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Jetting Question for the BST40 Crowd


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What jetting did you all end up going with? I am at 500 feet, but routinely ride at the 3000 to 5000 foot level. Trying to nail this at installation as getting this carb in and out of the 400 is a PITA...

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I was wondering the same thing! There is one person that posted in the "edison approach..." thread that has their jetting info in their signature? I just got mine this week - $54 to my doorstep! I can't wait to see the difference!

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The only reason I'd remove the carb is for a needle swap. I did all my jetting with it mounted in the bike. 2 screws and you can pull the bowl right out of there. I've got huge hands too so take a look at it you might find it an easy trick.

750 elevation, the only other place I've had it is silver lake MI and that's 750 too.

Stock mixture setting has been perfect, 1 1/2 turns. The stock 42.5 pilot has been spot on too. I'm running the stock needle. When I bought the carb it had a DJ needle, the slide mod was already done and it was way too much. The DJ needle would be good for an Ethanol setup. I have a KTM adjustable needle but haven't installed it because the stock needle works great with the slide mod. In the Edison thread I see I posted a 145 main. This winter I needed to richen it a little and discovered that I was running a 155. I'm now running a 157.5 main and even when it hit 80 a week ago it still ran great.

Pilot Jet 42.5

Mix screw 1-1/2 turns

Needle Stock

Slide modded

Main Jet 157.5

Bike mods: Aftermarket filter, snorkel removed, header welds cleaned up, 4" Supertrapp w/ 11 discs. Other then auto decomp removed internals are stock in engine.

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I am running similar to ATJ except I have a 45 pilot and 150 main. I think main is good but as it warms up the pilot I feel is too big.

I am running a KTM version of the carb and I had alot of trouble getting the bike to rev cleanly above about 3k rpm when first installed. I tried main jets from 147.5 to 155 with no effect on the stumble I was having at lower rpms. I then tried both 42.5 and 45 idle jets and the 45 made the low rpm stumble better but did not cure it. I eventually raised the needle one notch from stock (second from the pointy end) and that cured my missing and stumble at 3-4k rpm.

Could have been a fluke or maybe I have an air leak but it runs great now if not a little rich in the warmer weather. I think I am going to drop back to the 42.5 pilot in the coming weeks.

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Good info!👍

I would have preferred to know about the larger main jets last week when I placed my order from KTM.🤣 I ordered a 145, 147.5, 150 & 152.5 thinking these would cover it, because no one said they were running larger than 147.5.

I guess I'll start with the 152.5 and see if I need to make another order:foul:

Perc

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What do you mean by this? Did you have to change the slide?

Perc

The combination of the slide mod and the Dynojet needle was too much. The slide mod alone is perfect. I just had to remove the DJ needle.

I would have preferred to know about the larger main jets last week

Perc

I went to go back to the Edison thread to my post but the edit option is long gone. Sorry dude!

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The combination of the slide mod and the Dynojet needle was too much. The slide mod alone is perfect. I just had to remove the DJ needle.

I went to go back to the Edison thread to my post but the edit option is long gone. Sorry dude!

Ahhh, just a $30 set back in my quest for something that is better!

Since I have already drilled the 140 jet out to 0.059" and this is about the size of the 152.5 jet (which was too small). I just drilled the 140 out to 0.063" and will try this out when I get a chance. (Probably too big)

On a different note: I noticed an offset inside my slide where the needle goes through, causing the needle to cant forward. When you push the needle back to get it inside the needle jet, the needle is rubbing against the engine side of the needle jet because the high/low inside of the slide... Sound confusing enough?

I'll try to get some pics later.

Perc

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I tried out the 0.063" main jet and it was better, but still had some flat spots.

So I drilled to 0.067" and still better yet!👍 This time it will pull the front wheel off the ground with a throttle roll on, in second gear.🤣

I'll keep playing until I find the limits. Next up... 0.070":thumbsup:

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  • 2 months later...

Just wanted to bring this thread back to life since we lost the month of may!

I have finished tuning my BST40 with...

Pilot Jet 42.5

Mix screw 2 turns

Needle Stock

Slide modded

Main Jet drilled out to 0.076", (Dynojet Jet Size Comparison = 180)

I don't know why my main jet is so much larger, maybe we'll figure that out some day? But this thing runs so much stronger that I run out of gears too fast and 1'st gear is just too low! (14/45 3.21 ratio). Swapped back to the stock 15/45 (3.0 ratio) this is much better for my new power.

Perc

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  • 4 weeks later...

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