XS2 stutters at half throttle

[clabedan]

hi
I just had my throttle cable changed for a double cable one, all done by a professional mechanic and very neat job...

However the bike won't go over 40miles an hour and starts stuttering at half throttle. My first guess was that the main jets must be clogged up and indeed, they had a thin layer of white powder covering them... Carbs are original BS38 solex

Great, put them through my ultrasonic bath, came out really clean but unfortunately the problem remains. Everything else in the carbs is fine.

A few months ago I did replace jets with a standard pack (jets, gaskets etc)... i then realised that I should have ordered bigger jets since I am running with UNI pods and megaton exhausts... But still, the bike was running ok, nly stuttering a bit when reaching 75% throttle.

So my questions are:
What size main jet should I order with UNI pods and megaton exhausts

What could be the reason for the existing problem (bearing in mind that the bike did not have such a big problem after replacing the jets and gaskets)... And that I can't think the double thottle cable can be responsible (both sides responding in sync)

Apologies for the long message but I amslightly puzzled...
Thanks
Christophe

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[5twins]

Usually for pods and pipes, you go up one to three sizes on the mains, but only actual testing is going to show you what size is best. In other words, I can't tell you exactly what size to use, your bike will have to show you.

Keep in mind that a carb is a total system consisting of 3 circuits. You have one for idle (slow speed/RPM operation), one for midrange, and one for high speed/RPM (main). The three circuits overlap slightly and influence the one or ones next to them. That means larger mains may bleed over into the upper midrange enough to make it too rich. Leaning the midrange settings to compensate may also lean the upper part of the idle circuit creating a flat spot just off idle. That's why the usual set-up is larger mains, leaner needle settings, and larger pilot jets. Changing just the mains usually doesn't get the job done.

 

[clabedan]

Thanks... Will have to do some tests then...

But any idea why it would stutter at half throttle? It didn't use to do it with the existing jets

Usually for pods and pipes, you go up one to three sizes on the mains, but only actual testing is going to show you what size is best. In other words, I can't tell you exactly what size to use, your bike will have to show you.

Keep in mind that a carb is a total system consisting of 3 circuits. You have one for idle (slow speed/RPM operation), one for midrange, and one for high speed/RPM (main). The three circuits overlap slightly and influence the one or ones next to them. That means larger mains may bleed over into the upper midrange enough to make it too rich. Leaning the midrange settings to compensate may also lean the upper part of the idle circuit creating a flat spot just off idle. That's why the usual set-up is larger mains, leaner needle settings, and larger pilot jets. Changing just the mains usually doesn't get the job done.

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[5twins]

Maybe your carbs aren't as clean as you think they are? Or maybe a diaphragm has gone bad, developed a hole or tear.

 

[TwoManyXS1Bs]

Yes, ditto on the diaphram(s). Are you familar with the 'diaphram drop test'?

 

[clabedan]

Ahhhh the mechanic mentioned a hole in the diaphragm that he fixed with some sort of glue... Could be this

How do you test them ???

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[TwoManyXS1Bs]

Go thru this Carb thread, download the 'carb guide'. Instructions and voluminous info in there.

http://www.xs650.com/forum/showthread.php?t=43

 

[yamahama]

Are there any good aftermarket diaphragms for the original BS 38 VC's? Just the rubber diaphragms.

 

[cra-z1]

Check out this company
http://jbmindustries.com/Yamaha650.html

 

[yamahama]

TY. I did see that from a youtube video,however, the website was flagged as an unsecure URL so I started looking elsewhere. I can't believe what I'm seeing for $prices of the rubber attached to a new slide....prohibitive, to the point of passing on a fixer bike purchase. Having worked on and modified SU carbs for some 20 years...I just got lost in some of the reading material noted above....I'll go back to JBM and look again. Trying to get a mental handle on early BS 38 total rebuild costs prior to purchase on long sitting bike.

 

[650Skull]

This site is flagged as an unsecured URL every time i open it up........Never had any problems with JBL site and i have used it.

 

[yamahama]

TY..I see that. Little trick on this (xs650.com) site if your browser flags unsecured, just type https:// in front and (mine) switched to that. Not so on the above site. That said , last year I ordered a couple of sets of perfect, correct Campagnolo replica, gum rubber brake lever hoods from a shop in Netherlands. They actually were getting them from Italy. Finally arrived give or take 4 months after paypal payment....they were way back ordered and just ran out when I ordered. Thanks for the above for when I rebuild....and save another bike.