QWhen I engage forward drive first thing in the morning, the gearbox works perfectly well but after about an hour it does not engage or is intermittent Reverse works fine. I’ve checked, oil level, cables and selector movement they seem good, it has the clunk that seems to affect these gearboxes; can you help please? MIKE HASTE
ATONY REPLIES: This appears to be a mechanical gearbox using cone clutches with a degree of self servo engagement. That in itself will tend to explain the clunk on take up and I would not worry over much about it. My own mechanical box of similar design has been doing for at least 20 years without any problems.
Your symptoms are fairly typical of a worn ahead clutch cone. When cold the female cone will have contracted so it can still grip, when it gets hot the female cone expands so may not grip. There may also be wear in the self servo arrangement. Whatever the fault the box will probably need stripping for inspection. However just to be sure try operating the little lever on the box by hand with no cable connected. If the box works properly when hot it indicates a maladjusted cable that has insufficient movement. This could be because one of the outer cable anchors has worked loose or the trunnion in the control or the gearbox lever is worn or in the wrong hole.
Also make sure you are using the correct oil in the gearbox. This manual says use ATF (red automatic transmission fluid often sold as Dexron).
Do not use engine oil in it. If the oil is not red then drain the box and refill with ATF.