bst jetting guide

8/11/2019 BST Jetting Guide

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SPECIAL X-MAS EDITION!!!!

KTM LC4 640 jetting for Mikuni BST40

By: FLANNY


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KTM LC4 640 jetting for Mikuni BST40Disclaimer Bull-Shit: Do any of this stuff at your own riskit took me two yearsto learn all this the hard wayif it doesnt work for you, thats too damn bad-dontblame me! If you know better, then make your own damn guide!

Main jet

Change stock 142.5 to one between155 and 170 depending on exhaust& altitude.

Jet type is N100/604

Change this jet last, and use dyno

testing or clocked WO runs to getbest time to determine jetting. Forstock exhaust with 6 extra disks,start with 157.5 and experiment fromthere. For full race exhaust (straightthrough), start at 165 and work fromthere. 170 should be MAX size for arace can.

These are for near sea level. Adjust down for altitude

NOTE: You can change the main jet in a jiffy on the bike by simply removing the17mm bolt on the bottom of the float bowl. BE CAREFUL NOT TO LOSE THESMALL BRASS SPACER THAT THE JET SITS ON!!!

Needle & Needle Jet

Stock needle is fine.Stock Needle Jet is fine.

Change stock clip position ofmiddle position to secondposition from the bottom toraise the needle.


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Slide & Spring Modifications

In order to improve mid-range throttle responsewe need to make the

slide lift faster, andsooner. This isaccomplished byreducing the spring pre-load on the slide to makeit lift at lower venturidifferential pressure. Wealso need to reduce theair spring in the slide(which acts as a damper)by increasing the size of

the holes in the bottom.

THIS MOD MAKES ANOTICEABLEDIFFERENCE!!!

So:Remove SlideSpring from slidebody.FOR

SUPERMOTOONLY:Cut FOUR coilsoff the top of thespring. DO NOTCUT MORETHAN FOUR!!! FOR OFF-ROAD:I have had goodresults with TWOcoils cut off for off-

road use, withouttoo muchbogging in thewoops due to slide lifting unwontedly. I would recommend this off-road atyour own risk, as others have reported bad slide bogging problems in thewoops.If I had to do it again, I would still cut two coils off.


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Drill Slide Holes

Using a sharp 1/8 drillbit, slowly drill the twoside holes on the bottom

of the slideBe careful not to blowout the inside of the holeas it goes through.Drill speed high, feedrate slow!

GETTING JETS:

I have had great luck with www.carbparts.com and others swear bywww.sudco.com


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Pilot Circuit

To improve 1/8 to throttle response, we needto richen up the pilot

circuit. This is done buychanging the stock 1.2Pilot Air Jet to one jetricher size 1.1.

Mikuni Jet BS30/97 size1.1

Note Some people claimthat changing the PILOTFUEL JET (the one inside

the float bowl) from a 45one step richer to a 47.5INSTEAD of the PILOTAIR JET works better - Idont know they are twoends of the same cicuitand should yield similarresults (one controlsmaximum air, while theother controls maximumfuel).

Do one or the other, butnot both .

Lately, I tend to agree thatPILOT FUEL Changesmay be best.


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Setting Idle Mixture Screw

The LC4 is EXTREMELY sensitive to the idle jet. Set it as followsget the bike HOT by going out for a longing spin (No use jetting the bike

for when its cold)With the bike idling beside you make the screw leaner by tightening it untilthe idle speed just starts to drop.STOP and note how many turns you are at.Now richen the mixture by loosening the screw until the idle speed startsto drop again.Note this position.Go back about a third of the way between the two points (I have found theLC4 to prefer a bit of a richer bias).

Usually somewhere

around 2.5 turns isprobably the norm

If you dont reach anoticeable change inidle speed at either endof the spectrum (>3.5turns out, or


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Air Box Mods

I have been advised on by some tuners in Germany to leave the snorkel on, anduse a plate like the one pictured. This procedure is thanks to SCHLINKY fromKTM TALK.

Remove SideAirbox coverCarefully pry offfoam insert fromCoverTrace old insertonto a sheet of0.062" Aluminum.Mark centers ofholes with center

punch. Then markdesired cutoutopening and cutout.

Drill Corner Holes

Epoxy Glue meshscreen onto inside

Bolt into place withstock cover (foaminsert removed ofcourse) and go!

Use tape in inchincrements over thehole to fine tunepart throttleperformance. (Thishole shouldprobably 1/3covered for bestperformance)


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Emission Controls

Follow this link to the best description of how to remove the emission controls.

http://www.squigglethegundog.co.uk/KTM_LC4_TUNNING/EMISSIONS/emissio

ns.html

It took me a full season to get my bike dialled before switching to theTM40..So be patient!

OK Well that is it Good luck!

Ohhere is what makes this the X-mas edition!

There you go. Merry Ho.Ho!

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