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MG MGB Technical - Trouble shifting gear box

My 1977 mgb roadster intermitently gives me problems shifting it into gear.The transmission is a 4 speed w/ LH type over drive,the fuid level is at proper level, the slave cylinder level is proper and has no leaks.My problem is when I start it in neutral and let it warm up a little and try to shift into 1st gear it won't go in,if I start it in gear with the clutch depressed it either jumps like its engaged or it won't let me shift it into gear (even after I've pumped up the clutch pedal).This does not happen all the time but its getting more freaquent.EXAMPLE: Last Sunday I let it warm up in the garage and had to shut it off,engage it in gear and drive off.When I went to shift into 3rd gear it would not go in. I returned home and let it sit for about 20 minutes.I tried again and it drove flawless the rest of the day.I do notice a slight whine I believe is from the throw out bearing. Any idea's on what the problem could be?
Thanks
Steve
S.E. Silvia

Clutch system, if it jumps forward when you start it in gear.

This can happen from at least two causes - one is that the hydraulics aren't pushing the release arm far enough, the other is that it is, and the release bearing is moving the cover plate diaphragm springs as it should, but the friction plate is still sticking to the cover plate and/or flywheel. Intermittent sticking to one or the other would be unusual (unlike having been parked up for months or years), unless it is jamming on the splines which is holding it against the flywheel enough to cause the baulking when you try to put it into gear. Another possible cause is the spigot bearing sticking with the same effect.

My V8 tends to do this, but as it is slight and occasional usually when I haven't used it for a while I'm leaving it be for the time being. If I can't get first I can usually get second, and if I can't get that I can usually get 3rd which is enough to get it rolling then I can select second. Trouble is if I need reverse! Yours sounds much worse than that if you couldn't get 3rd when rolling.

I tend to think it is the main seal in the master intermittently leaking back. Unlike the secondary seal or the slave seal this gives no visible leaks since it just leaks back into the reservoir. The only way to diagnose it further would be to rig up some kind of indicator in the cabin that will show how much the release arm is moving, like an old throttle etc. cable. On the occasions when it does seem to work, try sitting for a while in gear, engine running, clutch pedal down and see if the car starts creeping forward. If it does then that is definitely clutch master main seal.
Paul Hunt

Thank you very much your comments are very much appreciated.I am going to try to take a look by mid week. thanks for your help
steve
S.E. Silvia

I know, it's 3 weeks later, but you may not have solved your problem yet.

I had a similar thing happen years ago on a VW Golf. Turned out to be a slight oil leak onto the friction plate causing the plate to stick to the flywheel. Not enough of a leak to cause slipping.

Herb
H J Adler

Friend has just this problem after a long layup for a rebuild. Sometimes fine, sometimes baulks in a forward gear and grinds in reverse, tends to jerk the car forwards when starting it in gear with the clutch fully down. Funnily enough the latest issue of the MGOC mag has a comment on this happening with a new master as well as after a long layup where fluid might have drained out of the master, and recommends pumping the pedal up and down rapidly for as long as your legs (you have two, remember) can stand it, the theory being it will put some heat into it and soften the seal to give a better, well, seal.
Paul Hunt

This thread was discussed between 11/10/2009 and 04/11/2009

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