Gearchange Problems
Problem Number 1 : Jumping out of 1st gear.
So why do you get this problem in the first place?
When 1st is engaged from a standstill that non-moving gear is effectively rammed into slots in the mating gear, hence the big 'clunk' when you do it.
Also, because these gears are actually 4th and 2nd gears, when changing gear while riding the dogs are rammed into slots on the moving mating gears.
This has to cause wear on the dogs which, as they wear, gradually forces the gear back, shift after shift, into the selector fork causing the wear pictured on the fork. Eventually the wear on both means that the gear engagement cannot take the torque, the dogs jump out and the shift lever gets bent.
You may still be able to get it to go into gear at a standstill but as soon as you apply torque it will not want to stay there.
One good reason to not to just softly put the transmission in to first gear. Don't let the gears grind going in, but still feed the gear in firmly. Try to leave the clutch disengaged for a couple seconds before engaging first gear to allow the rotating gears to slow or stop..
When you replace the gears (yes, you need to replace both mating gears!) make sure to check the hardened splined washers that locate the gears on both shafts as they can wear too.
The clutch adjustment is part of the problem. If the clutch has drag on it through poor adjustment that will make the problem worse, since the temptation is to boot the gears in hard.
Understand that due to the way the engine is constructed the whole engine, apart from the right hand head , needs to be torn down to fix this. This is not a 10 minute fix! Nor a cheap one!
Here is a microfisc showing the position of the these gears:
You need to replace #11 and #17 gears. Note that you are replacing 4th and 2nd gears. These are for the 75-77, the 78-79 are different.
23440-371-000
23471-371-000
You should also replace the thrust washer #19.
Also, you will need to carefull inspect the selector forks for wear, as above. Any sign of wear at all is a no-no.
Needless to say, all these parts are now obsoleted by Honda. So it is a matter of hunting down N.O.S. parts or good used ones. The later 78-79 models featured slightly heavier selector forks; worth looking for these if replacing.
Also, realise that it makes sense to go through the engine and renew all the oil seals, timing belts if they have not been changed recently along with their tensioner springs, possibly starter chain, check the water pump, oil pumps and the clutch plates too. Basically just check every wearable thing, as you really don't want to be doing it again!
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Here you can see how the trouble builds up. The gear on the right is just starting to show wear on the leading edge ot the dogs. The first and second picture show a gear that has worn so badly it would not stay in engagement.
The selector fork on the left has been severely worn by this gear being forced out of engagement.
There is no remedy for this other than replacement of the worn parts. The rest of the gear train will also have to be carefully inspected.
Badly worn dogs on this gear.
See how the dogs are rounded off