This weekend on my way home from a 300 mile ride, I had the annoying crosswind engine stumble that mimics fuel starvation.  When the wind starts blowing, the engine stumbles and hesitates.  When coming to a stop at idle, it can sometimes die completely.  This happens notably at highway speeds (60-70MPH+)

I did an extensive search, and have found that this could be due to atmospheric pressure changes in the carb vent tubes not allowing fuel to flow properly.  Only happens on windy days.  Calm days, does not happen and the bike is perfect.

How to I prevent this from happening?  I do have a lunchbox filter therefore changing the vent tube routing slightly.  I've studied the carb hose routing diagrams for a while, and this is frustrating.  Has this happened to anyone else?   

Thoughts?

I would say that it's more likely negative pressure on the gas cap vent than positive pressure in the carb vent.  With air moving quickly past the vent, the pressure on the fast moving side is lower than the non-moving side.  Clean out your gas cap vent and see if it that helps the problem.   The only way I could see positive pressure occurring in the carb vent is if it facing directly into the wind.  Maybe try taping it down on the back side of the lunchbox filter facing downward?  This should prevent any positive pressure effect.

This is all just speculation, but those are the two points in the fuel delivery system that I could see being affected by wind.

I wasn't thinking fuel vent initially.  I stopped several times, and opened up the gas cap listening for the "woosh" sound.  Got none.  Gas was full.  I'll clear out the tube with compressed air tonight.  May also try and route the other vent tubes differently and see if that helps.

I'm no expert though...

If the gas was just filled up, this would lend more to the gas cap vent theory in my mind.  There is less volume of air that needs to be sucked out to achieve a low air pressure in the tank.  Maybe a fluid dynamics engineer can correct me on that, but it seems to make sense in my mind.  There may not be enough volume left in a full tank of gas to fill with air and get much of a whoosh sound when the cap is opened.  Also with a high level of gas, you would expect more fuel pressure in a steady state at the needle valves.  So it would take more positive pressure at the carb vent side to resist the carb bowl filling.

Quote from: jestercinti on June 18, 2012, 01:19:27 PM
May also try and route the other vent tubes differently and see if that helps.

Which other vent tubes are you talking about routing?

Most likely it's the sideways blast of air into your lunchbox filter. It would take a lot of vacuum on your carb vents to make the motor stumble.

Quote from: Big Rich on June 18, 2012, 02:00:49 PM
Most likely it's the sideways blast of air into your lunchbox filter. It would take a lot of vacuum on your carb vents to make the motor stumble.

I didn't think about that.  I suppose that could impact air fuel mixture.

I have 3 hoses.

1: gas tank vent.
2: battery vent from Yuasa OEM battery.
3: carb vent from 'T'

Stock air box gone so that one is not there.

Did not think of the lunchbox air condition. Hmmmm.

I have the F model by the way with all plastics if that helps.

Battery vent will have zero effect.  So will the one from the gas tank.  That tube is actually a drain for fuel overflow so that you don't dump gas on a hot engine.

The gas tank vent is really small and it's integrated into the cap.  There is a thread on how to clean that out floating around here somewhere.

Copied from a post of mine dated about 2 years ago, might be of some interest to you:


A letter in the service section of the Jan issue of CW mag said:

...............I own a 2000 Suzuki GS500...............Most of the time it runs flawlessly, but it's had a quirk...................When I encounter exceptionally windy conditions, the engine often stumbles and sputters as if starving for fuel................The moment the gust lets up, the engine immediately settles back down and runs fine.........................The bike is absolutely stock........

To which CW gave a long winded answer about being too lean and rejetting, etc, etc...........

Follow up in latest June issue of CW:

In the ...Jan issue, TA described a problem with his GS500........I've had this same problem with two of my bikes........82 GS850GLZ........84 CB700SC.  I resolved the problem on both bikes by rerouting the carb float bowl vent tubes to keep them out of the wind..............

There you go!  Just run that carb vent behind the battery box and make sure it stays there somehow.

I'll try rerouting the hose behind the battery box. I may need a longer one, but no worries.  I have a few inline couplings and some extension hose.

Problem solved.  Re-routed hose, and all fixed.  There were some crazy crosswinds the other day, and nothing!

Just be sure that the hose has slack and does not kink.  Otherwise you may be late for work and have to explain to the boss who knows nothing about engines/bikes/carbs why you were late to the staff meeting.  Ask me how I know this.   

Good work!  Did you route it so that it terminates just behind the battery box?

Quote from: bombsquad83 on June 20, 2012, 12:28:43 PM
Good work!  Did you route it so that it terminates just behind the battery box?

Yes.  Just make sure that there is not a kink.  Makes sense why, but I was racking my brain on the highway for 10 minutes when my engine would not run! 

I had this happen, read the link GSJack had on another post, and made sure the vent hose was routed so it wouldn't catch wind. I haven't noticed any issues after that. I had this happen on another bike, also. I would lose power when it was very windy. Nothing I could do about that one, though.