Plugs different colour each clyd

[tandemtod]

If the crank was twisted even slightly, the engine would vibrate like a crazy thing which obviously is not happening on your engine. So don't waste your time looking for spooks where there aren't any. If you must, you can check crank orientation through the plug holes as sujested in a previous post, but you won't find anything.
The different coloured plugs are perfectly normal and most air cooled Rotax have this and the reason is very simple and straight forward. THe engine is cooled by a fan at one end only and the first cylinder gets cooled slightly differently to the next cylinder in line. This gives a slight difference in temperature between the cylinders which accounts for the ever so slight difference in mixture which accounts for the colour difference. When you have two carbs, you can compensate for this condition but with one carb you are stuck with it so rather go and enjoy yourself flying and worry about fixing the engine when the book tells you to.

[nickjaxe]

Hi when you say check for........crank orientation........through the plug hole could you explain what I should be looking for, sounds interesting and could maybe save me a lot of work.

Nick.

[Duck Rogers]

Hi when you say check for........crank orientation........through the plug hole could you explain what I should be looking for, sounds interesting and could maybe save me a lot of work.

Nick.

Ok, do it this way.....Remove the plugs. Take a welding rod (nice straight piece of wire) and stick it into a plug hole to rest on top of the piston (any piston first, will do)
Turn the engine over slowly to bring the piston up until the rod doesn't move up anymore. Make a mark on the rod level with the top of the cylinder head. Without moving the piston up or down, stick another rod into the other plug hole and again mark the rod level with the cylinder head.
Now turn the engine over slowly (by hand of course) until the piston that was at the bottom now gets to the top. Swop the rods around and the marks made on the rods should line up evenly with the top of the cylinder head if the crank is ok. If it's twisted, one of the marks won't line up.
This is not an exact science since there is a minute amount of play in the crank when the pistons are at TDC and BDC respectively. Just feel it out by hand and you'll see what I mean.
Be careful not to jam the rods into the top of the cylinder head when moving the pistons up and down. Make sure they sit nice and loose in there.

I'm still betting that the crank is ok

[nickjaxe]

And I hope you win your bet, thanks Duck.

Nick.

[tandemtod1]

Ducks method with the wire/welding rods will give you a good indication as to the axial alignment of the crankshaft but as he says, it is not 100% accurate. Here is an aid that will make it a bit easier for you. Take an old sparkplug and break off the earth electrode with a pair of pliers. (thats the piece of metal that runs from the side to the centre electrode where you set the gap at the business end of the plug. Now put the plug in a vise and break off the porcelain until it is level with the metal part of the plug. Pull out the core wire that will be sticking out and using a suitable punch, drive out the centre electrode of the plug and you will be left with a nice guide with a 3mm hole in the centre that will screw into the cylinder head. Make two of these and then continue with Ducks method in order for him to win his bet. It is usefull to oil this guide to make the wire/welding rod slide nicely inside. One word of caution, make sure that you only rotate the engine always in the same direction or you will get two different TDC positions.
Good luck

[nickjaxe]

Hi guys I actually have a dial gauge that screws into the plug hole, I took the engine off the a/c yesterday and spent a few hours with it on the bench today but its not an easy thing to check but it looks to be within 1 deg not sure what the max out of tolerance is.

Nick.

[tandemtod1]

The dial gauge will measure distance so i'm not sure how you are converting that into an angle. However, the difference that you are reading could easily be a bit of carbon or a mark on top of one of the pistons. Either way, if you can measure no more than one degree of axial misalignment then you need to pay Duck his wager. The maximum axial misalignment for a production crankshaft is 0.2mm measured on the crankwheels between centres. For racing purposes, we realign a production crankshaft to within 0.05mm or better. If the crank runs out by more than 0.2mm then you get substantial vibration which consumes power (you need power to produce vibration) which obviously means less power to do the work and the vibration will put undue stresses on other components of the engine and airframe. You have never stated that your engine vibrates excessively and that is the real test, so therefore I am certain that your crank is straight enough. Almost every fabricated crank that I have ever checked has run out by at least 0.1mm and this does not pose a significant problem. In order to measure a crankshaft properly, it needs to be removed from the crankcase and held between centres and measured with a dial guage. The measured runout then determines whether further action is needed. In your case from what I have gleaned on this thread, I would consider this a waste of time and effort and completely unessesary. As I have said previously, go and enjoy flying and do the book maintenance, the little Rotax is as bulletproof as they come.

