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Hi People,

Motorcycle is a 1962 CB250. A very nice example that has had a full restoration including motor rebuild so pretty much the works. Although it is not mine I may be purchasing in the near future but for the moment I have been tasked with sorting out the fuelling issues. I will do almost anything on a bike maintenance wise but only really worked on fuel injection bikes.

First issue - the fuel petcock only supplies fuel to the carburettor switched to reserve. There is no flow in the ON position and the tank is full.
Secondly - the bike only really runs with the choke on so I am guessing the carbs are bunged up and I need to clear the jets etc. Also maybe a vacuum leak between the carb and cylinder head.

Carb or Carbs?? Manual says one but looks like two so will have a look tonight when home. Can anyone recommend a good cleaning solvent and or method? Actually any advice pertaining to the model fuel system. I think it is a model C77? Picture for reference of the bike.
700687285[2].jpg
Cheers Nick
 

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Nice looking machine. It's a CB72, cousin of the CB77(305cc.). 22mm carbs. You can pull the petcock off the tank(two screws) and disassemble it for cleaning(two more screws).
 

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Discussion Starter #4
Nice looking machine. It's a CB72, cousin of the CB77(305cc.). 22mm carbs. You can pull the petcock off the tank(two screws) and disassemble it for cleaning(two more screws).
I read somewhere last night that the "ON" position requires vacuum to function which is the reason why I wouldn't see any fuel flow without the engine running but wasn't sure if it applied to this fuel tap.

Thanks
Nick
 

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No vacuum petcocks in 1962, if gas doesn't flow something is plugged.
 

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Discussion Starter #6 (Edited)
No vacuum petcocks in 1962, if gas doesn't flow something is plugged.
Sweet, thanks for that Mike. Be easier to remove the petcock if it did not have the connection tube to allow gas from the RH side to get to the carbs. I could remove the tank first but I guess I need to drain from there first after siphoning as much as I cam through the filler.

I have removed the LH carb and noted (attached pictures) that it appears there is a leak, which would explain the oily pink film covering the top of the crankcase cover under the carbs. The inside of the carburettor looks nice and clean. The nuts securing the carb to the cylinder were not tight but a just a little tighter than finger tight but not by much.

The other carb appears to be secured better. Would one loose carb cause the bike to need choke on to run?

Thanks for the help Mike. I do own two bikes, A 2007 Aprilia RSV1000 factory and a 2003 Ducati ST4s that I have had since new. Probably the best bike I have owned in my memory is a 1981 CB900 Bol'dor :). I do all my own maintenance on all the bikes I have owned.

For some reason I cannot put pictures on this post but the Carb showed fuel gum on the lower side of the mount flange. the spacer/seal was very wet with petrol on the lower side
 

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There is an issue with attached pictures being visible at the moment as the website goes through a transition to another server. The admins are aware of it and working on it.
 

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Unless the stand pipe inside the tank is plugged there really isn't any need to remove the petcock from he tank. Drain most of the fuel and tip the bike way over to the right and the lever/seal can be removed for inspection. There are three holes behind the rubber disc, one hole goes into the bottom to fill the sediment bowl, the other two go to the "on" and "reserve" openings on the top of the petcock assy. You could blow air or carb cleaner into those holes to clear out obstructions(loosen the gas cap first).
The rubber seal in the petcock is a fairly common part, used in numerous Honda models in the sixties and early seventies, should be relatively easy to source.
 

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Discussion Starter #9
Unless the stand pipe inside the tank is plugged there really isn't any need to remove the petcock from he tank. Drain most of the fuel and tip the bike way over to the right and the lever/seal can be removed for inspection. There are three holes behind the rubber disc, one hole goes into the bottom to fill the sediment bowl, the other two go to the "on" and "reserve" openings on the top of the petcock assy. You could blow air or carb cleaner into those holes to clear out obstructions(loosen the gas cap first).
The rubber seal in the petcock is a fairly common part, used in numerous Honda models in the sixties and early seventies, should be relatively easy to source.
Thanks Mike, It looks pretty obvious to me that there is an o ring missing between the carb and the spacer plate as there should be one that sits in the groove of the carb flange. What about the cylinder/spacer join. Is there anything supposed to be included to help the seal?
 

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On the slightly later CB160s there was a paper gasket that went between the head and the black spacer. I do not know about your model but if you can find a fiche on cmsnl.com it would probably confirm that idea
 

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The O-rings should be enough to make the seal(one in the carb and one in the spacer). Don't go overboard, tightening the mounting nuts, it's easy to warp the carb bodies by overtightening things. Enough pressure to lightly compress the O-rings is just right. There should be small diameter hoses, attached to brass fittings on the bottom of the flat bowls, that go down behind the rear motor mounts to keep gas stains off the engine.
 

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Discussion Starter #12
On the slightly later CB160s there was a paper gasket that went between the head and the black spacer. I do not know about your model but if you can find a fiche on cmsnl.com it would probably confirm that idea
Yeah looks like an O ring either side of the spacer. Thanks for the link.
 

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The O-rings should be enough to make the seal(one in the carb and one in the spacer). Don't go overboard, tightening the mounting nuts, it's easy to warp the carb bodies by overtightening things. Enough pressure to lightly compress the O-rings is just right. There should be small diameter hoses, attached to brass fittings on the bottom of the flat bowls, that go down behind the rear motor mounts to keep gas stains off the engine.
Yes the hoses are there Mike. The o ring in the spacer is that old it has a square section. The o ring for the carb was non existent which where all the fuel is leaking from and gathering the film on the motor. New o rings this morning, play with the pet cock and should be good to go. Now I need to find out how to balance the carbs. More google me thinks.

Thanks again.
 

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Discussion Starter #15
Ok so a bit of an update. Managed to get it all back together and upon firing the bike up, the LH cylinder was not firing. Traced that to a bad spark plug. new plugs and she fired up on one cylinder for a while, only the LH cylinder running...go figure. After a short while it clears itself and runs fine as long as you do not let sit for more than 3-4 hours.

Runs absolutely perfect apart from this but if you let it sit the RH cylinder does not fire until you have ridden down the road 500m. Any ideas?

Thanks!
Nick
 
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