which carbs to use? mikuni bs38's, bs34's or buy new?

[BiffTirkle]

im sure this answer is in here somewhere but through about an hour and a half of searching i havent found it.


for christmas i gave my brother an xs650 project bike. which was pretty much a frame engine wheels etc....

so we're trying to figure out which set of carbs to use, i have a set of mikuni bs38's and bs34's. i will give either set of these to my brother and all he has to do is rebuild them. i told him he could just get a set of 34mm round slides and make life a bit easier, but its a pricey solution.

so here i am asking the masses what their thoughts are. rebuild, and if so which set, or buy new?

 

[jim540]

If the carburetors you have are rebuildable then that would be the best way to go. I had a set of 38's that were worn beyond repair so I went with the aftermarket. Check what you have first, you may have some carburetors that only need a good cleaning.

 

[cros36]

if they are rebuild-able and run-able no reason not to use them. if not get with hugh on some lectrons, if the stars align at tax time ill be getting some.

 

[XSLeo]

Clean what you have. You may find that all the parts in your carbs are ok to use and replacing the parts unnecessary. Have you read this? www.amckayltd.com/carbguide.pdf
This guide will tell you just how to tear down, clean, reassemble and test and tune your carbs. Do both sets.
The BS34's have better throttle response, the BS38's a bit better full throttle power.
Leo

 

[BiffTirkle]

thanks for the advice, i have torn down and rebuilt many carbs (i did forward that link on to my brother though), im just trying to figure out whats best for my brother who has very little experience with troubleshooting carbs and problems that can arise from an old set, both sets that i have are complete, and will probably work just fine. they will need to be rejetted for sure we're running open exhaust with k&n filters.

and what are these lectrons you speak of. i vaguely remember reading about them on here somewhere, they piqued my interest.

 

[littlebill31]

In my experience, aftermarket carbs can be harder to tune then stock. Gotta know the basics of what the stock bike has and how to use them before using modified parts. I'm pretty good with carbs and the VM's bugged the heck out of me. Probably why Yamaha used the BS34/38's.
The lectrons are very nice, a little pricey, but very nice.

 

[BiffTirkle]

I have a set of 34mm round slides. But I'm hanging on to those for 1 of my projects. Decided that we will use stock carbs on my brother's project. Now which ones do you recommend. The bs 34s or 38s?
Both are complete and rebuildable.

 

[alfredo]

It is my opinion that the BS34's are the way to go. I have not owned a pair, but I'm on my second set of 38s and I just can't get them right. My nod for the 34s comes from the hours of reading this forum and the opinions of the many. I myself, will now start my search for a pair of 34s.

 

[XSLeo]

If everything is working right the BS38's will give you a bit more top end power.
The BS34's Might not give you quite the top end, but the idle to mid range and mid to top transitions are better. This gives you better drivability.
In the off idle to mid range is where you will spend most of your time any way.
I have had both the BS38's and BS34's on my 750 kitted 75.
The K&N filters are a good filter on a butterfly or slide type carbs. The CV carbs don't like them as well. I don't recall the exact model number but the UNI-Filters perform better. Some thing about the way air flows through the filters gives the CV carbs a bit of off idle stumble that can be a PIA to tune out. THe UNI-Filters don't flow air the same and can be easier to tune to.
On rejetting. You will have to let the bike tell you what it needs. The carb guide helps there. Most likely one up on the pilots and two or three up on the mains will work.
I have one up on the pilots and two up on the mains and it runs well. I have UNI-Filters, Mike's 1.5 inch headpipes, Emgo shorty mufflers. Straight through, pretty much glass packs. Louder than stock but not stupid loud like open headers.
Leo

 

[Gordon]

Both sets are good carbs, and the 38's are easier to tune stock. The slide needle is adjustable, where the 34's are not. The 34's do have parts available to make them allot more tunable, like adjustable slide needles, diaphram spring's ect., but you have to spend some money on them, where the 38's probably won't need much more than some jetting to run the pod's and open pipe's.
As Leo stated, Merc has the Uni filters that have the flow to open up the diaphrams faster to reduce off idle stumble. Merc is, of course, Mike Morse at 650 Central.com.
BTW, 'open' pipes only create headachs, loss of power and L.E.O. attention.

 

[BiffTirkle]

since this bike is my brothers i'll give him the 38's, ive got another set of those, i only have 1 set of the 34's.

thanks for the jetting tips too, i was going to do some searches on that next. the pipes will be simple straight through mufflers, and k&n filters,

 

[jefft]

Pricey Lectrons? About how much does a set cost?
jefft

 

[littlebill31]

Retail is about $300.00 each. That engine can only turn so fast and my 34's do just fine, lol.
I would love to have a set, just can't justify it right now.

 

[XSLeo]

Gordon, you left a space in MMM's site. Should be 650central.com
Leo

 

[BiffTirkle]

i prefer to buy from mmm when possible and convenient. i bought my set of round slides from him.

 

[jefft]

What part of these 38's wears to the point that they can no longer be rebuilt?
jefft

 

[5twins]

The throttle butterfly shafts, or more correctly, the holes in the carb body that they turn in. Too much play here will not allow the shaft seals to work any more.

 

[jefft]

Thanks. I wonder if anyone here has used the Lectrons and how easy they are to tune? Those VM's were the rage for a while, but from the hundreds of questions here and elsewhere, as well as many of my own, they did seem a bit difficult to tune for most.
jefft

 

[54inches]

IS the consensus still 34s over 38s as I have both.

The 34s look freshly rebuilt and the 38s look like they need a rebuild.

The 34s are SO much lighter.

Thanks in advance for any additional comments.

 

[nj1639]

I'm a fan of the 34's and will convert whenever.