Topic: is it not ok to run without carb bowl overflow tubes?

[lostmykeys]

 I just read an intresting thread where a forum member of this sight was losing power at  50 mph in 5th gear.
 Turned out that his overflow tubes were routed wrong causing some kind of vacume problem at higher speeds.
 Im asking because I removed mine as I thought they were just to keep gas from dripping on the engine case.
 I am trying to figure out what is going on with my bike because at wot in the higher gears it cuts out at about 80 to 90 mph.when I let off the throtle for a second it runs fine again.

[Bodi]

I've never had an issue with this, I don't have tubes either and just accept that gas dripping on a hot engine is a bad idea.
What you describe is more likely to be either fuel flow restriction or the tank vent is blocked.. Try staying on the throttle and opening the gas cap a crack when this happens (not with a full tank that may spray fuel all over you from the wind)?
If you have an inline fuel filter, check for clogs or an airlock in the fuel line. These can let fuel through fast enough for leisurely riding but not for maintained high speed work. If not, check the in-tank filter sock for clogs.

[Duke McDukiedook]

I don't see how the routing of the overflow tubes could affect fuel flow. Look elsewhere for problems in the fuel supply system.

[lucky]

I just read an intresting thread where a forum member of this sight was losing power at  50 mph in 5th gear.
 Turned out that his overflow tubes were routed wrong causing some kind of vacume problem at higher speeds.
 Im asking because I removed mine as I thought they were just to keep gas from dripping on the engine case.
 I am trying to figure out what is going on with my bike because at wot in the higher gears it cuts out at about 80 to 90 mph.when I let off the throtle for a second it runs fine again.

Some people make up stories to justify a bad running motorcycle. They are just guessing.
It won't matter as long as they do not leak.
The problem is not just that gas can get on the engine. The problem is that if there is enough gas and under some conditions it could catch fire.

Just put some drain hoses on those drains.
Don't be foolish.

[Deltarider]

Some people make up stories to justify a bad running motorcycle. They are just guessing.
It won't matter as long as they do not leak.
The problem is not just that gas can get on the engine. The problem is that if there is enough gas and under some conditions it could catch fire.

Not so, some 500/550 K3 owners had trouble with the vent tubes, because that's what they are in the first place. If the vent tube is blocked, it's more difficult for the gasoline to fill that bowl.

Just put some drain hoses on those drains.
Don't be foolish.

Although my vent tubes are still hanging there and waiting to be attached, out of laziness the last 10 years or so I didn't. I've spilled gasoline on a hot engine on a few occasions, but the engine for some reason never caught fire. But you're right maybe and I should attach them, to be on the safe side.

[flybox1]

clean your carbs. keep your drain tubes
(auto windshield wiper hose is cheap  )

[TwoTired]

I just read an intresting thread where a forum member of this sight was losing power at  50 mph in 5th gear.
 Turned out that his overflow tubes were routed wrong causing some kind of vacume problem at higher speeds.

The thread I remember is where the tubes were picking up water in the rain and putting it in the carb bowls.  A reroute of the tubes cured his rain cut out issues.

The other one I recall is where the overflow tubes/ vent tubes were routed to the air box.  The tubes are to vent to outside atmospheric pressure, and the air box is NOT that.

[lucky]

The purpose of the drain tubes is so that if the float bowl overflows for an reason the gas is drained away from the hot engine.

Float bowl vents are a different hose. Not the same thing.

[TwoTired]

The purpose of the drain tubes is so that if the float bowl overflows for an reason the gas is drained away from the hot engine.

Float bowl vents are a different hose. Not the same thing.

They outlet into the exact same space inside the carbs.
Some of the Cb750 PD carbs have no dedicated bowl vent or tube at all and rely on the overflow tubes for the atmospheric pressure source.

[bollingball]

The purpose of the drain tubes is so that if the float bowl overflows for an reason the gas is drained away from the hot engine.

Float bowl vents are a different hose. Not the same thing.

They outlet into the exact same space inside the carbs.
Some of the Cb750 PD carbs have no dedicated bowl vent or tube at all and rely on the overflow tubes for the atmospheric pressure source.

TT Don't get lucky started on PD carbs he does not like them because he can not get them to run correctly with pods. 

[lucky]

The purpose of the drain tubes is so that if the float bowl overflows for an reason the gas is drained away from the hot engine.

Float bowl vents are a different hose. Not the same thing.

They outlet into the exact same space inside the carbs.
Some of the Cb750 PD carbs have no dedicated bowl vent or tube at all and rely on the overflow tubes for the atmospheric pressure source.

I have a 1978 with PD carbs and it has a dedicated float bowl vent tube.

[TwoTired]

The purpose of the drain tubes is so that if the float bowl overflows for an reason the gas is drained away from the hot engine.

