Get to 30mph, put it in fifth and put your foot down a little. If the engine noise skips about , it be slipping!
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mine used to succesfully do that. cruise along at 40-50 in 5th, dip clutch, rev the knackers of the car, dump the clutch. If the engine just keeps racing the clutch is shagged. If the engine slows gradually and then clutch bites its fine.
You'll know when it's a problem. Because it will gradually get worse. With my 1.3 I realised when I did racing starts and on changing to 2nd the engine note seemed to rise more quickly than my speed.
The other easier and less damaging way is to put the handbrake on put it in first and try adn drive off. if the engine dies, your okay. If the revs increase(and your not spinning your wheels!) its slipping!!
There are two things no man will admit he cannot do well: drive and make love.Stirling Moss.
so 40mph? dip the clutch....rev the engine....let clutch out quickly.....if it does anything else but bite its shagged?
Or...Find a steep hill and put it in a high gear like 4th at 25mph and then floor it.
If the clutch is fine the engine will try to rev and won't be able to and strain, if its slipping the revs will rise.
Needs to be a steeeep hill though, one that you could never climb in 4th gear.