gear selection problem

G'day,
4 speed 1300 78 model.
What is happening is, finding third and top selection not positive, have to search for them and at times hit reverse or second. now and then have to go to second and move the car to get first. I'm thinking maybe engine supports allow the trans to move and change the selector position slightly or worn selectors.
I have found that on making an adjustment. it goes fine for a few changes even maybe half an hour of driving and then gradually becomes less positive.

When I brought it the ball was missing from around the gear stick, I found half of it in the tunnel but cannot get access to find if the other half is in there. I had two halves turned up using the one found as a profile.

Anyway, I'm having trouble adjusting the selector rod, and did find a comment that said to tighten the bolt that connects the flat flex coupling to the selector shaft out of the gbox. I did that as well as the centralizing the stick adjustment procedure. I think now that it needs to be set at a sliding fit
Thanks for reading. Cheers Neil

Your initial conclusion is likely to be bang-on. If the lower engine mount (the large doughnut that holds the gearbox in place) is worn it can misalign the selector rod under different engine loads. The only solution is a new engine mount if that is the case.

The big problem you face is that once the synchros start to wear on these engines the rate of wear will increase. Loss of 2nd gear as a result is not uncommon. On a later 5-speed it tends to reverse and fifth that are completely lost with 2nd crunching all the time.

I would suggest sorting the engine mounts out (all of them - if one has gone then the other two won't be far behind) and swapping the gearbox oil for a really high quality oil - I suggest one of the Redline oils (MT90 is the preference) for this purpose as they really, really do make a difference. If a lot of metal comes out with the old oil though it is worth getting the 'box fixed first as the Redline oil is not cheap and definitely not the sort of thing you want to just throw away if the synchros are past saving.