jetting question

[drybranch]

How long did it take you all to get your rides jetted to run right after you put aftermarket pipes on? I am getting a Sparks and b/c of my elevation they gave me a few suggestions on the main jet and needle position. This is going to be my first jetting endeavor but I have cleaned some carbs and think I will have no problem but just wondering what kind of time it should all take because I want to get it ready to go riding pretty quick after I recieve the pipe, but I want to make sure that I get it right.

[jsmith2232]

When i got my pipe, k&n and rev box i just went with what fmf reccomended and it was about perfect, it ended up being a dynojet size 146 which would be the same as a kehien 160

[GPracer2500]

Originally posted by drybranch
...jetted to run right...

In a sense it's never ending. Changing temps and altitudes require new jetting to keep the fueling spot-on. Now, it's usually pretty easy to just set it up and leave it (unless your riding conditions change significantly) and your jetting might just be a little off here and there. But many (most?) folks have trouble distinguishing between small jetting changes anyway so it's no big deal.

As far as I'm concerned, the real goal is to learn what ever-so-slightly lean and ever-so-slightly rich sounds and feels like so you know when a small jetting change could do some good. That just takes time and experience...

Main jet changes only take a few minutes. Needle and pilot jet changes take--oh I don't know, maybe 15-30 minutes. You should be able to get things dialed in pretty darn close within a couple hours give or take an hour.

[drybranch]

One more ? Is the jetting process shown in the FAQ forum very similar to that of the 300 ex? I figured that it is very similar.

[ryanh250ex]

what GPRacer said. i've seen him post in a few jetting threads, and he knows his stuff. ;)

doesn't take too long, but it really is a never ending process. what i do is jet it so that it might run maybe a tad rich in the summer- it does get hot out here in Jersey :D , then for winter , i can just throw the airbox lid on. if i am still a little lean, i could always through on my stock air filter (i run a UNI).


good luck, post if you have any troubles.

[300exOH]

Originally posted by drybranch
One more ? Is the jetting process shown in the FAQ forum very similar to that of the 300 ex? I figured that it is very similar.

The carb jetting article in the "articles section' is basically the same for the 300ex. It is very easy to do but takes some patience basically because of the small parts involved. You should be able to get very close by using the pipe manufacturer's jetting recommendations. The job should take about 1-2 hours your first try.

Here's a good article I found on the web somewhere:

Jetting

PLUG CHECK & READING SPARK PLUGS
The first step is to replace the old spark plug with a new one. Make sure it is the proper plug for your machine, and that it is gapped properly. An improper plug gap will throw off the readings. If the gap is too small, it will not burn properly. If it is too large, it will cause added stress on other ignition components, causing premature failure. You should have fresh fuel at the proper octane level. Most small engines run best with 92-94 octane fuels. It is also important that you have a clean air and fuel filters for the tests.
Plug testing is the easiest way to help you dial in the jetting of the carburetor after adding performance parts and for different altitude or weather changes. The color of the spark plugs is used as an indicator and lets you know what way to start re-jetting the carburetor.
There are three basic ranges that a plug test can be used to check.
3/4-Full throttle, which is controlled primarily by the main jet
1/2 throttle, which is dominated by the taper angle of the needle and the clip position.
1/4 throttle, which is mainly, controlled by the strait diameter of the needle but also the slow circuit.
Testing the Main Jet
Once you have a new plug installed and gapped properly, start the machine and let it warm up. Take the machine for a VERY short ride to bring it up to normal operating temperature. Find an open spot of road and get it up to top gear. Roll on the throttle and hold
it wide open; 2 strokes for 3-5 seconds. 4 strokes 10 - 15 seconds. While you are still wide open you must stop/kill the engine using the kill switch while pulling in the clutch at the same time, all the while still holding the throttle wide open. Let the bike coast to a stop with the clutch still pulled in .Do not let the clutch out until you come to a complete stop.
**** It is very important that you do not let the engine slow down or idle at all. You must do it this way for every test!****
Now you need to remove the spark plug and check its condition. You want to look and take note at the white insulator part of the plug only, not the metal tip or electrode.
The insulator of the spark plug must be a light grayish-tan to light brown color. If the insulator is white, it is too lean, meaning the main jet is too small. If the insulator is black or wet, it is too rich, meaning the main jet is too big.
The steps for checking the other ranges are basically the same. If you want to check the 1/2 throttle range you want to hold it steady at 1/2 throttle in top gear for about 20-30 seconds. The same for 1/4 throttle testing, hold it steady at 1/4 throttle. Pull in the clutch and shut if off at the same time while holding the throttle at a steady speed.

