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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.

Maddog

Sounds more like worn synchro teeth on first motion gear, or 3rd/4th selector hub or a combination of both. These can get damaged/rounded off when the baulk ring is not slowing the gear down enough when changing up. Baulk rings often get replaced but people overlook inspection of the synchro teeth on gears and hubs.
Other areas to look at if box removed

-Synchro hub springs/balls worn
-3rd/4th selector fork worn

Things to check before the box is removed:
-Selector rod detents&springs worn or seized 1st/2nd & 3rd/4th can be checked in situ) - so definitely worth a look before removing the unit.
- check all external components for gear change(knuckle joints, diff housing bushes, shafts) for wear and secure fixing.

Regards Jason

Jason, many thanks for your suggestions. Your professional expertise is greatly appreciated. I had thought of checking the detents, but I do suspect like you that it is something inside the box that is at fault sadly. It just seems a bit odd that he test drove the engine and box and it was apparently fine, and now dropped into a different car it has developed this problem apparently out of nowhere...
Regards
Maddog

No problem Maddog

Fingers crossed that something is loose/worn that can be resolved without removing the unit. Does the gearstick feel positively locked in 4th(engine not running). With the stick in 4th see how much movement there is when gently pushing towards neutral. Worn forks/hub springs will be indicated by the stick.
Keep us posted on progress……

Didn't we have a similar querie a couple of years ago - or TEARS ago! It turned out that the engine was rocking slightly and the gear lever hole in the recipient car was not compatible with his new box and gear extension. It needed to be modified to suit as the lever just didn't quite get the gears into mesh. Just as Jason mentions.

Worth checking this.

Lot of really good suggestions as to what the problem might be.
One other thought......since this was a good working unit prior to the car swop I might be inclined to look for an exterior source of the problem.
It is possible that 4th is not being fully selected on the upshift.....if that is the case the gear will slip back to a neutral state when you lift off the throttle. There is a lateral force inside the gearbox when you are going forward....when you lift off...that force goes in the opposite direction....an improperly selected gear will then jump out of position.

So, I would remove the rubber boot around the gear lever and inspect that whole mechanism.....check that the lever can fully move backwards all the way when you select 4th gear I am assuming the lever and the retaining bits were removed during the swap over.

If a gear is fully selected...the hub is completely locked in place with the spring-loaded balls......takes a bit to move those hubs.....that is why I would make sure the gear is fully selected before you go pulling the whole unit out of the car.
Take it for a drive....check that the lever really feels like it has fully selected 4th....maybe put a bit more effort into the lever to get it all the way.
Just my opinion.....

Thanks everyone who has posted suggestions here. If and when we get an answer I will try to remember to update this thread to see who was on the money.
Cheers, Maddog