Carbon & Exhaust Valve

[dkenney1]

Re: Carbon & Exhaust Valve

Thanks again for all your comments and info. To answer a question, It appeared my AI did much of what you describe above. His little kit included rope which he put inside the cylinder as he had me slowly turn the prop in various direction. Then there was an adapter of sorts that screwed where the spark plug goes. It was adjustable with a pivoting head and he would put various tools and picks through it and would turn it. Several of the AP AI gathered around to watch because as he said, "we are going old school now" and they wanted to see. He mostly whistled happily once in awhile saying, "that's good, feels right, oh guide is clean and smooth, there it goes got it now, compression should be in 70's now". Some of the carbon came out with the tool. Wish i had got a picture or video of it, but I was too nervous to watch much.

I've flown it three hours since at higher rpm and did compression check yesterday, all still in high 70's. Per Hank's suggestion I got one of those infared thermometers and checked the cylinder temps after 2 hours of running hard. Granted it was after shut the plane down, but valve covers were 180 degree range with the front cylinders about 10 to 15 degrees less, if measure the cylinder near spark plug around 260 with front cylinders 15 degrees less.

Per comments/advice from here, now don't idle as much after startup but get in the air quicker, before shut down, let it run for one minute at 1000 rpm and then kill the mags, and store it with oil filler cap open and off to the side.

[Scott]

Re: Carbon & Exhaust Valve

I think I understand what you're comparing and I'll offer some relevant info:

A two stroke is inherently inefficient at burning fuel, in fact at most RPMs a significant amount of un-burnt fuel (and oil) ends up in the exhaust. Because of the configuration of the transfer ports, the fuel charge is introduced up and over the the combustion chamber leaving a "ball" of burnt fuel/oil/air (exhaust) on top of the piston. To try and eliminate this exhaust on the piston top, the exhaust port is raised (cut in the cylinder at manufacture) so that the first 40-50% or so of the compression stroke is used to push this exhaust into the pipe. Inevitably a good deal of the fresh charge (fuel/oil/air) goes out the exhaust as well. This is undesirable in terms of fuel efficiency, but good as far as lubricating the piston in the hot exhaust port area. Note that the only reason the oil in this ejected fuel/oil/air charge is of benefit to the engine is because it has bypassed combustion. In terms of power, well we don't care too much about fuel economy. This fundamental reality of the two stroke cycle is why they burn far more fuel and are becoming a thing of the past in terms of environmental responsibility.

Four stroke engines, because they have valves, are much more precise in terms of creating a pure fuel/air charge in the combustion chamber. In addition there is a whole stroke of the piston which is used to purge the combustion chamber of exhaust gases. There is some "valve overlap" but in a normally aspirated engine very little of fuel/air charge enters the exhaust port. Note that the exhaust valve, even in our low-tuned continentals will be glowing cherry red or hotter while in cruise. Any oil in the combustion charge is burnt either in the combustion chamber or on the surface of the hot valve. Typically when an engine is burning oil, carbon buildup increases significantly both in the combustion chamber and on the exhaust valve, specifically on the stem between the guide and the seat. Adding oil to the fuel, synthetic or other, will increase carbon build-up. This is a certainty. The oil also reduces the effective octane meaning the fuel charge burns faster, further reducing the likelihood of unburnt additive getting to the exhaust guide. So.... Isn't hoping that some tiny percentage of an oil additive can actually make it to the valve guide, and if it does can actually have a positive impact a bit of a stretch? Maybe it's enough for some.

I think the marketers of fuel additives have done a good job taking advantage of the "mysteries" of combustion engines. Fortunately for them there is enough intermitantcies in the types of problems they claim to solve that positive anecdotal accounts emerge, and there is considerable time before the negative impacts of using their products reveal themselves, and of course there are enough people looking for a quick fix and/or piece of mind to spend a view bucks and give it a try. Good news is, on our low tech engines, there is not much short term negative impacts. Using them on boosted engines, now that's a different story.

[senior]

Re: Carbon & Exhaust Valve

Why I use a small quanity of sythetic 2 stroke oil mixed with my 91 oct ethinol free gas in a small continental.
Some what contradictory to one of the above opinions.

