Carburetor jetting help required

[Fleksta]

Hi guys,

I purchased a 1979 XS650s last week and need some help regarding tuning. The bike is equipped with the standard 38mm carbs. The only modifications are K&N filters (OEM style, not pods), XS1 style header pipes, and Dunstall British style slip-ons.

The problem is that the bike falls completely flat just off idle; to the point where more often than not it stalls it out. The previous owner had only said that he went up one size on the main jet.

I'm looking at the list of jets on Mikesxs.com and have no idea where to begin with selecting sizes. Also they are listed as being sold in pairs, is one pair enough for both carbs? I would also assume that the pilot jets need to be changed as well?

I would like to make the attempt at rejetting the carbs on my own but have no experience doing so. Any input would be greatly appreciated.

 

[littlebill31]

Best thing to do is, first, read www.amckayltd.com/carbguide.pdf to get a better understanding of these carbs. Every carb is different. There are too many variables and there is no set recipe.
Example: These carbs work off of throttle postion, not so much on engine speed. That being said you need to understand that the pilot jet controls idle to 1/4 throttle. Jet needle and needle jet 1/4 to 3/4. And the mains from 3/4 to full open throttle.
So if your bike stalls right off idle, then you know it is pointing to the pilot circuit, which is the pilot jet. Probably a lean mixture telling you to up the size of the pilot jet.
So make sure you read that guide or you'll be chasing your tail trying to figure it out.
However, you need to do a little work before, always before, you mess with carbs.
In this order, make sure the cam chain tensioner is adjusted, valve lash is adjusted, timing is spot on, then, and only then, move to the carbs!!!

And you will need to have 2 of everything. i.e. jets, needles, ect when ordering. It's always good to use 2 new of the same kind/size, obviously, when changing parts. Never just throw a part in the carb without knowing what size, condition, correct model.

 

[5twins]

If the P.O. lowered (leaned) the needle setting to compensate for the larger main, that is probably the cause of that off idle flat spot. You usually need to increase the pilot jet size when you lean the needle. This is particularly true with the '78-'79 carb set.