I have a 98 Banshee that is moderately ported, bored, Wiseco pistons, Coolhead with 19cc domes, dual 35 mm Air Stykers, CPI Drag Pipes, etc.

I jetted it at 175 and it seemed to run good. Just for kicks, I tried to fine tune the jetting so I went down in size to 165 then 168. The engine sputtered out at maximum throttle with both. I put the 175's back in and the problem went away.

The strange part is the plug color never changed. Both with the 168's and 165's the color was light tan (coffee color), the same as the 175's. I conducted the plug test at full throttle, third gear, wound all the way out, release throttle, kill engine, check plug color.

My question is do I want to hit the jetting at the point it just quits sputtering out (175 mains) or should I go to a larger jet to allow more fuel? Will providing more fuel (ie: larger than 175 mains) make it quicker? Or is it best to get the jetting right at the point before it begins getting too lean?

Thanks for the help!

 

Rob: The best way to know that you are jetted right on the $ is with an EGT(Exhaust Gas Temp guage) With one of them you will never have to wonder if your jetted right.... the guage never lies.It sounds like you had a lean pop when you dropped jet sizes. When doing a plug chop you want to hold the throttle pinned when cutting the motor off and holding it there until you come to a stop. I was told once that you can lean a motor too much by closing the throttle like that(I may be wrong).
When you did the run w/ the 165/168's did you ride it for a while first? Or change to new plugs? It will take more than one pass to change the color of a plug. Have you tried a plug between the 168 and the 175? Most of the time you want to jet high enough so that the motor cant rev on top and then drop down 1 size at a time until it cleans out. From the sounds of it your jetted real close. I am currently running 165's in my 35pwk's.
On my web site I have jetting sheets that may help you to understand a lil more if your confused.
walter
88 Banshee370
95 Dodge Ram 4x4
99 Quadbuddy

 

Walter:

Many "thanks" for your response!

I orginally had Paul Turner pipes on it and had it jetted with 158 mains. The plug and performance seemed perfect when jetted there.

When I put the Drag Pipes on, I increased to 175. I tried going down to see if it made a difference at 165 and 168. Here, it sputtered out when the revs maxed out. However, the plug color was fine (I installed new plugs at each jet change).

I put the 175's back in and the sputtering went away, but the plug color never changed. My question is how do I know when I have hit the correct jetting considering the above? Is an increase from 168 to 175 significant?

What is an EGT? Is it something for a novice like myself? It sounds expensive???

Thanks for your help!

 

Rob: The EGT's are somewhat expensive but if you look at it from a diff angle..... what will it cost you after you burn up a top end and or crank by running it lean? Go to this link http://www.exhaustgas.com/ and this one http://www.oc2net.net/exhaustgas/Pro...c?CategoryID=8 Your best bet would be to contact them through email or phone and explain to them what kind of bike you have....
Another tip would be to stop over at http://www.macdizzy.com/2stroke.htm and there is a link somewhere on there about an EGT. If not post a short msg on his forums and I am SURE you will get more info on it from Rick himself or someone else.

walter

 

Rob: here is a link that may help you a little
http://www.macdizzy.com/egt.htm

walter

 

175 sounds small-i have a 96 banshee bored 30 over, dg pipes, toomey 2:1 filter, boost bottle, 12 tooth front sprocket 41 back (3.42 gear ratio), 280 mains and 30 pilots, yamalube 32 to 1

rick

*********oops-sorry didn't read all the post, different carbs*********

 

Apples and oranges, Rawze. You run Mikuni jets in your Mikuni Carbs, Rob is running Keihin carbs, so the jet numbering system is different.

Rob: You're right on the edge of a perfect mix. You may need to change them as temp change within the day's time. AM more jet, denser cooler air, PM less jet, hotter less dense air.

The difference in plug color. Try looking down within the spark plug along the center electrode insulator. Down about a 1/4" or so from the electrode itself will be a "smoke ring". This is probably the best place to read a spark plug from. The smoke ring will faintly register the same series of colors that will eventually begin to appear over time on the rest of the insulator. A jump from a 175 to a 168 CAN be a nice gain. Example my wife's R. it sputtered and spit with a 175 but ran well with a 170 main jet. Two size differences made a world of difference. Now could this be your case...maybe. Try experimenting some more. You may and most likely will be able to run the smaller jet on shorter length runs, less heat builds. But longer WOT runs WILL require more jet as the heat builds and builds and...So.

You may find that if you are wanting to drag race only you can possibly get away with smaller jets. But then swap to larger jets for rat racing around WOT with everyone else. Your engine is telling you what it likes for fuel, you're lucky, some engines tell you that by the scattered parts. Be careful and diligent, it sounds like you're right on the edge and well within the "sweet spot".

EGT's. Exhaust Gas Temp. gauges. A probe is placed in your head pipe(s). The guage then reads the exhaust gas temp and it is displayed. You still have to do plug reading to establish a base line to go from. But then changes in EGT's from changing conditions can be tuned to. Timing changes WILL require that you re-do your plug reading. Sometimes changes in fuel type(manu's even) and oil will sometimes also require you to reestablish your base line. EGT's do really make it easy, after you do your homework to get your baseline set.

Good luck!!!

 

Backcountry:

Excellent response! Thanks!

So, was the "sputtering" with the 168 most likely caused by a "lean" condition? If it sputtered at 168, but cleared up with a 175, should I go to a larger main jet?

Also, I am still not clear on the outside temperature. I thought the hotter it is outside, the larger jet you need (ie: 175 jet at 75 degrees = 178 jet at 100 degrees).

Is this right?

Since it was very cool on the day I ran the 175's, should I go up another size (178) since I usually run in +90 degree weather?

 

You have it reversed. The colder it gets the bigger the jets, the hotter it is the smaller the jets. Dave

 

Ruling out any other possibilities, yes you were most likely just over the edge of being lean.

Dave's post below is correct. Temp swings of 20 or so degrees will most likely require a slight rejet. But...if you leave your richer AM jet in and don't have any rideability problems later in the day, then unless you want to be absolutely positively spot-on, ride on!!!