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 Post subject: The BST40 Surging Dance
PostPosted: Mon Oct 09, 2017 4:08 pm 
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I'm posting this just to vent (no pun intended)! I know what the solution is but just for $hits and giggles….

Aside from replacing the BST40 with the precious pumper carb, is there truly a solution to the BST40’s incessant (or inherent) sensitivity to winds?

I think I’ve reached the “pinnacle” of BST40 surge protection. I’ve eliminated 100% of the surging from my BST at ALL RPM’s except when I’m moving between 75-80 mph. I have a ½” heater hose running from the tip of the vacuum slide filter vent, to the very back of the bike, with the end of the hose terminating right below the seat (behind the regulator/rectifier and covered by the side plate. It’s about as protected as it can be back there, and still effectively vent, yet I get the slight pulsing at 75-80 mph when I’m riding against a headwind! Below that speed it’s sweet. Above that speed, no problem. Calm conditions, no surging. But when there is blustery conditions, the pulsing remains. I guess I’m at that point where I’ve run out of options except to upgrade.

Disclaimer: I have an opened up airbox, top cut/side door removed, jetting just about perfect, 50 mpg average fuel efficiency.


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PostPosted: Mon Oct 09, 2017 4:12 pm 
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Tried reinstalling the side door, and rejetting accordingly?

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PostPosted: Tue Oct 10, 2017 10:03 am 
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The unfortunate thing about the BST is the slide just floats there. :pardon: It's only stability comes in the fight between the pressure that the vacuum diaphragm is applying to the slide spring. So atmospheric surging is possible from not only the diaphragm vent but the airbox itself. I know you like the sound of the airbox cover off... but you could at least try putting it back on and see if that helps at all in the more windy conditions.

Years ago when I was messing with the BST I tried every known hop up trick on the carb. Tested multiple different spring renditions, needle tapers, etc. I was running an 02 sensor so I knew the surging was not a lean condition. It was atmospheric in nature and really showed it's ugliness on windy days running down the freeway. Obviously this is the place you are going to notice surging the most.... smooth road, constant speed and throttle setting. The only way I was able to just about eliminate the atmospheric surging (besides shielding the vent filter from cross flow) was to increase the preload on the stock (unclipped) slide spring. But then you loose the snappiness that the clipped spring provides. This is the reason I never drilled the additional hole in the slide because I knew it would make surging worse in my riding conditions.

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PostPosted: Tue Oct 10, 2017 11:50 am 
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Location: San Diego, Mexifornia
DesertRogue wrote:
I'm posting this just to vent (no pun intended)! I know what the solution is but just for $hits and giggles….

Aside from replacing the BST40 with the precious pumper carb, is there truly a solution to the BST40’s incessant (or inherent) sensitivity to winds?

I think I’ve reached the “pinnacle” of BST40 surge protection. I’ve eliminated 100% of the surging from my BST at ALL RPM’s except when I’m moving between 75-80 mph. I have a ½” heater hose running from the tip of the vacuum slide filter vent, to the very back of the bike, with the end of the hose terminating right below the seat (behind the regulator/rectifier and covered by the side plate. It’s about as protected as it can be back there, and still effectively vent, yet I get the slight pulsing at 75-80 mph when I’m riding against a headwind! Below that speed it’s sweet. Above that speed, no problem. Calm conditions, no surging. But when there is blustery conditions, the pulsing remains. I guess I’m at that point where I’ve run out of options except to upgrade.

Disclaimer: I have an opened up airbox, top cut/side door removed, jetting just about perfect, 50 mpg average fuel efficiency.


Move to an area with no wind??? You are welcome! :howdy:

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PostPosted: Tue Oct 10, 2017 12:19 pm 
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mxrob wrote:
The unfortunate thing about the BST is the slide just floats there. :pardon:



I see what you did there.

:BevButt:

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PostPosted: Tue Oct 10, 2017 3:25 pm 
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signit98 wrote:
Move to an area with no wind??? You are welcome! :howdy:


Let me think about this for a New York minute.....

:lol:


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PostPosted: Tue Oct 10, 2017 3:43 pm 
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mxrob wrote:
The unfortunate thing about the BST is the slide just floats there. :pardon: It's only stability comes in the fight between the pressure that the vacuum diaphragm is applying to the slide spring. So atmospheric surging is possible from not only the diaphragm vent but the airbox itself. I know you like the sound of the airbox cover off... but you could at least try putting it back on and see if that helps at all in the more windy conditions.

Years ago when I was messing with the BST I tried every known hop up trick on the carb. Tested multiple different spring renditions, needle tapers, etc. I was running an 02 sensor so I knew the surging was not a lean condition. It was atmospheric in nature and really showed it's ugliness on windy days running down the freeway. Obviously this is the place you are going to notice surging the most.... smooth road, constant speed and throttle setting. The only way I was able to just about eliminate the atmospheric surging (besides shielding the vent filter from cross flow) was to increase the preload on the stock (unclipped) slide spring. But then you loose the snappiness that the clipped spring provides. This is the reason I never drilled the additional hole in the slide because I knew it would make surging worse in my riding conditions.


