With the colder temps, our 90cc mini is hesitating and loosing power.

The carb has an idle screw and an air screw. I was wondering if turning in the air screw will help performance (my experimenting seems to say no) or is changing the needle jet needed.

Any thought of similar experiences welcome.

 

First of all let me say that I have no experience with the 90 cc models, but I would think the same fundemental principles would apply to them as well as any other 2-stroke. Colder air is more dense than warm air so there is more " air " in a given volume of cold air than there is in the same volume of warm air. Because of this you have to increase the amount of fuel the engine is getting to rebalance things with the increased amount of air.
Turning in the air screw will enrichen the idle circuit somewhat so you are on the right track there. In order to also enrichen mid-throttle you should move the needle-clip down one position on the needle, this will move the tapered needle up slightly and enrichen this circuit. To enrichen full throttle you should increase the main jet by 1 or 2 sizes to rebalance things.

hope this helps some

 

Thanks for the help.

I may try adjusting the Needle jet.

Can all needle jets be adjusted?

 

I am not familiar with the carburetors on the 90 cc 2-strokes, but it should be relatively easy to check to see if they have adjustable needles. On the larger 2-stroke motors the needle is located on the bottom of the throttle slide. You don't need to remove the carburetor from the quad access the slide. To access the slide you must unscrew the threaded cap on top of the carburetor with the throttle cable coming out of it, and then just pull the slide up and out. The slide is actually just a small cylinder that moves up and down within the carburetor bore to increase or decrease the carburetor opening. Note the orientation of the slide when you pull it out as it has grooves on it which correspond with locator pins on carburetor body. In other words there is only one right way for the slide to fit in the carburetor body. You should see the tapered needle hanging out the bottom of the slide. There should be a small metal plate on the top side of the slide where the throttle cable attachs to the slide. Remove this small plate and the throttle cable. The needle can then be removed from the top of the slide. You should see a small retaining clip on one end of the needle. Use a pair of needle nose pliers and remove the clip while noting its position. If yours is adjustable it should have several notches on the needle where the clip can be placed ( my carburetor has 5 positions ). If you want to enrichen your midthrottle mixture you should lower the clip 1 position ( this is what you would need with colder weather ) and then when it warms up again in spring you can raise it back up again. What moving the clip does is raise or lower the tapered needle in the needle jet, thereby changing the amount of fuel which can flow between the needle and the needle jet. The needle jet is just a fixed sized orifice that the needle resides in. If you think about it it for a while it will make sense.( lowering clip will raise the needle, raising the needle will enrichen mixture because the needle is tapered and smaller on the bottom than it is on the top )
Then just put the needle back in the slide. Put throttle cable end back into slide and then put metal plate back in slide. Put spring in slide and drop slide back down into carburetor body and screw on cap. Your done.
To replace your main jet you must loosen the carburetor from the airbox and rubber airboots. You don't need to do anything with the throttle cable or choke cable, just leave them on. There should be enough slack in the cables to pull the carb to one side or the other where you can work on it. The main jet resides behind the float bowl drain plug. If your carburetor has no drain plug you might have to remove the bowl screws and bowl. You will see the main jet ( small brass part with tiny hole in center of it ). Remove jet with socket (6mm or 1/4" I believe) and replace with larger jet. Jet should have a small # stamped in it. Larger # means larger jet. Reassemble carburetor and reconnect airboots and airbox. You're done.
Remember when it gets hot again you might have to change your jetting back to what you had initially.
Always check your airfilter too to make sure it is not getting plugged as this can have a effect on jetting and engine performance also.

I hope this information helps some as it is how my 1993 350L is setup, hopefully yours is similiar enough for my instructions to be of use to you.

Doug

 

Instead of all the jetting changes, why not cover a small portial of the intake opening with a piece of duct tape? You will effectively be richening your air/fuel mixture up. When the weather warms up, take the tape off and viola! Correct jetting.