AIR FUEL MIXTURE RATIO

[Guest J430]

Andy,

 

My guage is 100PSI base and in %leak, so you are looking for a result that is better than say 56-58/80. Factory say 60/80 also but they run fine a bit below that, but usually you will find the 60-65 will be stable and common, if you have a leak issue from valves or rings, it will be really low, more like a 65%leak or 28/80.

 

It takes a human hair to give you that kind of leak. So make sure you have run the engine in flight, bring it back and give it a good blast at 2000 for 15-30 seconds, then straight away take off the cowls and one plug each and test it.

 

Some fol could be worrying over nothing unless you check properly.

 

You will also need at least a 12CFM compressor or bigger, the baby SuperCheap compressors will not cut it!

 

J

 

 

[Guest Andys@coffs]

After speaking with the L2 today Im told that the piston mark is in fact normal and is the indicator that the piston is in the right way.... Shame the mark wasnt made in such a way as to look like it belong there rather than a manufacturing defect (ie symetrical) in any event a good outcome from that perspective.

 

Andy

 

 

[facthunter]

Valve leakage test.

 

With the kero test, I would suggest you place the head with the two ports uppermost, and pour about 15 Mls into each port ( one at a time) and you will observe the leakage into the combustion chamber, far more effectively, than doing it the other way around.

 

If you have had the heads reworked ( valves & seats) a good test before fitting back on the motor is to apply a small amount of pressurised air into the port and spray some WD. fluid around the valve heads. If there are any bubbles evident investigate why.

 

nev....

 

 

[Guest disperse]

ive done part of my automotive apprentiship and im just wondering if a standard compression test is of any use in aviation instead of the leak down test ?

 

THANKS Troy

 

 

[Guest J430]

Do both, keep a record of it! All data is good if analysed correctly.

 

With a leak down you get a chance to assess where the leaking is, exhaust (pipe) rings (breather), inlet (out thru carby)

 

J

 

 

[Guest Andys@coffs]

Hi Guys

 

Does anyone have an effective valve compressor they use to remove the collets on the valves in a jabi head? The standard compressor from supercheap auto is a tad oversize and I'm looking to see what others have used.

 

Andy

 

 

[Guest brentc]

I placed the head(s) into a bench drill thing. One of those ones that sits on the bench and has a big handle on it to lower the drill bit. In the drill bit / holder, I placed a strategically chosen piece of metal from the scrap bin that applied pressure to the top of the valve. There is probably a better way though, but this had been used on a head before so it was easy to visualise. Stability of the head is important so you don't have bits flying around your garage when it goes wrong.

 

 

[Yenn]

Has anyone any experience of using a vacuum gauge on an aircfraft engine as a diagnostic tool. I used it regularly in my years as a motor mechanic, before the electronic gear became all the rage.

 

 

[Guest Andys@coffs]

Leak down problem=== Outcomes

 

For those that were interested in the outcome.

 

After removing the #2 head I removed the inlet and exhaust valves to asses the seal between valve and seat. The lev2 was unable to come out so I ended up doing the work myself (some of the benefits of a 19 reg).

 

What I saw was that there was some minute amounts of carbon on the exhaust valve and seat that probably contributed to the problem. The geometry of the seat and valve was fine and as such I didnt think I needed to reface the seat and have the valve refaced. I used valve lapping compound (course then fine) to remove any of the carbon on both the exhaust and inlet valves. I had been told that on successful completion the valve held closed with finger pressure should seal against kero pored into the port. I couldn't get to the point of having no leakage, but did get to the point where the leakage was almost non existent.

 

Put it all back together , flew a few close in circuits and on landing measured 80/65 in that cylinder.

 

In retrospect I believe that I had 2 problems. the carbon buildup, that may well have been solved by just flying the engine hard for 45 minutes or so, or perhaps using "Sea-foam" a decarboning fluid the outboard boat motor builders produce. The second problem is that the head bolts weren't all torqued up correctly.

