XS2 right hand carb idle problem.

[DelXS2]

The problem is that he right hand cylinder is not firing at idle.
It will fire at higher revs when the throttle is advanced/engaged.
Situation: both carbs cleaned and adjusted but not balanced and rebuild kit installed in right carb - no change.
Compression good, cam chain adjustment good, timing set - points system.
Carbs from neighbour's xs fitted to the xs2 and bike idles reasonably well.
XS2 carbs fitted to neighbour's bike and right cylinder won't fire at idle.
Right hand carb cleaned again ( ultrasonic, carb cleaner, compressed air).
The carb body does not show any structural damage or corrosion.
I would think that with the work to date that the problem is definitely the carb.
Is there any secret or trick with the Mikuni Solex BS 38 that could cause this situation??
Thanking you in advance for any help.

Cheers,
DelXS2

 

[TwoManyXS1Bs]

Hey, DelXS2. Sounds like it's time to look for the unusual.

I've run into one where the needle jet (long center aerator) wasn't fully seated, bad o-ring. Previous owners simply snugged-up the bowl screws, seemed to work, but intermittant lean-outs from air wicking in the middle area of the bowl gasket.

The snugging-up of the bowl screws warped the corners of the body slightly (where the bowl screw threads are), and the bowl wouldn't fit snug enough to seal the important central area.

To check for this, remove the bowl gasket, put a small dot of thick grease somewhere in that central zone, near where the needle jet fits. Then fit the bowl, no gasket or screws, to the body, remove, and see how much the grease dot is 'squished'.

It IS possible to re-flatten this interface, but great care is needed...

 

[gggGary]

Find out if it's not firing from lean or rich. Let it idle, then pull the spark plug, what do you see?

 

[DelXS2]

gggGary:

Running rich - right hand plug is wet.

New Developments: I picked up a second right side XS2 carb and cleaned and installed it. There was no appreciable improvement. The bike only runs if the choke is engaged and still only the left cylinder.
Did another compression test and got 130 for the left and 140 for the right.
Any ideas??

Thanks and cheers,
DelXS2

 

[RatyXS]

Seems to be the same as mine, I still havn't figured it out. Bike runs like a scalded dog when you ride it, but will kill a hole at idle some times. Carbs are set to jetting specs, are spotless, even swapped from left to right, up the heat range in the plugs with better results for sure, but still rich as hell on the right side. Compression on mine is also great 155+ in both holes, but right plug gets wet.

 

[gggGary]

Sorry I don't have a hard answer the o-ring around the emulsion tube can get brittle and allow fuel to be sucked up causing a rich condition. But I'm not sure that rears it's head at idle... The choke is on LH carb isn't it? is there's a cross over tube for the choke? I have only built one early set of carbs so far and they are running fine on my 73. Have you changed spark plugs, checked for strong spark at low RPM??

 

[5twins]

Yes, you may be chasing an electrical problem through your carbs. Maybe you have a weak spark on the right side, possibly a bad coil, plug wire, or plug cap. I would swap the coil assemblies (coil, wire, cap) side to side and see if the problem follows. The points set or condenser for that side could be bad as well. Swapping the condenser pack leads side to side would be a simple enough way to test them. For the points, if they're old and used, I would just get new ones.

 

[DelXS2]

Members:
I have gone through all the normal procedures one would expect with a carb problem as described in my earlier posts - thanks to the members that posted suggestions - but to no avail!
In desperation the bike was sent to a Yamaha dealer for investigation/review and the dealer was unable to identify or rectify the problem.
The bike runs well with my neighbors 1976 linked carbs but still falters on the right side when the individual carbs (stock arrangement for 1972) are installed. I have purchased and rebuilt 2 other right side carbs and still no improvement.
The Yamaha dealer suggested buying a set of linked (1976 through 1979) carbs and rebuilding as required to try and solve my problem. A motorcycle salvage yard, that has a set of the 1976 carbs, is suggesting, in spite of loosing a sale, that the purchase of a new set of carbs may be the most economical solution in the long run.
Any thoughts or suggestions regarding this situation would be appreciated as well as any suggestions for suppliers of new carbs / rebuilt carbs.
Thanking you in advance for any suggestions.
Cheers,
DelXS2

 

[pamcopete]

Check compression.

 

[DelXS2]

pamcopete:
Compression is good - the Yamaha dealer confirmed my numbers.
The bike runs well with the neighbours linked carbs so I a thinking the right hand carb is the problem.
Thanks,
DelXS2

 

[TwoManyXS1Bs]

Hey, DelXS2, here's some info to dig thru. (2) PDF's of the XS1 and XS2 carb parts fiche, and (2) pics of the XS1 (256) and XS2 (306) carbs, translated, thanks to the Dutch XS club.

Another member here experienced similar problems, caused by having the bowl vent tubes linked, not venting independantly. Check vent tube, part #54, in the XS2 part PDF.

The bowls look amazingly similar between different models, mix-ups can happen. One way to know for sure would be to post crisp/clear pics of all sides of the disassembled carbs and bowls. There are various little 'tells' that help to confirm part identities.

For these early carbs, the idle jets should have the fuel orifice at the bottom of the jet body.

Scotty says the transporter's down, so pics may be the best option here...