Topic: Airbox Tube and Gastank tube where do they go?

[spaceboy412]

1978 CB550K, okay so i need to know where this tube goes, does the other end connect to anything? its the tube coming out of the plenium between the carbs and airbox, i do not mean the rebreather tube coming from the valve cover. also where does the tube coming from the bottom of my gastank goto, i figure its an air intake, does it just wind down to where all the other tubes exit near the swingarm?

[merc2dogs]

not sure on the 78, but if it has the trap door type tank, it should be the spill tube, under the door you'll see a small hole where overflow is spposed to drain out, just routes under the bike by swingarm.
 
 on the plenum tube, no real idea.

 the 750A has a tube that comes from airbox to a junction and connects to a fast idle solenoid in front of the carbs. ( 'tween carbs and jugs)

Ken.

[TwoTired]

#17 on your diagram simply routes between trans and swingarm, as does the tank well drain tube.

Cheers,

[spaceboy412]

okay, so 17 goes out the back, is it capped or open at the end, it just seems weird to me to have another place for air to get in besides the air filter.

[TwoTired]

I agree it is "odd" to have that alternate air source.  But, that is the way Honda did it.  That is a a gas drain by the way.  If the petcock is left on and a float bowl valve sticks open, that Plenum box fills with gas.  Honda must have felt it was better to have it drain onto the ground rather than fill up the engine crankcase.

It is a pretty small hole compared to the carb inlets.  If that is of any comfort.

Cheers,

[Loas 78 CB750K]

I dont know about the 550 but the 750 has a hose that comes out of the top of the air box and runs to a breather valve. There are also tubes that run from the the bottom of the carbs to dump excess fuel, if the float bowl sticks, in front of the rear tire.

[m00ntan]

I don't get it.  It's unfiletered air.  Is ther a check valve?  If the carb overflows aren't blocked is there a reason I wouldn't cap this? 

CB550K1, same issue.

[TwoTired]

Yes, it is unfiltered air. No check valve.   But, the stock tube is a molded tube with reduced I.D., so the inlet is small and the tube runner length makes for very small air flows.

Block this off if you can guarantee that the carb overflows will never block or the drain tubes will never have mud wasps block the exits.

If you check your oil before every start to make sure the oil isn't gas contaminated, no worries.

If you aren't worried about sucking in a pool of gas and hydrolocking a cylinder, no worries.

It will probably work fine with that inlet blocked for a long time, and then one day...

Cheers,

[m00ntan]

Well thank you.  I'll add this to my "B" list of parts to replace.  I hope "one day" comes long after I sort out the "A" problems.