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Sticky valve question for gurus ?

Larry DeCamp

Well Known Member
Today I reamed a sticking valve on Mellinium cylinder with 250 HRs. There was only carbon in the reamer and the valve stem had some bronze scuffing. The valve and cylinder look perfect with a scope.

I only use no alcohol mogas and lean ROP. Oil is XC 20-50 and camguard. Plug insulators are white and Pmag checks on ground and air are no differenc.

So, what is the carbon source, CAM GUARD ???
 
You must aggressively lean when on the ground - as in just before cutout.

I run LOP 95% of the time and strive to keep CHTs in the 350-400 degree ranged as stuck valve preventative measures. I refer your to Mike Busch’s Webinar on stuck valves: https://youtu.be/7ohsVvYbAaQ

Carl
 
From above link;

The relatively new, mid-octane unleaded fuel now becoming available matches the 89-octane rating of leaded fuel. But it lacks the equally important lubricating qualities of leaded fuel.

"Lead acts as a solid lubricant," said Jim W. Garthe, a fuels specialist in the agricultural engineering department at Penn State University. "When lead burns in the engine it forms white deposits . . . (with properties) sort of a like a graphite."

This was something I have heard for many years, the link was just something I found with a quickie Google search.
 
Hey Mike
I’m not getting anything relevant from that link. It is my understanding that the sole purpose of lead in leaded gas is as an anti-knock agent and that the valve lubrication thing is a myth. Curious to know more if possible.

Erich

Yes, octane boost is the primary purpose, but the dry lubrication is a side effect. The valve faces and seats were once softer and when lead was removed from auto fuel, the valves had reduced life due to face wear. Valves never seat around the full circumference of the seats/valves resulting in some movement to align them when seating. For longer life and lower cost availability of materials, and vendors aviation engine manufacturers migrated to the same benefit. So, since this took place 60 yrs ago, it may be legitimate to call it a myth.

Someone asked where the carbon comes from. When fuel or oil cook hot and slow, both will deposit carbon. For exhaust, it is likely just the oil. Although . . during starting everything is cold and A/F is rich, fuel could get deposited then. Also, oil leaks past rings when sitting and could add a film to the exhaust stem from the hot end each start.
 
IMO the only thing that causes stuck valves is dirty oil. I do not change oil based on hours or calendar time. Gets dark brown, almost black...change it. My last oil change in my Comanche was at 44 hours.
 
Today I reamed a sticking valve on Mellinium cylinder with 250 HRs. There was only carbon in the reamer and the valve stem had some bronze scuffing. The valve and cylinder look perfect with a scope.

I only use no alcohol mogas and lean ROP. Oil is XC 20-50 and camguard. Plug insulators are white and Pmag checks on ground and air are no differenc.

So, what is the carbon source, CAM GUARD ???

I suspect the engine oil is the source of the carbon, what CHT's you run? (I suspect they are on the high side)
I probably have seen more stuck valves in superior cyl's than others.

I think sometimes it the luck of the draw, if you get cylinders with tight stem/guide clearances you are much more likely to get a stuck valve.

I change oil every 25 hrs and lean aggressively, I have really good CHT's (generally 300-350) so tend to run very close to peak EGT/max power on the rich side (65-75%). Never had a stuck valve (I know never say never). Always had ECI and now Titan cyl's.

If you're having to clean lead out of the bottom plugs every time you pull them that's not a good sign.
 
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Another opinion:

I believe that the black carbon in the guides is coked (i.e. burned / oxidized) oil. The lycoming suffers from poor oil flow qty to the valve area and exhaust valves get a good part of their heat removal from oil. Not much can be done about this beyond adding squirters up there.

While Bob seems to have interesting emperical data about oil contamination, I am not sure it makes sense to me. Sticking exhuast valves in auto engines were quite uncommon back in the leaded gas days when the engine oil got pretty dirty. The primary difference between the lycoming and the old auto engines are water cooling and better oil flow to the valves. Both of those point to lack of cooling causing oxidized oil, which is known to create jet black carbon deposits, as opposed to deposits, such as lead, coming out of suspension. We have all seen lead deposits on the plugs and know that it is grey in color, not Jet black like what comes out of the guide.

It would be interesting to hear if anyone with roller lifters has experienced sticking valves. While I have never seen a LYc roller lifter, I suspect they may have put bypasses in them to solve the anemic oil flow issue. The lack of a bypass circuit in the Lyc hyd lifters is what causes the anemic oil flow to the rocker box.

Larry
 
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roller lifters

I have roller lifters on my TMX IO-360, built by Mahlon's guys at Mattituck a while back. Perhaps he could chime in with his view on if there is extra oil flow up there.
 
Conflicting possibilities

Thx to all for the input.

Mike: maybe the mogas users with high hours can share valve stick experience. Maybe I am in a minority experiencing lack of lubrication from no lead.

Carl & Rocket Bob : coked oil makes sense, but my chts run 290. This suggests my stem clearance may be too tight for ANY burned oil to accumulate. I plan to block the cowl exit to get cht up to 350. Also, I rarely apply full throttle and typically cruise around at 2300 rpm max. Could this contribute ?

LR172: Valve oil supply shortage effect ? Bob says oil gets bad before 50hrs. I have bragged that no lead gas keeps my oil so clean I cannot read the dipstick. HOWEVER, when I drain the sump, the oil is black. AND, the oil dripping from the push rods was black at valve disassembly for reaming. So it appears the rocker box gets black oil. I will start changing oil at 25 hr.
 
My 2ct...

I've had my share of stuck or sticking valves over the years, 3 on the smaller Contis, and one on a Lyc 320. Happened in flight on the Contis, and on the ground in the form of morning sickness on the 320. These happened over the course of several 100h.
Each one of them was due to coked oil. Now the Contis were run pretty hard on either AVGAS or MOGAS, but the Lyco rather less and only on AVGAS...
Having coked oil is, I would think, a rather good sign, proof that said valve guide is getting oil in the first place.

I now abide with Lycoming's SB388C, though still uncertain what material my guides are made off. Checked them last year and all had a play within the required tolerance.
 
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