I was given a 1990 Sportster as a project bike. Now I don't have a service manual for it, but I do have one for a 2003 model, Are there any major differences? I'm really looking for headbolt torque procedure and torque values.
First of all, your bike will have a chain drive instead of the newer belt drive. The belt drive is quieter, requires less maintenance, is more expensive to replace and is a pain the ass to install a new one, as the tire and swingarm have to come off to thread the new belt around it.
The chain on the other hand requires constant adjustments as the chain stretches as it ages. It also requires manual lubrication. The benefits of the chain over the belt are that it can be repaired on the road, should you have a chain failure. (I have a '90 and carry a spare connecting link, just for this purpose). In addition, the chain being narrower than the belt, allows you to go to a wider tire than belt would normally allow.
Another major change is that the '86-'90 Sporties have alternator/stator setup behind the clutch basket. The constant heat of the clutch assembly loosens the magnets over time. They start to break loose and grind up in the clutch/primary drive, not only screwing up the charging system but all too often, taking out the clutch assembly in the process. Eventually, your charging system will not be able to keep up with the bikes demands and you will have to replace the clutch basket ($300-$400), which has the magnets glued inside them and most likely the stator as it will be destroyed by the loose magents grinding around in there. After '90, HD realizing this was a constant problem, moved the alternator assembly forward in the primary assembly behind the main sprocket. The reduced heat has virtually eliminated the problem.
Another major change after 1990 for Sporties is that HD went to a 5spd tranny, whereas yours is only a 4 spd.
These are just a few examples of the differences between your '90 and the newer Sporties. There are many more, but I will not go into all of them here.
Bottom Line: I strongly recommend that you get yourself a Harley Davidson Service Manual specifically for the 1986-1990 XLH models. It will be the most effective investment you will make for your bike. DO NOT use the '03 manual that you already have. It is not accurate for your bike and you'll go crazy looking for stuff on your bike that isn't there.