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How to confirm your main jet?

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After a few years of going stock I'm venturing into the world of typical mods with my 2000 KTM LC4E and its Mikuni 40 BST CV carb. I've read various threads and searched here at ThumperTalk, ADVRider, and KTMTalk, followed a bunch of external links and still have some questions when it comes to selecting and setting jets for the big circuit.

I'm not looking for advice on what to choose (I've read enough to have a good starting place) but how to choose. There seems to be at least six methods used to find the right main jet and set the jet needle:

1) There is the new plug, warm bike, full throttle run, cut the engine and pull the clutch and stop, and read the plug (or is it tea leaves?) method;

2) Mikuni's warm bike, full throttle, roll-off an 1/8th method;

3) The cold engine/warm engine, which pulls harder comparison method;

4) Add an oxygen sensor (wide, not narrow range) to the header and use a gauge while riding;

5) Take it to a shop with a dyno and exhaust analyzer and have them do it for you; and

6) Ask an expert and do what they recommend.

I've now got a full selection of MJs handy and almost have the bike back together after doing the slide drill, air box cover mod, jet needle drop a position, 47.5 pilot, SXC "sparky" silencer, and "S" bend pipe replacing the pre-silencer.

So, when I get the bike back in one piece and install the main jet recommended by most LC4/BST experts, how do I go about knowing which of the main jets I've got in little baggies is the best one for my LC4?

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1) Pointless to do a plug chop with todays fuels. Maybe 40 years ago it worked.

2) Nope

3) Yup, the best way. Expensive to do. Unit cost $300~$400 and you need the bung in the header. Though on subsequnt bikes, all you need is the $10 bung. This is the method I use.

5) Sort of, problem is, does the operator really know what they are doing and can they inset the probe a minimum of 18" in?

6) May be dead on balls accurate, may be way off. Only really good as a starting point.

Without a wide band, you can do it by process of elimination. It probably will not be absolutely perfect but you will be close enough that it does nto matter. Even if you get it perfect that day, the following day, the air density will be different (Air pressure, humidity temperature).

This is what I do:

Starting with the recommended main, remove the airbox door and go for a ride (bike fully warmed up). Is it better or worse?

If it is better, you need a smaller main.

Go down one size, replace the airbox door, ride. Remove the airbox door and test again. Better or worse? If better, go down a size again. Keep repeating this till the test with the airbox door is worse.

If it was worse with the airbox door removed, tape over 1/3 of your airbox opening, test (airbox door on, of course).

If it is worse now with the tape and was worse with the airbox door off, your main is just right. You are done!

If it seems better, you need to go up a size in main jet. Then test it again (remove the tape). Replace the tape, test again. If with the tape on it is better, go up another size in main. Keep repeating this till having the tape on is worse than with it off.

To finish up and ensure you are set accurately, retest the bike with the tape off (airbox door on), ride it, then remove the airbox door (tape off, of course). Best performance should be with the airbox untapped, airbox door on.

Remember, the main only operates at WOT. Ideally, you want to be in 3rd or 4th gear doing the tests, hitting max revs (just shy of the limiter) for at least 10 seconds to get an accurate representation of the jet status.

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Alright, we now have a #7. This sounds the easiest and best so far without investing in instrumentation. Just to make sure, I'll restate it the way I understand it to see if I’ve got it right. It is all based on this theory. Removing the airbox door (or snorkel?) will result in a leaner air/fuel mixture at high RPM and WOT. Restricting the airbox with tape will result in a richer air/fuel mixture at high RPM and WOT.

So when Goldilocks takes her warmed up thumper out on a deserted road, gets into fourth or fifth gear at high RPM, and cracks the throttle wide open, this is what she finds:

The original main jet is too large (rich at WOT): Testing with the airbox door off will lean the air/fuel mixture closer to ideal and the bike will run better than with the airbox door on. Reduce main jet size and retest.

The original main jet is too small (lean at WOT): Testing with the airbox door off will lean the air/fuel mixture even more and the bike will run worse than with the airbox door on. Testing with the airbox opening restricted will richen air/fuel mixture closure to ideal and the bike will run better than non-restricted. Increase main jet size and retest.

The original main jet is just right (ideal air/fuel mix at WOT): Testing with the airbox door off will lean the air/fuel mixture from ideal and the bike will run worse than with the airbox door on. Testing with the airbox opening restricted will richen the air/fuel mixture from ideal and the bike will run worse than non-restricted. Main jet size is correct. Now go find the ideal jet needle position and read the sticky on confirming your pilot circuit.

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Alright, we now have a #7. This sounds the easiest and best so far without investing in instrumentation. Just to make sure, I'll restate it the way I understand it to see if I’ve got it right. It is all based on this theory. Removing the airbox door (or snorkel?) will result in a leaner air/fuel mixture at high RPM and WOT. Restricting the airbox with tape will result in a richer air/fuel mixture at high RPM and WOT.

So when Goldilocks takes her warmed up thumper out on a deserted road, gets into fourth or fifth gear at high RPM, and cracks the throttle wide open, this is what she finds:

The original main jet is too large (rich at WOT): Testing with the airbox door off will lean the air/fuel mixture closer to ideal and the bike will run better than with the airbox door on. Reduce main jet size and retest.

