Fouling lots of plugs

[dddford]

I just replaced the piston on my son's '96 KDX200 but it continues to foul plugs like crazy (daily). We are mixing fuel at 32:1 using valvoline 2-cycle motorcycle oil. The plugs are NGK BR9ES. We recently bought the bike used so I don't know the jetting situation. Any suggestions?

 

[Rhodester]

Try Mobil mx2t @ 40:1 to get a situation that is proven to give a clean burn to start with.

The 200 is jetted way rich from the factory. That's almost a given to what's fouling your plugs. Your cold plug doesn't help the situation either. Try BR8ES or BR8EG (my pet plug) and that'll help. If you're running a 160 main, try going to a 155. Your pilot will need to drop from the factory 48 to a 45 or 42 (depends on elevation and temp....in both the main and pilot choice). Start with your air screw @ 1 1/2 out and fine tune from there. Start with the needle clip in #3 or #4 and go from there. You'll be amazed at the difference in power as well as how it runs in general.

 

[mnnthbx]

I'm in Knoxville, so I'm familiar with jetting for the area. While each bikes differant, Rhodester's pretty much right on. It's all a jetting thing. I don't know how stock the bike is, but I'd head toward a 158 or 155 main, and a 45 pilot. I'd say start with a 155 and 42 if it were warmer, but you'll have a tough time starting it cold on the 42 around here now I bet.

I am not claiming these jets will give you your best performance, but I'd bet they're close enough to solve your problem, and aren't too lean.

Let us know what comes of it.

 

[Fred T]

The BR9ES is too cold a range plug, first try the BR8ES and that whould do it. The BR8ES is the stock plug on that year KDX so it shouldn't have a 9 in it.

Other things that help with fouling is the airbox mod and jetting, but try the plug first, it's a simple fix.

 

[dddford]

Thanks for the help

Thenks for the help, great information on jetting.

 

[NM_KDX200]

I use a BR7 and it seems to burn a little cleaner. It's almost certainly a jetting problem (and that includes plugs).

FWIW, I have _never_ fouled a plug. Never. Not in my XR250, not in my YZ250wr, not in the KDX200. I'm still using the same BR7ES that I put in the bike last year when I did a top end. I never fouled plugs even when I rode a '76 RM125 way back when!

 

[canyncarvr]

Have you been to CDave's site?

The basic information you need to get started on jetting is there. It is linked in the 'Every kdx rider should read...' link stickied to the top of this forum.

Yes, a 9 is too cold. Note that changing premix from 32:1 to 40:1 will make the problem worse..you are making the air/fuel mixture more rich with that change. Plugs are fouled by a too rich an air/fuel mixture, NOT too much oil (within reason of course...and many other things being 'good').

A 7 is close to too hot. Well, depending on how you ride and how you're jetted. Making jetting changes while using an ill-chosen heat range plug is a recipe for disaster. Changing plug heat ranges to make up for improper jetting doesn't accomplish anything.

The kdx 2-hunny will run forever on an '8' plug if it is jetted 1/2 fast right. That jetting is not only bike-dependent, but rider-dependent, too. What works well for rider 1 may not work well at all for rider 2. Let alone elevation, humidity and temperature differences from place to place.

 

[Rhodester]

Of course, CC, you are right. Assuming that he stays with the stock jetting he'll indeed have a richer air/fuel mixture at 40:1. I would suggest giving 20:1 a try if no jetting changes were suggested....and maybe a BR7ES too. I was just a little suspicious of the generic Valvoline 2 stroke oil he's been using. I thought his jetting results might be a little easier to read if he tried a good clean burning oil at a very typical and proven ratio (at least that's my experience with mx2t).

 

[canyncarvr]

re: 'I was just a little suspicious of the generic Valvoline 2 stroke oil he's been using..'

He is doing himself no favor running that oil...but, I didn't want to get into that part. Opinions and everybody has one and such.

It will work...just not very well. ;)