Some rotax advice

[Rodr]

Would app some thoughts on 3 issues I have on a new 912 ULS with 6.5 hrs TT.

 

First one is that on a mag check I get a miss and fart on one mag under 3000 revs however on throttling up it disappears !

 

Secondly on checking plugs I find that both plugs in no1 and no2 are both extremely black and nos 3 and 4 are nice and grey!

 

Thirdly at 6hrs I can now feel a small vibration throughout the full rev range wasn't there at the start , checked prop all good.

 

I would assume that one of the ignition sensors maybe failing but not sure? also the richness of the front cyl indicates that the carb may set too rich as they are the furthest cyl from the carby?

 

And the vibration may be a combination of all these abnormalities ?

 

Any ideas would be welcome

 

 

[cscotthendry]

Rod:

 

Not sure what the miss might be, but the roughness could be carby balance. Rotax engines operate like two independant engines tied at the crankshaft. Any differences between the carbies makes the two engines fight each other. This can come from having different lengths of throttle cable or the balance not properly set up.

 

If it is a new engine, did you hook up the soft start connections to the ignition units? The soft start retards the spark timing for about 8-12 seconds after startup when it is wired up. If it isn't wired to anything, that is an open circuit input which could pick up electrical noise and trigger the soft start function. The easy way to check if your engine has the soft start modules is if they have yellow part number labels rather than white.

 

For new engines, don't worry about the different indications on the spark plugs. That is normal until they're well run in.

 

 

[Rodr]

Thanks Scott the soft start was wired and works a treat, having a Savannah with 912 I have just wired that one too and makes starting a breeze, the Savannah has done 360hrs and I have not had yo touch it , this one is in a replica tiger moth but I may not be so lucky and wondered if anyone has had to replace the ignition coils sensors as the mag checks are telling me that all is not well however at high rpm you wouldn't know!

 

The vibration wasn't there on earlier runs but seems to be progressing so I guess it could be carby balance I haven't checked that as yet,thanks

 

 

[Downunder]

Money's on the carbs out of balance I recon. First thing to check anyway.

 

Black and sooty plugs are normal. (After idling)

 

Carb and induction system is designed for 5000 to 55000 running, so this is where the egt's start to balance out and it all "comes together".

 

 

[alf jessup]

Rodr,

 

Mag checks should be done at 4000rpm, you will probably find it won't miss and fart, as for the plugs it is the norm, you can check the idle air screw making sure they are both out 1 1/2 turns as per Rotax specs, vibration could very well be carby balance if the prop is ok as you say

 

Cheers

 

Alf

 

 

[Rodr]

Cheers guys, using the two out of one throttle made for rotax from aircraft spruce they are great for keeping equal movement however as you all mentioned the carby balance, I remember removing one of the carby throttle springs to reduce the full power pull on the throttle and checking this while stopped in different throttle settings found to be equal , maybe not while running so will replace the spring and test.

 

Also the mag test on the Savannah 912 at all revs shows no difference, but this one does only as I mentioned at under 3000 rpm maybe a newer electrical format in the ignition system but seems strange!

 

Cheers