Dolt
Thanks for the info. This is a project bike (fuel injected) which will be stripped down for paint this winter. While it is being painted, I'll change the engine sprocket. I have plenty of torque and would expect slightly better fuel economy with the switch. The bike has 30,000 miles and is probably on the original belt, so a change would be good. Planning a 4,500 mile trip up the East coast next Spring.
PS where to get a 141 tooth belt?
First let's get the lingo straight. The pulley you are referring to as the "engine sprocket" is the trans pulley. Motor sprocket and main shaft sprocket are inside the outer primary and drive the tranny. The trans and wheel pulleys are exposed and drive the rear wheel.
Unfortunately, I don't believe anyone makes a 141T trans pulley and if you do find one, it will be aftermarket which I do not recommend; too many get broken. The OEM belts are more expensive, made by Gates but to the MoCo specs. For example, there is a 140T offered by JP Cycles and the manufacturer is listed as "Gates by BDL", so I don't know if this belt is manufactured to the MoCo specs; it sells for $141. The OEM PN for that belt is 40024-09A and it sells, from a discount dealer, for $220. Given the choice and considering the work involved in replacing a belt, I would pay the premium for the HD belt but that's me.
The tooth pitch on the trans pulley is 14mm, so two teeth = 28mm. Your current belt is 139T (disregard the 137T to 139T comment in my previous post, I was thinking FXSTD and not my FLHT). Anyway, adding one tooth to the belt reduces the difference between the 28mm added by the 34T pulley to 14mm by the added 14mm on the belt. A bit of bad news though, if the 140T belt (or a 141T if you can find one) will only be available in a 24mm belt. Doesn't mean you have to change the wheel pulley but it would be a good time to do it which would allow up to a150mm tire on the rear in the future. A new rear wheel pulley is going to run near $200.
A larger pulley is going to require that the axle move forward at least 14mm, plus some additional for slack to fit the belt over the wheel pulley, let's say another 8mm just for sake of this discussion. So, if you currently have say in inch of space to move your axle forward, the 140T belt will work; if not, it won't.
I am going out on a limb here and suggesting to you that you forego the trans pulley change for a couple of reasons:
1. I doubt that the 140T belt will fit but maybe it will. I suppose you could order one, try to fit and return if it doesn't but that's a lot of work only to discover that a 141T belt is required and may not even be available.
2. If the 140T doesn't fit and you can't find a 141T, you have to put everything back together again and punt or decide to change out the 24/37 primary to the 25/36 to get the same results as you would with the 34T trans pulley.
3. There is an option to modify the swing arm for more adjustment travel which adds cost and downtime but is an option and would allow the use of the OEM 140T belt.
4. You will need to spend another $100 plus for a speedometer recalibration module to correct your speedo which is affected by changing the secondary gearing.
I suggest that you carefully weigh the trans pulley change against the primary drive change. Much less labor involved; you can get it done in a couple of hours. Cost will be about $500 for the motor sprocket and the clutch shell& sprocket/ring gear/bearing and retaining ring; no need to change chains. No need to recalibrate speedo for primary gear changes either. You retain your secondary drive; trans pulley, wheel pulley, belt and replacement belts, should you ever need one, are OEM and readily available.
Just food for thought; good luck on the project.:s