I'm probably beating a dead horse but its a new question to me so here goes.
decided to rebuild the stock keihn carburator, I noticed during the rebuild that the main jet is a # 165.
my shop manual says it should be # 155
I know the difference is the size of the orifice but how does it go? bigger number bigger orifice?
and does anyone know if the 165 is acceptable for my 84 iron head 1000?

 

I have an '83, and I think a 165 main jet is stock. The larger the number, the larger the jet. Ibelieve these are measured in millimeters, so a 165 has an orifice of 1.65mm. I'm running a 170 in mine, with slip-ons, a Screamin' Eagle air cleaner, and the choke plate removed. I've even run a 175 when I used a velocity stack. The size is really dependent on your intake/exhaust mods, but if a 165 works, use it.

 

As XLX83 wrote correct jet size depends various differend thinks on your bike: exhaust, aircleaner, airbox, cams, cylinderhead porting,CR, elevation.... It can vary anything between 150-190 notice different carburator manufacturers has different standards to mark their jets... Mikuni by flow, Keihin by mm, S&S by inch so comparing different manufacturers setting might be hard...