when to de-coke?

[Bruce Tuncks]

After 400 hours, I took the heads off and there was a lot of stuff to clean out.

 

It was between .5 and 1mm thick on both the piston and the head. It was a mixture of black carbon and white stuff, possibly lead oxide.

 

There appeared to be enough to raise the compression ratio and to create hot-spots, although I have to say there was nothing in the way the engine ran to indicate any problem. But it didn't look good and I'm wondering about the wisdom of running on avgas .

 

And if I keep using avgas, how often should I do a de-coke? Or does the deposition happen so fast that some sort of equilibrium happens and so the de-coke doesn't help?

 

Any comments? .... Bruce

 

 

[old man emu]

If you have found deposits of gunk on the crown of your piston, and the surface of your hear, guess what! You haven't had any detonation problems. Detonation will blow the crud away leaving you with nice clean parts - and a stuffed engine.

 

OME

 

 

[ross holz]

Some swear by avgas some by mogas. Iam a mogaser (98 shell or bp doesnt seem to matter) at around 50 to 70c cheaper a ltr and cleaner burn its the better option in my opinion. Ive seen similar deposits to yours in our club aircraft. Even the old holden running on super was never that bad with all the oil it burnt

 

A local flying school did 1000 hrs on mogas and cylinders etc looked as good as new with as new compressions.

 

Nothing wrong with mogas as long as its not too old, 4 to 6 wks in my experience and you make sure your fuel lines dont induce a vapour lock by having high points along their route. I leave the electric fuel pump on all the time just in case.

 

Have heard of rings getting stuck in their groove from high lead deposits but no other problems that I know of. The octane of avgas is quite high.

 

Like most things each has its good and bad points to consider. As far as decoking goes, in my opinion, I wouldnt worry unless oil consumption / compression was an issue.

 

 

[Old Koreelah]

Somewhere I read a comment that an engine using avgas had to run fairly hot to burn off the excess deposits. My Jab 2.2 runs quite cool, so should I go back to mogas?

 

 

[facthunter]

Lot of questions there.

 

In a two stroke the oil used is probably a major contributor to carbon build up. It is most dangerous in the ring grooves where it can cause ring breakage. You should run a two stroke a bit on the rich side if you want it to survive. This will promote carbon too. Some of the material in the build up could be dust particles as some of the aircleaners are not very effective. Thick carbon could incandesce at the edges where it might start to lift off.

 

Regarding avgas ( lead) effect with temperature, cool running engines like the liquid cooled Rotax's (4 strokes) don't get hot enough on the valve seats for the avgas formulation to work correctly and sometimes there is an excess build up on the seats which can allow gas to blast across the valve face and cause damage if it happens for a significant period. Avgas is basically designed for air cooled aero engines, which do achieve the temperatures to make the lead compound become plastic and flow slightly much as a flux does when you are brazing metals. Nev

 

 

[Old Koreelah]

Regarding avgas ( lead) effect with temperature, cool running engines like the liquid cooled Rotax's (4 strokes) don't get hot enough on the valve seats for the avgas formulation to work correctly and sometimes there is an excess build up on the seats which can allow gas to blast across the valve face and cause damage if it happens for a significant period. Avgas is basically designed for air cooled aero engines, which do achieve the temperatures to make the lead compound become plastic and flow slightly much as a flux does when you are brazing metals. Nev

Thanks for that Nev. My CHT readings are usually on the low side of optimum, so is it likely that the spot temperature on the valves could be high enough for lead deposits to "flow"?

 

 

[facthunter]

OK hard to be sure but the valve/ exhaust port area is pretty hot on the Jabs so I reckon you would be ok, even if your CHT's are a bit low. You measure at the spark, plug do you not? There is probably a variation one to the other. Nev

 

 

[Old Koreelah]

OK hard to be sure but the valve/ exhaust port area is pretty hot on the Jabs so I reckon you would be ok, even if your CHT's are a bit low. You measure at the spark, plug do you not? There is probably a variation one to the other. Nev

I tapped my probes into the tiny holes between the plugs, so I guess there could be a variation in temperature between there and the valve seat.

Thanks for the info, FH.

 

 

[G-OMER]

I have the same issue.

 

I treat myself to a new camera, a draper CAM7 endoscope. this is 5.5mm dia with a right angle adaptor that goes on the end and maintains 5.5mm dia. the unit can take mpegs and jpegs. i had a look in each bore and at the valves, seats open and closed number 1 and number 3 looked realy good with mild carbon on the top of them and only a slight trace on the head, pistons had brown/black carbon on them to be expected. when i opened the valves i could see the stems ok and the seats, I could also see the gap between the valves had no burning or erroding going on. this confirmed my leak down on number 1 is going past the rings, 65/80

 

number 2 and 4, the valves were again good and seats etc but they have a layer of carbon build up on them valve tops and the pistons were heavily covered in white carbon? the head of number 4 had quite a bit of this white carbon build up near the valve. my plug tips are slightly white on number 2 and 4,

 

so how do I ritchen this side of the motor up as it looks to me these are running leaner than the other 2. I dont have EGT sensors on. CHT is on No4 and is always within limits. engine has done 480 hrs, first 340 on avgas and the rest on 95 ron mogas I have some pics of this I will try and post

 

Cheers

 

Gary Omer

 

 

 

 

 

 

 

 

 

[facthunter]

Carbon exists in 3 forms. diamond (rare) amorphous carbon (soft black soot) Graphite flakey/slippery (black.) Used in so called "lead" pencils. The white stuff would not be carbon it would have to be dust or some fuel additive. I saw some scratch marks on the bore there I think. These seem to be fairly common on jab cylinders and would indicate lack of oil to the surface or mismatch of materials in the rings to bore. ( possible oil ring material to bore material compatibility or just the action of the oil ring too severe}. Tends to happen in the really hot part near the exhaust port. That white stuff on the piston looks as if it has been quite hot. Do you have the latest jetting? What are your fuel consumption rates and power settings?Nev

 

 

[Guernsey]

You pour De-Coke into each cyclinder and leave overnight, you will then have nice shiny cyclinders. .