"The differences between a 2-stroke and a 4-stroke engine are significant, and those differences demand different properties from the lubricant. Lubrication of the 2-stroke system requires a fluid that will form a tenacious film that will not be evaporated and will create a protective layer of oil over the entire piston surface. Unlike a 4-stroke engine, there is no oil submersion in a 2-stroke machine, so the film strength is one of the more important properties of a 2-stroke oil. It needs to withstand the heat and combustion of the fuel without burning and also needs to be able to clean the parts that may have carbon deposits or gummy residues left over from poor combustion or less effective lubricants. Since the thin film of lubricant is the only thing to stop the piston rings from grinding into the cylinder walls, these lubricants must be capable of protecting against high levels of pressure during the stroke of the piston."

Taken from...http://www.belray.com/2-stroke-lubrication

So in effect, the unburnt 2 stroke oil flowing out the exhaust & over the exhaust valve stem would provide some lubrication & perhaps even a bit of cleaning action.

[M Towsley]

Re: Carbon & Exhaust Valve

So, Alcor TCP is snake oil? Just curious as what I have read it works.

This article is one of the ones I have read regarding TCP and MMO: http://www.pipercubforum.com/marvel.htm

[N96337]

Re: Carbon & Exhaust Valve

Well said, Scott. That's what I've been preaching for years....
John

[M Towsley]

Re: Carbon & Exhaust Valve

Tom,

Remember when I stuck a valve on my L-2 in Rantoul, geez, seven years ago?

After your help in unsticking it, when I got her back home I did exactly the way you did. Piece of cake.

[3Dreaming]

Re: Carbon & Exhaust Valve

I have always done it without removing the exhaust. I push the valve into the cylinder, and pull the valve stem through the sparkplug hole to clean. I clean the guide, then use the mechanical fingers through the sparkplug hole to position the valve back in the guide.

[Scott]

Re: Carbon & Exhaust Valve

Sitting here nursing a sore back so here's my 2c;

In my experience, which is admittedly old experience, old continental engines through to O-300 experience periodic sticky valves. I have never been convinced that adding any form of oil to the fuel on a four stroke engine is a good idea. If you thing about it, in order for it to get to the exhaust valve, it somehow needs to survive combustion, otherwise all its doing is adding to combustion by-products ie carbon. I've built tuned and raced a few two stroke engines and even running ridiculously high compression ratios, 120 octane fuel, computer controlled ignition,leaned to within an inch of its life, the oil in the fuel still causes carbon build-up. Additives in these low teck, low compression, low RPM, rich-running continentals, not a good idea IMO

Now I know that there are a bunch of guys who'll provide a bunch of accounts of how they ran their favourite additive and cleared that sticky valve right up. What we can't determine however is what would have happened if they hadn't used the additive. My contention is there's a high probability that the outcome would have been the same (or maybe even better, we'll never know). Also I'm willing to bet that, in the absence of a proper fix, the problem eventually came back.

Exhaust valves in a running engine are or should be too hot for carbon to build up on the seat. Carbon on the seating surfaces is an indication of poor combustion/cool exhaust valve. Its a bit of a chicken and egg thing but in my view there is a high probability the carbon is a result of the sticky valve which is the real problem.

The good news is sticky valves on these engines are relatively easy to fix cuz all you need to do is clean the valve guide which can be done without removal of the cylinder. The procedure has probably been posted here before, but basically removing the exhaust is the most difficult part. You need the appropriate valve spring compressor and a length of rope to feed through the spark plug hole and hold the valve using the piston. With the keepers and spring removed the valve can be pushed in and held through the exhaust port with some mechanical fingers. At this point you can do a proper inspection of the valve, seat and guide. Ream the guide with the right size reamer (ideally one with a taper in its first half inch or so as they're easier to keep straight), clean and oil and re-insert the valve. Through the spark plug hole, you can put a small dab of fine valve grinding compound on the valve, slide the valve all the way in place and connect a length of hose to the stem where it emerges from the guide (use a small hose clamp). Now you can use the hose to turn the valve and clean the seat and valve sealing surfaces. Give everything a thorough cleaning using rags moistened with WD40, re-inspect the valve/seat sealing surfaces as after the compound it will be easy to see if there's a good seal. Reassemble and go. There's a good chance this valve won't stick again through TBO.

Scott

[N96290]

Re: Carbon & Exhaust Valve

When I had a stuck valve my AI told me to work it free and put a tablespoon of MMO in the cylinder. I then turned it through several times with mags cold and repeated the turnig through for the next several days. When I did start it it ran normal. I now do not run 100LL if possible and mix MMO with either car gas or 100LL if I have to use it. Also I was told to run it like I stole it to help keep the carbon and lead deposits burned up.

