John,
Sounds like your linkage is out of adjustment, bushes worn or the springs holding the gate in place are missing / broken / gone weak... maybe all three...I also suffered from the 3rd gear lottery, and when I stripped the mechanism found one of the springs was missing so essentially there was no gate for third/fourth. You just had to push it forward at the correct place, no chance!
After fixing the springs I also replaced all the bushes at the lever and at the coupling and then followed the adjustment procedure below. All done in a lunch hour and cost of about £25 from Roger Bray for the bushes. With worn bushes I couldn't get enough forward movement to fully engage 3rd, 1st and 5th, and then still get 2nd and 4th. Now it's transformed, you get the gear you go for every time! I'm currently rebuilding it but that's only to fix a very noisy third and a couple of weak synchros.
Good luck! 915 boxes don't have to be a pain.[
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<Borrowed from Pelican Parts Tech Info pages, also to be found in the Haynes manual>
Follow the factory shifter adjustment procedure. It is as follows:
a) Take off the cover between the rear seats which exposes the shift coupler. Pull up the shifter boot, particularly so you can see the lower part of the shifter lever where it bends from "angled back" to more vertical.
b) Loosen the retaining bolt which pinches the shift rod on to the spline on the shift coupler. Let it be very loose.
c) WITH THE TRANSMISSION IN NEUTRAL, rotate the shift coupler clockwise when viewed towards the front of the car. You should be able to feel the coupler rub against the various shift stops as you rotate it back and forth. Rotate it to the furthest clockwise position, as seen when you are facing forward. Don't pretend you are Tarzan and turn it with huge force....lightly is all that is necessary..
d) Keep your hand on it and hold it there. Don't let it wiggle, if it does, turn it back to the furthest clockwise position.
e) Move the shift lever so that the more vertical part of the lever is vertical (ie 90 degrees to the level...sticking straight up so to speak). Move it so that it lightly touches the side of the shifter housing which is on the 1st/2nd gear side.....closest to the driver. Again, gently.
f) They should now be properly aligned.....lever closest to the driver with bottom part vertical, coupler clockwise in the neutral plane.
g) Carefully tighten the pinch bolt. Make sure it is quite tight.
h) Check things out. You must be able to engage reverse clash free (give the gear a little bit of time to stop after you stomp on the clutch), shifting should get to all the gears easily when driving.....things don't work as smoothly when stopped and lastly, there must be a little bit of rotational play when 5th gear is selected. This is checked by shifting into 5th, and feeling whether you can wiggle the shift coupler with your hand. It should just click back and forth slightly...not much, but clearly discernable play.
i) Assuming all is well, put all the covers back. If it is not well....my fervent suggestion is that you START OVER at b. ....I have never had any luck fiddling with the linkage.
