Xs650 carb swap...

I had these on my Xs:

Genuine Mikuni VM34 right hand carburetor, I had brass tops for em, throttle body spacers and air filters from mikes.

I could never get them tuned right with my new pipes So I switched to some stock ones I had. Not sure how it runs now....it's to late to start it up lol. Anyone have experience with either or? The vm34s kept fouling plugs and the idle was always jacked. Any advice for the stock set up or the 34s?

You can always take it to a shop with a sniffer and have them tuned.

I've done that....twice.

Maybe you had a bad set? Usually VM34s run great on these things. I've had my eye on a pair of those Lectrons Hugh seems to like though...

They ran great until I swapped pipes and had a shop rejet it...then it was no good. I know they would redo it but it's 30 mins away and a pain to get a truck to get out there...

Man I would not go back to stock.

I have BS38s (76-77), ran them on my tired motor and they got the job done. Then I tore it down, rebuilt with 707ccs, added shell cam, and now the thing simply won't idle unless I run a stupid huge PJ (137.5). I can get it to crank first kick, feels fine at WOT and midrange, but the motor will pop stall randomly at idle throughout my ride if I don't run an obese pilot jet. That's when I bit the bullet and picked up VM34s.

I would work with the VMs. They are simple, and from your post it doesn't appear that you've taken the time to learn how they work (no offense). Pull off the float bowls, unscrew the lids, and check your jetting. XS650 specs are as follows:

Needle jet: 159 series, P5 or P6
Needle: 6F9
Pilot Jet: VM22/210 series, sizes 20-25
Main Jet: 4/042 series, sizes 180-200
Stock vm slide is a 2.5, works well for XS.

If you haven't inventoried your jets, here is how they come from Mikuni bone stock:

Main jet: 4/042 series, size 260
Pilot jet: VM22/210, size 35
Needle: 159 Q-2
Needle jet: 6dH4

My point is that these carbs are a custom application and the old racers have already figured out the best jetting formula, and it's completely ass backwards from how the VM34 comes from Mikuni out the box, so if you're not sure what's in there it's all just a game of cat and mouse.

Another point to consider is that every single "XS650 jetted" carb kit out there save for 650central and CycleX is jetted too lean. To make these run right on the XS you've got to have them jetted per the specs I posted above.

Fouling plugs you say, tells me maybe you're still running the stock 35 pilot. Or perhaps your floats are outside of the specified 22-24mm height.

Also, read this shit front to back:

http://www.amckayltd.com/vm34-36.pdf

http://www.mikuni.com/pdf/vmmanual.pdf

Thank you for that...now that they are off the bike I will def crack em open and see what's in there...my guess is also stock...I've taken it to a shop twice as I no NOTHING about diving into carbs...I doubt they stock jets for these and only "tuned" what was in there...I guess it's time to put my big boy pants on and try it myself lol. Thanks again dude.

No problemo man.

For brass parts, I usually buy from nichecycle.com since they have all genuine mikuni brass and are very reasonable with shipping for these small items . I believe they are out of the 6F9 needle and 25 pilot, but you can find those on ebay.

Make sure to buy genuine mikuni parts if you buy elsewhere.

couple of other tips for tuning carbs:

Make sure your battery is full charged
Make sure the charging system is good
Make sure your valves are set
Make sure your cam chain is adjusted

I know it sounds like that has nothing to do with carbs and it doesn't, but it really does. I have vm34's on my cb750. V stacks too. I've had to clean them a couple of times and tried rejecting them , tinkering here and there. Pretty happy now. Pulled the needle up one to richen the flow.

I also had fouled some plugs. Pretty sure I had the idle set to low and was probably just dogging out my plugs at idle. Seems to run better with a healthy idle. Just doesn't sound as cool.

Thanks!...a buddy pulled my stock carbs apart and cleaned everything...put those back on and it seems to run great now. I may fuck with these 34s for the build I'm doing now or just off em lol

Could you show a photo of the stock ones you are going to be running? if you are going to be running the stock ones instead of the ones you showed a picture of, I have the stock ones on my 1978 motor, and they are pretty nice, as I am a honda person, and i really got used to the kehins that the cb 750s have, the round tops that is, and the one you are showing a picture of is like the round top kehins, and to me, they are much easier to tune to the bike rather than the mikunis, but, my mikunis, are really tuned, and i cannot complain, they really are dialed right, and i am very happy with them, I just wonder why it is difficult to get it tuned to the motor you rae running, Since i am finally feeling better, i took a ride today, and man!! what great feeling, of course it was on my xs650 with the mikunis!!! My favorite chop of all!!! tomorrow, i will be riding my favorite honda, the shortest chop of all of my chops, along with my longest chop, because i feel much better, and riding really makes me feel even better!!! I am tearing it up!!! and loving it!!!! I truly thank God for this ability!!!! Sorry, if I am typing weird stuff, it just feels great to be out again!!!