Carb adjustment

[Ma77]

there is a adjustment screw on the carb that is on the bottom of the carb and screws in vertically next to where the carb joins on to the intake manifold....

does anyone know what it adjusts.
im assuming its the idle mixture and if so which way is to go richer and which way leaner....many thanks

[SpeedSouth]

I think maybe you're confusing two different adjustment screws. There is an idle adjustment, which is probably what you're looking at, and it simply increases or decreases the idle speed. Then there is also a air/fuel mixture adjustment screw.

Here's a good read - http://www.iwt.com.au/mikunicarb.htm

And for Keihin - http://www.4strokes.com/tech/carbover.asp


I'm far from an expert, so maybe those links will help....or maybe someone else will chime in.

[forchetto]

Does the carb look like this?:



The vertical screw is the idle mixture screw and in this case you turn it anti-clockwise to richen.

In most other slide-type carbs the screw is fitted horizontally near the idle speed screw, and they work the opposite way, anti-clockwise to weaken the idle mixture.

The rule of thumb is that if the idle mixture screw is on the air filter side, it richens clockwise. This is because it works by controlling the air to the idle jet. The more air you let in by unscrewing it, the leaner the idle mixture becomes. If you screw it in you cut off air to the idle circuit making it richer.

On the other hand, if the idle screw is on the engine side, the screw acts like a petrol tap or valve. Unscrewing it lets more gas through and richens the mixture.

CV (Constant vacuum) carbs invariably have the idle screw on the engine side, sometimes underneath and sometimes on the top, and are turned out anticlockwise to richen the mixture.

[Ma77]

yes thats what it looks like.....real akward to get to the adjustment screw, some one has put a starter motor too close to it and a hot exhaust in the way :evil:

i have bogging when going to accelerate slightly after maintaining a steady speed or decelerating slightly...what you reckon?

[forchetto]

I would first try unscrewing the idle mixture screw about 1/4 turn to richen the bottom end a bit. This adjustment only affects carburation up to 1/4 throttle, so if it doesn't make any difference try raising the slide needle one groove by removing and replacing the circlip one groove further down.

For easier manipulation of this sort of screw I used an old electricians screwdriver and cut most of the handle and blade off. Then I re-ground a new tip on it. The total length depends on how much (or how little) space there is between the adjuster screw and any other engine bits.

[chriseck]

Heres what you need to use on this screw so you can adjust. This screw drove me nuts as nothing I had short of extended long nose plyers would even let you turn it alittle, and that was after removing the tank fliaring and exhaust shield. My air compressor came with two extra filler nozzels a blue and red plastic type. Simular looking to the blue Bic ball point pen cap. I removed the carb, recleaned it out then slid this narrow cap which fit very tight over the screw with some glue for extra holding strength. I cut the cap off some so it wouldnt hit the starter after checking for clearance issues. I drilled a hole thru it horizontialy and pushed a finishing nail thru the hole. I cut the nail only to leave just alittle stickin out on each end. I now am able to use a screw driver to push on the nail ends to rotate the screw for adjustment with flairing etc still in place. No more burned skin Rode the bike some and found the sweet spot for the mixture screw. No more bogs off the line or surging at crusing speeds. The key here is to find something that extends that adjustment screw down enough so you can turn it where you want. This is what worked for me. Nice pic of the carb and info on whats what. Will be making a copy of that for my ever growing china bike folder. Chris.

[forchetto]

That's a good idea. Some people are selling extended idle screws for other types of carbs at a price...$35 for this one:

[Frog]

$35 8O

Well I suppose it is gold.............

Frog

[pimptron]

my bike was set way too rich and would not fire easily for months.

the clip going down made it worst for me. it set it back to the middle notch. and leaned the idle MIX screw way leaner like 1 whole turn at least.

now runs and starts good. i think alot of people dont recognize flooding easy like me for one :roll:

[Ma77]

yes...thats what i found.

The bike started running worse after running it with a K'n'N style filter and a straight through exhaust so i thought it was running lean. after moving the needle clip down one notch, it was pretty much the same so i moved it down another...it was still not runnig right as i have described but now there was a lot of carbon deposits on the can (the pipe end and end cover are pop rivited to the main body so it is not properly sealed) and some splattering on to the swing arm.

i have moved the clip back to one below the middle notch and leaned off the idle adjustment and it is now running really well.

[pimptron]

ma77 what is your elevation? what region are you in? just curious for needle settings to compare

[Ma77]

Im in the south coast of the UK so pretty much at sea level