The chain is broken off the bike We bought a new one but need a diagram of how to weave the chain through Thanks

I suggest you google for fitting a new bike chain and you will find loads of instructions and videos though having another bike to copy might be the best way to success.

It is not the chain routing you will find difficult I speculate, but finding the right length of chain to fit. The chain you have bought will probably need a few links removing and then once fitted the ends will need rivetting together securely.
You will find that a simple matter when you have seen it done. Adjusting a chain length by adding or removing links is difficult without a chain link extractor tool. Such a tool is inexpensive and an essential addition to the tool box of anyone concerned with chain driven vehicles or equipment.

Thicker chains have the benefit of being able to fit split links and half links but even if they are available they should not be used with a derailleur gear.
Good luck!

Replace the chain

Why don't you just take off the old chain and take it with you to the bike store, since you have to return your new chain anyway and buy a new one. You don't even need to know the size, because they will match the old chain with the new chain. No guesswork required.

very likely that chain links not "fitting" cog teeth . if both chain and cogs are brand new, then skipping should never happen. more often than not, a repair would require both new chain and new rear cogs, and possible new fron t chain wheel. all depends which item are worn. I would probably get a new chain firstly, then see if this fixed the problem. nigel

http://www.parktool.com/blog/repair-help/chain-installation-derailleur-bikes

Or it could be the dreaded mismatch of new chain with worn drivetrain parts...

http://sheldonbrown.com/chains.html

if the bike has gears it does not have a master link, that is for single speed bikes only.

You will need an 8spd chain for your bike (1/2" x 3/32), these will fit any 6,7 or 8 spd bike. This requires a special tool to install ($8-$25), I would recommend taking into a bike shop and have them do it if you have no experience with this type of chain.

Side note: if you do own the tool, you can repair a broken chain for a temporary fix in the future, so may be worth buying. Check around, many bike shops offer basic repair classes and cover this.

Both the 1985 honda xr250r and xr350 look loke they have self adjusting cam chain tensioners. You may have a broken tensioner or tensioner guide. With the age of the bike you may need a new cam chain. There is nothing to adjust so you need to just figure out if the chain is stretched , if a guide is broken, or if the tensioning mechanism is malfunctioning.

Below are generic things to look for if getting a used bike.

Does the engine start easy and idle smoothly? Does it accelerate without hesitation?
Does the clutch engage and disengage smoothly?
Is the clutch lever easy to pull?
Any oil or coolant fluids leaking after a ride?
Do all gages work? Lights, horn and flashers?
Any red lights on the gages lit up? Any computer codes flashing?
Do front and back brakes operate and stop smoothly?
Is the motor oil pretty clean or really dirty? (Sign of how well the owner has cared for the bike).
Is the oil "white-ish"? (water in the oil. If motor is water cooled it's usually bad water pump seals).
Any funny sounds from the gear box? Anywhere else?
Does it shift smoothly up and down the full gear range? Neutral light come on properly?
Sprockets still have rounded tips not pointed? Well lubricated chain?
How much adjustment is left in the chain?
Good tires? How old is the battery? All lights work properly?
Is the upholstery good? Plastics cracked or broken?
Gas tank badly rusted inside? Do the wheels run true or weave?
Do the forks look bent from a side view or front view?

I found an old used bike and salvaged the parts. So do not need any more help. Thanks.

Forget new guide wheels. A new rear derailer is the real fix. You will need to order a new one by the number of gears on the rear gear cluster. How the derailer is mounted, Which way it moves when by the spring direction etc... By the way, get a higher quality one than the one you have got. If you go megawheel on the lower guide wheel; which I recommend, you might need a couple more segments in your chain. Yes, you need a new chain. If you wore out a part on you derailer, your chain is getting old. I recently bought a new rear derailure for my wife's bike for 29 bucks at a much better quality than the old one. It does not have problems of getting between gears, clicking etc... like the old one did new.