Microlighters

Hi guys been working on my highish EGT on WOT 1200f just on the max limit, and decided to fit new plugs today, the old ones puzzled me when I took them out as the 2 from the PTO end are dark brown and the 2 from the MAG end are light brown going on to a tan colour,

I have made sure that the carb is at 90degs to the crank line being bolt upright,

Runs sweet and starts well, its a 503 SCDI electric start, its on a trike, but doing a mag check give a 250rpm drop which I think is within Rotax spec, I can hear the drop as well as see it on the tach,

Any ideas guys.

Nick.

Sounds like a slight mixture difference between the cylinders Nickjaxe.
Being a single carb motor there is a remote possibility that you have a very slight leak on the manifold gasket of the leaner cylinder. I would replace those and see how it goes.
Are your jets correct for your average operating density altitude? Search the net, I have seen a correction chart which will give you guidance for the exact jet and needle sizes and needle groove setting you will need for your operating altitude and temperature, I do not have the link but perhaps someone else can point it out.
Correct jetting will definitely get your EGTs to where they should be.
Also please bear in mind that correctly cleaning and lubricating the air filter is also very important to maintain the right mixture control on a 2 stroke. If you have the right jets but run your filter dry there will be more air in the mix and therefore run leaner.
Good luck
Mogas

Hi and thanks for your ideas,

I am going to take the manifold off today and have a good look at it although it was fitted with new gasket about 12mths ago,

I serviced the K+N filter a few weeks back and have been experinment with jets, but I would have thought that if the jetting where wrong it would effect both cylinders equily but odd things do happen,

I was suggested that the electric starter I have its an M4 can cause cranks to get a twist in them (scarey) and have the effect of running with different ignition timing on the 2 cylinders I wonder anybody heard of this.

Nick.

Nick,
By your description nothing seems out of the ordinary. I would not be worried at all. 250 mag drop spot on the norm.

I should have said I get a 250-300 drop on one mag and only a drop of around 150 on the other.

Nick.

The manual says with engin at 3000 rpm - 3500 rpm, the magdrop should not exceed 300 rpm. I have never had the magdrop exactly the same.Always a difference ranging between 100 - 300 rpm respectively.

"As for colour, again the following from the manual:
Light grey : overheating
Black: Fouling, rich mixture, too cold
Brownish : Normal"

and more applicable to you ?

"on engines with a single carburator, one sooty sparkplug usually indicates a bad plug or faulty ignition system. If both plugs are sooty, check the carburation".

But again, does not seem to apply to your descriptions.

Roel

Nick, I think the weather's getting to you. Stop fiddling...
Those plugs seem just fine to me. I'm with Kloot Piloot on this one. I doubt if you'll get it 100% spot on. You're just gonna frustrate yourself in the process.......

I know what your saying duck but there is such a difference between the 2 cylinders,

It has been suggested that the crank may have a sight twist putting the ignition out on one cylinder.

Nick.

nickjaxe wrote:I know what your saying duck but there is such a difference between the 2 cylinders

No way, there's not SUCH a big difference.

It has been suggested that the crank may have a sight twist putting the ignition out on one cylinder.

Ok then. Remove the engine and get to the magneto. Mark the mag and the casing at a point where one cylinder is at TDC (top dead centre) and the other should be at BDC (bottom dead centre). Swing the crank through 180 degrees to bring the "bottom" piston to TDC and see if the marks line up again. If they line up when the bottom piston is now at TDC, then the crank is not twisted. Be aware that there is a little play in the crank at TDC. This is normal.

If you have 2 EGT gauges, then they will read MARKEDLY different if the crank was twisted and your plugs will read black/brown and not brown/lightbrown. The engine is gonna run rough. Check through the exhaust ports at the tops of the pistons and look for signs of detonation.
I'm willing to bet that crank is NOT twisted.

I must admit Duck it does run nice and starts well but just seems odd as both cylinders are fed off the same single carb, have you came across odd colour plugs before like mine.

Nick.

Nick, mine's like that most the time. Slight difference but in the same brown/tan range. Not a great difference, but different nonetheless. Must add that mines a DCDI though.
Remember also that fuel, once through the carb, could be dispersed slightly more/less to a cylinder (for the SCDI)

It's the weather, I tell you...

what EGT temps do you get in the climb out and cruise Duck are are they matched from pot to pot,

I only have 1 EGT unfortunately.

Nick.

nickjaxe wrote:what EGT temps do you get in the climb out and cruise Duck are are they matched from pot to pot,

I only have 1 EGT unfortunately.

Nick.

Mine sits at about 950/980 respectively in cruise and 1050 at climb. 2 seperate EGT's. I start stressing at 1150.
There is a mathematical way to calculate if your probe/s is correct. I have the formula on my other PC at home. Currently out of town. I'll get it to you next week so you can check the accuracy of your probe, which I think is incorrect because at 1200 it's on the hot side and theoretically you should be able to see it on your plugs, but yours is showing normal temps.

Hi Nick

Analyzing machines for a living deciding what and if there is something wrong with them is what I do. I take info rework it and get an answer. Now I’m not suggesting that there is nothing wrong BUT taking the info available, (you provided it here) knowing engines ( saw them in a movie once) I’m very confident that you should go, assemble the engine put it back into the plane and fly it. That is of course if you have stripped it out and down by now.

Secondly post a pic? Maybe we don’t realize the extend of the problem. In my mind now the discoloration is SLIGHTLY different between the two. Just my 2c


Sanna

hi guys, duck the mag end plugs are verging on white which make1200f seem about right, the pto end is the correct colour at mid brown well from what I know mid brown is ok,

I did find a chart on line telling how to test the EGT probe by heating with a torch which I have done, the chart has a colour range for heated metal that also give the output voltage of the probe in mV at that heat colour, I got it to the colour for 1200f and it was putting out 26mV which again = 1200f my gauge probe is on the the mag end branch so it does look like I maybe running at around 1200f on that pot WOT,

To put my mind at rest I am going to have the motor off and check for crank twist, if I dont do it now it will just keep nagging me, not that I want to but what can I do.

Nick.