Xs400s 1980 carbs help

Caden D Quillen

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So first things first. I'm new to carburetors. I pulled the float bowls they're clean, but when I throttle to start the bike the floats, I think, they don't open with the butterfly valve. I can push them into the float bowl and they drop down at about the same speed more or less. Do they need fuel pressure to raise and lower? I need a Mikuni carburetor crash course lol
 

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So first things first. I'm new to carburetors. I pulled the float bowls they're clean, but when I throttle to start the bike the floats, I think, they don't open with the butterfly valve. I can push them into the float bowl and they drop down at about the same speed more or less. Do they need fuel pressure to raise and lower? I need a Mikuni carburetor crash course lol
This video might give you some hints.

Note that your carbs don't have the accelerator pump so disregard that minor part. I'm sure with a little more digging one could find a better video.
 
https://thexscafedotcom.wordpress.com/2011/01/10/xs650-carbs-mikuni/ This is for the mikuni bs34 carb. There is a lot of this on google. Your look to be for a 82 sohc xs400. All 80-82 sohc xs400 carbs are the same other than the 81-82 used foam floats where the 80 used brass ones. The float heights are set different. Brass are 26mm and the foam are 32mm.

For a xs650 but very close to the same.
 
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I may have miss understood the OP's question. Now I'm thinking he is referring to the throttle slide and not the floats. As he is expecting them to move when the butterfly is actuated. He also says he can push them up and see them move.
 
hi
i think you are describing two different things
the float and float bowl are situated at the bottom of the carb - the float moves up and down depending on the fuel level in the float bowl.
the throttle slide is upstream of the throttle butterfly and can be moved manually up and down but not into the float bowl which is a separate part of the carb. this slide relies on differential pressure between the engine and the air box/atmosphere being exerted on the diaphragm (situated on top of the carb ) which causes the slide to move. the butterfly valves vary this pressure so being simplistic changing the throttle position alters the butterfly position which in turn raises and lowers the throttle slide - it does not depend on fuel pressure.hope this helps
 
just to add - if the slide is not moving when the engine is being revved , assuming the slides are normally free to move then you have a diaphragm problem - either a hole or not seating correctly
hi you guys hope yous got the issue resolved. my problems are just beginning. lol.how much should the float needle be open before fuel enters the float chamber.or does it not really matter as long as the needle shuts off the flow of fuel when the floats rise to there set level?thanks
 
how much should the float needle be open before fuel enters the float chamber.or does it not really matter as long as the needle shuts off the flow of fuel when the floats rise to there set level?

The latter - you adjust the floats so that the valve closes at a set level in the bowl.
 
It works like a toilet tank. Once the liquid is at the proper height, it shuts off.

The proper fuel level is about 2mm below the gasket surface.
 
hi.thanks for your reply.didnt realise the fuel level would be as high. I took the float bowls off carefully with the fuel still in .they were only half full of fuel ?I must have set the level to low.bike is refusing to start .going to try rejetting it with larger jets and dropping the throttle needle one position. this is because I'm using those bloody pod filters .hopefully get there .
 
It works like a toilet tank. Once the liquid is at the proper height, it shuts off.

The proper fuel level is about 2mm below the gasket surface.

you can’t tell if fork tubes are bent prior to rebuilding them LOL. You can’t see 2mm from bird shit. Leave this poor man alone.
hi.thanks for your reply.didnt realise the fuel level would be as high. I took the float bowls off carefully with the fuel still in .they were only half full of fuel ?I must have set the level to low.bike is refusing to start .going to try rejetting it with larger jets and dropping the throttle needle one position. this is because I'm using those bloody pod filters .hopefully get there .

for your pods- silicone the outside at cap, fill each little seam up. I silicone the inlet and outlet at the seams to the cap/outlet rubber, if you wish to restrict even more air- safety wire a shop vac internal filter sock over your pods. They’re like $4 each. You probably have 37.5 or 40 pilot jets but you need 42.5 or 45.

with a warm engine with choke off- when you rev up in RPM and chop it off- it should not “hang on” - it should accept the throttle being off and not hesitate to idle.

if you still have issues- your buttlerfly assembly should show the light of day with slide fully seated.

also- your engine breather- go big with the tubing. I run a 351w PCV valve to a can because racecar but you’re fine going to a breather / atmosphere.

also check your voltage to the coils and make sure you don’t have voltage loss from the key to the coil.

Take a infrared covid style thermometer and shoot at the flat surface at each spark plug. You want 208-212°F at idle/warm engine (even under load). Hotter = lean. Colder = rich.

