float bowl vent tubes?

[gearheadE30]

If you don't want to read the whole thing:
What is the purpose of the float bowl vent tubes, and do I need them?


The whole story:
I have K&N pods and a Kerker exhaust on a 1979 GS750. I've pretty much gotten the jetting hammered out and working with my setup, but I keep coming across conflicting information regarding whether or not to remove the float bowl vent tubes. Without the airbox there, there isn't really a place to clip them. I had them tied off to a frame tube, but eventually found out that that was causing the bike to stutter badly at highway speeds. Relocating them to the battery box using longer tubes worked wonders for that; the bike is nice and smooth now.

What others have been saying is that with the tubes in place, the bowls can't fill quickly enough at WOT. Is there any truth to this? The bike never feels quite perfect after a run up through the gears at WOT, 9k rev shifts, up to about 85 mph. When I back off, it feels a bit lean, though it may be placebo from not having as much accelerative force. Removing the tubes brings back a little bit of the hesitation.

[Guest]

Conventional wisdom says you don't need them if you are running pods...

I'm running K&N pods with a 4:1 on my '81 1100E and don't use them - no issues.

[GelandeStrasse]

Great! I was just about to post a question on fuel starvation and came across your thread.

I'm having similar sounding symptoms as you Gearhead, I think. Maybe!
Mine's a 1000cc, CV carbs with pods + dynojet stage 3 + exhaust. It runs fine and pulls well until 5k then it sputters like it's hit a rev limiter and won't rev any higher. Feels like it's not getting enough fuel. I've checked all the usual suspects without any success.
I have the breather pipes joined with a Y connector just behind the carbs then I ran the single pipe down over the back of the engine with the other vent pipes. I guess that's around 2ft of pipe AND they're joined.

Could my breather pipe routing or length as described be causing a fuel starvation type problem?

Be an easy fix if that's all it is!
Cheers

[1980GS1000E]

The vent tubes should exhaust facing towards the rear near the back of the gas tank. Too long, left to hang in the airstream or below float level can result in fuel starvation.

[Steve]

It's not so much that the bowls can't be filled while a WOT, as there is precious little air movement through those tubes. When the fuel level drops just a bit, the float valve opens and fuel enters. When all works as it should, there should be NO air movement through the vent tubes, because the fuel is replaced as it is being used.

The real purpose for the vent tubes is to provide a stable atmospheric reference. The air that is sped up going through the venturis has a lower pressure. Lower than what? Lower than the pressure of the atmosphere inside the fuel reservoir (the float bowl). That is what draws the gas through the jets.

In the stock configuration, the vent tubes go over the airbox, and end in a pocket of still air. When you remove the airbox and install pods, the air is more turbulent in the area where the vent tubes end. If you can extend them to a pocket of still air, like ... (pick one) under the seat, near the battery, under the tank ... they would work as normal. You can also remove the tubes, leaving just the "T"s. Because they are between the carbs, directly behind the engine, they tend to be in relatively calm air and will provide a stable-enough reference.

It really has nothing to do with how much air can move through the tubes, it's all in the stability of the air at the end.

.

[GelandeStrasse]

Doh! So hanging down under the engine in the airstream like mine are isn't ideal then eh!
Thanks for the explanation Steve

[gearheadE30]

That explains why moving mine to the battery box appeared to cure my issue. I think I have kind of a compound issue at this point with the pilot circuit being too lean, because it still has a stumble off of cruise ~10% throttle at 35 to 55 mph. I will probably put some 17.5 pilots in (factory is 15) and see how that goes, and then see if I can get away with removing those tubes. As I can tell a (negative) difference when I pull the tubes, I'll probably try to keep them. Thanks for the great explanation guys.

[Guest]

That explains why moving mine to the battery box appeared to cure my issue. I think I have kind of a compound issue at this point with the pilot circuit being too lean, because it still has a stumble off of cruise ~10% throttle at 35 to 55 mph. I will probably put some 17.5 pilots in (factory is 15) and see how that goes, and then see if I can get away with removing those tubes. As I can tell a (negative) difference when I pull the tubes, I'll probably try to keep them. Thanks for the great explanation guys.

Do you have a stage 3 jet kit installed?

[gearheadE30]

My GS is a '79 GS750, so no jet kits for me. needle in the 4th position, 122.5 mains, stock 15 pilots, 1 turn out on the fuel screws, high idle on the airs, etc etc. It runs great except for that little cruise leanness that I'm trying to get rid of.

[AJ]

It's not so much that the bowls can't be filled while a WOT, as there is precious little air movement through those tubes. When the fuel level drops just a bit, the float valve opens and fuel enters. When all works as it should, there should be NO air movement through the vent tubes, because the fuel is replaced as it is being used.

The real purpose for the vent tubes is to provide a stable atmospheric reference. The air that is sped up going through the venturis has a lower pressure. Lower than what? Lower than the pressure of the atmosphere inside the fuel reservoir (the float bowl). That is what draws the gas through the jets.

In the stock configuration, the vent tubes go over the airbox, and end in a pocket of still air. When you remove the airbox and install pods, the air is more turbulent in the area where the vent tubes end. If you can extend them to a pocket of still air, like ... (pick one) under the seat, near the battery, under the tank ... they would work as normal. You can also remove the tubes, leaving just the "T"s. Because they are between the carbs, directly behind the engine, they tend to be in relatively calm air and will provide a stable-enough reference.

It really has nothing to do with how much air can move through the tubes, it's all in the stability of the air at the end.

.

This is correct.

The tubes in the stock configuration "sample" the air pressure very near the inlet to the air box. Having the tube end in an area of turbulent air, will result in erratic performance. Removing the tubes has the same result, as the air is extremely turbulent at the carbs, directly behind the engine.

Tube length is not an issue. Virtually no air "flows" through the tubes. Also, the viscosity of air is comparatively low and the air pressure will adjust at more-or-less the speed of sound.