Evening all, today I was trying to help a mate with a gearshift problem. He has a Mk2S engine and box (4 synchro remote) in a Mini K. He has just got the car going here in Tassie, but he bought the power unit in Queensland and was able to test drive it there, where everything was working fine. Since getting the car running here it is now jumping out of fourth gear.

I had a look today at the remote shift assembly for him in case it was a simple fix. It felt tight and changes gears nicely. However, I did note that the rear bracket on the remote only had two bolts instead of three, and one of the two was barely finger tight. So, there was not compression on the rear mounting bush. We dropped the shifter housing, moved the bracket and tightened everything up to get good compression on the rear mount.
Unfortunately, when he test drove the car, it still jumps out of fourth when you back of the throttle. He did say the gear shift felt better.
Engine sits on new engine mounts and top steady has poly bushes. So, I don't think it is an engine rocking that is knocking it out of gear, especially now the remote housing is properly fitted up.

I suspect he probably has a synchro problem but the shift does seem fine, not like a worn synchro. Could be a poorly shaped new synchro, or a type that is incompatible with the mating surface on the gear. I have no details about what parts are in the box.

Does anyone have any thoughts? I know it is next to impossible to diagnose via forum, but just after some theories. He is gutted at the prospect of having to pull the motor out to strip the box, but I suspect that is the likely outcome. A shame as he has done a good job and from what I can see the workmanship on the engine and box has been done by someone who knows what they are doing. Nothing looks bodged and it is very tidy.

TartanRed

Check the knuckle joints on the top of the diff housing and also the pinch bolt under the the gear stick on the shift rod in the remote extension housing

I had a 4 synchro remote box that did this on 2nd gear. A new shift fork and detent springs didn't fix it. Drive dogs on the gear and in the synchro sleeve had worn at a slight angle. I changed both and all good then.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

I don't think a synchro ring will cause that - it can only be if the gear isn't engaged properly

So..

1. the knuckles haven't allowed the selector fork to push the hub across properly - it can happen if one of them isn't in the right position on the splines - but it really should pop out of 2nd gear too
2. the selector fork is worn to the point where it can't push across enough - the 3rd/4th gear selector pushes from side to side, if the side that pushes 4th gear is worn out then it won't push across
3. like Dr Mini says, the teeth are rooted and it's just slipping off

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did I tell you that I won a trophy?

Also check for wear the 2 sintered bronze bushes in the diff housing.

Esp the top one just below the knuckle joint.

It is highly likely that the synro hub and or dog teeth on the 1st motion are buggered.
The shift fork could be worn and not throwing the full distance.
Multiple slop in all the linkages. A bit here and a bit there adds up to a lot.

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1963 Morris 850 Smoke Grey
1963 Morris 850 Shadow Blue
1965 Morris Mini Smooth Van
1966 Morris Mini Van
1969 Morris Mini Deluxe Resto
5/72 Clubman Gt

I'm thinking that its the selector fork

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The bitterness of poor quality (or Crappy Workmanship) remains LONG AFTER the Sweet Taste of Low Price is forgotten.

Thanks everyone for the advice and suggestions here. Seems the smart money is on this being a problem internal to the gearbox (which was broadly the same conclusion I reached as a non-gearbox expert). I will aim to update this thread at some stage if and when we have resolved what the actual cause is.

Cheers, TartanRed