Quote:
116 main jet seems waaaaaay to low for that pipe...
sdjohn hit the nail on the head.
the 120 AC means it may be right on.
I've run tuned engines with similar.
There are variables - and you may know all about them - but here is a primer if helpful.
(if this is redundant - just ignore).
When you "O tune" a cylinder (involves larger ports and increased flow through the piston), you certainly need to upjet.
A pipe will also demand more upjetting.
A simple rule of thumb could be to add 10 points to your main for each of these alterations.
So if it came with a 105, you would go to 115 for the pipe and 125 for the O tuned cylinder.
However - and this is a big however: the carb can't feed much more than about a 120MJ, without modification. This is where the 120AC comes in.
The specific carb modification needed is to enlarge the hole in the bottom fo the float chamber.
Otherwise, this hole chokes off gas flow and limits you to the above mentioned 120 or smaller main.
I mean - you can put a 160 main in - but you are only going to get the flow of a ~ 120 or so, due to that hole size.
(Side note: modifying the carb is a pretty simple operation).
One way to avoid having the flow choked off is to change the jet stack to using a 120AC.
This shifts the range of needed main jets down significantly.
Example: you could be running a 160AC, with your carb bored out, and find that it tunes in at say a 130MJ. That exact same carb, might need only a 120MJ if it had a 120AC.
Jetting is a bit of an art - and I would personally never trust a new bikes jetting without checking.
Whoever jetted that was probably wise to use the 120 AC in this case - but you can't know how good they were at jetting, or if their final jetting was truly spot on or a little lean.
Other's like Rowdy are encyclopedic on what it would have come with stock - maybe they can comment on whether it started life with a 120.
My assumption was no (may or may not be correct).
If it were me:
1. look at the plug. you can post pics if you like. it will stlll have some evidence from last run. mostly, here you are looking for any little balls of aluminum that suggest lean, or any whiteness vs brown/blk. (brown good. white - not good).
2. if you have a little one of those scope cameras - shoot a pic of the top of the cylinder - or just pull the head and have a look. Top of the piston will be VERY telling. Post a pic or two.
3. If it looks good - then move to running it and use the above methods to confirm it is tuned in. Pay special attention to how it returns to idle (quickly is good) when you rev - as old seals can leak - and leaks create lean - and lean doesn't return well to idle...
Hope helpful.
-CM