XS360 Carbs in a XS400 model - jetting?

[zeen]

Greetings from Poland!

I have a XS400 Special (4G5) on my workshop with an intention to make her a nice cafe racer. Instead of rebuilding her i have made a few modifications so far (cut the frame, installed pods, cut the muffler etc.).
Now that she is almost completed i've tried to tune her and found out that my carbs comes from XS360 model!
Seller claimed that this one is an "open" with 38hp but i'm not sure if SOHC can be "open".

My jetting specs:
Pilot jet - 17.5
Main - 135
Needle position - 3
Jet needle - 4FP21
Needle jet - X-6

I'm sure i need some jetting with custom pods and muffler but maybe i need to upgrade all of jets and needle at first to XS400 standard then go to "overclock her". Is it possible? With stock carb from XS360 i think i never unleashed whole power of this larger engine.

I really appreciate your opinions and expertise in this matter.

thx

 

[Pork Chop]

G'day Zeen,

How does it run now?

What size is the airfilter side of the carb? You could have bs32 carbs.

Post up some pics so we can see you fine work.

PC

 

[zeen]

Hey PC,

arghh... there are lots of problems needs to be fixed.

I believe i set a proper timing and valve clearance.
I synchronized carbs a little visually, tomorrow i'll do it with home made vacuometer.

While on a test ride i noticed problems on high revs, engine is punctuating.
Also there is a popping noise coming from one of cylinders and strange sound like choking with water... i really don't know what it might be.
Firing from mufflers on reving down also occurs.
And few other problems that needs to be fixed (slowly decelerating or stalled revs)

I have an exact carb from the first picture in below manual, i believe it's BS34.

http://xs400.net/wb/media/faq_anhaenge/Unterschiede_Vergaser_V1_1.pdf

I'll post pictures after tank painting

 

[xschris]

I use the xs360 carbs on my 80 xs400 sg and they work great. I run it mostly stock. 17.5 pilots with the mix screws 1 1/4 turns out the needles set on the third clip and 140 mains. I think mains are a little to big. 137.5 or the 135 would work better I just need to swap them out at some point. The bike starts on the first kick and runs smooth up to red line with no popping or surging. The bike has a set of vintage K&N RC 130 air filters (these are oiled with the proper filter oil which make a big difference in flow compared to the knock off filters you see today) with the stock H-pipe and a vintage winning performance 2-1 exhaust with a free flow muffler. Thats what I use with my set up. You might have to do something else with what you have. Just make sure the carb is synced with a manometer or vacuum gauges, valves and timing is correct and the carb is super clean and in good repair

 

[zeen]

Don't you think that stock xs360 carb limits performance of xs400 somehow?

But first, i'll make all the tuning you mentioned and post the results.

 

[xschris]

No. The xs360 carb is a non-restricted version of the bs34 mikuni carbs. Remember there is only 33cc difference (358-391) between the xs360 and xs400. The xs360 used a lighter crank. The xs360 had a higher compression engine (9.6:1 for the xs360 and 9.3:1 for the xs400) A stock 77 xs360 made the same or more power than a stock 77 xs400. The xs360 carbs are a much richer running carb than any stock xs400 set made. I know because I have run them all on the same bike. I have run them on my 80 for about 500 miles now and love them so much I sold all of my xs400 sets and bought more xs360 sets as spares

 

[zeen]

Yeah, you've made my day with this news!

So according to your setup i should be getting sightly bigger main (137.5 or 140)?

This one (from xs650) should fit my BS34? - > http://www.mikesxs.net/product/48-0120.html

 

[xschris]

No. The jets you want are these http://www.mikesxs.net/product/48-0216.html The 76-77 bs38's are almost the same as these carbs for most parts (jets, gaskets, bowls, floats and o-rings) avoid the mikesxs ones because they are china made junk. Use mikuni made ones they will cost more but will make the bike run better. I have used both and you can tell.