the state-line" from "Running on the Main Jets" to "Running on the Midrange Jet Circuit." When your throttle crosses that threshold, the engine becomes a different animal. If not jetted correctly, there will quite possibly be a large change in EGT's. The most common scenario where seizure happens is when a pilot has been flying around fast at high throttle and "Running on the Main Jets" where the engine is getting enough oil. Then they reduce throttle to land and when the engine enters the mid-range jet circuit, EGT's start to rise. Your airspeed decreases to land so cooling air is less abundant. Heat increases because the lean engine has more friction and BAM! It seizes. The pilot usually is able to land and when the engine cools, the seizure disappears and the pilot cusses his two stroke and contemplates a 912. He goes flying again and the ruined engine runs well at full throttle but again, when power is reduced, another seizure occurs. The correct pilot action would have been to fly with the richest midrange jetting possible without "loading up" at midrange throttle settings and "cautiously sneaking-up on midrange jetting" that is correct. THERE SHOULD BE A CORRESPONDING DECREASE IN EGT's AS THROTTLE IS DECREASED AND THAT TREND SHOULD NOT REVERSE WHEN YOU CONTINUE DECREASING THROTTLE INTO THE MIDRANGE.
 
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