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Old 08-01-2005, 07:04 PM   #1
RedWarrior OP
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New musings on BST40 bog

I know this problem has been broached before but after having some bogging problem in the bumpies this weekend, I got to thinkin'...


Some have theorized that the bog comes from the slide bouncing up and down obviously bringing the needle with it causing a rich condition. I don't think this is the problem. However, I think it is possible that the needle is jumping a bit off it's seat on the slide when being bounced around. The needle isn't being held in place by anything other than gravity. I was thinking that, if indeed this is the problem, a drop of some adhesive material (I'm thinking a tiny drop of red loctite) to secure the needle to the slide would cure this problem. I'm going to give it a shot and test it out this weekend.

Good thinking or am I full of crap?

Red
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Old 08-01-2005, 07:58 PM   #2
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You can never be full of crap if your thinking about why something doesn't work well. You can be wrong, or right, or even "close"... but the fact that you have a theory can only be a good thing.

Consider this...

Let’s say for a moment that the vacuum piston "bounces"... and with it, the needle is hopping up and down as well within the vacuum piston.

Depending on throttle position and air velocity thru the venturi, the theory you pose could have some validity to it.

A small throttle opening and high RPM will create a substantial low (negative) pressure signal in the venturi. A movement of the needle relative to its "normal" location in the vacuum piston, could cause a rich condition.

A large throttle opening and low RPM would create the opposite effect in that the pressure signal would be inadequate to pull fuel from the bowl, regardless of the needle position relative to the vacuum piston, in a useable ratio with the volume of air... causing a lean condition.

To be honest, I don't know how a vacuum piston which weighs well less than an ounce and captured in a relative position by substantial positive and negative pressures, with the constant force of a spring against it as well, could bounce up or down to any significant degree under the forces of gravity and motion in the first place.
But hey... I could be wrong.

Back to you RW...
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Old 08-01-2005, 08:18 PM   #3
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One caveat over and above Creep's essay...

The needle tends to float in the slide so as to maintain it's centered position in the needle jet. If it's secured by an adhesive it may well bind as it enters the jet. This would result in a slide that hangs up, a condition well known to any Brit car buffs famliar with the Skinner's Union CV carb.... just a thought.
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Old 08-01-2005, 08:26 PM   #4
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Quote:
Originally Posted by ChrisC
One caveat over and above Creep's essay...

The needle tends to float in the slide so as to maintain it's centered position in the needle jet. If it's secured by an adhesive it may well bind as it enters the jet. This would result in a slide that hangs up, a condition well known to any Brit car buffs famliar with the Skinner's Union CV carb.... just a thought.


Good point Chris.
If you did "lock" it into position with an adhesive, I'd do it only long enough to test the theory.
If you could increase downward pressure on the needle ('course, that might be a bad thing too) without increasing pressure on the vacuum piston... that might be an alternative.
Any method of locking the needle down might create it's own set of problems.
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Old 08-01-2005, 08:34 PM   #5
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Quote:
Originally Posted by creeper
Good point Chris.
If you did "lock" it into position with an adhesive, I'd do it only long enough to test the theory.
Me thinks that unless the needle was perfectly aligned before the glue treatment, it would wear itself, and the needle jet, out with great haste.
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Old 08-01-2005, 08:41 PM   #6
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Quote:
Originally Posted by gaspipe
Me thinks that unless the needle was perfectly aligned before the glue treatment, it would wear itself, and the needle jet, out with great haste.

Yep... wonder which would take the wear first... the hard anodized aluminum needle, or the needle jet? I'm not enough of a DIY metallurgist to know.
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Old 08-01-2005, 08:43 PM   #7
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Me either....
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Old 08-01-2005, 08:46 PM   #8
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Quote:
Originally Posted by gaspipe
Me either....
Brass has some self-lubricating properties... Kinda. Modern hard anodize is some serious tough shit. We'd still have SUs and Strombergs on cars if they had the process back when Chris was... still pretty old.
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Old 08-01-2005, 08:52 PM   #9
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Quote:
Originally Posted by creeper
Brass has some self-lubricating properties... Kinda. Modern hard anodize is some serious tough shit. We'd still have SUs and Strombergs on cars if they had the process back when Chris was... still pretty old.
Ya know, I just try to add something to the conversation that Mr. Hickson overlooked and there he goes, attacking a senior citizen.

Has he no shame?
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Old 08-01-2005, 08:54 PM   #10
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Quote:
Originally Posted by ChrisC
Ya know, I just try to add something to the conversation that Mr. Hickson overlooked and there he goes, attacking a senior citizen.

Has he no shame?
I got shame... or something that passes for it. See?
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Old 08-01-2005, 08:59 PM   #11
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Who loves ya, Crappy?
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Old 08-01-2005, 09:07 PM   #12
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Originally Posted by ChrisC
Who loves ya, Crappy?
My daughter, wife and probable future son-in-law do. Well, OK... he could be lying.



Oh yea... my dog kinda likes me, sometimes.



And I know you do Chrissy Poo.



Sorry for the hijack RW... it always goes down the shitter like this eventually.
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Old 08-01-2005, 09:33 PM   #13
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The BST40 needle is held under the slide spring tension.

I've not had a problem with the BST40 stalling after bumps, and thankfully so as I fall down enough as it is.

I've read through some threads and some claim that venting is finicky, in particular try to keep fuel from clogging them so perhaps loop the bowl vent line upwards. The bowl vent is sticking out on the right side. Mine says "made in Japan" next to it. Looking at it now, I don't believe that bumps could splash fuel that high, though if you fell over to the right side fuel could plug the line.

The other vent which feeds into the brass elbow has a filter on it. That helps provide atmosphere pressure outside of the slide diaphragm. It also adds a bit of air to the choke circuit. I imagine if that filter were sufficiently clogged, it may affect slide operation.

There is also a fine filter sticking in the end of the carb's fuel spigot. I suppose this could be clogged or whatnot and slow fuel delivery.

People have described the BST40 bog as causing them to stall, but with careful throttle control they can recover. Interestingly I didn't see a lot of complaints about hard starting after a stall. Surely, if it stalls due to richness, it will be very difficult to start. I know when I fall over and thus flood the carb bore, it takes me several minutes to get it to fire over. So also consider that this may be a fuel starvation issue.. either due to poor venting, or poor bowl filling rate.

If the BST40 simply can't keep up with demand during the whoops, you can buy a bowl drain plug with an extra fuel area. I don't know if this would help though. It just has a little extra for the main. I have one which came with a yost kit for my carb.

This kind of thing:


I saw them for sale somewhere cheap, but now I can't find it. I guess you could see how much Bob will sell one at yostperformance.com.

I do see advertisements for larger float bowls for other carbs, with claims to eliminate the bogging associated with jumps and whoops. So there may be something to that.

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Old 08-01-2005, 10:20 PM   #14
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Back from work now and I think I know what will happen if there is needle/jet misalignment. For one reason or another (of which I really have no clue) I had a needle wear out on my Buell in a Keihin carb. It was an aftermarket needle I got in a jet kit from Yost. The needle actually wore such that it looked like a unicorn's horn. Obviously it was rotating around in the jet but I could never figure out why it wore like that. The jet was in perfect conndition. I continued using it with another needle and it hasn't had a problem since. Weird.

Anyway, it's been a while since I've had the BST apart and seem to remember that Potatoho is right. There is some kind nylon bushing as a base for the spring putting pressure on the slide/needle assy. as a whole.

Red
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Old 08-01-2005, 11:08 PM   #15
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bookmark. keep it up ya carbilosophers
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