I just got a 1990 fifty less then a week ago and my 3rd and 4th gear won't engage like if it's on nuatrual but my other gears will go catch. And when I hard shift 1st and 2ND won't go in it will grind but if I shift it soft it goes in there with Lil grind 5th ND reverse goes in smooth with no problem. What can cause this to happen?
I don't know how you feel about the DYI thing when it comes to gears, some people don't like the idea of working on transmissions. If the DYI transmission fix idea isn't something that you would rather leave to someone else, here are some resources:
I did a Tremec 3550 rebuild, and it wasn't that difficult. The video was an immense help, and I would recommend viewing it. At $20, it will give you an opportunity to look and decide for yourself if you think that it is something you want to try to do.
The tricky part is that you will need some shims to adjust the endplay in the two shafts: the new bearings will change the clearance. There is no way of telling what it will be until you get to the point that it is time to measure and check the clearances. Halon has the individual shims if you can wait for them. If you can't afford the wait, the auto parts store can sell you some shim stock and you can make your own, or have a machine shop make them for you. The shim stock comes in brass (easy to cut) and steel, in varying thicknesses. That's where the dial indicator & base come in - there is no way to get the clearance right without it.
Getting the car jacked up high enough to get the trans jack in and out from under the car was somewhat challenging. Since I didn't have a helper, getting the trans out of the car and back it was one of the more difficult things about the job. I built a trans jack out of 2x6's, some angle iron and a $25 floor jack. If I hadn't done that, I would have needed a helper or to rent a commercial trans jack. At 100+ pounds the Tremec 3550 was far too heavy for me to lift by myself.