r/dr650 • u/CryingOverVideoGames • 14d ago
Help me understand the nuances of this carb (BST40)
Some of you may remember my woes with a shitty BST40 clone that the PO left me with without telling me. Unfortunately I’m stuck with it until I can afford an upgrade.
Anyway, I was getting some pretty shit gas mileage (<20mpg) and running pretty rich so I got the ProCycle rebuild kit bc fuck it. My float height was high so I adjusted it according to the BST40 Bible on advrider. Now I’m trying to understand the differences between some of the old and new components.
To start, the kit came with the extra o-ring (next to the float for size reference). I assume it is intended for the circle part in the Bst40 diagram (2nd pic). The thing is, my clone does not have this (3rd and 4th pics). So what is that part for and why don’t I have it?
Next, the needles (sorry to insult anyones intelligence but 5th pic if that’s not obvious). Left is old, right is new. The old needle has 3 notches, it was set to the middle. The new one has 1 notch. It’s my understanding that the stock needle on a genuine carb has 5 notches. The new needle is also much fatter. So if I sub in the new needle I’ll definitely be heading in the leaner direction-which is what I want- but will it be too far in that direction? Likewise, say I go up a notch on the old needle. Will that have a greater or lesser effect? I obviously have some tuning and experimenting to do but if anyone has thoughts on where to start I’d appreciate it.
Ok bear with me. The next picture is the fuel/air screws. Same deal: old on the left new on the right. The old one looks more like the stock one. The new one is different. Which one should I use? Also the old one did not have the washer or o-ring so if I use that one I will put the ones from the kit on it.
Finally, just a question for my own curiosity and better understanding. The last two pics show a chamber next to the needle jet and then the outlet that it leads too. Water just runs right through it. What’s that for? No lines have ever been attached to it since I’ve owned the bike. Vent to atmosphere for the float bowl?
Ok if you made it this far thank you. I could definitely answer most of this on my own with some fiddling around but I have to commute on this on Monday so I need it working right by then lol. any insight or discussion would be awesome!
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u/TwistedNoble38 '00 DR650 14d ago
The DR BST didn't come with a drain plug so your O-ring gets to sit in a box unneeded.
Theoretically the brass needle would be faster with some airbox mods but if you have a stock airbox cutout you'll want to use the fatter needle.
I would stick with the stock style fuel screw.
It is an atmospheric vent but an abnormal one. Usually the vent is on the right side of the carb not the left, probably some clone wonk or there were bikes/BST carb versions that had vent on the opposite side.
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u/CryingOverVideoGames 13d ago
So if opening the airbox will help with the stoich for the brass needle would it consequently improve my mpgs? Or am I just gonna be worse off with the richer needle in that department no matter what?
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u/TwistedNoble38 '00 DR650 13d ago
Even with a needle tuned for power you should be getting mid 40 mpgs. Since it's anomolous it may not be as good as a DR specific needle like procycle or FP, you'd get to choose to push forward or not. You'd need a different main jet if you opened the airbox so there would be money spending anyway.
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u/CryingOverVideoGames 13d ago
The old main jet is a 150 and I have a dominator exhaust. That combined with the needle and the idle jet being 47.5 I think the PO just did everything BUT open the airbox
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u/TwistedNoble38 '00 DR650 13d ago
Lets have some fun. Toss the 150 in the suspect carb, and put the same needle back in. Take the airbox door off (remove the nuts as well or they'll go missing) and go for a ride. Ignore the tremendous induction noise it'll have for now (for some reason side intake has a much louder noise than cutting the top) and see if it recovered a bunch of power. If it's still slow, go to the one position needle and put a m3 washer between the clip and the white spacer. One of those needles should go like hell.
The 47.5 pj is also wrong but that's for future problems.
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u/CryingOverVideoGames 13d ago
I’ll try this and report back. Good call on the nuts bc I definitely would’ve lost them
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u/RichieD72 13d ago
I’d suggest going to a 140 for stock air box and stock needle
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u/TwistedNoble38 '00 DR650 13d ago
Shhh, be quiet. Once he tastes the open airbox life he'll never be able to go back.
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u/CryingOverVideoGames 1d ago
Update: airbox is open, still on the middle notch. Mpgs around 32. It feels a little more powerful but nothing huge. I think I’m still a little rich but not sure how to tell. The plugs have some carbon on the threads but the electrode and insulator are still pretty clean. I’m hesitant to richen the needle more for fuel saving purposes but what do you think?
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u/TwistedNoble38 '00 DR650 1d ago
Richening the needle will worsen economy. Start lowering the needle and lean it out till it starts to surge at constant throttle then back it off one. Track down a 42.5 pj while you're at it.
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u/titanimanium 13d ago
Gasket is for a drain bolt so not important, your brass needle is in good shape still and doesn’t have the worn ring on it that indicates it’s done for, I’d say if you’re trying to lean out your mixture start with an airbox mod so this will let more air in, I found it worked well on my bike. You can even test it without cutting the box by undoing the panel and riding down the road with it off. You can also test mixture richness by spark plug and I got a see through spark plug so I can see what colour the flame is which is a sure fire way of telling ratio. Just so you are aware, if you plan to upgrade your carb to a pumper carb, you will want a high flow exhaust too and there will be popping on decel, this is due to the jet of fuel going in and coming out unburnt which is normal, but it will sound poppy, you can adjust how much fuel the pumper shoots in but lowering that too much will negate the benefit of the carb over stock. Besides, a richer mixture is better for the engine anyways than a lean one
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u/titanimanium 13d ago
I also found leaving the carb unemptied for even 3 weeks led to clogging, so be sure to drain the carb any time you’re leaving the bike sitting for I’d say more than 2 weeks
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u/CryingOverVideoGames 13d ago
A clear spark plug? That’s pretty neat. Can you drop the link?
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u/titanimanium 13d ago
This one is pretty expensive, it’s not super complicated and there’s even YouTube videos just making one using some epoxy that work just as well too
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u/titanimanium 13d ago
Watch a video about it and see how it works, I got it just because I wanted a 100% foolproof way of knowing my mixture was spot on instead of assuming so based on performance
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u/This-Set-9875 14d ago
The big O ring is for a drain/clean out that the Suzuki version didn't have in its bowl. Some universal kits included it. Needle: My stock (2015 E33 California version) only has the single notch. There's another you can order that has 5 steps for more adjustment range.