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I am working on a 80 model yamaha maxim 650 that i got not running. The carbs were leaking causing it to instantly foul the plugs. Compression was ok so i rebuilt the carbs. it runs now but runs rich. I closed the idle circuit off and only let it idle with clean plugs to see if it was the idle or just the main circuits. sure enough idle only it soots the plugs and runs rough. I blipped the throttle and sure enough black smoke. I checked manifold vaccume and the guage bounces hard like a valve problem but i know that running bad from being rich could cause those readings so i just put that info in my back pocket. float level is set via the clear tube method and i atually had the floats probably 5 milimeters low. so im pulling carbs back off and want some advice on what i might be missing. Right now iam thinking about putting the stock jets back in even though the aftermarket ones are at least marked the same size as stock calls for. My curiosity is can too low of a float level somehow cause a high bowl presure or some other wierd annomily or should i be looking elsewhere while i have them back off. Floats seem to be working properly and i cant see any extra fuel leaking while looking through the throats while its running so i am just stumped. the choke seems to be working properly also.
 

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Aftermarket jets do not follow the stock jet sizes. What brand of jet are you running.

IE
Kehin sizes are by the size of the hole
mikuni run by CC's of fuel per min.

Dyno jet are all 10-20 sizes bigger per same size as kehins
Factory pros are different as well

I forgot the exact numbers but look for a chart on the net to compare them.

Does that have the funky Hitachi carbs? some mid 80's Yamaha has them.
 

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dude i have a 1985 yamaha maxim xj700, WORSE mistake I ever made with that bike was trying to change the jetting on it. I put on a mac 4 to 1 exhaust and pod filters and tried everything with the jetting. I ended up putting the stock jetting, exhaust and air box back in, and got it running fairly good!
There is a forum called xjbikes.com to search on try there!
Mine has hitachi carbs on it ?!?! couldn't find a carb kit for it cause they only made the bike model for 2 years!
Good luck, I hate working on that bike!!!
 

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Discussion Starter #4
Yep its got Hitachi's. The jet kit is actually for the bike. I forget what the brand was. Im gonna go back to factory jets. Thanks
 

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I am working on a 80 model yamaha maxim 650 that i got not running. The carbs were leaking causing it to instantly foul the plugs. Compression was ok so i rebuilt the carbs. it runs now but runs rich. I closed the idle circuit off and only let it idle with clean plugs to see if it was the idle or just the main circuits. sure enough idle only it soots the plugs and runs rough. I blipped the throttle and sure enough black smoke. I checked manifold vaccume and the guage bounces hard like a valve problem but i know that running bad from being rich could cause those readings so i just put that info in my back pocket. float level is set via the clear tube method and i atually had the floats probably 5 milimeters low. so im pulling carbs back off and want some advice on what i might be missing. Right now iam thinking about putting the stock jets back in even though the aftermarket ones are at least marked the same size as stock calls for. My curiosity is can too low of a float level somehow cause a high bowl presure or some other wierd annomily or should i be looking elsewhere while i have them back off. Floats seem to be working properly and i cant see any extra fuel leaking while looking through the throats while its running so i am just stumped. the choke seems to be working properly also.
ohhh the wonderful life of carburetors.....

in my opinion I would get your float level perfect first... having the float sit 5mm to low will allow much more fuel to enter the float bowl thus causing an excess of fuel in the bowl which could cause this problem...


did you adjust the needle height or the air/fuel screw??? i dont know if you have a fuel screw or a air screw but you can control the idle mixture with those I'm sure you know but are you adjusting the right way???

if the screw is located on the side of the carburetor closer to the air filter thats a air screw, closing off this circuit will make it run richer since your blocking off the air and making it suck just fuel = rich

if the screw is located on the side of the carburetor closer to the engine that's the fuel screw, closing off this circuit will make it run leaner since your blocking off the fuel supply it will run = leaner

if you already knew those things I didnt mean to try to lecture you on carburetor set up's at all just helping....



one other thing.... when its idling if you pull the choke what happens??? also what kind of choke is it, do you have plates that close on the carburetor or does the cable run into the side of the carburetor "fuel enrichment circuit"

p.s. where are you located?
 

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had 80 maxium that ran 11.40, 123. 820 750 motor, decked head,421 mega cams, ported head. most fun i ever had on bike, ate gs1100 and gpzs for lunch, kill them in a roll on, could not kill that thing. one thing,ended up running bone stk carbs, and they were perfect. a little adjusting on air screw,thats all.anything in jetting or needle changes hurt. very sensitive cv carb. ckd comp., pull cover,and ck valve clearance , do that first. remove spark plug boots and cut bk wire till you see clean wire and ground to ck spark.carbs are touchy, slides are vacum and must move in sync, pull air box rubbers and sight as you give throttle. pull slides out, make sure no tear or pinch in diaphram rubbers, if hard soak with wd40 to make pliable again. there is a shim thatfits around needle, easy to put on bottom raising needle making rich. pilots must be removed,and cleaned. hitachi jets only, think i still have some stk size. get valves right, spark strong,seal intake rubbers, strip carbs totally, clean slide, slide body, ck needle position, shim position,reassemble. push slide up,should feel same resistance on each, then synch. old cv carbs are difficult, but will work. call me 4043047464 if i can help with jets.
 

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Tuning carbs are a real PITA for sure, but an excellent learning experience!

Keep at it. Just start rich and work your way lean. I tune carbs this way to be on the safe side.
 
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