 

At the 100hrly I didn't re-torque the heads as I didn't have a 1/4 Allen that was long enough to go through the hole in the rocker area. That was a mistake as the head bolt in that under the rockers position is the one that seems to be most heat affected and as a result was less that the requisite 20ft/lbs. (In fact the bolts most likely to be loose are all in the lower 1/2 of the cyl head, lower when viewed as fitted to the aircraft). The long allen key required was simply a kit of ball ended long allen keys that came from super cheap auto, and then had the short end cut off with a dremel cut off wheel. the resultant long shaft fitted nicely into a 1/4 socket which them went straight on to the torque wrench.

 

Anyway, after 10hrs or so of work the outcome is fine and the engine once again is serviceable. The only new parts required were the exhaust manifold bolts and the inlet manifold gasket, which I manufactured myself from paper based gaskit material. The manifold bolts were seriously corroded and as such I expect to replace the remaining manifold bolts at the next service, which is due in under 10hrs.

 

regards

 

Andy

 

 

[Guest J430]

Andy, this is not a slag at yourself at all, but far too often your typical problem is attributed to some thing like.......head bolts not tensioned due to no tool, or a similar such thing.

 

Glad you have it all sorted. We do our head tensions and tappets evry 25-35 hours, basically a 100hrly 3 times in 100......no it did not stop carbon on ring grooves but our heads, valves and seats were pristine.

 

Can't say the same for valve guides on the inlets, but that we had no control over as I can see it.

 

Go enjoy your plane now!

 

J

 

 

[Guest JRMobile]

Finally got around to building and using the leak down tester: (http://www.xs11.com/tips/misc/misc3.shtml )

 

Results are as follows-

 

1- 62/80

 

2- 68/80

 

3- 70/80

 

4- 60/80

 

5- 65/80

 

6- 65/80

 

We had been using the leaner jet recommended by Jab for 105 hours (JSL002-2 update installed 10 hours). All cylinders appear OK. Was surprised at the amount of difference between readings, any comments.

 

Cheers John

 

 

[Guest J430]

Numbers look all good, while they are above 60 its good, I reckon even a bit under 60 is still workable, but when you get numbers below 40/80 you know its leaking pretty well.

 

Couple of thoughts, was the engine hot, must be flown for a few minutes get heat right through everything, and then quickly get into it.

 

Also you need a compressor of at least 12CFM capable of keping up a constant supply pressure.

 

check to see where the majority of the air is escaping.........oil filler (rings) exhaust pipe (Ex Valve) Carby (inlet valve) and I have heard of a head on a J200 not bing torqued properly that leaked around the head/cylinder interface. This was lapped in and all was good.

 

J;)

 

 

[Guest JRMobile]

"Couple of thoughts, was the engine hot"

Funny you should mention that - engine was hot when I started the job but by the time I got her ready and worked out a way of steading the prop (working on your own :;)6: ) the temp was down to warm. J - thanks for the thoughts.

 

 

[jetboy]

Leakdown test results on my 2200 @ 200 hrs. This has run with the lean&mean economy jetting for the last 150 hrs, 14 L/hr

 

80 / 64

 

80 / 71

 

80 / 63

 

80 / 64

 

air was only heard escaping from oil filler. Its probably better to put up with a little blowby than pulling the cyls for a hone and ring job with another 50 hrs run-in and no guarantee it will be any different. Been there done that with an 0-200.

 

Engine was warmed prior by the time I finished checks the prop would stay put at TDC like its allways hard to turn when warm. (but I stay clear of it anyway)

 

I rejetted per the latest revision

 

Cruise EGT now too low, using 17 L/hr

 

put back original needle jet .276, looks like 15 L/hr

 

EGT WOT 640, max in cruise 679 @ 2850 rpm

 

It appears the original economy tuning kit has not ruined the valves (yet), so I'm off to buy a more suitable prop and start rechecking mixtures again!

 

Ralph

 

 

[Guest J430]

Ralph, those numbers are OK. When they get to something way less you have a ring issue or something else.

 

I still run the econ kit in the 3300, the bigger jet makes it run rough as......on a well maintained and nil induction leak engine its not a problem. The richer jet was to cater for the lowest common life form!

 

Like most things!

 

J

 

 

[jetboy]

Dont know if this is the best thread to ask but is anything known about the 14 valve failures being reported on the UK site?