BASIC CARBURETION
To be able to properly jet or re-jet a carburetor you must first know how they work. Most ATV and off-road motorcycle carburetors work basically the same way. The two most popular models are the Keihin and Mikuni.

These carbs have two primary fuel circuits; the slow/pilot circuit and the main circuit, which have several parts or components that will effect the mixture at different throttle positions. There are 4 throttle positions that are used to determine what component is effecting the mixture. They are idle/off idle, 1/4 throttle, 1/2 throttle and 3/4 to full throttle.
It is important that you remember that the main fuel circuit is only part of the entire fuel delivery and any changes you make to the slow circuit will affect the main circuit as well, because the slow/pilot circuit is delivering fuel from idle to full throttle. So you must start with the slow circuit and work your way up to the main circuit.
The Pilot/Slow Circuit
The pilot/slow circuit has two major components that affect the mixture. They are the the pilot/slow jet and the air mixture screw. The pilot/slow jet has a fixed diameter hole that controls the amount of fuel being delivered through the slow circuit. The mixture screw is an adjustment screw to control the amount of air being mixed with the fuel from the slow jet.
The Main Circuit
This circuit has 4 components that effect the mixture from about 1/4 throttle to wide open/full throttle. These parts are the throttle slide cut-away, jet needle or needle, the needle jet and the main jet. The throttle cut-away effects the amount of air but can be left alone. The needle primarily controls the mixture from 1/4 throttle to 3/4 throttle. The needle moves up and down inside the needle jet as you open and close the throttle. The diameter of the needle is what affects the mixture at 1/4 throttle. The taper of the needle and the clip position effect the mixture at 1/2 throttle up to 3/4 throttle. At 3/4 to full throttle, the mixture is controlled mostly by the main jet.
Starting From The Bottom
The first throttle range to dial in is the idle/off idle range. Any time there is a weather change or you are riding in a different area you may need to adjust the mixture screw for a clean and crisp engine response. The two main settings are the air mixture screw and the pilot/slow jet. The correct setting is the one that allows a crisp engine response with the air screw about 1 3/4 turns out. If you need to turn the air screw out more than 2 1/2 turns, then it is too rich and you need a smaller pilot jet. If you need only to turn the mixture screw out 1 turn, then it is too lean and you need a larger pilot jet.
1/4 Throttle
The mixture in this range is mostly controlled by the diameter of the needle. A lean mixture will cause the engine to hesitate and a rich mixture will cause it to sputter while keeping it at a steady speed. If there is hesitation, then a smaller needle is needed. If it is rich a larger one will need to be installed.
1/2 Throttle
This range is controlled by the clip position and taper angle of the needle. The correct clip position is often all you need to dial in this range. The correct position is the one that will allow the engine to rev cleanly from 1/4 throttle to 3/4 throttle without hesitation or sputtering. Raising the needle clip will lean the mixture and lowering the clip will richen the mixture. If you find that you are all the
way to the top or all the way to the bottom positions and the jetting is still off, you may need a different needle jet (if available) or a needle with different dimensions.
3/4 to Full Throttle
This range is controlled mostly by the main jet. A correct main jet will allow the engine to rev cleanly all the way until it drops off the power band with out sputtering or hesitation. A lean mixture will have power in the lower RPM's but will get sluggish and/or hesitate in the higher RPM's. A rich mixture will rev ok but sputter in the higher RPM ranges

[ohsobad_chevy]

It took me about 3 months to get mine "spot on" I was so anxious to ride when i put my new pipe on, I just got it "close" I put a 152 main and a 42 pilot in my 400EX, with the needle at the 2nd pos. from the top. I am fairly new to four strokes but I can say from experience that they are VERY VERY picky as far as jetting goes, I'm sure some of these other guys can second that. :D

[ryanh250ex]

Originally posted by ohsobad_chevy
It took me about 3 months to get mine "spot on" I was so anxious to ride when i put my new pipe on, I just got it "close" I put a 152 main and a 42 pilot in my 400EX, with the needle at the 2nd pos. from the top. I am fairly new to four strokes but I can say from experience that they are VERY VERY picky as far as jetting goes, I'm sure some of these other guys can second that. :D


I couldn't agree more. Ride a bike that has spot-on jetting, versus a similiar bike that is 1-2 jets too lean/rich (lean especially), and there is a noticeable difference.

[400exrider707]

I couldn't disagree more....haha, 400's seem to be one of the least finicky ones to jet, but that also depends on the carb setup. I've heard that the FCR's on the 400's are a PITA to get dialed in correctly.


Also ohsobad_chevy, if your needle was second from the top you leaned it out instead of going richer. Just FYI.