I've got a WES single box to go on my TY, and plan on openning up the top of the air box - what size needle should I move up to to account for the additional breathing?

Normally, if necessary (not always the case), pilot and main jets are changed, not the needle. Needle position can be altered up or down with the needle clip. Lifting it up richens the mixture and vise versa.

Ok, well I've managed to avoid messing about with carbs so far, so figured I'd ask - I know this is a common mod on TYs so thought someone could possibly just rattle off a part number or two from memory

There are two different standard TY175 carbies. One has a stamping 525 - ***** and the other is 1N4-*****. They have different jetting and are that way because there are two different TY175 end muffler designs and the 1N4 carby bike comes with a rubber snorkel on the airbox lid.

Have a look at your carby and see which one it is.

If you want the alternate jets on hand for when you test ride the bike, you can take the pilot jet and main jet out of the carby and read the sizes that are stamped on them and maybe buy jets one size bigger and smaller. You can buy genuine Mikuni jets from a Mikuni agent or aftermarket jets from multiple sources. The non-genuine jets are sometimes not size stamped.

Pilot jets for those carbies go up in intervals of 2.5. For example 22.5, 25, 27.5, 30, 32.5, 35 etc

Main jets for those carbies go up in size in intervals of 5. For example 130, 135, 140, 145, 150, 155, 160, 165 etc

Both jets are commonly available types. You can look at Mikuni parts listings on the internet. Look for Mikuni VM type carby parts.

You can probably also buy them as Yamaha TY175 parts from a Yamaha shop but that would be quite expensive.

You may not need to make any changes to the jetting

Awesome, thank you! 

Hi Guys, The Haynes Manual for the TY range of bikes gives the details for the 175 Carbs as follows....

Mikuni VM22ss  , Number  52561 ,  22mm Choke size.

Main Jet  240 , Air Jet 2.5 , Pilot Jet 25 , Needle Jet 0-0 , Needle 4L6 , Clip Position 4th from top.

Pilot screw 1.5 turns out for an idle speed of 1100 rpm

All the TYs I have looked at seem to be as above, but one seemed to run rich at full revs.

The small O ring on the needle jet should be changed regularly, as they wear and let fuel pass.

.

Thanks, that's also handy info   (I'll be honest, I hadn't really thought about checking the Haynes for this ... as it stands I've decided to keep the stock airbox for the moment, so hopefully it'll run ok with minimal tweaking to allow for the exhaust and I'll worry about the carb later.  I might even go crazy and try a flat slide carb!