Carb Jetting queries

[ghosty]

Hey guys, just trying to sort carbs out, running BS38s ( 76 xs2 ) pod filters and megaphone exhausts with no baffles and +1.0 oversize bores. I have 40mm needle, #42.5 PJ and #130 MJ. Can I get away with these or is it best to go up a size? I don't know a damn thing about carbs. Have downloaded the carb guide but trying to do this quicker than my reading patience as the Burt Munro challenge is happening here next weekend and that's always been the deadline for this rebuild....Also would a lean mixture cause my timing mark to wobble under the timing light? I can't get the bastard to stay still and it's making timing a real bitch. I'm pretty sure my mechanical advance springs are sweet...it seems to advance as it should past 3K rpm.

 

[funky]

Ghostly I wish i could give you a quick answer with the jetting the truth is it takes time.
the timming mark wobble is standard.
when re jetting start with the main. and work back to the needle jet then the the pilot and air.
Pilot and air pilot are for starting the bike up about 1/4 throttle
needle jet is for mid throttle you have to adjust the needles to weaken or richen the mix there are five groove on the blunt end of the needle top groove is lean and each groove down richens the mix 1/4 throotle to 1/2 ish
main jet is full throotle 3/4 to full.
but all these circuits overlap so its trial and error it takes time.... theres no quick way apart from Dyno tune at a garage which costs...
I have posted on this before and all this has to be done before re jetting ....so click the link here and follow all the best
http://www.xs650.com/forum/showthread.php?t=43007

The link below is about jetting it may help
http://www.xs650.com/forum/showpost.php?p=427992&postcount=24

 

[gggGary]

Timing; Cam chain tension set properly?
76 XS2 um that's pretty confusing. XS2 is 72 and 72 carbs share little with 76 carbs.
72 carbs have 2 throttle cables one choke with a cross over and raised venturi floors, 76 carbs are held together by brackets and use a single throttle cable.
Search pod filters, pleated pods are not such a good idea.
A pic or three might help us sort out what you have.

start by verifying you have the correct brass in your carbs.

True megaphones?

 

[ghosty]

Yeah got that wrong, it's an XS-B. Had xs2 engine cases lying around from a 72 and transplanted internals into those so I get confused. She's made of all sorts from various boxes I've picked up around the place. Not sure what a true megaphone is but couldn't think of any better description. Original engine numbers say '74 and bike previously registered as a '76. I put a recon kit into the carbs and that consisted of #42.5 BS30/96 pilot, #130 main and 40mm needle. Idles ok with encouragement, when I set the timing to the right place it runs like crap and kicks back like a bastard when I kick it over. Thanks for your help guys...

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[gggGary]

Couple of things
Check if your timing light has an advance knob see if it's set to zero.
check that your TDC matches the piston rise a coffee straw in a plug hole will get you pretty close. Stock alternator or PMA?

need to see better shot to positive ID the carbs, remove an air cleaner shoot a pic looking in the throat?

 

[ghosty]

Running a new PMA, and pretty sure TDC is good as made a piston stop and shagged around with it for a while. Timing chain tension good...started easily when set new PAMCO ignition in the midrange as per installation instructions. Not so good when timing it.

 

[gggGary]

So I see a RH idle screw and no raised venturi floor 74-75 carbs sounds about right.
Since some brass has been changed best to check all of it, many "kits" have the wrong parts in them.

Go back to the rough setting method if it runs better there. Something wrong with how you are timing.

 

[5twins]

You can see in that black hole? lol. If they are '74-'75 carbs without the raised venturi floor, and by 40mm needle, you mean measured from the top clip slot to the tip, that needle is way too short and rich. You need one 45 to 46mm long. You also need to re-think your choice of pods. Those don't work on CV carbs very well. Even with the right parts in the carbs, you will probably still have all sorts of tuning headaches.

 

[grizld1]

Ghosty, I know you don't want to hear this with an event coming up, but the best and only advice I can give you is that there are no short cuts to this work. As Gary says, you'll get nowhere fiddling with the carbies if the ignition isn't right. As 5twins says, jet kits can be relied upon to contain at least some brass that's wrong, and you can be sure that the needle out of a kit will be incorrect. If you must use pods, get a pair of big foam filters before you try to tune further. With your setup, start with 1 step up from stock on PJ's and 2 steps up on mains, and tune from there.