Don't know what thinking about this

[Guest 4aplat]

the sparks plugs and the oil are 25 h old

 

the piston rings 25 h old too

 

cylinder 01

 

 

cylinder 02

 

 

cylinder 03

 

 

cylinder 04

 

 

 

[Guest Walter Buschor]

Hi 4aplat

 

Judging by the photo's cylinder 1&3 are running too rich , number 4 is good and number 2 in between.

 

I don't have any experiance with jab engines so I cannot help. For what it's worth if I was running a jab of any sort I would fit the fuel injection kit to eliminate those uneven fuel mixtures. It might even fix all the other problems too.

 

Thanks for the pictures but unfortunately I cannot offer any ideas other than the one mentioned above.

 

fly safe

 

Walter

 

 

[Tomo]

What was the Leak down pressures?

 

 

[Guest 4aplat]

Hi Walter.

 

I hope it's just because 1 & 3 are too rich, but there is only one carb for all the cylinders......... and I'm afraid that the valve guides or/and the piston rings are out !

 

For tomo : sorry but i'm not very good in reading/writing english. What do you want to say with "Leak down pressures"

 

I will try to connect a pressure gauge at a spark plug place and give the "compression"

 

unfortunately, this test is supposed to be done when the engine oil is at the running temperature and I'm not able to run the engine till las saturday (problem with the carb choke i guess)

 

MicheL

 

 

[Tomo]

Ok, sorry. Compression is what I meant.

 

Have you had the valves lapped in recently?

 

Vous faites bien en anglais, mieux que moi en français!

 

 

[Guest 4aplat]

English is an international language....... French is not !

 

nothing have been done on the valves in the past 20 hours

 

A lot of french 2200 jab engines had got some problems with valve guides (I broke the engine in another aircraft 5 years ago)

 

and most of us change the valves with brand new one

 

see this article

 

JABIRU-ENGINE-400hours

 

Bonne nuit !

 

MicheL

 

 

[brilin_air]

You might have given the clue with the carby choke, if that is the case then No4 is running lean.

 

Brian

 

 

[jetjr]

Jabiru have a well talked about problem with fuel distribution. For some reason fuel mixture doesnt flow to cylinders evenly

 

Those with EGT and CHT on each cylinder see this clearly and fly accordingly

 

I think this is the heart of some reliability problems

 

There are a few small ways to help but without all cylinder monitoring its a guess, even with this it is hard to find a solution using carb

 

Leak down is testing how much leak the cylinder has when all valves etc are closed. It tests how worn the rings and valves etc are and can pick up many problems before they get too bad. This can be done hot or cold, but cold gives worse readings (making engine condition seem worse)

 

It requires special tester with 2 guages and orifice between connected to cylinder. Compressed air is applied and the pressure lost out of the cylinder is indicated by comparing guages.

 

Talk to Aircraft Engineer, standard practice on aircraft engines

 

 

[geoffreywh]

Looks like a lot of oil on most of the plugs..I would guess either the rings are not bedded in yet ( #'s 1 and 3 ) or the bores are glazed from pussy footing around?...LONG ago I did a course at Champion reading plugs . The rule of thumb for getting the heat range correct was:- The first four threads should be discoloured after a reasonably short time.( I can't remember how long) This has always proved to be correct... From the threads on your plugs ( in the foto) they look to be running too cool.

 

 

[Guest 4aplat]

The results with the compressiometer are :

 

cylinder I III and IV : 10.25 bar

 

cylinder II : 10 bar

 

MicheL

 

 

[facthunter]

reading plugs.

 

Heavens Geoff, reading plugs has NOTHING to do with what the threads look like. It is the colour of the INSULATORS. What are they teaching you these days???

 

 

[geoffreywh]

compressions look very good, I think that the bores are glazed and the oil ring has not bedded in. Looking at the rings should tell you...Simple fix...Just hone the barrels and run in bores...I thinks it needs to be done quite quickly, for example a climb to altitude at full throttle ...with one wary eye on temp. meters...Don't get it too hot!....

 

 

[geoffreywh]

As far as reading the correct heat range plugs You are very blinkered in your approach... Perhaps you might explain to all how You would judge that you have the right heat range, With lead discolouring the electrode and oil contamination masking the situation ...After 50 plus years in racing motorcycles you would think that I might have some clue how to go about reading a plug.. The course at Champion was in the 70's and was concering reading plugs in endurance racing....During the 72' Barcelona 24 hours...so "what are they teaching these days" is so much hot air.. The electrode condition tells heaps>..BUT is very quickly changed in colour by coasting with low throttle settings.Like in a circuit, or landing.For example if the idle was set VERY rich when you landed the plug might show a dark electrode..The colour of the electrode is a momentary "photo"...while the discolouration of the threaded portion will tell a longer term story....How much heat the plug is exposed to over a longer period.. It WILL NOT tell you much about the mixture ... it tells ONLY if the heat range is correct for that particular carburettor setting AND at Full throttle, so look at your plugs again and see if they are of the correct heat range.........

 

 

[facthunter]

Correct plug temp range.

 

I agree with the second half of what you say. Any comments that I have made previously on this matter, have emphasised the uselesness of checking the appearance of the insulator, IF you have done a glide approach and/or a long taxi. You would only get results on a ground run with an engine cut.

 

The heat path from the insulator to the head is up the insulator and into the metal body of the plug and then into the head. Aircraft plug washers are solid copper. The car plug washers we use are not.

 

The temperature at which the head runs will affect the choice of plug. If the plug is not tightened sufficiently it may overheat or come loose. Aircraft plugs are torqued to a set tension which varies from engine to engine.