[astjp2]

Re: Carbon & Exhaust Valve

Fly it for 1 hour with oil temps above 160, to get rid of the carbon and water vapor in crankcase. Idling it that long could be giving you high cht's because of not enough air flow while you are waiting for oil temps. I try to get in the air within 2~3 minutes from engine start. Just get it leaned for max power prior to take off.

Wow, pretty impressive response rate and thank you everyone for the information. In an effort to answer some of the questions asked of me, I’ll give some more background.

My Tcraft has always been very slow to warm up, even on a 70+ degree day. So that may be one fault because I often spend 10 minutes or so running at 900 rpm to try to warm the oil before putting the boots to the engine. Tuesday was a 55 degree damp drizzle day but good enough to fly 12 miles away for an oil change. I know, do it yourself but I had a couple of things I wanted checked since the annual 3 months 26 hours flying ago. Oil was clean and screen very clean, no carbon no metal, A&P and me happy. Top and lower plugs on all cylinders still looked new, none wet. Did a compression check, 78/76/25/76. My heart sank and visions of money flying out the door and then thoughts that I was lucky to get there without blowing up the engine. The shop owner (senior) came over had my turn the prop and listened around, assured me he doubted it was a bad cylinder, it was 74 26 hours ago, “it’s probably carbon on your exhaust valve, it’s definitely something with the exhaust, relax, we’ll start simple before we unbolt it and send it away”. After boroscope it he could see rings looked good, no oil in the cylinder, piston looked good, but top of inside cylinder had a lot of carbon deposit. Valve operated properly but exhaust valve had a ridge of carbon on one side that didn’t let it fully seat on one side. His father who built these engines for Continental in the 40’s and 50s taught him a few things as he said, let me get his tools. He came back with an old wood box and told me to take valve cover off. Did his magic and we checked compression again, 40, he fiddled some more and was ZERO which he predicted, filled again and said “we’ll be in the 70’s now”, and it was 76. Put everything back together and flew it hard for half hour landed and checked again 78/76/78/76. The engine seemed smoother and stronger in flight especially at high rpm.

I questioned what causes the carbon is it the fuel, how I run it? He told me usually it’s 100LL that does it more. He felt I was not doing the engine any favors babying it like I was. I had been running in the 2100 to 2300 range and as I said with long idle/runup. He told me to run it no less than 2400 for awhile with spurts at 2500+ and try not to let it run at low rpm for long time on the ground. He told me there was a good chance that if I had flown the plane after the low compression check at high rpm for an hour the carbon would have burned off. “Don’t forget you idled on the ground longer than you flew to get here, much of that in the pattern at low power, when you shut down the oil temp was only 130 you didn’t heat it up to burn the carbon”. Another factor could be the cylinder is not getting warm enough and we may need to tape the intake grill some. The engine is show no sign of getting hot and the baffles are in good shape very tight. Even at 2400 rpm on a 80 degree day I never see oil temps beyond 185, usually more in the 160 to 170 range.
I was going to start running a few tanks of 100LL through it for winter storage, but not wondering if the lower octane fuel is better. The lowest octane MoGas I can find without ethanol around here is 91 or 93. I can get 87 octane with 10% ethanol.

So how best to I prevent this from happening again, or am I making too much of it? Also when you say add MM is that Miracle Marvel Oil? If so how much, etc? Should I run the high octane no ethanol or low octane 10% ethanol? Incidentally I tested one batch of ethanol free and it was not ethanol free and it never went in the plane.

Thanks

Re: Carbon & Exhaust Valve

What did he do to the cylinder?

Change the pushrod?

Clean the valve stem

Dave

[M Towsley]

Re: Carbon & Exhaust Valve

First off, if you are trying to pump ethanol-free mogas from a single line gas pump you are going to get ethanol. I pump about a gallon into my car to clear the line. Do not run ethanol in the plane, your STC will specifically state you should not run ethanol in the system. Some guys may do it, but I do not.

As for MMO, Marvel Mystery Oil, I run what is indicated on the bottle. I believe it is 2 ounces per 5 gallons of fuel. I utilize MMO with the 100LL and it seems to keep the plugs clean for me. MMO is not something that is 'approved' but I have read it is similar to TCP. MMO is a solvent, really, and allegedly loosens carbon deposits, etc.

