The International CBX Owners Association

Recently in a conversation on old bikes, I heard that the rubber overflow drain lines on CBX's can create a vacuum problem for the carbs at speed and should be removed. Comments?

Thanks

Great forum

What year is your CBX ? If it is a '79, getting rid of them is not a bad idea for other reasons (hydrolock).
Best,
Phil

It is an 80 and has 79 carbs.

Are you talking about the 6 tubes that run off the bottom of the float bowls?
Never heard anything about them causing a problem.
Maybe he was referring to the 2 air vent tubes that run off a T between carbs 2/3 and 4/5?
There are many threads about replacing them if required.

Welcome to the site.

Unfamiliar with the air vent tubes referred to. The lines I am talking about hook to brass nipples on the float bowls. These fittings face forward and the lines T into a larger 1/4 inch line behind and centered on the cylinder bank.

Never heard of this being an issue on CBXs, but have experienced it on an NT-650, aka, Hawk GT. Only in a strong cross wind.

BTW, welcome...

I believe that Mike Nixon has written on the '79 overflow tube set up. As long as everything is working perfectly, they are OK. However, lets say that you leave your
fuel valve on and have a sticky float valve while your bike is on the side stand , the 6 into 1 manifold for the drain lines is higher than the #1 float bowl and instead
of the overflow going out the drain line, it will fill the #1 cylinder with fuel. Ka-Pow, the perfect storm for hydro lock !! It is recommended to use drain lines from a later
model CBX, run individual lines, or just leave them off completely.

Very Best,
Phil

Thanks Phil! As an old brit bike owner, I always turn off the petcock and don't trust the sidestands on the CBX as they look kinda puny....

And here i thought i knew EVERYTHING there was to know about these brutes. haha.
Thanks for the info, Phil.

That was a money tip for sure Phil. I like the tidiness of the manifold, but hadn't considered this potential.

As Phil noted, Nixon has written about it extensively. Honda recognized the design flaw very early on - and in fact changed the design the very next year. I believe that Honda replaced quite a few engines (hole-in-one) under warranty and it got their attention. So, it was a problem from the beginning.

When I did my carbs, I left the drain manifold thingy off.

What Honda did to address the issue was including a vacuum valve between the petcock and carbs with the 1980 model. They also offered it as a retrofit for 79s with only one fuel outlet.

EMS wrote:What Honda did to address the issue was including a vacuum valve between the petcock and carbs with the 1980 model. They also offered it as a retrofit for 79s with only one fuel outlet.

You are soooo correct on all of that Mike (of course), and they also changed the configuration of the overflow drain lines.
Very Best,
Phil

FalldownPhil wrote:

EMS wrote:What Honda did to address the issue was including a vacuum valve between the petcock and carbs with the 1980 model. They also offered it as a retrofit for 79s with only one fuel outlet.

You are soooo correct on all of that Mike (of course), and they also changed the configuration of the overflow drain lines.
Very Best,
Phil

Yes, Phil, they did, but theoretically, you could still have an overflow from carbs 2 and 3 into carb 1 with the revised line configuration without the vacuum petcock. I believe the overflow line change had little to do with the hydrolock problem. It was more of a fuel management change that came about with the dual inlet. Only 6 separate drain lines would remedy the overflow issue into carb 1.

If you're happy, I'm happy
Regardless of 1 or 2 fuel inlets, all 6 bowls are full of fuel.
I submit that IMHO, all of these changes had to do with hydro lock.
Kind of a fail safe cure.
Phil