As far as the floats- unless you’re flooding out OR you are looking for a little more because WOT and racecar- check your float levels. The float level (if somewhat in spec- can be off and run fine) will only matter during extended periods of 1/2 or more throttle and you are feeding through lets say 3.74mm total which is 1.87mm total of ID of the holes fuel can travel through TO the cylinder head FROM the float bowl. Moves through (1) 1.4mm home and (1) .4mm hole for each carburetor. Adjust the tang / floats as needed and go from there. Yes- the jet sizes have the mm silent so you can buy brass plugs of the same thread pitch and buy micro drill bits and make your own or I can sell you some.
 
you can’t tell if fork tubes are bent prior to rebuilding them LOL. You can’t see 2mm from bird shit. Leave this poor man alone.


for your pods- silicone the outside at cap, fill each little seam up. I silicone the inlet and outlet at the seams to the cap/outlet rubber, if you wish to restrict even more air- safety wire a shop vac internal filter sock over your pods. They’re like $4 each. You probably have 37.5 or 40 pilot jets but you need 42.5 or 45.

with a warm engine with choke off- when you rev up in RPM and chop it off- it should not “hang on” - it should accept the throttle being off and not hesitate to idle.

if you still have issues- your buttlerfly assembly should show the light of day with slide fully seated.

also- your engine breather- go big with the tubing. I run a 351w PCV valve to a can because racecar but you’re fine going to a breather / atmosphere.

also check your voltage to the coils and make sure you don’t have voltage loss from the key to the coil.

Take a infrared covid style thermometer and shoot at the flat surface at each spark plug. You want 208-212°F at idle/warm engine (even under load). Hotter = lean. Colder = rich.

As far as the floats- unless you’re flooding out OR you are looking for a little more because WOT and racecar- check your float levels. The float level (if somewhat in spec- can be off and run fine) will only matter during extended periods of 1/2 or more throttle and you are feeding through lets say 3.74mm total which is 1.87mm total of ID of the holes fuel can travel through TO the cylinder head FROM the float bowl. Moves through (1) 1.4mm home and (1) .4mm hole for each carburetor. Adjust the tang / floats as needed and go from there. Yes- the jet sizes have the mm silent so you can buy brass plugs of the same thread pitch and buy micro drill bits and make your own or I can sell you some.
thanks for your reply .its getting too technical now lol.some of your advice I can do but some I can't. I'll try setting it up as you have pointed out .just waiting on new jets and pilots arriving then I'll give it a bash .tbh I'm ready to take a match to it .
 
you can’t tell if fork tubes are bent prior to rebuilding them LOL. You can’t see 2mm from bird shit. Leave this poor man alone.
Funny you should say that
I can sell you some.
You can't see the dude is in Scotland? Your trying to make a buck is gonna break him on shipping. They sell jets over there, calm down.
 
I took the float bowls off carefully with the fuel still in .they were only half full of fuel ?

That isn't how you check the fuel.

You do it with a clear tube on the drain, opening the drain screw, and checking where the level in the tube is.

Like this:

ADCreHdLSCCYt9qys8VwKMKoKUHYXU1Zp_GjMgKLEGuX_YMdfqYBMZrzodJbkgQF0UVBYLatodzrHVDyy8GxWnoX3kca0wSc1LGJGiPIc20AcmGvqdK6jbpv0l8aTJDALMWeGFkJpvHspKeek3UIgRddiFoO39ZwLqbllgZizfqpJisRqKujRBlwzCs1IitTffnVPrqZq2-sIkwipI8MFNfLXDW19AD28a1i-xV_HcBZ8hZVMxF4YO90_Qug9HSuJzELmDR18RkKuC_HbNgZsgEXxzqUTXyifw-kNIZEexPcurzodar_YiyufzAafhYy2hd3piMMMYIw8xW3GDG7uCFwXFkisvE7Y_A8_xKKXrFPLT848Iy2YTmwHHGfKlTngwRGNwZqZnXkRFbsYv-ilbl4gHJQeQ3Y1ZEqDWGX5LBrbn_UsY-Vb81kqmdoho_E8OoDWCHE3zeGcvy2h2Z0B_1q0qvyHBr1e-k7716LyoRPl99EjtYbMgFi_2luCYyk2J-Pf8j_dv1z02eJ27-6yEtWKxgVll3_Kul39BhpS5cdmyuaBUg3w8Zc9Vgnt4SNIm1BacIXxGqFGYKCZxocziFkhmZ9yzmug4Uy3qrh1fMsgcRSjdxKDx0_swf3Rm2jrbPvd9Vc3fUxDHf8MEPQkB53nUUI8sysWAAKvRhETjepG21zKhp-xrKw-VEwoo8jY15BD0kVVj-hk18skEUStOXJJmwQNTmISr948LCyD11PbacVUVsMX-5YxqUvfPH_x4oQf98kJpm1T7xsrA5nqSb7TbEbDWDopU7n_r8P0gfa794iIR-ryadMA5OZhnLZQh_xT7KLvkTuorQ_sy7hvbClEu7sW4AAbXZo6JDcVVTtJlXVO5-pOkHN-rHWVE3A5DJNf7vz5094x4ygSgUuwcGty-I=w701-h931-s-no
 
Then levels are all right.
thanks .I had already taken them bk off and started messing with them again.trying to fix something that's not broken.lol.when I get them bk on bike I'll do this test again. I'm just hoping to get it started. wish id left it running the way it was .waiting on bigger jets and took your advice and ordered foam filters to.
 
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