Mikuni carb on MSO bike

[Mack]

There are a multitude of jet sizes for a carby. Basically the tiny hole in the little brass thingy is the actual 'jet'.

The needle on the main jet meters the rate of fuel flow from the jet as the throttle is opened but at the end of the day this is only a fine adjustment, the size of the little hole has to be correct to begin with.

The pilot jet controls from idle up to 1/4 and beyond to some degree. The jet itself is delivering a set amount of fuel dependant on the little hole size. The screw your mucking with has a point on the end and adjusts airflow into the pilot system, and is only able to be screwed in (blocks air supply therefore mix becomes rich) or out (lets more air in so becomes leaner). Now even if the mixture is correct, if that jet is too big (which is often the case) no amount of turning the air screw is going to lean it out enough and same for if its too small.

So when people talk about jetting a carb, they mean getting those jet sizes right before anything else.

Now the problem is of course, which way do I go? Up or down in jet size. That is where you must perform a few tricks to give you a clue. A simple one to try that may give a clue is to make sure the bike is warmed up properly and then engage the choke only slightly. This will richen up the mix. If the bog is worse then it could be an indicator that you should try going down on the pilot jet size. If it improves a little then your pilot could be running lean and wants to be bigger. Once you get close to the right size pilot the more refined techniques of using the air screw to determin one up or one down can be used.

EDIT- Don't worry this is no easy task sometimes. It can take a lot of mucking around to get it right. There are other factors which can affect all this like running too large a carb to begin with. Or valves not seating correctly all sorts of things. That is why the tuning manuals state to make sure the engine is in good mech. condition before you begin. Sometimes you can change a jet and its perfect. Other times the best tuners could pull their hair out trying to work it out. The problem with the china bikes is importers just chuck a bigger carb on a bike to match what the others are running just to make a sale. I'm pretty sure they go on all types of motors with the same standard jets. Its not until someone gets a certain size carby jetted correctly for a particular engine that they can then let the rest of us know where a good place to start from will be as regards jet sizes.

 

[deekay]

mack you know you stuff, good article. One of the repliers in this thread refers to lever up for choke and down for no choke, this is different from any other mikuni I have dealt with in 40 years of riding, are they correct or as I suspect incorrect, den.

 

[Sidney]

So is the idle screw the screw that ajusts how high or low it revs when... it's idleing obviously?

And the mixture screw ajusts how much oxygen is mixed with fuel? So when this is set perfect it should perform much better right?

 

[Mack]

gotta love it when someone digs up old threads...saves time going over same topic for new members...
Anyway, yes the idle screw you will generally see at the base of the throttle slide, it has a pointed end too and all it does is raise the slide inside the carby, therefore throttling it up abit.

And yes, the mixture screw adjusts the air to the pilot system. But like I said above, fiddling with it will not have the desired effect unless the jet itself is the right size first.

Another point to add is that you needn't bother jetting your carb until you finish building your motor to where you want it. The reason being that as you change the way your engine breathes your jetting requirments change also.