Xs400 with mikuni bs38s?

[Matt Sour]

Newbie here. Still working on my first XS400. Its winter in chicago so prefect time to build something. I've only worked on 2 stroke Mopeds so far. So motorcycles are a new ball game.

So my question. My mikuni carbs are trashed and I need new ones. I want to just find replacement mikuni bs 34s (77-79) but looks like its hard to find used ones for cheap. So I was wondering if I could fit on mikuni bs38s which seem like parts are easier to come by.

Also it seems like I could gain more top end from the 38s? I hear the bottom to middle throttle response is wacky but I came across a kit to resize the slide to fit a different needle to fix that response problem.

Are the 38s to big for the xs400? Am I creating a mess for myself?

Thanks, Party on!

 

[bcware]

I have never heard of anyone doing that swap. I am going to bet they simply do not fit.

One member has successfully retrofitted DOHC carbs on an SOHC engine, but it did require modification.

What exactly is wrong with your carbs?

 

[xschris]

The bs38 wont fit or work for a 400. The 76-77 bs34 used on the xs360 uses a lot of the same parts as the bs38 but are hard to find. The best bet is a set of 80-82 bs34's there a lot of parts for them and I find them to make more power than the 80-82 after basic mods Prices for carbs are not cheap and the the cheap ones are often junk. What is wrong with the ones you have? Post some pics so we can see if they can be fixed.

 

[Matt Sour]

Cool. Guess that wont work. Well the brass mixture screws were both over tighten the tips broke off in the carb bodies. The only way to remove them after trying different methods was to drill them out which now makes them a little hard to tune. I was hoping to replace the bodys and screws but havent been able to find replaces. They are mikuni bs34 (77-79).

xschris your 34's are looking pretty good right now. Also do you mean the 80-82s make more power than the 77-79s?

What kinda mods can be done to the 34 80-82s?

 

[xschris]

The needle jets from the 77-79 carbs are adjustable where the 80-82 are not (you can put the 77-79's in the 80-82). Also they have a wider slide than the 77-79. From the factory the 80-82 had caps over the mix screws. When you remove them they are fully adjustable. I have run both on my 80 and the 80-82 seamed to work much better. I am now have a set of 76-77 360 carbs on but cant test until spring Have you tried ebay for carbs? Sometime you can find a set for cheap if all you need is the bodies.

 

[xschris]

One of the members on here has a complete set of 80-82 http://www.xs400.com/forum/showthread.php?t=9574

 

[Charles]

Just slap a set of these on there

And BTW, anything can work, if you have the ability to fab up brackets and have the time to tune.

 

[Matt Sour]

What are those babies? Also what kinda options do I have for aftermarket carbs? Total Noob here. New stock vs Old stock?

 

[phate]

What about these http://www.mikesxs.net/product/48-8102.html and I believe you are referring to New Old Stock (NOS) which is usually eBay lingo for parts that were made when the bike came out but were never used (generally not even open) that are now for sale. This is good stuff but will cost an arm and a leg for bikes like ours (~40 years old)

 

[Matt Sour]

your right. What I meant to ask is whats better, buying some new carbs that'll fit or trying to find some old carbs. What is the performance difference? seems like I should just track down some used bs34's.

 

[Charles]

It depends. If you can find a nice set of used stock carbs. A set that only needs to be rebuilt/cleaned, no new parts, get them.
Here are 3 for $85.
http://www.ebay.com/itm/1979-Yamaha...Parts_Accessories&hash=item4d09511480&vxp=mtr

Once you get her running and understand carbs and fueling and what you want from the bike, reassess your carb choice. You can always sell the BS34s to someone here and probably make a profit.