The original main jet is too small (lean at WOT): Testing with the airbox door off will lean the air/fuel mixture even more and the bike will run worse than with the airbox door on. Testing with the airbox opening restricted will richen air/fuel mixture closure to ideal and the bike will run better than non-restricted. Increase main jet size and retest.

The original main jet is just right (ideal air/fuel mix at WOT): Testing with the airbox door off will lean the air/fuel mixture from ideal and the bike will run worse than with the airbox door on. Testing with the airbox opening restricted will richen the air/fuel mixture from ideal and the bike will run worse than non-restricted. Main jet size is correct. Now go find the ideal jet needle position and read the sticky on confirming your pilot circuit.

I think he means the actual door to access the filter and when you "tape it off" you are covering the snorkel opening.

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The door is the large access side panel as most have already removes any snorkle and have enlarged the air inlet. You have to have a way to vary the amount of air available to the engine. So you can remove the airbox access door or cover a portion of you air inlet.

But other than that, you have it.

Fuel Screw/Pilot Jet

Fuel screw settings in the 'book' are recommended starting points. Every bike is different, as is the temp and altitude. Set the screw according to this method.

Gently turn the screw all the way in. Now back it out two turns. Start the bike and fully warm it up, go for a 10 minute ride. Set the idle to speed to 1,500~1,800 RPM as best you can (I know, without a tach this is tough, just set it to were it idles relatively smoothly). Once warmed, slow the idle to the lowest possible speed.

*** When turning the fuel screw, keep an accurate 'count' of the amount you are turning it and record it in case you have to reset it for some reason. Makes life easier when you can just set it from notes Vs. going through the procedure again.***

Turn the screw in until the idle becomes rough or the bike stalls.

if it stalled, open the screw about 1/4 more turn. Restart it and slowly screw it in till you can just perceive a change.

If the screw can be turned all the way in and the bike still idles perfectly and does not stall, then you need to go down a size in pilot jet.

Now very slowly, open the fuel screw till the idle is smooth. Blip the throttle, let the bike return to an idle, wait say ten seconds. Confirm it is the same smooth idle.

If the screw has to be opened more than 3 turns to get a smooth idle, you need to go up a size in pilot jet.

If you find it does not stall with the larger jet but has to be open more than three turns with the smaller pilot jet, put the larger one in and set the fuel screw at 1/2 turn.

If the idle speed increased, adjust the idle speed knob to return the bike to a real slow idle speed. You must then re-visit the fuel screw. Keep doing this till the fuel screw is opened just enough to provide a nice steady idle at the lowest possible RPM. Once this is done, increase the idle speed to the normal one for your bike, typically about 1,800 rpm, but go by the spec in your manual.

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The door is the large access side panel as most have already removes any snorkle and have enlarged the air inlet. You have to have a way to vary the amount of air available to the engine. So you can remove the airbox access door or cover a portion of you air inlet.

But other than that, you have it.

The KTM 640 LC4s have a little different "standard" airbox modification than most. After opening up the exhaust flow from stock, the standard fix is a vented side access panel (KTM even sells a screened replacement side panel) and leave the snorkel in place. You just have to be a little more careful with water crossings. I don't have a clue why this is the best solution for this particular bike but I'm sure there is a good reason.

The nice thing about the standard KTM airbox mod combined with this method of selecting a main jet is that one can quickly take off the screened side cover for more flow and cover the screen for less flow and never have to get under the seat. Thanks for all the help. I'm looking forward to giving it a try.

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LOL, does not matter how or where you cole off part of the opening for the test, just that you do it, typically by about 1/3 of the total inlet area.

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William1,

I tried to follow your method but did not have great luck. Every time I ran the bike with the inlet covered by a third it ran slower no matter what MJ installed. When I left the side cover off it ran slower with different main jets. So, I tried something else.

I found a straight stretch of road with an uphill grade of about 5%. I figured out a distance that allowed me to start in 4th gear at 50 MPH (~5,000 RPM, max torque) and accelerate to 80 MPH (~7,500 RPM, max HP) full throttle in about 13 seconds. I started at 50 +/- 0.5 MPH and recorded the terminal speed with a GPS (max speed) at the end for three runs. I then calculated an average max speed. I went through this using a range of main jets from 150 to 157.5.

Even though the difference between the slowest average and highest average with the different main jets was only about 1 MPH (80.17 MPH vs 81.20 MPH), the 155 MJ consistantly recorded the best average.

Here are my results: 150=80.17 MPH, 152.5=80.25 MPH, 155=81.20 MPH, and 157.5=80.57 MPH. I ran through the whole bit a second day with similar results. I assume the best A/F mixture = most power = highest terminal speed. Anything wrong with my methodology?

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No, nothing wrong with it. You basically are making a ride like a dyno. Only thing is it can be more difficult to know which direction to start testing, leaner or richer. But numbers are always better than the Butt-O-Meter. Good going! :thumbsup:

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