Idle and main jet

Hi there

I have just installed a DR 75cc kit to my 50n, along with a new exhaust. Also a new 16.16 carb and polini manifold. 

I started it up and to be expected it revved high and took a while to settle back to idle. (quite a scary experience for the first time). I am assuming that this is to do with the jet, or at least I am hoping. It ran ok before I installed the new kit. 

I am going to try out putting in a larger main jet, something between 66 and 70, (read this on a post here somewhere), also changing the spark plug.  

Do I need to also change the idle jet to a large one. I read on that the jetting for a 75cc kit on a 50n (1965) should be main jet: 76 and idle jet: 42?

Any help or advice would be great


It doesn't matter of you don't see any air leak... The best way to find out if the manifold totally enter the carb or not, is trying with manifold and carb in your hands, without the bushing intake manifold. Try to connect manifold and carb and you immediately realize (also by looking internally throug the carb) if the manifold does not enter fully because it gets stuck before the end. In that case you have to rotate the carb forcing it against the manifold till the latter fully enters; repeat the operation until it is no longer necessary to be "Hulk" to make the manifold fully enter the carb, so that when you install the manifold on your vespa, you are also able to insert the carb correctly without excessive force.

HI there, thanks for the tips...

I have checked the springed skrew, its 1.5 turns out. But I did have a look at teh carb to the maniflod. The carb is not totally on the manifold but I cant detect any air leaks from it. Its really wedged on there and I cant move it. 


Would it be an idea to heat it up a little to try to push it on all the way?



Oh may also be that you did not install properly the cylinder kit or, more probably manifold-carb, so that the engine inhale air (it happens quite often with new carbs because it is hard to make the manifold fully enter the carb...).

Hi, did you first try to regulate the lateral screw? it should be on the left of the carb, with a spring, in some carbs it is butterfly-shaped. That screw allows to balance the ratio fuel/air as if changing idle jet, and the problem you describe is tipical of pour carburation (too low idle jet or lateral screw too screwed). Because you should have already the right idle jet, probably you have that screw not sufficiently unscrewed. Usually it shoud be about 1,5-2 turns (1 turn=360°) unscrewed from the position totally screwed. Be careful never force screwing.