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my xs400 is REAL boggy at low throttle maybe less than 1/4 from zero. sometimes i don't think its gonna make it. as long as my rpms are up its real punchy, hell i think the old hoss is getting close to be able to pop a wheelie as long as the rpms are ramped up. but from a dead start up its just awful.

pods, jet up on main jet only, 2-1 exhaust are the only engine modifications so far.
is this normal? is it lean on the idle jet maybe?
 

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Coreyjdl;

I found a really nice article about mc carbs at www.hondanighthawks.net.carb17.htm. Another article can be found at www.salocal.com/sohc/tech/carb/asmpg_mgs/jetgraph.htm Both articles are slanted to 4-cylinder Mc's but it should provide information about carbs in general.

According to one article (the 2nd one) zero to 1/4 throttle is main controlled by the air screw and slow jet (idle jet). In some carbs, Turning the air screw out increases the amount of air thru the main jet for other carbs the reverse is true. Other areas to check is the float level setting for your carbs. Also doing a plug "read" will also give you clues on what your engine is doing. If it's real dark - you're problem is too rich, too much of an ash color, you're too lean. Desired color is a nice tan to light brown color.

Hope this helps.
 

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is it a dual carb? are they synced together? maybe not and only one cyl is opening when you give it 1/4 throttle and less.
 

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With all the previous in consideration, what type of carbs are on this bike???

If it's a CV type, check for a vacuum leak or clogged low speed jet. You could try shooting the low speed jet with carb cleaner. If it's a regular carb, you may want to try to move your main jet needle cir clip setting up one level. It could be getting too much gas in the low R's flooding out the motor a little.
 

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Those early 80s Yamahas were jetted very lean on the pilot jet to start with. I would replace the pilot jet with one size larger anyway. It will probably end your bog and make it start easier, too. The mixture screws are possibly hidden under some little aluminum plugs that will need to be drilled and pulled out to get to them. Check the manual for your particular year if it doesn't look obvious. If the mixture screw is on the engine side of the carb it usually meters fuel. If it is on the intake side it usually meters air. I suspect you are too lean from too small a jet and a clogged fuel passage.

Ken
 
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