Carbon & Exhaust Valve

[dkenney1]

Not well versed at engines as I'd like to be I have questions regarding carbon deposits on an exhaust valve. I've done some online research but most deals with the deposits causing an exhaust valve to stick. That is not the issue I'm researching, but when the deposits don't allow the valve to seat properly or fully close. What causes these deposits and what can be done to decrease or eliminate the buildup? I would assume 100LL fuel could cause more build up. Obviously the C85 was designed for 80 octane, but good luck finding that these days. Basically my options are 93 octane ethanol free MoGas or 100LL AvGas.

I've read stuff saying the cylinder is too hot, another that it's running too cold and causes carbon, run it hard, don't run it hard. It gets confusing.

Thank

[Guest]

Re: Carbon & Exhaust Valve

No way that you found confusing info on the internet it's the source of all truth.

My experience has been that when I had carboned up exhaust valve stem it was because I had worn oil control ring and the cylinder was burning oil and the excess oil was burning to the stem. Cylinder compression was actually ok. That engine never appeared to run hot. I recall the lower spark plug was wet too. I removed the piston & cylinder and the oil ring end gap was way out of spec. Replaced the rings and all was well.

Mine never progressed to the point of valve not seating and loosing compression.

The running hot suggestion could make sense too maybe combined with a worn guide as well.

Sticking and failure to close may be just degrees of the same issue; a carbon coated stem.

If you are saying failure to close and mean a dirt seat or valve surface then that may be a different scenario than I am thinking about.


FYI there are others ways to make a valve not seat. Are you sure its a carbon issue?
Dave

[Ragwing nut]

Re: Carbon & Exhaust Valve

carbon coating or coking is left over fuel and lead deposits that was not burnt during the combustion stroke. Main culprit is the 100 avgas and the excessive lead in it. get an auto fuel stc and run auto gas, or mixture of. Marvel in the oil will help free up carbon deposits. if your running hot, check your compression as combustion gas will get by the rings and heat up the oil. Also check and make sure your baffling is tight.

[Guest]

Re: Carbon & Exhaust Valve

Not well versed at engines as I'd like to be I have questions regarding carbon deposits on an exhaust valve. I've done some online research but most deals with the deposits causing an exhaust valve to stick. That is not the issue I'm researching, but when the deposits don't allow the valve to seat properly or fully close. What causes these deposits and what can be done to decrease or eliminate the buildup? I would assume 100LL fuel could cause more build up. Obviously the C85 was designed for 80 octane, but good luck finding that these days. Basically my options are 93 octane ethanol free MoGas or 100LL AvGas.

I've read stuff saying the cylinder is too hot, another that it's running too cold and causes carbon, run it hard, don't run it hard. It gets confusing.

Thank

C85 has a min octane of 73 so you could use 87.

http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/755e0696cb285fdf8625797a007399d0/$FILE/E-233%20rev%2018.pdf

[astjp2]

Re: Carbon & Exhaust Valve

You can have a few things wrong, too low of an idle causing incomplete combustion, oil getting into the cylinders, too rich when taxiing ( do you lean the engine and keep 800+ rpms prior to shut down?) these are just a few things that can cause excessive carbon. Hot cylinders will burn the carbon off typically so if you are running at temperature when flying, that is one way to get rid of some of the carbon, but not always. Tim

[senior]

Re: Carbon & Exhaust Valve

You can have a few things wrong, too low of an idle causing incomplete combustion, oil getting into the cylinders, too rich when taxiing ( do you lean the engine and keep 800+ rpms prior to shut down?) these are just a few things that can cause excessive carbon. Hot cylinders will burn the carbon off typically so if you are running at temperature when flying, that is one way to get rid of some of the carbon, but not always. Tim

Without meaning to contradict what you say since it's true, but I would think the minimum amount of time your idling or running to rich while taxiing should burn off quickly when flying, blowby now is a diff matter, that will obviously cause gumming problems.
The biggest cause to our little low comp engines is 100 av...IMHO, wish I could remember but it's either 4Xs or 10Xs the amount of lead that 87av is. I used to mix one gal of 100av to four gal of 91 auto.
When I pulled a cylinder the valves & head were clean as could be running that way. I now just run ethinol free 91 auto with 200mls of 2 stroke oil to help out the valve guides.

