Carburetor, what screws are these and what do they change

[Davis]

 

[Snaptrax]

First one is the idle screw. Second dont touch that adjusts the slide i believe and the last one is the air fuel mixture.

 

[Davis]

Ok thanks, so i take it that adjusting the screw in the last picture either richens or leans out the carb? Just trying to get a starting point to tune the carb are there any other screws that adjust anything on the carb? The research i have done says i should start at two turns out on the air/fuel screw and work from there is that correct? What about the idle screw where should that start from before adjusting?

 

[my67xr]

see if it starts and idles,
if it doesn't screw the idle speed screw (1st pic) in until the bike idles at a decent rpm,

 

[Davis]

Yeah it idles but when i blip the throttle it idles faster and then slows down, just took the spark plug out and its covered in black gunk on then end, does this mean its running two rich? took it for a quick ride about 50m and when i open the throttle it past about half it boggs down and dies? Is this a fuel float problem?

 

[my67xr]

so does it take a little while to come back down to normal revs once you blip it ?

 

[Davis]

yeah probably a couple of seconds maybe 5 seconds max, changed the spark plug seems to have helped a little although i have some what appears to be oil over the swingarm after going for a couple of 5min rides. Also there is some smoke coming out of the exhaust after i have run it for a while, could be cause its new though.

 

[my67xr]

you really need to tune it properly by adjusting the mixture screw first

but being slow to come back to normal rpms is caused by a too small Pilot jet,
or there is a blockage in it, or possibly in the the air bleed ports that feed the pilot jet

 

[Davis]

Ok ill have a look, couldnt see any blockages when i cleaned it out last time a couple of weeks ago. Would the pilot jet be to small as it comes from the factory it has never been changed in the carb. I believe the carb is a mikuni 30mm on the Pitpro 140cc bigfoot.

 

[my67xr]

it could be, depends on what jet is in it.
if it has a #15 in it,
it may need a #17.5, or #20, they are around $5 ea from DHZ iirc
they control how much fuel from idle to 1/4 throttle it gives,
and if they are too small it will hesitate /tumble, and can even cut out and die when you blip rev it.

 

[Davis]

Ok ill have a look and see what size jet is in it, the jets have the number on them i assume otherwise how do you tell what size you have?

 

[my67xr]

see the Pilot jet on the right, you can just read 15 on the bigger end