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loss of compression

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Dick Maddux

Joined: 24 Jun 2008
Posts: 516
Location: Milton, Fl

Posted: Sat Aug 08, 2009 4:50 am Post subject: loss of compression

Well, it has finally happened to me. I have read about this problem happening to a few members and now I witnessed it.

  My Kitfox had not flown in about two weeks so I decided to fly it yesterday. I checked the oil quantity and of course it was below the dipstick (as usually happens when the aircraft sits for a while) I started turning the prop thru its normal ritual when I noticed one of the cylinders had no compression. Great, now what do I do. Well, the tank had not  "burped " so I continued turning the prop and eventually the compression came back on the one cylinder and I flew the aircraft. The engine performed fine.

  I am assuming I had a stuck valve and it "unstuck" itself. I have been using 87 auto fuel with no alcohol. No avgas has been in the engine. The 912 UL has 210 hrs since new. The only thing I have done differently recently (I know this is sacrilege) is added Marvel to the fuel, along with fuel stabilizer. I have read and talked to people that have had this problem in the 912 and they have indicated that Marvel seemed to help with sticking valves and that is why I started adding it to the fuel. I used it in my Pacer and prior aircraft with no problems. I have had problems in the past with sticking valves until I used the Marvel (specifically a Cessna 120 which ended up with a stuck valve and bent pushrod)

  My plan of attack now, is to try and rotate the prop every day whether I fly or not in order to move the offending valve.

   If this happens again and the compression loss does not "fix itself" should I run the engine or not? Did the addition of Marvel loosen carbon sticking it in the valve guide? Should I not use it? I personally think it probably had no effect at all on the valve sticking.

  Any ideas would be greatly appreciated. I know this has happened quite a few times to other guys with there 912's. My oil temp has been running about 190-200 in cruise and coolant temp about 185.

                                                                  Dick Maddux

                                                                  Kitfox 4

                                                                  912 UL

                                                                   Milton,Fl

 

  [quote][b]

Roger Lee

Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

Posted: Sat Aug 08, 2009 6:09 am Post subject: Re: loss of compression

Hi Dick,



Why do you think you have a stuck valve? Many Rotax engines will feel less compression or none at all when they have sat a while and or they are cold. (I have experienced the same thing. Run the engine like you normally would up to operating temps and then after that see if it feels the same. My guess it will be back to normal. If the plane has sat a while a lot of the oil has drained away from the areas that help seal some of that compression, plus the cylinder walls and pistons are not at their optimal diameter because they are cold and different metals.

Go taxi it around or just park it and run it up to 4000 rpm on the ground to temp. bet it goes back to normal.

I know some people swear by Marvel Mystery Oil and it may not cause an issue, but it may not help either. No one really knows in a Rotax. We don't know for sure if it may have a odd burn with one of the engine oils or leave a burnt deposit where you really don't want it. We just don't know for sure. 5 million Rotax's running out there and the majority don't use Marvel Mystery oil and they have never had any issues and some are in the 3000 hr range. I'm not saying don't use it, but more or less, why?

Sticky valves aren't very common and is usually caused by something we did to the engine or didn't do. Using a good fuel has cleaners already in them.

_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056

dashwood

Joined: 24 Jan 2009
Posts: 73
Location: sw ontario canada

Posted: Sat Aug 08, 2009 6:51 pm Post subject: Re: loss of compression

I may not be understanding your plane but if i had no oil on the dipstick i would add some first before hand prop to ensure i didn't just pump air into the system from an empty reservoir.  ???? My 912 has very little compression after sitting a few days and takes several turns to burp. after a 24hr sit it turns harder at all strokes and burps after 2 or 3 compressions

arno7452(at)bellsouth.net
Guest

Posted: Sun Aug 09, 2009 12:48 am Post subject: loss of compression

Hey Dash,

About how much would you add?