 

ONLY JOKING...ONLY JOKING...ONLY JOKING.

 

 

[G-OMER]

fuel is 8 to 10 ltrs per hr. #2 #4 are lean as plugs white, the other two are black rich, its not jetting its the distribution through the plenum box I think, more is going to 1 side than the other. i have searched the net and I think a cobra head with the cross in it or a plate baffle should improve the distribution to these pots going by others and various things I have read, this is my next move. any one had experience using these?

 

 

[jetjr]

Yes, it does help but this problem is a ongoing issue for many

 

You can tilt the carb slightly, top toward the lean side which also helps

 

 

[jetboy]

On my 2200 I had some early issues with EGT, made a few different 'smooth' bends and cobra head type angle, by far the most effective fix was an aluminium sleeve on front of carb containing cross vanes making 6 segments, the vanes about 50mm long ( 1 straight across, plus 2 half height pairs, riveted to the tube wall). The tube clamped to carb, and to scat hose. No further issues - did try Sceet hose as well which made no difference except the worry that the internal layer might come adrift and terminate flight one day

 

Ralph

 

 

[Guest Andys@coffs]

On my 2200 I had some early issues with EGT, made a few different 'smooth' bends and cobra head type angle, by far the most effective fix was an aluminium sleeve on front of carb containing cross vanes making 6 segments, the vanes about 50mm long ( 1 straight across, plus 2 half height pairs, riveted to the tube wall). The tube clamped to carb, and to scat hose. No further issues - did try Sceet hose as well which made no difference except the worry that the internal layer might come adrift and terminate flight one dayRalph

Photo?

 

 

[G-OMER]

I read on jabiru430.com that you tilt the carb away from the hot side? so which 1 is correct?

 

I have bought the long cobra head, so thinking of putting a plate in it before I fit it

 

 

[Guest Andys@coffs]

I read on jabiru430.com that you tilt the carb away from the hot side? so which 1 is correct?I have bought the long cobra head, so thinking of putting a plate in it before I fit it

The problem with descriptions of "away" or "towards" is that for it to mean anything you have to identify the point of reference and the point of refernce needs to be unambiguous. The front of the carby could be the air inlet side (makes sense to me from a functional perspective if not a location perspective) or the vapour side because thats closest to the front of the aircraft.....

So, all that said I dont know the answer. I could speculate that the vapourside of the carb if pointed towards the lean side may well end up with that side getting more vapour but I as I said specualtion.

 

Andy

 

 

[DonB]

Tilting the Bing to deliver more fuel to the lean side of the engine can be thought of this way; Fuel enters the airstream from the atomiser and needle at the 6 0clock position. By tilting, you are moving this discharge of fuel closer to the side that is lean, basicly, fuel bowl (bottom of carb), towards the hot side. The reason why this works is because there is a divider in the intake manifold and by moving the atomiser to either side will bias the fuel into the airstream versus straight up plumb position.

 

 

[G-OMER]

so top of carb to the rich side

 

 

[DonB]

Yup...

 

 

[jetjr]

Yup - not wot I said sorry

 

You cant tilt them much anyway as it strikes the mag I think but it can help.

 

I believe the air swirls down the intake tube further biasing the left right bias, trouble is that the swirl can depend on air speed through the carb so sometimes its only a problem at certain RPM

 

 

[G-OMER]

Bugger, mine is tilted the other way

 

 

[Bruce Tuncks]

my spark-plugs show that the distribution of fuel between cylinders is not too bad, so I wonder if the old 32mm carb (which I have ) was better. But I'd also like more details from Ralph in NZ.

 

What you have described sounds like a flow-straightener, while I would have thought a swirl-inducer might have been how to do it.... please tell us more.

 

Bruce

 

 

[G-OMER]

was a long time to fit. I took the air box off and radiused the part were the out let pipe goes inside as this was sticking through, I did a trial fit, it could of done with a little bit more of an up sweep as quite a bit out of line to the airbox. I re cut the scat hose using the middle bit of the old 1, fitted the head, quick measure up then fitted the scat to the airbox, I could then not get the cobra head on the carb, so after another 2 cuts to the scat I managed to get it on. I nipped up the clip to the cobra head carb end but the head still remains slack on the carb, there is no saw cuts in the head. I did not want to add saw cuts so I stripped it again and put a bead of loctite black rtv type silicon (hight temp fuel resistant) around the carb then reassembled the whole lot again, this time the cobra head is sitting on a bed of silicon, not sure on how important it is to get an air tight seal in this area, but is should be OK. next time its removed I will prob beef up the cobra head in this area and add some saw cuts in it the beefed up bit should stop the saw cuts migrating.

 

 

 

 

[jetboy]

I described an airflow straightener earlier which I made with 6 vanes and fitted years ago. have no photos but it was modeled from a similar one pictured here

 

http://groups.yahoo.com/group/jabiruengines/photos/album/867968529/pic/list

 

there is plenty of other mentions around the net

 

http://usjabiru.com/Gallagher_601.html

 

Ralph