[markku]

Hi, it´s the first time I post in this fantastic forum, which I have followed for some time and find the most knowledgeable of the karting community.

Perhaps you can help me with this one.

My brand new Rotax DD2 engine (I raced in this category last year also, so I have prior experience with this engine) misfires badly when it reaches 10,500 rpm in second gear.

We have changed plugs, have tried different carburettor settings (main jet and air screw, we have to try with the idle jet too), we have changed the ignition coil and we have checked the gearshift cables.

We also made a gasket for the ignition pick up (we didn´t have the one that Rotax supplies to solve this kind of misfiring).

We couldn´t measure the battery charge that day (I´ll test with another battery tomorrow), but the kart started allright all day (more than 10 times), so I don´t think that´s the problem.

We plan also to change the fuel pump, put the proper Rotax gasket in the ignition pick up, try another carburettor and the idle jet of 30 (now it has the 60 one).

Any ideas?

Thanks in advance.

P.S.: interestingly enough the engine misfired in both of the two long straights, but didn´t do so in a flat left? Carburettion related?

[Mikko Nassi]

Hi Markku

You're definitely doing the best thing with switching over components - carb, battery, fuel pump etc.

I have zero experience with the DD2 so don't know any specific nuances of it... however things that you didn't mention checking are the float levels - check that and maybe change it a bit to see if it changes it. A very rare problem is also a having a leaking float (fuel in the float). Additionally check the exhaust to make sure it doesn't have a broken baffle (although if it's new quite unlikely).

If your kart is also new check the entire fuel line including inside the fuel tank for blockages and to see that it can pick up fuel properly.

One more unlikely but possible cause - if your throttle isn't lifting the slide all the way up (but does in a left-hander because your leg is pushing the cable further) it might effect it. Very quick to check to rule that out.

After changing out everything possible there is always the possibility with a new engine that something is internally a bit off, and you might need to open up the engine to have it looked at. Although because it works in a flat left like you said it does seem carb related.

[markku]

Thanks for the advice (I finally found my password, ho, ho...)

Eventually we cured the problem.

But don't ask me what was wrong.

We followed your advice and performed some other changes (the ones specified in my former message).

When we changed the water temperature probe of the Mychron 4 (which started giving 250º c readings, and it was clear that the engine was not overheating) the problem solved.

Maybe it was a coincidence and we inadvertedly changed some other thing at the same time which was the real cause.

Maybe the engine needed a little more time to 'loose itself', specially at high rpm with load.

[Jules]

by 2009 - Mikko would have some experience with the DD2 too.....cos he raced it twice at least.

he would recall I had similar issues.......and in end, I even swapped karts with him!


would be good if you could tell us how ur fax was......but for me, I think the fuel pump did the trick.