I would not worry about the octane level of mogas you are using, just ethanol free is what you want. Don't get high octane 100LL and 91 octane mogas confused, one has lots of lead and one has no lead. As stated before, in winter I would utilize the 100LL as it seems to work better in colder temps. Also, 100LL has about a year shelf life as opposed to about 6 months for mogas.

[dkenney1]

Re: Carbon & Exhaust Valve

Wow, pretty impressive response rate and thank you everyone for the information. In an effort to answer some of the questions asked of me, I’ll give some more background.

My Tcraft has always been very slow to warm up, even on a 70+ degree day. So that may be one fault because I often spend 10 minutes or so running at 900 rpm to try to warm the oil before putting the boots to the engine. Tuesday was a 55 degree damp drizzle day but good enough to fly 12 miles away for an oil change. I know, do it yourself but I had a couple of things I wanted checked since the annual 3 months 26 hours flying ago. Oil was clean and screen very clean, no carbon no metal, A&P and me happy. Top and lower plugs on all cylinders still looked new, none wet. Did a compression check, 78/76/25/76. My heart sank and visions of money flying out the door and then thoughts that I was lucky to get there without blowing up the engine. The shop owner (senior) came over had my turn the prop and listened around, assured me he doubted it was a bad cylinder, it was 74 26 hours ago, “it’s probably carbon on your exhaust valve, it’s definitely something with the exhaust, relax, we’ll start simple before we unbolt it and send it away”. After boroscope it he could see rings looked good, no oil in the cylinder, piston looked good, but top of inside cylinder had a lot of carbon deposit. Valve operated properly but exhaust valve had a ridge of carbon on one side that didn’t let it fully seat on one side. His father who built these engines for Continental in the 40’s and 50s taught him a few things as he said, let me get his tools. He came back with an old wood box and told me to take valve cover off. Did his magic and we checked compression again, 40, he fiddled some more and was ZERO which he predicted, filled again and said “we’ll be in the 70’s now”, and it was 76. Put everything back together and flew it hard for half hour landed and checked again 78/76/78/76. The engine seemed smoother and stronger in flight especially at high rpm.

I questioned what causes the carbon is it the fuel, how I run it? He told me usually it’s 100LL that does it more. He felt I was not doing the engine any favors babying it like I was. I had been running in the 2100 to 2300 range and as I said with long idle/runup. He told me to run it no less than 2400 for awhile with spurts at 2500+ and try not to let it run at low rpm for long time on the ground. He told me there was a good chance that if I had flown the plane after the low compression check at high rpm for an hour the carbon would have burned off. “Don’t forget you idled on the ground longer than you flew to get here, much of that in the pattern at low power, when you shut down the oil temp was only 130 you didn’t heat it up to burn the carbon”. Another factor could be the cylinder is not getting warm enough and we may need to tape the intake grill some. The engine is show no sign of getting hot and the baffles are in good shape very tight. Even at 2400 rpm on a 80 degree day I never see oil temps beyond 185, usually more in the 160 to 170 range.
I was going to start running a few tanks of 100LL through it for winter storage, but not wondering if the lower octane fuel is better. The lowest octane MoGas I can find without ethanol around here is 91 or 93. I can get 87 octane with 10% ethanol.

So how best to I prevent this from happening again, or am I making too much of it? Also when you say add MM is that Miracle Marvel Oil? If so how much, etc? Should I run the high octane no ethanol or low octane 10% ethanol? Incidentally I tested one batch of ethanol free and it was not ethanol free and it never went in the plane.

Thanks

[N96337]

Re: Carbon & Exhaust Valve

I used to haul fuel for a living. It was hilarious...loading at an Exxon rack, with Chevron, Shell, BP, Texaco and us, Unocal all loading out of the same spout....the only difference...about 2 liters of "additive" for Texaco and Chevron per 10,000 gallon load.
John

[PA1195]

Re: Carbon & Exhaust Valve

Yeah, no alcohol, but that oxygenated crap in the winter is a bitch on carbs and fuel lines

Tim, no MTBE here for years. It was bad when we had it mixed in. Everyone at the pumps got a quick contact high.

Marty, Chevron specs their fuel and additives up here just like the rest. I suspect the main benefit is the Techron which is an effective proven fuel additive. My experience is to avoid premium blends and alcohol additives which tends to affect rubber components like gascolator O-rings. My exhaust runs cleaner with the Chevron regular mogas mix than other available fuels of the same labelled octane.

Gary