 

A long thin ceramic insulator runs hotter and a short thick one runs colder. IF you use a plug which is too hot you get detonation at high temps and power settings. This would be the main thing to avoid, as it can break the engine. Too cold and you will get lead and soot building up on the insulator and shorting the plug out. If you are in doubt it is safer to run the colder option however.

 

IF the motor is using a lot of oil it is hard to work out anything. That condition should be corrected first..Nev

 

 

[Guest 4aplat]

Hi guys

 

thanks for the answers. Your english is a bit too much "technical" for me but I thing I'm able to understand the main line.

 

If it could help, all my landings are made with a glide approach (idle to the minimal)

 

I use the same gaz as in my VW beetle (mogas that is an unleaded gaz)

 

in 25 hours of flight I use 1.6 l of oil

 

And I'm about to machine some new valve guides (the engine is 300 h old)

 

MicheL

 

 

[jetjr]

With aircraft, the long idle for landing and taxi can darken spark plugs.

 

This will give false reading of spark plug colour.

 

Best to do higher rpm run just before shutdown and spark plug check

 

That oil use is a worry - looking like glazed cylinders to me too.

 

 

[facthunter]

Bore Glazing.

 

The name Bore glazing seems to cover anything where rings don't bed in. There is a recognised phenomenon in diesels where there is a carbon build up above the top ring of the piston and this carbon polishes the bore and you get problems with blow-by.

 

A place where I worked used a process of mirror finishing the bores of their reconditioned engines. This was a complete failure as we ended up with a heap of scuffed pistons. I believe the highly polished bores didn't have any pores/grooves for the retention of a film of oil.

 

The earlier air cooled Porsche engines were alloy barrells with chrome plated bores. These were provided with small dimples all over the bore surface to retain oil.

 

Modern design ring and piston technology goes to a lot of trouble to reduce AIR POLLUTION and there is a great emphasis on this (coming out of California mainly) I would question whether this technology is appropriate for aero engines that are not designed with stop-start polution considerations in mind. The finish on NEW Jabiru cylinders is a very fine hone finish. Once there is polishing of this finely finished surface, I can't see what running the engine a bit harder will achieve. You would have to remove the cylinders and put the honed finish back on the bore surface. I would try to make it a bit coarser, but you would still have to do it well. As the bores are not nitrided they will wear to polished again fairly quickly. Every time you hone the cylinders they get a bit larger, and your piston clearance becomes greater. Nev

 

 

[geoffreywh]

Yes , exactly. The only cure for "glazed bores" is to remove the cylinders and re-hone. Those "ball brush " type hones are ideal and work very well....Gives a nice coarse finish.( in metal finishing terms) .But the rings will have to be bedded in quite quickly. When I worked for Ducati we had a lot of trouble with glazed bores when they introduced "Niguasil (hard nickel) plated barrels. The only solution was to re-hone the barrels and then, While the customer was led away, Give the bikes (851's) the absolute berries around the track a couple of times then a quick oil change ( it comes out like silver paint!) and, hey presto! no more oil consumption...Same for a Jab engine (in my opinion!) Maybe warm the oil even to minimise ground running. Then a fast climb possibly in steps so as not to over heat the engine..and I think the rings would bed in very quickly....

 

 

[Guest terry]

The simplest thinks kept me happy, I've enjoyed reading this thread very much. keep it up.

 

Terry

 

 

[Guest 4aplat]

Hope I understand what you say.

 

Is this tool good enough to re-hone the cylinder ?

 

 

Do you think I'll have to change the rings ?

 

thanks in advance and for your help

 

MicheL

 

 

[stanzahero]

Hi 4aplat,

 

No that is a brake cylinder hone...

 

It could cause inconsistant cross-hatch and may scratch the bore.

 

It is difficult to get even preasure on the bore with anything less than 4 pressure pionts keeping it steady.

 

The item they are talking of is a 'flex-hone'.

 

flex hone - see here

 

 

[stanzahero]

As David said they can either be total disaster or a God-send.

 

This is the way I used to use them...

 

Depending on the grit size (finer grades are better for finishing, courser grades for de-glazing) I lubricated the cylinder bore with either cerosene (?) or a light mahine oil for the courser grades.

 

Using a large drill (needed for the power) place the hone inside the bore while it is stationary. Slowly spin the hone inside the bore while moving the hone up and down the length of the cylinder.

 

The optimal cross-hatch angle (the angle between the up lines and the down lines) is between 60 and 90 degrees.

 

If the angle is too great, the piston rings tend to spin on the pistons causing abnormal wear on the ring lands and rings. Too little causes bedding in problems and reglazing.

 

It is important not to let the drill spin in the one spot as this could do serious damage to the bore.

 

This was my technique foe automotive engines...

 

Stanzahero.

 

 

[Tomo]

Speed - don't go too fast with the hone.

 

 

[Tomo]

Do you think I'll have to change the rings ?

Yes, usually when you hone the bores, you replace the rings. Ensure rings are fitted the correct way around, top to top etc...

 

 

[jetboy]

4aplat,

 

If you are removing the heads etc. to work on this, then you should carry out the machining required to the cylinder base nuts at this time. Jabiru issued a bulletin last year, and it is described in some issues of Jaba Chat, on the website, and you will also discover the service bulletins and details of leakdown testing there.

 

Looking at your plugs, I think its an oil problem, either valveguides or more likely stuck oil rings in 2 of those cylinders. Jabiru has acknowledged the original cylinder basenuts can distort the barrels and cause other problems.

 

It would be good if you can get the help from another Jabiru owner who has aready done this type of work.

 

Ralph