replacing 2nd gear fork

I have an'85 MaximXJ700N that won't stay in second gear. I replaced the shifter springs but it pops out of second on an upshift. downshifting it will stay in second, but then when starting out it locks up in first til I let the clutch out and then it pops out of second. I was wondering can I work on the shifting forks without pulling the whole engine apart. Thanks

 

It's probably not the forks. The gear dogs on 2nd are likely worn.
There is a way to undercut the dogs by flipping the bike over and working through the oil pan, but I'd be concerned about not being able to make the cuts correctly.
The best fix is to split the cases and install a replacement 2nd gear (or have the dogs on your existing one undercut).

If the problem does lie in the shift forks, then you do have to split the cases. There is no workaround for that.

 

Do I need to take the upper end apart to split the cases?

 

No you do not, but you should remove the oil pan first in order to inspect before proceeding.
If you have a dental mirror (or better yet a flexable borescope) you can remove the shift cover and insect through the large oil passage that lies between the transmissionand the shift lever.

If you determine that the cases must be split it is advised to have on hand both the Yamaha FSM, and a copy of the Haynes service manual. Both miss a few improtant things, but together they have all the information you will need for the job.
It is also advisable that you replace the alternator chain guide, and rebuild the starter clutch while the cases are open. Both are somewhat prone to failure (the guide due to age, the clutch due to roller wear).

The following link covers the case split (other than bore and stroke, it's the same engine), and as I saud you do not need to strip the top end in order to split the cases.
XJ750 Engine removal and split - Pictures.

The workaround to undercut the gear while only having to remove the oil pan was first done on the XS1100, and can be found at xs11.com (sorry, I don't have the link for that saved).

 

I had a broken shift fork that I replaced. Luckily, the broken segment went straight into the pan and no other damage resulted.
K-moe is correct. You can work on the fork dogs via the pan. It is difficult, frustrating and straight up sucks but it is doable. The entire top end remains "as is".
I went with pulling the engine, flipping it over on my bench and splitting the bottom of the case. I replaced the shifter cam and all the forks, rebuilt the alternator/starer clutch and replaced the timing chain guides all in a weekend. The following weekend I reinstalled the motor and was riding on Sunday afternoon.
Splitting the cases apart sounds daunting but it really isn't. If you do split the cases my best advise is to make sure each of the 35+ bolts that lock the two case halves together go back into their respective location. They vary in length and must return to their designated bolt hole. Use 3bond sealer on the mating surfaces that are spotlessly cleaned and prepped with brake parts cleaner on a rag. Get the FSM and follow the bolt tightening sequence with proper torque settings. The bottom of the case has numbers on it for each bolt designation so you don't get mixed up while cinching everything together.
There honestly isn't a better, more controlled way of doing this fix. It is time consuming but your end result will be a solid, long term repair that won't come back to haunt you.
You got this...

 

I'd like to add to what Larry said.
I make a cardboard template for all of he case bolts to go into. I number the template so the bolts can go back into the same numbered holes on the cases (the numbers are cast into the cases).
Instead of Threebond I like to use Locktite 515 or 518. Both are anerobic case sealants, and any seepage will mix harmlessly with the oil instead of curing into goobers that can damage the engine.

 

My xj900 used to jump out of second quite often

yes had to split the casings as it was wear on the gear dog mentioned above