What should the timing be on a 4-port SP 13b with Mikuni carbs? I do not know what the timing now is, but it doesn't run right. If I know what the timing should be then I can change it and hopefully make it run good.

 

bump

 

Disable vacuum advance and set the timing to the first mark on the pulley which is 5° advance..
At least, on the s4 and above 13bs, it is 5° advance.. (not sure about the older ones and GSL-SE versions) You might tinker around that area of the pulley, my 12a still likes some advance at idle as well.
The key is disabling vac. advance on the distributor. Be sure to do that and reset your timing with your carb setup.

 

Do I need to set it to 5* advance at idle? This doesn't matter anymore since the advance is desabled right? How do you disable the advance? Are there any drawbacks when disabling the advance?

 

Yes, set it at idle. The distributor has mechanical advance built in which adds a total advance of 20° and it should be fully advanced by 4k RPM, if I remember correct. What you are disabling is vacuum advance, not advance all together.
So, 5° advance at idle yeilds a total advance of 25° at 4k and above RPM. You really don't want to get more advanced than that, and you should be in a good state of tune before you set that, as well..
Plug the vac. ports on the manifold, and leave the vac. diaphram on the distributor open, then set timing at idle with 5° advance.(this is what I'd recommend anyways) Leave the timing light on the pulley and rev it up to note which way 'advance' is(away from the trailing mark on the pulley). You will have to eyeball how much 5° is also, try and gauge the difference between the marks for the trailing timing mark on the pulley, which I understand to be 20°.
You know, there ARE threads about setting timing.. AHEM*AHEM :P