Replacement carburetors for XS650?

[jim540]

I have a 1978 XS650 and the carbs have delt me a fit ever since I puchased the bike this summer. I have taken them apart, adjusted and cleaned but they still won't idle correctly. They had several vacuum leaks in them and I have slowly tracked them down and repaired them. The latest were the throttle shafts. I tore the carbs down and replaced the seals but it appears that the housings are worn and still allowing air to get by the seals. Very frustrating indeed. My question is, has anyone found a suitable replacement for the stock carbs?

 

[tonyc]

You will be better off finding a good set of 38's original. Saying that I have VM34's on my bike with an aftermarket exhaust system. I bought them from M.M.M. at 650Central. The carbs were jetted right on even the floats were pre-set. Only a few adjustments were needed. Good luck TonyC.

 

[CDNTX650]

No one seems to like bs-34's so there is tons of them out there. i am running a freshly rebuilt set right from bodies up on my resto modded 74, only mod to them is up one size on the main jet now. (pods and repro stock exhaust) and the bike pulls pretty damn nice with no popping etc.

My 75 brit style strutted bobber which i will start next winter, i will be running a single bs34 with a xsjohns needle in it and appropriate jetting (just finished the carb rebuild today)

Bs34's are cheap and there is a ton of them out there for far cheaper than the 38's.

i do have 2 sets of 38's to rebuild now too from the newest xs's added to the fleet.

 

[XSLeo]

Personally I don't like the 78-79 carb set. I didn't have much luck with them.
I have a 76-77 set that work pretty well, bad diaphrams i think are the off idle problems with that set.
I currently have a set of BS34's from an 82 that need a bit more tuning, they don't pull quite as strong, but better throttle responce. Might pull better with a bigger main, just up one size now.
As others have said and I agree, try another set of BS38's or a set of BS34's.
Every now and then a carb set comes along that just won't work right.
MMM's carb sets can run a bit more than some others but they are very close to start and he will help you get them right.
Leo

 

[Patches]

BS34's are actually my carb of choice if I were to choose from the XS factory carbs. They are easy to tune, parts are readily available and personally I think they have beter throttle response then the 38's. Just my 2 cents. I think there are many others around here that will agree that the BS34 will become the preferred stock carb of choice for the XS. Not aftermarket...... that's a different beast...

 

[ReycleBill]

I'm trying to figure out port fuel injection. I've got a throttle body and 2 injectors from a 1500cc Toyota 4 cylinder I'm think I might try to use. I think I can use a Microsquirt for the electronics. I'm a very long way from having it done, mostly still in the parts collection phase.

 

[Robhall]

From what I understand Lectrons are very good, they will probably be my next set if I can find a good deal.

 

[jim540]

As far as throttle response these 38's work great. I have cleaned and adjusted them. Replaced the diaphragms and raised the needles one notch. It appears that the housing is worn out on one of the carbs and allowing air to be pulled in around the throttle shaft. I have been doing some research and I think I am going to go with the carb setup that 650 Central has. I think I am done with the CV carburetors.

 

[grizld1]

The usual issue with leaks at the throttle shaft is the rubbers seals at each end; unlike us, they get harder with age. The throttle shaft rides on brass bushings, and the bushings sometimes wear. Throttle shafts seldom exhibit wear. If you don't feel any radial slop, seal replacement will cure the leak.

The bushings were never offered as replacement parts, but it's on my to-do list to extract and measure the OD of a bushing and check into availability of usable stock.

 

[Ghetto]

I like 34's myself. Like Patches said, easy to tune.

 

[gggGary]

Yes any of the stock carbs 76 up will work fine IF completely overhauled AND the throttle shaft bushings aren't worn out. Here in the upper midwest that's not a common issue but I have gotten southwest carbs with completely worn out bushings, I'll let you guess why that is.

 

[jim540]

I already replaced the throttle shaft seals. That cured the vacuum leak in the right carburetor. The left one is still leaking. When I spray starting fluid through a tube to direct it to the throttle shaft seals, the inside seal or bushing shows signs of leakage, engine revs.

 

[jim540]

Well it's official, the bushings in the carb body are worn. Took the left carburetor back apart to check seals, shaft and anything else I could find that looked wrong. I was able to band-aid the inner shaft seal. I remembered that refrigerant oil will swell normal "O" rings and seals. Put some oil on the seals and fixed the inner one. The outer bushing is just too far gone. Looks like a new set of carburetors is in order.

 

[CDNTX650]

why dont you just change the seals....it took me a day to do mine on my bs34's. I just bought some Viton Orings and double stacked them to squish them a bit....passed the carb cleaner test.

New carbs is probably more fucking around than changing the seals.

 

[jim540]

Already changed the seals. Too much play between throttle shafts and carb housing.

 

[grizld1]

Extracted a throttle shaft bushing yesterday. It's a helluva tight interference fit; I threaded the bushing and ran a bolt into it, set a drift against the bolt, and hammered, and the sucker wouldn't move until I put heat to the carb body.

The news isn't real good. For some perverse reason, the outer half of the bushing and the outer half of the hole are splined and of larger diameter than the inner half. It might be worth trying to use bushing stock with OD sized to the inner half of the hole; looks like there may be enough material there to support and seal, especially if you followed up with a penetrating locking/sealing agent like green Loctite. Other possibilities would be to machine the holes to a uniform ID or to turn a step in the OD of the bushing. A tight interference fit is needed, so you can reasonably expect that the bushing would need to be reamed after installation to correct any deformation.

 

[gggGary]

Thanks for the report G1
Wonder if they are cast in place? I am trying to envision how to center the shaft hole correctly. I think bushing replacement is not going to be a viable option till some far distant time when the restorers are working with piles of rust formerly known as motorcycles like the guys do with teens through 40's bikes today....

 

[grizld1]

Gary, the bushing seats have a machined appearance, and my best guess is they were line bored; I doubt the bushings were cast in. A guy who can do his own machine work wouldn't be challenged, but the job would take more sophistication than fooling around with a drill press, and more bucks than it's worth to have a pro do the work (unless said pro was working in batches to cut setup time per unit).

If anyone wants to hunt for material, I got 10.15 mm. as the OD of the outboard section, measured across the splines, and 9.98 mm. as the OD of the inboard smooth section.

 

[jim540]

I have been reading all the posts I can find on carburetor/intake options. I have read the thread TeeCat started on using a Joe Wiseguy intake and single Mikuni VM36 carburetor. Actually I read it twice. Sounds like a viable option. Also have been checking out the Mikuni VM34 setup that MMM offers at 650 Central. Also a very viable option. From what I have read and researched either setup would do me well. Price wise it appears that it is going to cost me around $400.00 either way I go. Decisions, decisions. Guess I need to figure out which way I want to go.

 

[ReycleBill]

I have been reading all the posts I can find on carburetor/intake options. I have read the thread TeeCat started on using a Joe Wiseguy intake and single Mikuni VM36 carburetor. Actually I read it twice. Sounds like a viable option. Also have been checking out the Mikuni VM34 setup that MMM offers at 650 Central. Also a very viable option. From what I have read and researched either setup would do me well. Price wise it appears that it is going to cost me around $400.00 either way I go. Decisions, decisions. Guess I need to figure out which way I want to go.

I think I would like a single carb but because of 20-30 degree temperature differences between cylinders, these bikes need to be tuned differently on left and right cylinders and I can think of no way to do that with a single carb. Anyone?