De-coke of Agas Engine

[Bruce Tuncks]

When I took the heads off my 500 hour avgas Jabiru 2200 engine, I was shocked at the buildup of deposits.They were about 1 mm deep on the top of the pistons and the underside of the head.

 

I reckon engines like this should be de-coked about every 200 hours.

 

Engines and Props

 

 

[Old Koreelah]

Agreed, Bruce. The buildup of crud must greatly reduce the volume of the combustion chamber at TDC, so that must mean increased compression ratio. I've asked LAMEs about this but never received a convincing answer.

 

 

[spacesailor]

The New "decoke" is by aerosol can, Subaru have their own brand, and other cans are on the market. Sib's are alloy. but I did an old ford then had to remove the head, so checked it was clean.

 

spacesailor

 

 

[facthunter]

Carbon build up is not good for the motor. Some parts can get very hot and cause pre ignition and bit's can detach and get stuck under an exhaust valve seat. Water/alcohol injection I believe eliminates build up. Some of it can be very hard and difficult to remove. Pistons running hot are part of the cause of hard carbon build up. Some of what looks like carbon is actually dust incorporated in it. Nev

 

 

[Old Koreelah]

Using some product to clean carbon out might cause problems: dislodged bits getting under valves, etc.

 

How do we prevent carbon buildup from happening? A lead-free AvGas is still not available and I don't trust Mogas. Running pure ethanol has its issues, but should ensure the combustion chamber stays pristine.

 

 

[Bruce Tuncks]

The stuff I think is lead oxide residue. It is light brown with darker bits but not much carbon black. Maybe I shouldn't have used the word coke because that word implies carbon.

 

Space, I like the idea of getting rid of it chemically.

 

People who operate with mogas apparently don't have the problem, so I wonder if a couple of tankfuls of mogas would clean it up too.

 

 

[facthunter]

There's a chemical in 100LL (Bromine) that's meant to clear out lead residues. That gives the light grey to white colour inside some exhaust pipes. I personally don't believe in running a jab much over 400 hours without cyl head removal. Plenty of other (older) aero engines need that level of attention. The life (average) of the Gnome et Rhone early models was less than 10 hours average, back in their day, but then they were a top of the line fighter engine. The springs in the piston valves were the main cause of failure. The Monosoupape (single valve ) engines were more reliable. Later version.110 Hp Le Rhone. Nev

 

 

[Nobody]

There is a good discussion of the fouling problems associated with lead here:

 

Lead Fouling

 

 

[old man emu]

The stuff I think is lead oxide residue. It is light brown with darker bits but not much carbon black.

Using the colours of the deposits on the plugs in the attached photo, which colour is closest to the residue on your heads? (Disregard the labelling at this stage)

 

I think that before getting too deep into avgas -v- mogas we should decide if the problem relates some other controllable source.

 

 

 

[Downunder]

The stuff I think is lead oxide residue. It is light brown with darker bits but not much carbon black.

I thought lead contamination/buildup was light grey?

 

 

[facthunter]

That's the lead in the exhaust pipe that's been scavenged from the engine. My post #7. White or light grey. Dead giveaway you are running avgas. Nev

 

 

[Bruce Tuncks]

It was the colour of the lean sparkplug in the pics OME posted. And definitely beige or brown rather than grey.

 

 

[facthunter]

The colour on a plug can indicate it's too hot a plug (or too cold). Long thin insulator is HOT running and short thick is cold running as well as an indication of mixture being correct. To get a mixture indication of any value the engine should be "CUT" after a reasonable period on full power. This isn't very good for it and inconvenient also. Taxiing is low load and will normally result in some soot being on the insulator so devalues the whole effort. A too Hot plug may look as though it's lean and may cause detonation or engine running on after switches are cut, if you do that as a test. A plug not tensioned enough may run too hot also. Nev

 

 

[old man emu]

Hot or cold plug - Are the plugs the type and rating called for by the engine manufacturer?

 

Plugs tight enough - Check that plugs are torqued to correct value

 

The pictures I posted are from a car source, so the fuel used by them would have been mogas.

 

Assuming that the colour of the deposit is similar to the "lean" plug, then an examination of the mixture strength would be a sensible step. For starters, are you fuel consumption figures LOWER that similar age angines and the manufacturer's published claims?

 

OME

 

 

[spacesailor]

Most of the "coke" in old engines is oil burning,.

 

My previous post has a spelling mistake "Sib's are alloy." should read" Sub's are alloy"

 

That's better, takes care of the decoker chemical harming the metal.