Ken



do not archive





---

Dick Maddux

Joined: 24 Jun 2008
Posts: 516
Location: Milton, Fl

Posted: Sun Aug 09, 2009 4:11 am Post subject: loss of compression

Roger,

  Thanks for your response! Your discussion makes perfect sense. It had not even occurred to me that the rings and loss of oil around them might have caused this. My only thought was a sticking valve as that has happened to me before on other aircraft. 

  I will discontinue the use of Marvel also.

  After flying the aircraft yesterday my old nemesis came back. On shut down with the throttle all the way closed (aprox 1400 rpm) the engine stopped and then turned about 3 or 4 revolutions backward. I restarted the engine and then shut it down the same way. It did it again! I restarted then increased the rpm to aprox 2200 then closed the throttle and turned off the mags at the same time (a method mentioned by another owner) that seemed to work on this one occasion.

  After a little thought I decided I better check for possible air in the lifters, because of the backward rotation, so I pulled the valve covers and checked the valve clearances. They were OK.

   I have called Lockwood about this and I have decided that I am the only guy in the country to have this problem. It is much like an old car used to do when you shut it down and it keeps running. I am also one of two in the country that has had an oil filter loosen up by itself and almost spin off. I tightened that Rotax filter absolutely in accordance with Rotax instructions and torque marked it. I now safety wire the filter in place with a large clamp (Adel rubber under it)

  Back to my engine "run on " I have been using 87 auto fuel as specified for the 912 UL. Last summer I went to premium when this happened but now premium is not available without alcohol. I guess I will bite the bullet and use it. It will all be that way anyway. 

  I had thought that my engine shut down technique might be the problem but I guess that octane plays a bigger role in this summer (95) heat.

    Thanks again Roger !!

                                                            Dick Maddux

                                                            Rotax 912 UL

                                                            Milton,Fl

  

 

  [quote][b]

daveaustin2(at)primus.ca
Guest

Posted: Sun Aug 09, 2009 4:18 am Post subject: loss of compression

Where is the oil draining to?   The outlet from the dry sump goes into the 

top of the tank.  The outlet from the tank goes from the top of the tank 

(with tube to bottom of tank) to the oil cooler/pump inlet, so how is the 

tank draining?  Do you have it hooked up correctly?

Dave Austin  601HDS - 912

Dick Maddux

Joined: 24 Jun 2008
Posts: 516
Location: Milton, Fl

Posted: Sun Aug 09, 2009 4:38 am Post subject: loss of compression

Dashwood,

  My Kitfox sits high on smaller bushwheels. The oil tank is above the engine and is fixed as low as it will go. The oil siphons back into the engine case as it sits until it goes below the dip stick (over a few weeks) When I "burp 'the engine it is all full again. If I were to put more oil in, oh boy what a mess !!!

  I really don't know how far it drains down. I suspect if I lifted the tail up to where the tail is level I would see some oil on the dipstick. If it drained all the oil into the crank case I am sure I would have a massive hydraulic lock (which I haven't)

  I have asked this same question on the Kitfox forum many times and no one has an answer. (they have it too)

  My oil filter contaminant and oil analysis checks have all been normal. If there was any abnormal cam wear I am sure it would show up. I have done the lifter check many times and it is all normal.

  The plane was built in 2002 so has had many years to develop a problem over this but hasn't so I guess at some point the siphon stops.

                                                                

                                                                   Dick Maddux

                                                                   Rotax 912UL

                                                                  Milton,Fl

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Roger Lee

Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

Posted: Sun Aug 09, 2009 7:40 am Post subject: Re: loss of compression

Hi Dick,



The way I understand the situation is you are flying shut down to 1400 rpm the kill the engine while flying and it wants to keep going or kick back some?

If this is true then your problem isn't that strange. You are better off having that idle set higher like 1650-1750 rpm to have a smoother run on the gearbox. The 912ULS with more compression do better with 1800rpm.

