KDX250 blowing spark plugs

[trevor morgan]

My 92 KDX250 keeps blowing plugs, it gets through a plug every 6 to 10 hours of riding. The standard plug was a BR9 which would last a couple of hours so changed it for a BR8EV racing plug which I think runs hotter and does last longer but I'm sure they should last longer than this. The bike does get ridden hard as I only use it on motocross practise tracks. Does anyone have any advise, should I try a different grade of spark plug like a BR7EV ? or would this be too hot, don't want to hole the piston. Thanks.

 

[dirt bike dave]

For trail and duaI sport I used a BR7ES in my KDX250 with good results. But your big problem is the stock jetting is way way too rich. You will want to go leaner on your main jet, pilot (slow speed jet), needle and if the budget permits change your slide, too.

There is a member, I think his name is duc748, that has a web page where he has assembled tricks and tips for the KDX250. Search for his name, find his webside and follow the jetting recommendations.

 

[trevor morgan]

I've used his web page quite a lot and have carried out most of the mods like 160 main, 38 pilot, ground 1mm off the slide, air box mod. I've also added a DEP sport front pipe and a tail pipe from a 92 KX. Done a plug chop and it looks a good colour. What ever mods I've done it has always fouled plugs the same. I am a bit nervous about going a grade hotter on the plug due to the track I use having a straight on it which is nearly 3/4 mile long involving holding the bike flat out in top for a good 20 seconds at about 75mph, worried it may melt the piston. How about the oil i use, silkolene comp 2, is that known to foul plugs? I mix at 40:1 and still quite smokey.

 

[G. Gearloose]

A 'good color' plug chop is still too phat. Also you mentioned everything 'cept the needle.

 

[trevor morgan]

I have lowered the needle but I haven't changed it. The plug never seems to breakdown when I'm out on the track and working the engine hard, but when I'm either in a slow woods section or just ticking over and then go to open her up the plug will foul, not every time, but if its going to go it will happen at low revs. I know what your saying about jetting etc but I really feel I've got that spot on because it runs fantastic. I will have another mess if you think thats where the problem lies, just wondering if there's anything else which may cause this?

 

[G. Gearloose]

trevor are you 'cleaning' your plugs or getting new ones?
Also, if you were indeed spot on, you shouldn't remark 'quite smokey', don't assume its the oil's fault.

Drop the needle til it falls flat, then bring it up. If you are out of clips or one intermediate throttle position is going lean markedly before the rest, you likely need a different needle profile.

 

[trevor morgan]

I replace the plug with a new one each time, the old ones will not work again even when cleaned and gapped.
I will try the needle position, it will take a while to see if this works as the plug could last only a few hours or may last up to 5 or 6. I am also going to move off the silkolene and try Motul 600 fully synthetic, quite expensive but supposed to smoke less than other oils.

 

[dirt bike dave]

How many turns out is your airscrew? It has quite an impact on low speed running.

If you have your owner's manual, the needle you want is the optional one with the highest #.

When I first got my KDX250 it had stock jetting except for a slide with 1mm more cutaway. It fouled a plug in the woods my first ride. Cleaned up the jetting and it very rarely fouled a plug on me after that. Many of the jetting specs on duc's page are what me and my buddy developed for our bikes through trial and error.

 

[trevor morgan]

Thanks for the input, I know for a fact the needle is in the 3rd clip from the top so I will drop it to the 2nd when I get chance at the weekend. The following weekend I'll be on the bike for about 5 hours so that should give it a good test, will take a spare plug just in case!

 

[trevor morgan]

The air screw is 1 3/4 turns out, seems to run best like that, have played about with the air screw a few months ago for half a day whilst practising. Maybe when I drop the needle I will have to readjust the airscrew anyway and it could be these two things contributing to the fouling problem.

 

[G. Gearloose]

Yes, Set the air screw each ride for best off-idle power. Anything then amiss is then some other circuit's responsibility.

If your changing oil anyway, consider Mobil Mx2T. Verify the airbox has been modified for better airflow.

 

[Matt90GT]

Here is what I ran on my KDX250:

Jetting: 38/158 , R1367H needle 3rd clip from top, #6 slide

Then used the BR9EIX plug. Never fouled one after doing all that to the carb. I ran Amsoil 2000 @ 50:1 with premium and sometimes a 50/50 mix.

 

[trevor morgan]

Thanks for that Mat90gt, I will order a new needle. I have tried to get a smaller main jet before, mines a 160, but 160 is as low as they go at a Kawasaki dealer over here, does a jet off another bike fit?