Float bowl vents are a different hose. Not the same thing.

They outlet into the exact same space inside the carbs.
Some of the Cb750 PD carbs have no dedicated bowl vent or tube at all and rely on the overflow tubes for the atmospheric pressure source.

I have a 1978 with PD carbs and it has a dedicated float bowl vent tube.

I didn't say all. I said some.  Pics were posted on this forum.  I hadn't seen that in person before.  But, there it was, and there is no reason why it can't work, provided ALL the carbs aren't overflowing at the same time.

[750K]

My 77 with PD's doesn't have vent nipples, or at least I can't find the damn things. Maybe I'm blind but I can seem to find them, I was trying to track down a wet weather stumble. Interesting to know some weren't equipped with them.

[lucky]

My 77 with PD's doesn't have vent nipples, or at least I can't find the damn things. Maybe I'm blind but I can seem to find them, I was trying to track down a wet weather stumble. Interesting to know some weren't equipped with them.

The vent hose is right next to the fuel line filling up the float bowls.
If front of it actually.

[lostmykeys]

Lucky,
 This thread was talking about the hoses that come off the bottom of the carb bowls not the one hose next to the fuel inlet.He also had an older bike so no pd carbs.
 Out of curiosity I went out to the garage and pulled a bowl off another set of carbs and blew through the drain and air does flow both ways with the drain srcew closed.The air came out of the brass pipe that comes from the bottom of the bowl that sits between the jets when the carbs are assembled.

[750K]

My 77 with PD's doesn't have vent nipples, or at least I can't find the damn things. Maybe I'm blind but I can seem to find them, I was trying to track down a wet weather stumble. Interesting to know some weren't equipped with them.

The vent hose is right next to the fuel line filling up the float bowls.
If front of it actually.

Lucky,

I'll have to have another look, I've had the tank off a few times in the last month dialing in the carbs and setting the valves. I have yet to find the vent tube anywhere and trust me I looking hard for it. Haha, if it's not there I'll post some pics.

[Don R]

If you want more high speed fuel in the bowls, route the tubes into the bottom of the air box. the vacuum in the air box will cause the bowls to flow slightly more fuel at high speeds. It's in the Honda bible. Makes sense that also the turbulence caused by venting the carbs into an area of mixed up air might possibly cause an issue with fuel filling such as you have reported. I can't say for sure the air is smoother under the bike but behind the engine it is buffeting.

[TwoTired]

If you want more high speed fuel in the bowls, route the tubes into the bottom of the air box. the vacuum in the air box will cause the bowls to flow slightly more fuel at high speeds. It's in the Honda bible.

Wait a minute.  Are you advocating applying vacuum to the carb bowl chambers?

Have you considered what happens to the fuel jet flow if you apply vacuum to both ends of the carb's internal jet tubes?

[Deltarider]

Not so, some 500/550 K3 owners had trouble with the vent tubes, because that's what they are in the first place. If the vent tube is blocked, it's more difficult for the gasoline to fill that bowl.

I stand corrected. Excuse me, must have been tired and mixed up drain tubes and vent tubes.
There have been reports on K3 models with their vent tubes in heavy rain/on wet roads. Cutting them by 2 inches seemed to have cured this. All vent tubes must be free of course to make it easy for gasoline to fill the bowls. The drain tubes are another thing. I have them unattached. This is because after winter sometimes in spring a bowl will overflow and this way you can identify immediately which bowl it is. So far I have not been punished for my laziness, but I don't recommend to follow my conduct. Better be on the safe side and now that I think of it, I'll bring them on again.

[bollingball]

Delta on my 78 the bracket by the swingarm will keep the hoses in line just like the carbs so if the hose on the left were to leak I know it goes to the #1 carb. Maybe yours will or you can tweak it.

Ken

[Duanob]

If I do any carb work I like to leave the hoses off until I'm sure none of the over flows are leaking. If they do I know exactly which carb it is immediately. Once I'm sure all the carbs are working properly then I attach the vent tubes.

[mrrch]

PD's for the 77 model do not have the upward facing vent nipples, 78 model PD's do.

[Spanner 1]

The specific problem the member ( Johnie ) had was a flat spot @ 50 mph..... not bad running above 50mph. The cure was to re-arrange the 4 drain/vent tubes and , I think, cut the ends at an angle 
P.M. Johnie, he can refresh our memories on this question.

[lostmykeys]

thats the right thread spanner,thanks for finding it.

[Deltarider]

Delta on my 78 the bracket by the swingarm will keep the hoses in line just like the carbs so if the hose on the left were to leak I know it goes to the #1 carb. Maybe yours will or you can tweak it.

Good tip. I'll check if I have the same arrangement on my '76 K2.