How tell if gear dogs are worn?

Howdy everyone. Been a lurker for a while, now a question asker (will try to be a responder in future too). Got an old IT490J ('83) with third gear prob, seeking advice over. Stick it in third, and engine can rev freely but rear wheel locks. Have split cases - two shift forks obviously worn and being replaced. But wonder if actual cause is something else. How would the above symptoms occur? Presume something to do with wear on gear dogs (but would that be only if slipping out of third?). So, advice sought on what actual cause might be. Then, if it is dogs, presume should replace third gear on countershaft plus the cog it joins to to hook up the power?
If not dogs, perhaps worn dowel pins not moving third gear far enough away from previous gear and gets stuck in between or something?

Thanks in advance,

Gerard

WaltCMoto - thanx for your advice. Sounds like must be dog problem. Tell me - do you replace the third gear plus the gear it mates with on the countershaft (as opposed to the gear the actual teeth on the countershaft third gear mesh with on the mainshaft? As the engine can rev freely when this prob occurs, me things the problem gear/s are on the countershaft (which locks the back wheel). I'm guessing go with the countershaft gears (third plus the one it mates with to connect power to the back wheel).

Gerard

WaltCMoto said:

I replaced the 2 gears (with the dogs) and 2 forks that were shiny, but bearly bent. $300 later and some late nights putting her back together and that solved my 3rd gear problem.

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