 

In reply to: Enrico 's message, "AAIB and exhaust valve failures" on 17:12:29 10/21/07 Sun

 

Enrico – I’m the start of the AAIB investigation. On the 12th September I had a wonderfully day flying from Rochester to Manchester, on into the Lakes and then headed back for Rochester.

 

With 45 min left to Rochester and at 1500ft I got sudden serious vibration. I reduced power and started to look for a good field, as I did so the engine stopped. Not my best ever landing (no smart comments please) but only minor damage to the aeroplane and no damage to me.

 

In response to your questions

 

Has anyone experienced this failure?- Yes, oh yes

 

What is thought to be the cause? – One of my exhaust valves broke and was then rammed back out the inlet. The AAIB are still investigating and so far have taken valves from my engine, from a thruster, from a J400 3300 engine and several other new and used valves for examination.

 

What can, if anything, be done to prevent this failure? – Nothing to do, my engine is hydraulic tappets and therefore no adjustments. Practice forced landings and maybe avoid long water crossing until we know what caused the failure.

 

I have been flying behind two other Jab2200 engines after mine failed including this weekend and if anyone out there wants to offer loans then don’t by shy. I have 130 hours on type with only one crash landing so far.

 

By the way the 3300 and 2200 share the same valves.

 

In reply to: Eamonn 's message, "Re: AAIB and exhaust valve failures" on 08:57:45 10/24/07 Wed

 

Congrats Eamon on a good dead stick ,you walked away from it ,a learning curve for all of us , today I had a conversation with a director of the BMMA and an inspector of same ,he told me information he had was their had been 7 failures of the Ex valve nature todate , 4 UK ,2 Aus ,1NZ , you have 100 plus hours on engine ,but how old from new , and would it be possible to ascertain what fuel you were using ,the old chestnut has come to the surface regarding the use of MOGAS .

 

In reply to: Howard 's message, "Re: AAIB and exhaust valve failures" on 1024 10/24/07 Wed

 

In France there have 7 problems with broken exhaust valves. All these problems have been with the hydraulic tappet motors. We are currently investigating solutions using increased lubrication in the cylinder heads and have installed valve guide seals. We think that the guides have been wearing rapidly. We have found that this is particularly so on the front cylinders with the tail dragger aircraft

 

Ralph

 

 

[Yenn]

At least with manually adjusted valves you will hopefully get some warning of impending valve failure by the closing of the gap when you check it at each 25 hours.

 

I keep a very good eye on mu valve state and always do a full pull through before starting to feel the compression.

 

 

[facthunter]

Valve guide wear?

 

Rapid valve guide wear has to be wrong geometry or incompatible metals. If the guide is loose then the valve head does not hit the seat correctly and the stem is subject to bending, and ultimately fails. Nev

 

 

[Guest brentc]

It's the heat in the Jab engines that's most likely doing the damage. I suspect that the geometry and design is perfect, but the lack of oil and high CHT temps are causing the valves grief.

 

 

[facthunter]

Wear.

 

You will need to examine the surface of the stem of the valve to work out what is going on . If it is NOT shiny smooth and polished, I would suggest that it is galling. Galling is where some of one surface, partially welds to the other, producing a destroyed finish, and consequential rapid wear. Unless you have an abrasive present, it is not possible to get rapid wear any other way. This can be wrong metals. (Some wear well in combination, some don't) or bad finish (not likely really), or overload, (wrong geometry, or dimensions too small to take the load)

 

High temps make it harder for the oil to do it's job, but I'm not convinced that the HEAD temps, (as distinct from the valve temperatures), are any hotter than any other air-cooled aircraft engine. If the exhaust valve temperatures are elevated, the metal will become "hot short", lose it's tensile strength, and stretch or break the head off.

 

If valve guide oil seals are fitted, the amount of oil finding it's way up the guides is b***er all anyhow.. Continental and Lycoming have spent a lot of time and money over the years trying to get the "TOP" of their piston engines to last through the TBO period with mixed results. Arguably, Lycoming are more successful, but they generally run sodium- cooled (hollow stemmed) ex. valves, and THEY don't come cheap.... Nev..