When flying with a little speed it takes very little with the UL for the wind over the prop to want to keep it in motion. When it finally gets so slow that the compression wants to stop it and the wind trying to turn it you can get the backlash at times. Even on the ground at idle if the engine is so slow and compression starts to impede the stroke then shut down makes it act strange and it may try to kick back. Try setting the carbs at 1700-1800 rpm and do a good balance on them. A bad carb balance can cause this. I bet your issues go away. And while I'm thinking of it when was the last time you checked the carb float arm height? Supposed to be 10.5mm from the carb lip to the bottom of the float arm when the carb is setting upside down. Also make sure the carb vent tubes are in the same area, i.e. in the Rotax airbox, and if not there, not in a jet stream of air.

_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056

Dick Maddux

Joined: 24 Jun 2008
Posts: 516
Location: Milton, Fl

Posted: Mon Aug 10, 2009 3:52 am Post subject: loss of compression

Roger,

  If I pull back on my throttle all the way on the ground it will go back to about 1400 rpm. This equates to a minimum of aprox 1800 in the air due to the airflow. I have it set this way so that I can slow down and land the airplane. Previously I had it set higher and it was difficult to get the aircraft slow enough and out of ground effect. I don't allow the engine to idle at 1400. 1800 is my minimum. I use a Vernier throttle to control it The only time I was using 1400 was at engine shutdown at the end of a flight (and that was on the recommendation of one of the 4 Rotax distributors in the country) I am aware of the damage that can occur to the gearbox with continued low rpm settings. I saw the results on some gearbox parts at my Rotax class (many years ago). The 1400 shutdown is for a very short time in an effort to stop the" kick back". (but I will use the higher rpm method and higher octane fuel)

  My carbs are in sync. I did that about 8 hrs ago BUT I did not check the float heights. I will give that a shot!

   Thanks again for your tips!

                                                Dick Maddux

                                                Rotax 912 UL

                                                Milton,Fl 

  

 

  [quote][b]

Dick Maddux

Joined: 24 Jun 2008
Posts: 516
Location: Milton, Fl

Posted: Mon Aug 10, 2009 4:04 am Post subject: loss of compression

Dave,

  If your oil tank is high (which mine is) then the oil will tend to siphon into the crankcase when you shutdown the engine. They mentioned that in my Lockwood Rotax class which I attended years ago.( and in Kitfox bulletins) The location is the only place to mount the oil tank on the early Kitfoxes so I have to live with it.(changed on the newer airplanes)

  The lines are full of oil so the hot oil just starts a small reverse flow. It takes awhile for a lot of oil to drain into the crankcase but that is the reason you turn the prop thru before you fly the aircraft. You are moving this oil out of the crankcase back into the oil tank.( also to check the actual oil level)

                                                                          Dick Maddux

                                                                          Rotax 912 UL

                                                                          Milton,Fl

 

  [quote][b]

daveaustin2(at)primus.ca
Guest

Posted: Mon Aug 10, 2009 4:49 am Post subject: loss of compression

Hi Dick,

 My oil tank is just above the dry sump.  Maybe  that's why I don't get the syphoning.

 Maybe a check valve is the answer to your  situation.  How many turns of the prop does it take to get the oil back  into the tank?

 Dave Austin  601HDS -  912

    [quote][b]

rparigor(at)suffolk.lib.n
Guest

Posted: Mon Aug 10, 2009 9:43 am Post subject: loss of compression

Hi DickNot flying my 914 yet but a few ideas on shut down kickback.*I have heard that when you get ignition modules hot, the first thing that happens is that when you ground them, they may not shut down, or partially shut down. Just for hahas before a very hot shut down at 1400 RPM give a mag check and see if you get any coughing on one or both mags.*On a hot engine there could be glowing carbon that could give a firing when you don't want one.  I had a 1972 yamaha RT-2 360 that I removed spark plug on a hot engine and proceeded to take a compression test, got a firing and trashed my gauge! You could try pulling choke for 15 seconds before and during shut down??*Did you adjust low speed mixture? You may want to adjust them on a very hot engine on a very hot day, or do as manual says find highest RPM and instead of richening (I think it is 1/2 turn, please check manual) go a slight bit richer.Ron Parigoris   [quote][b]

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