Well, not from first hand, but a friend of mine had an 1VJ:

- Overheating: there's two little 'hoses' just above the cilinder head that the intake air passes through. Above the hot head. Modification: plug them and remove the cover through which you remove the air filter for service. Throw away the cover. The engine will get cooler air, it will be a bit noisier as the intake sound is less dampened.
- Cil head bolts threads: they have been designed too short, which leads, in some cases, to stripping. Helicoling is possible, or install a second hand 3AJ cilinder head. BTW, the 34K designation is explained as follows. Yam part numbers start with the model type that the part was first used on. You will notice for instance on the rear rim the designation 5Y1, which is the XT550 model number.
- Slits in the front mudguard: if yours does not have any, change the 'guard or make them in it. In Germany I believe the slits are forbidden for road bikes, that's where the story comes from.
- Oil pump. There's 3 models, 100%, 150% and 200% capacity. The 150 was used used (I think) on the 1VJ. You can use the 200% from the 3AJ, which will improve lubrication. Also, 20W50 for temperatures down to 0 degree celsius is good.
- Fifth gear: yes, all XT's have that problem. The bangs from the one-cil engine are just too much for the gears. Pitting and eventually breaking in pieces will occur. To avoid, never run it below 4500 rpm. The engine has been made for higher revs. It's noisier, that's true, but a well-tuned XT driven in that range is a lot of fun.
If your gears are pitted, replace them for the modified gears from the 3AJ. It is not the entire solution but will make them last longer. The only way to inspect is to take the engine apart.

Succes!
Auke

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Auke
http://aukemarieke.blogspot.com