clint640 I have also drilled my slide and cut 2 coils out of the slide spring.
This helps the slide lift faster by taking out some of the pre-load.
Yes (before someone jumps in) is does make the spring stiffer, but only a
small amount because of the number of coils.
Heres a link to a site of a guys, that has done a lot of testing.
Carb mods
That link does not always work so try this one.
Better link
Click on english and then link and lastly Flanny's BST40 Jetting Guide (760kb)
Not sure what year your 640 is, so this may be no good to you.
That is if you do not have a BST carb.
What you could also try to get more power is.
Remove the front sprocket cover and you will see 3 sender switchers.
Now the middle one (will have to recheck on this) is the neutral switch and
the other 2 are for 2nd & 3rd gear.
Now these activate the solenoid switched vacuum bleed and some think
that it changers the timing to get the bike though the noise level tests.
By not letting the bike rev out in these two gears.
Just disconnect at try as it will cost nothing and let me know if it helps.
And tape up the ends to stop shorting.
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