 

spacesailor

 

 

[facthunter]

One reflection on coke build up. Consider modern two stroke oils. Today, you can run one for ages with minimum build up, or practically none. In the "olden days" when Castrol XXL was used (for example) in the mix , the exhaust port would block up in12 months of riding 20 miles a day to work, on a lightweight bike, like a BSA Bantam.

 

When you remove such build up it's extremely hard and it's easy to damage the ring lands trying to remove it particularly where the piston has been running very hot. The top ring groove is normally the most difficult indicating temperature rather than the amount of oil is more the cause..

 

There have always been "cold"solvents available in the engine reconditioning business but they are hard to get a permit for the more effective ones as they are extremely toxic . I use a hot bath for ferrous items, but alloy (aluminium) is not suitable as it will be corroded.

 

I would be extremely doubtful of any down the carby throat "instant" fix. Subaru did use a solvent in some of their high compression motors, which were a bit critical in that area. but I don't think It was done with the engine running.. An aero engine should not be running with lots of carbon build up. It can incandesce, and cause preignition or the particles breaking off can unseat a valve. Usually the exhaust is the one affected . A leaking exhaust valve tends to run very hot which can cause the head to break off.Nev

 

 

[Sloper]

Had a problem on a Holden V8 with carbon build up, raised the compression a lot, burnt out starters and finely backfired destroying the carbureter.

 

Then it managed to suck a piece of the carby into the engine, heads off.

 

Thats when l found it was carbon buildup that caused the problem, 3mm thick.

 

With that much carbon there was no damage to the piston.

 

Back together and it was all good.

 

l was blaming modern starters as being cr#p.

 

regards Bruce

 

 

[Jabiru Phil]

My old FE Holden had well over 200.000 miles (ex taxi) Woulnt pull your hat off.

 

A mechanic mate poured about 200 ml of upper cylinder lube (Redex) down the throat of the carby whilst operating the throttle to keep running.

 

Blew a trail of blue smoke for five miles.

 

Fantastic result. I kept that car for a few years more.

 

His decoke method I thought was genius.

 

Probably wouldn't try on an aircraft engine though.

 

 

[M61A1]

DECALIN RUNUP FUEL ADDITIVE from Aircraft Spruce

 

I'm sure that there is another additive, but I can't recall the name.

 

 

[facthunter]

Some aero engines used water methanol for enhanced power for take off or emergency situations (both pistons and turbo props) The FW 190 used it and the RR Dart series of turboprops. I can't see why it couldn't be given a try on a Jabiru engine under the RIGHT circumstances. I've injected several "very loose" non aero engines and on head removal were very clean in the combustion chambers. Since water is produced when Hydrocarbons are oxidised a bit more can't kill the idea completely. When you burn one gallon of petrol you produce 1.52 gallons of water. You can verify that easily. Mixing methanol (wood alcohol) with it cools the charge making it more dense and gives a bit more fuel to enrichen the mixture. The % you use can vary( W/Meth to fuel) ratio, and also the methanol to water ratio for how much you wish to enrichen the deal. Best to have the engine at working temp and atomise the added water meth . The cooling effect can be quite marked with the methanol and could promote icing in some circumstances. You need the extra power on VERY HOT days, mostly so...... nev

 

 

[M61A1]

I have had good results with Moreys upper cylinder lube, though not in a Jabiru. I have used it on pretty much everything I've had since the late eighties excepting 2 strokes. V-twin and inline four motorcycle engines, 4, 6 and 8 cylinder cars, even when still running the old "super", a couple of four stroke aircraft engines. Ports and chambers always clean, and no problems. Mixed at 650:1 so really won't upset anything.

 

 

[facthunter]

Some of these extra oils in fuel have been recommended for winter diesel where the pumps may be damaged by lack of "lubricity". It can end up in the engine oil through transference by blowby process. Unlikely to be a problem in a good condition engine where the oil changes are fairly frequent. Nev

 

 

[Old Koreelah]

Some aero engines used water methanol for enhanced power for take off or emergency situations (both pistons and turbo props) The FW 190 used it...

... and so did the Japanese, in their later model Zeros with the bigger engine. With their supplies of oil and other resources cut off by Allied submarines, they stripped their forests for resources.

 

 

[Old Koreelah]

I have had good results with Moreys upper cylinder lube, though not in a Jabiru...

Does this mean you didn't use it, or that you used it and it didn't help?

(I've been adding Moreys to my AvGas in the hope of doing some good to my Jab's top end.)

 

 

[M61A1]

Does this mean you didn't use it, or that you used it and it didn't help?(I've been adding Moreys to my AvGas in the hope of doing some good to my Jab's top end.)

It means that I've not used it in a Jabiru, if I ever owned one, I would use it.

On that note...how does the inside of your Jab's top end look after using it?