[M Towsley]

Re: Carbon & Exhaust Valve

Just a side note to the above comment, ethanol-free mogas is not always an available option as it is becoming more difficult to find. With tax breaks to fuel distributors more and more are eliminating their 'clean' mogas.

[Guest]

Re: Carbon & Exhaust Valve

Question regarding a few responses I saw here.

90+ octane non ethanol mo-gas keeps coming up.

That's "hi-test" (I think) I am in an agricultural part of the country and I can get the 87 octance mo-gas (or maybe it's 89) at the gas station. So I use that.

Why does the 90+ octane mo-gas come up in the conversation? Just curious.

Dave

[M Towsley]

Re: Carbon & Exhaust Valve

Dave,

Around here they only did premium in ethanol-free mogas and I live in Illinois, home of the corn growers. I have never seen, in recent history, anything less than 90+ octane ethanol-free.

[Guest]

Re: Carbon & Exhaust Valve

Interesting Marty.

I can get both. Tractors, mowers and airplanes can use regular. Chain saws and other 2 cycle use hi-test (yes its true, manual says so).

I tend to use hi-test in the airplane.

We probably pay 40 cents a gallon more for ethanol free.

Dave

[Nefj40]

Re: Carbon & Exhaust Valve

I just stuck a valve after about 25 hrs of running straight 100LL and mmo. I don't know if that's exactly the cause but it's what I'm going with for now. Next time I'm pouring mogas in and the local Airport-Know-it-All comes up and starts raising hell with me about running mogas, I'm not going to be as diplomatic as I have been in the past.

[Garry Crookham]

Re: Carbon & Exhaust Valve

Without meaning to contradict what you say since it's true, but I would think the minimum amount of time your idling or running to rich while taxiing should burn off quickly when flying, blowby now is a diff matter, that will obviously cause gumming problems.
The biggest cause to our little low comp engines is 100 av...IMHO, wish I could remember but it's either 4Xs or 10Xs the amount of lead that 87av is. I used to mix one gal of 100av to four gal of 91 auto.
When I pulled a cylinder the valves & head were clean as could be running that way. I now just run ethinol free 91 auto with 200mls of 2 stroke oil to help out the valve guides.

From the website of an auto fuel STC holder: Lead content in grams of lead per gallon:

100LL = 2
80/87 = 0.5
Regular Auto Fuel = 0.1
Unleaded (Premium or Regular) = .001

[M Towsley]

Re: Carbon & Exhaust Valve

Though I used to run mogas quite a bit, I have a tendency to run 100LL with MM more as autofuel additives change throughout the seasons. Each fuel has a different gravity which can affect the float level from what I have read. I had issues in a different T-Craft during winter where the mogas fueled engine would die on roll-out. No such problem with 100LL.

Dave, currently the only station anywhere near my city, (Hy-Vee), has a Premium ethanol-free that runs usually .80-$1.00 more per gallon than regular grade fuel. We didn't have any ethanol-free mogas for a couple of years before Hy-Vee moved into the area. They are definitely profiting from the exclusivity.

[PA1195]

Re: Carbon & Exhaust Valve

1 part 100LL to 3-4 parts lowest octane top tier mogas...I use Chevron with Techron to help clean the valves and combustion chamber. No booze in Alaska fuel. MM oil occasionally but especially during idle periods. Run the engine 2350-2400 and over 22" MP. Lean during all ground operations and prior to shutdown.

Gary

[M Towsley]

Re: Carbon & Exhaust Valve

Gary,

You may be disappointed to know that if you purchase a 'top-tier' motor fuel you may not be getting what is advertised. All low grade fuel is pretty much the same everywhere. The majors, (BP, Mobil, Shell, etc.), only add their additives to the mid and high grade fuels at the terminal. The lowest grade of fuel is bought and sold in the pipeline as well as the truck running down the road, no additives....just plain old low-grade octane fuel.

As a side note, if you ever participate in one of those 'Don't buy gas from BP on a Thursday' type of scheme you will be disappointed to know it affects them not one bit. The affected company will just sell their fuel to one of the other gas suppliers.