RECENT POSTS

Jetting - How scary is it really?

Joined
Sep 6, 2006
Messages
26
Likes
0
#1
After riding yesterday and fouling two plugs, with lousy 1st and 2nd gear throttle response when it was running, I am going to have to give in and jump into the world of jetting. I have read the links here but am wondering how tricky it really is to get the jetting just right? I am not afraid to get in and pull things apart to fix them, but for some reason jetting still seems rather intimidating.

Also, do I go out and buy a few jets in advance and then try them out to see what works best, or take it apart first and then buy one at a time and hope I get lucky early on? I have looked but was not able to find a jetting kit for my particular bike. I am riding a 99 yz250 if that makes a difference.

Thanks.
 
Joined
May 19, 2006
Messages
1,500
Likes
1
#2
Jetting is relatively easy, but very time consuming. You will need to pull the bottom off you carb and determine what jets you are currently running. There is a correct order to jetting. First you jet until you are fine with idle-1/4 throttle--this is the pilot jet. Then, you jet (change needle position or needle) until you are fine with 1/4 to 1/2 throttle and response. Lastly, you approach the main jet. Don't go to step 2 until you are done with step one. Don't bypass any steps or you will be chasing a neverending circle. You determine the proper jetting by color of the plug.
Sounds like you may be starting off rich, so get two or three pilot jets down a step from what you have and buy several new plugs. You can really only check your jetting by starting with a new plug after each change.
 

DougRoost

Subscriber
Joined
May 3, 2001
Messages
720
Likes
0
#3
...and to help determine pilot jet size you start with the air screw. It should be at 2 turns out to run crisp, plus or minus a half turn. If you can't get it right by adjusting this, you then change the pilot. Since this is metering air instead of fuel, if you have to turn it more than about 2.5 turns out to see improvement, then you're needing to lean it out, so go with a smaller pilot jet and repeat.

With a properly jetted 2 stroke, a 1/4 turn can make a big difference in bottom end snap when the temp or humidity changes. Once you get this right, you can get to the needle, which makes the biggest difference since it is where you spend most of your time riding.
 

tony91

Subscriber
Joined
Jan 30, 2002
Messages
493
Likes
0
#4
Use this method to get close on the pilot: Warm-up the bike, adjust the throttle (on the handle bar) to a "fast" idle, turn the airscrew in slowly until the bike almost dies, slowly turn the screw back out until you get the to peak idle. Do a search on jetting. I'm pretty sure I read the pilot method on here somewhere. There are plenty of great threads that will help. Also, Eric Gorr's books are excellent.

Almost forgot, find something other than pump gas that you regularly get your hands on and stick with it. C-12 and proper jetting were my 2 best "mods" prior to the overbore and porting.
 
Joined
Apr 21, 2007
Messages
68
Likes
0
#5
Once you understand how the carb works, jetting is actually pretty simple.
 
Joined
Apr 17, 2002
Messages
136
Likes
0
#6
Did your bike run fine in the past, and now it's running poorly and fouling plugs?

If so, there's likely something besides the jetting that's an issue. Low compression, dirty air filter, etc.
 
Joined
Sep 6, 2006
Messages
26
Likes
0
#8
I only got the bike late last year so I do not have much history on it. It did foul a single plug on the last ride last fall but no other problems that I can recall.
 

DougRoost

Subscriber
Joined
May 3, 2001
Messages
720
Likes
0
#9
Matt Fisher said:
Did your bike run fine in the past, and now it's running poorly and fouling plugs?

If so, there's likely something besides the jetting that's an issue. Low compression, dirty air filter, etc.
Not true if the weather or location was significantly different then vs. now. Temp, humidity, and elevation dramatically affect jetting.
 
Joined
Feb 3, 2005
Messages
475
Likes
0
#10
2strokerfun said:
... You determine the proper jetting by color of the plug...
Not true, you determine plug heat range by the color of the plug. Jetting is determined by the mixture ring at the base of the spark plug and you'd need a spark plug viewer to get a good look at it.

Here's the best thread I've seen for proper plug reading:
http://www.dirtrider.net/forums3/showthread.php?t=92818&highlight=jetting+101

Also,... I thought you were supposed to start with the main jet because if you change it last then all the jetting work down the line could be effected?
 
Joined
May 19, 2006
Messages
1,500
Likes
1
#11
I've always started with pilot as it affects about 5 percent of the jetting throughout the entire range. Main shouldn't affect idel, but--yes--it can affect needle setting (which I honestly hate doing more than a jet because getting my top on my carb is a bear)
 
Joined
Apr 17, 2002
Messages
136
Likes
0
#12
DougRoost said:
Not true if the weather or location was significantly different then vs. now. Temp, humidity, and elevation dramatically affect jetting.
True, though that's why I didn't say only mechanical items could affect it and added the etc to the end. I couldn't decide how technical to get with my initial post, I should probably have been more specific and listed more examples.
 
Joined
Sep 6, 2006
Messages
26
Likes
0
#13
Follow up question - Does it matter what spark plug I use when checking the jetting and doing the throttle chop tests? The bike specs for BR8EG plugs. I was thinking of using BR8ES plugs for the tests to save some money and then switching back to the BR8EG once I have it set up properly. Will this affect the tests? Thoughts??
 

Chili

Lifetime Sponsor - Photog Moderator
Joined
Apr 9, 2002
Messages
8,063
Likes
11
#14
bk250 said:
Follow up question - Does it matter what spark plug I use when checking the jetting and doing the throttle chop tests? The bike specs for BR8EG plugs. I was thinking of using BR8ES plugs for the tests to save some money and then switching back to the BR8EG once I have it set up properly. Will this affect the tests? Thoughts??
That will be fine.