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MG MGB Technical - Clutch? Hydraulics?

Hello all, wonder if anyone has any thoughts on a problem I encountered yesterday when racing my MGB.

Had the problem twice - and I'm presuming it is partly related to something gettign too hot - but I'm a bit stumped to be honest.

Went out in practice and the car was fine. after maybe 6 or 7 fast laps the transmission seemed to have a problem. It woudl shift down or up a gear without a problem - smooth engagment (although I double de-clutch whihc helps) but when the clutch was brought home there was a pause before i got drive - but with no flaring of the revs as you get with clutch slip or wheel spin. Also, as I lifted the throttle and then re-applied in (whilst in the same gear) it felt almost as if I had dipped the clutch brielfy.

When I returned to the pits and shifted down into 1st to trundle through it seemed to engae the tranny a few seconds after I'd released the clutch - went with a jolt.

I checked the car over and foudn no leaks and had fluid in the master cylinder. I bled it anyway just in case but there was no air. I truedled around the paddock and could not get the fualt - although the clutch is lighter than it normally is and the bite point is much lower.

During the race it was fine again for maybe 5 or 6 laps - then the fault returned. But the clutch is not slipping - I know what that sounds and feels like. No juddering either. Just a weird delayed reaction and this weird dipping the cluthc feeling. In some ways it was almost like the car was stalling when I braked and dipped the clutch and when I re-engaged the clutch it was bumpstarting itself - but I know the rev's didn't drop off the tacho between changes and the in-car video I filmed shows the engine still running(you can hear it).

I'm very confused by this - any pointers? (Sorry for the long post but it is quite hard to describe)

Thanks all,

Tony C

Tony, are you absolutely sure that this is a clutch problem and not the overdrive? Secondly, when you release the clutch pedal when the problem occurs does it have full retun load on it or does the pedal feel limp. If this is the case then there is a fair chance that the hydraulic flex line is acting like a check valve and needs replaced.
Iain MacKintosh

It may not be the clutch - I am stumped so willing to be proved wrong.
The overdrive is not currently wired to a live, but you are right it does feel a bit like being in overdirve, or when overdirve engages slowly (it's like this on my brothers tr4). Could it be somehow stuck in overdrive 3/4th?

Good point about the flex as well, the pedal did fell limp - it is normally quite a meaty pedal. The hose is one of the oldest parts on the car so may be a problem.

What am I looking for to test this, should the pedal return be very positive and with push to it? The pedal does fele like it has alot more wobble in it when you tab it with your toe - Ie, a lot of play before it seems to pickup any weight.

Eliminate the clutch first and foremost. If it feels strange then replace the flex and bleed the system and hopefully that will fix it. If not then is the time to look at the overdrive. Good luck.
Iain MacKintosh


I'd be interested if you sort this problem. My car (70 BGT) was on a 150 mile motorway trip at the weekend. When I pulled into the services I suffered almost exactly what you describe. It has happened before on my last long motorway haul. I assumed it was my dodgy clutch and changed it for a new one, but then the problem occured again this weekend. I normally do 20 miles each way to work every day, much of it at a reasonable speed, but I don't get the effect then. Only a long motorway haul at 80 or so seems to bring it on. There is quite a bit in the archives (need to be a member to see, but it's totally free) about OD slipping. I don't fully understand how OD works, but I believe this can effect a car whether the OD is engaged or not. I have just bought an OD box for 27 on eBay, so at some point I'll pull it apart so I can understand how it works. In the meantime I don't do many long trips so I'm just leaving it! But please let me know if you solve the problem as it will be a great help!

I D Cameron

Especially if it turns out to be hydraulics as this would be a far quicker easier fix!!

I D Cameron

Slipping overdrive? New one for me - I'll have a look in the archive - I'll have a go with the hose as well and poke around a few more things. I'm begining to think the clutch was not fully disengaging as even with the clutch depressed I caught a faint trace of hot clutch linings....

I've never actually run the car with overdrive yet, so I'm unsure of how much of a drop there is in third and 4th gear. DOes the car pull well from say 2500rpm in overdrive 3rd - or is it quite sluggish?.

It'll pull as well in 3rd OD at 2500rpm as it will in any other gear. In my gearbox 3rd OD is all but the same as 4th direct, so I only use it when accelerating hard (3rd, 3rd OD, 4th OD) or driving around twisty B roads (in and out 3rd OD rather than changing to 4th). Otherwise I only use 4th OD as a 5th gear.

In the archive a search for "slipping OD" with the "all these words" option selected pulls up plenty of threads on the matter.

If you are racing the car and don't need 4th OD could you not switch in a non-OD gearbox? I would have thought these would be fairly cheap as people don't really want them.

I D Cameron

Hmm, I think I have a mix of problems. I seem to have had an issue with the throttle cable as well: which may have explained the lack of go part way through the race. I found a few entries last night on the overdrvie and I've got it part dismantled at the moment to have a look - and I've been checking the hydraulics- only problem is that the car is not road legal so I won't know if it is ok until first practice on Saturday. Have to keep my fingers crossed :-(

It is the overdrive clutches that are at fault - Will need to get the box out to fix.

The lack of pwoer was due to the dizzy cap being damaged during the final laps of the race. A new one has fixed that fault.

managed to bag 5th at a wet oulton park race on saturday depsite the overdive box as well. :-)

This thread was discussed between 16/05/2005 and 22/05/2005

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