[Duck Rogers]

The dial gauge will measure distance so i'm not sure how you are converting that into an angle. However, the difference that you are reading could easily be a bit of carbon or a mark on top of one of the pistons. Either way, if you can measure no more than one degree of axial misalignment then you need to pay Duck his wager. The maximum axial misalignment for a production crankshaft is 0.2mm measured on the crankwheels between centres. For racing purposes, we realign a production crankshaft to within 0.05mm or better. If the crank runs out by more than 0.2mm then you get substantial vibration which consumes power (you need power to produce vibration) which obviously means less power to do the work and the vibration will put undue stresses on other components of the engine and airframe. You have never stated that your engine vibrates excessively and that is the real test, so therefore I am certain that your crank is straight enough. Almost every fabricated crank that I have ever checked has run out by at least 0.1mm and this does not pose a significant problem. In order to measure a crankshaft properly, it needs to be removed from the crankcase and held between centres and measured with a dial guage. The measured runout then determines whether further action is needed. In your case from what I have gleaned on this thread, I would consider this a waste of time and effort and completely unessesary. As I have said previously, go and enjoy flying and do the book maintenance, the little Rotax is as bulletproof as they come.

Amen.......

[nickjaxe]

tandemtod1 Are you talking about crank run out or crank twist??????

I can find a fig for runout in the manual but not twist,

Ok the way I did my measurement was find TDC for each cylinder with my dial gauge and mark the fan tower around the mag area, and then compare the 2 figs with a 360deg protractor that I have fixed to the end of the crank mag end, not perfect but thats the best I can do,

I am determined to get to the bottom of my differing plug firing colours cylinder to cylinder,

If I got the timing wrong on one mag last time I set it which was about 12months ago I wonder would that have the effect I am getting or would it just be out on both pots???

Must admit I found setting the timing a bit hit and miss with dial gauges into a cylinder on an angle, then having to use a compensated fig to allow for that angle, I wonder how the factory do it, from what I remember I ended up using my protractor on the crank end last time setting the timing to 16deg BTDC,

The book fig with the gauge in the plug hole is 1.47mm BTDC which I have been told equates to 1.697mm BTDC compensated,

I wonder what method you guys use to set the ignition timing on the 503DI, I would be most interested to know to see if I can get it better than the way I do.

Nick.

P.S. I have found that all 4 of my spark plug caps show open circuit with both of my multi testers, instead of the normal 5k ohm, I wonder if that would have an effect of the ign timing.

[tandemtod1]

Runout when measured on the CRANKWHEELS = crank twist = axial misalignment. This is recognised by severe vibration under NO LOAD condition and the vibration will vary dependant on engine speed. The manual will make no reference to this measurement. Axial misalignment will also show up as crank runout when measured at each end of the crank when measured with a dial gauge.
Runout when measured on each end of the crank is what the book refers to. Excessive runout can be symptomatic of either a bent shaft or axial runout. If the crank is within spec when checked at each end with a dial gauge then there is no problem.
Bear in mind that the crank is made up of several pieces and pressed together under production conditions so some axial misalignment is to be expected and is normal. You were initially refering to the possibility of the starter motor damaging the crank and causing it to twist (axial misalignment) Although I suppose it is possible, the crank would have to be a really crappy item for it to actually happen and you probably have to lock the output end of the crank and then operate the starter in order to try and make it happen. And Rotax don't make crappy cranks.
Different ignition timing settings on each cylinder will cause different combustion chamber temperatures which will show up as different colours on the plugs. Generally a slightly retarded timing will cause a temperature rise which will show up as LEAN on the plug. The difference in timing between the two cylinders would have to be substantial before it becomes important. Don't forget that I told you previously that the cylinders are cooled differently (the heated air from the first cylinder is used to cool the second cylinder) This has the effect of raising the intake charge temperature on the second cylinder effectively retarding the ignition timing on that cylinder. It is not adviseable for you to try and compensate for this. If you add all these little errors up then a difference in colour can be noted but it is still in the realms of normal. Even a long final approach at low throttle settings will change the colour of the plugs. The conditions that you have described are normal and in my opinion, no corrective action is required. Provided that your inlet manifolds and carb sockets are not leaking air and leaning the cylinder then the difference in colour of the plugs is of no concern.
I am beginning to think that you are scared of flying

[Duck Rogers]

I am beginning to think that you are scared of flying

Wrong. Nick flies a lot when the weather permits. Remember he's in the UK.
He also has a lot of time on his hands due to the weather conditions and his plane is "hangared" at home, that's why he likes tinkering with it.
He also doesn't have the support base we have over here.

Nice explanation on the crank though.

[tandemtod1]

Hi Duck,
It was said tongue in cheek to motivate him to stop fiddeling and do some flying. I didn't realise that he is in England and of course it's winter there . I hope he gets it to his satisfaction before summer comes.

[nickjaxe]

Its ok guys I know where you are coming from but in a way you are no to far off the mark, its not so much a fear of flying but a fear that my engine is not as good as I can get it,

Must admit I do annoy myself sometimes, I suppose it can become an obsession, its a good job for me that there are guys like you that are prepared to give advise to strangers and it is appreciated,

Yes hoping to get it sorted for the better wx over here,

What is your preferred method of setting the timing guys, dial gauge or timing protractor/disc.

Nick.

[Duck Rogers]

Dial gauge, feeler gauge and 2 eyes......