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Hi Folks, new member here. I know that this forum is for the much cooler "earlier" GS bikes not for the early 2000 GS500 I have but I'm hoping that you can help me with a carb problem I have. Bike is stock with stock pipes and airbox, clean filter, stock jets and needle, etc.
I’m a relaxed driver and hardly ever use WOT, however recently I had to drop a gear and go WOT to overtake a particularly annoying driver who was on their phone. However, as I got alongside the bike stumbled a few times accelerating between 6 and 7 k RPM so I had to abandon the overtake. Got quite a fright.
I have since managed to do this repeatable in testing afterwards on a quiet uphill back road. I can be in any gear and one can feel power drop out a few times when accelerating through 6-7k RPM. Stumble is almost non-existent if I close the throttle a little from WOT. Stumble is worse if under load (going uphill). Bike also seems to have more power if I close the throttle a little from WOT at various RPMs. Bike pulls strongly from 7 k rpm up to redline.
I understand the CV theory quite well but not how it plays out in practice. I am trying to understand two clues to decide to go leaner or richer (or something else) to try to attack this ( I don’t think it’s ignition as I can accelerate through that some RPM range no problem if the throttle is closed a bit from WOT). I understand that at the mid-RPM range where this problem is occuring I will be on the needle/needle jet range no matter where the throttle is, so the problem is probably their.
Clue 1: Closing throttle from WOT gives more power
I’m struggling to picture exactly what happens in the carb when this occurs - is the mixture;
- Getting richer due to the butterfly restricting the air a little or
- Getting leaner due to the butterfly restriction causing the pressure to drop and dropping the needle, closing the needle valve or
- None of the above
Clue 2: Stumble is much worse if I restrict the air intake a little
I placed some tape over the air filter intake at the airbox (covering maybe 5 to 10 % of the intake area) and in a test the flat spot had grown to cover almost a three times larger range (from about 5k up to 8 k RPM). I thought (thinking along my experience with “regular” slide carbs) that this meant that the "enhanced" flat spot was from being too rich as restricting the air means more gas in the mixture.
However, thinking in CV carb terms It might be that the restriction = less air and hence less air velocity at the slide which leads to lower vacuum at the venturi (Bernoulli). This, in turn, leads to lower slide/needle and hence less gas (leaner).
Currently, I'm working on the hypothesis that it's running too rich and will try to drop the needle to see if that helps.
Some research has indicated that another possibility is that the slide/diaphragm spring has gotten weak (can that happen?) and so the slide height for any given vacuum is too high? Also there is talk of opening or reducing the size of the vacuum channel in the slide to improve response but I don’t think this is relevant as I understand that this improves how fast the slide gets to equilibrium height, but does not affect the height achieved at that point, so I don’t think this is relevant.
Any advice is greatly appreciated (apologies for the long first post!)
Thanks
BMF
I’m a relaxed driver and hardly ever use WOT, however recently I had to drop a gear and go WOT to overtake a particularly annoying driver who was on their phone. However, as I got alongside the bike stumbled a few times accelerating between 6 and 7 k RPM so I had to abandon the overtake. Got quite a fright.
I have since managed to do this repeatable in testing afterwards on a quiet uphill back road. I can be in any gear and one can feel power drop out a few times when accelerating through 6-7k RPM. Stumble is almost non-existent if I close the throttle a little from WOT. Stumble is worse if under load (going uphill). Bike also seems to have more power if I close the throttle a little from WOT at various RPMs. Bike pulls strongly from 7 k rpm up to redline.
I understand the CV theory quite well but not how it plays out in practice. I am trying to understand two clues to decide to go leaner or richer (or something else) to try to attack this ( I don’t think it’s ignition as I can accelerate through that some RPM range no problem if the throttle is closed a bit from WOT). I understand that at the mid-RPM range where this problem is occuring I will be on the needle/needle jet range no matter where the throttle is, so the problem is probably their.
Clue 1: Closing throttle from WOT gives more power
I’m struggling to picture exactly what happens in the carb when this occurs - is the mixture;
- Getting richer due to the butterfly restricting the air a little or
- Getting leaner due to the butterfly restriction causing the pressure to drop and dropping the needle, closing the needle valve or
- None of the above
Clue 2: Stumble is much worse if I restrict the air intake a little
I placed some tape over the air filter intake at the airbox (covering maybe 5 to 10 % of the intake area) and in a test the flat spot had grown to cover almost a three times larger range (from about 5k up to 8 k RPM). I thought (thinking along my experience with “regular” slide carbs) that this meant that the "enhanced" flat spot was from being too rich as restricting the air means more gas in the mixture.
However, thinking in CV carb terms It might be that the restriction = less air and hence less air velocity at the slide which leads to lower vacuum at the venturi (Bernoulli). This, in turn, leads to lower slide/needle and hence less gas (leaner).
Currently, I'm working on the hypothesis that it's running too rich and will try to drop the needle to see if that helps.
Some research has indicated that another possibility is that the slide/diaphragm spring has gotten weak (can that happen?) and so the slide height for any given vacuum is too high? Also there is talk of opening or reducing the size of the vacuum channel in the slide to improve response but I don’t think this is relevant as I understand that this improves how fast the slide gets to equilibrium height, but does not affect the height achieved at that point, so I don’t think this is relevant.
Any advice is greatly appreciated (apologies for the long first post!)
Thanks
BMF
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