Question About '78 CB750K Fuel Tank

MagFlux

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Hey all,

Been searching for a while and I simply can't find any documentation or labeling about something I'm looking to identify on my fuel tank and was hoping you all could help me out.

Long story short, I recently bought a '75 CB750F which has a '78 CB750K fuel tank mated to it. Everything seems to run great on it but I noticed on the underside of the tank it has a second nipple (other than the petcock) which has nothing connected to it.

Looks like the fuel tanks for the F and K series bikes up to '78 only had one nipple (for the petcock), but for the '78 year there was another nipple added. The official parts list diagrams and shop manual make no mention nor label what this is, or where it's hose connects to (as far as I can tell).

I'm including some photos I've found online with the nipple I'm talking about circled.

Any help shedding some light on what this is would be greatly appreciated!

Regards,
Mag
 

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madmtnmotors

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I believe that is used with all models that have the fuel "door" on top of the tank, i.e. all CB750F 75 thru 78, 77/78 K, and 76 thru 78 A models. The fuel door creates a well where the fuel cap is and this provides a drain to redirect any fuel spillage into the fuel cap well and out the bottom. The drain tube typically runs to the back of the tank and kinda follows the battery overflow/drain tube down towards the center stand to keep the excess fuel away from the top of a hot engine.
 

MagFlux

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I believe that is used with all models that have the fuel "door" on top of the tank, i.e. all CB750F 75 thru 78, 77/78 K, and 76 thru 78 A models. The fuel door creates a well where the fuel cap is and this provides a drain to redirect any fuel spillage into the fuel cap well and out the bottom. The drain tube typically runs to the back of the tank and kinda follows the battery overflow/drain tube down towards the center stand to keep the excess fuel away from the top of a hot engine.
Ah, thanks so much for that!

Come to think of it I read that somewhere but I kind of didn't believe it cause it seemed like a lot of "over-engineering" to me and the context wasn't clear in what exactly was being talked about.

I guess I can test this out by just pouring some water or something in that well area (obviously not so much as to possibly get near the opening/cap) and see if it drains out that hole.

Thanks again, appreciate it.
 

moby1kanobi

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Ah, thanks so much for that!

Come to think of it I read that somewhere but I kind of didn't believe it cause it seemed like a lot of "over-engineering" to me and the context wasn't clear in what exactly was being talked about.

I guess I can test this out by just pouring some water or something in that well area (obviously not so much as to possibly get near the opening/cap) and see if it drains out that hole.

Thanks again, appreciate it.
Yes Madmtnmotors is correct. I just learned this myself on my tank refurbishing. When I rinsed etc it all drained into the hole near vent and poured out of the tube attached underneath.
 

madmtnmotors

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Ah, thanks so much for that!

Come to think of it I read that somewhere but I kind of didn't believe it cause it seemed like a lot of "over-engineering" to me and the context wasn't clear in what exactly was being talked about.

I guess I can test this out by just pouring some water or something in that well area (obviously not so much as to possibly get near the opening/cap) and see if it drains out that hole.

Thanks again, appreciate it.

If you remove the hose you can shine a light from one side and look through the other side. It is a straight-thru pipe. :cool:
 
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