I believe you MXRob. I've been reading a lot of your posts and responses to others and I was hoping I'd come to a different conclusion. I figured overall it has been a learning experience for me.

I don't want to put the side door back on. It's 90%-95% street ridden so I have no need to have it on and I do like the sound and the extra oomph I get. I recently removed the California mandated Fuel Vapor recycling system (that's basically what it is) and associated plumbing. For some reason, not having those rubber hoses and the support plate sitting in front of that side door opening has further improved the sound. I guess it's no longer reflecting back the intake noise as much. I followed your recommendation to use a short fuel hose for the fuel vapor inlet port on the carb, along with a bolt to seal the tip of the hose. I was going to use a rubber plug but I saw your warning not to. Thanks for that FYI.

Anyway, I plan to just bite the bullet and get the TM40 from Pro Cycle. It will be my Christmas gift to myself.

The bike runs so sweet outside of a windy day on the freeway. Getting rid of that last bit of pulsing at highway speeds+windy days would make me love this bike even more.


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PostPosted: Wed Oct 11, 2017 9:54 am 
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DesertRogue wrote:
mxrob wrote:
The unfortunate thing about the BST is the slide just floats there. :pardon: It's only stability comes in the fight between the pressure that the vacuum diaphragm is applying to the slide spring. So atmospheric surging is possible from not only the diaphragm vent but the airbox itself. I know you like the sound of the airbox cover off... but you could at least try putting it back on and see if that helps at all in the more windy conditions.

Years ago when I was messing with the BST I tried every known hop up trick on the carb. Tested multiple different spring renditions, needle tapers, etc. I was running an 02 sensor so I knew the surging was not a lean condition. It was atmospheric in nature and really showed it's ugliness on windy days running down the freeway. Obviously this is the place you are going to notice surging the most.... smooth road, constant speed and throttle setting. The only way I was able to just about eliminate the atmospheric surging (besides shielding the vent filter from cross flow) was to increase the preload on the stock (unclipped) slide spring. But then you loose the snappiness that the clipped spring provides. This is the reason I never drilled the additional hole in the slide because I knew it would make surging worse in my riding conditions.


I believe you MXRob. I've been reading a lot of your posts and responses to others and I was hoping I'd come to a different conclusion. I figured overall it has been a learning experience for me.

I don't want to put the side door back on. It's 90%-95% street ridden so I have no need to have it on and I do like the sound and the extra oomph I get. I recently removed the California mandated Fuel Vapor recycling system (that's basically what it is) and associated plumbing. For some reason, not having those rubber hoses and the support plate sitting in front of that side door opening has further improved the sound. I guess it's no longer reflecting back the intake noise as much. I followed your recommendation to use a short fuel hose for the fuel vapor inlet port on the carb, along with a bolt to seal the tip of the hose. I was going to use a rubber plug but I saw your warning not to. Thanks for that FYI.

Anyway, I plan to just bite the bullet and get the TM40 from Pro Cycle. It will be my Christmas gift to myself.

The bike runs so sweet outside of a windy day on the freeway. Getting rid of that last bit of pulsing at highway speeds+windy days would make me love this bike even more.

So give the extra preload on the slide spring a try just to see if that helps on windy days. I just used an appropriate sized flat washer up the diaphragm cap post for the spring. I don't remember how much preload I used but it wasn't just the washer thickness. The ID of the washer stopped on the post before it reached the top where the spring normally seats. Just make sure to use a stock unclipped spring as clipping the spring actually increases spring rate as Motolab mentioned. Not sure a clipped spring with additional preload would open the slide all the way. :s_dunno

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PostPosted: Wed Oct 11, 2017 3:37 pm 
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Thanks MxRob. I shall do that next to see if the increased spring preload will give the slide a bit of "backbone" and quit jumping around when windy. It should make some difference because I'm just as certain as you are why this sucker surges at certain riding conditions. I've got two months left until Christmas so I may as well play around with the BST40. A last dance so to speak.

Here's an interesting observation when i was riding to work on the freeway this morning: As usual, the section of the freeway sits low and the air is cold that pushes down on the warmer still-early-fall SoCal weather, creating stronger winds. Also, the freeway is full of cars spread around, which creates a lot of wind buffeting from vehicles in front of and beside me. Especially from larger vehicles. Under these conditions, the BST surging between 75-80 is quite noticeable. It's annoying in fact. There are moments where the engine feels like it's running out of fuel (probably because the slide drops slightly and "leans" the fuel mixture). Then it catches up again. It makes the 10 mile freeway hop interesting.

As soon as I get a clear freeway ahead of me with no vehicles within 50 feet....guess what? The surging almost disappears! Same speed....It's almost semi-entertaining to feel all this unfold.


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PostPosted: Wed Oct 11, 2017 4:03 pm 
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Thats the reason I chucked my stock carb for a FCR it would surge at 55mph. I noticed different bikes surge at different speeds like my 9q DR650 surged at 25mph


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