2008 BMW 320D

This may not apply to cars however I suppose some car knowledge can be applied. I have an Aprilia SR50R Carburettor model and since I bought I was very happy with the performance. Since I have added a Leovince zx exhaust because the old exhaust was rusty and I hoped to get some additional power. However, after I installed the exhaust, I was advised to rejet to a 65-69 jet and I chose a 69, but to my amazement the jet that was in there was a 78. I tried to run the bike with a 78 jet and new exhaust however the bike just revved and would hardly go past 10mph. I am now running the 69 jet and getting a max speed of 65 kmh and the spark plug is dark brown. What should I do!
October 2, 2012.

Does it smoke?

Yes, the back flow from the exhaust effects the needed jetting.

You may want a motorcylce shop to help you as they will change the jets as needed to get peak performance.


Yes it does a tiny bit which I think is normal for 2 strokes, but I can tell the bike is running rich because the exhaust pipe is all oily. I think I have the correct jet in but it may be a case of getting the carb set up which I will do at a mechanics. I am also getting a new belt put in because the old one is worn, is there any chance that the lack of performance could be due to the belt being worn?

Oct 3, 2012.
No, there is too much fuel from too big of jets.