Spark Plug/Mixture Problem?

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Jul 22, 2003
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#1
1992 KDX250 FMF FATTY AIR BOX MOD. Only owned bike for 2 months. Bike was performing excellent except for eratic idle and oiling up new spark plugs after approx 50kms. Dismantled carby and found float level too high around 13mm and needle on top clip. Running 150 main 35 pilot standard needle R1368H and slide 5. Reset float to 16mm as per manual and refitted. Now bike will not rev out in power band. Quarter to half throttle is better. Have cleaned air filter and tried needle in all positions. Fuel is 33:1. Bike starts first kick hot or cold. Cannot do plug chop test as bike won't rev out, it sort of misses at W.O.T. Any ideas on which way to go would be great.
Note: No oil in left hand engine cover.
 
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Feb 16, 2003
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#3
sounds like a rich mainjet. by changing the float level, you change the jetting of the other circuits. Try going leaner on the main jet

Eric
 
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Jul 22, 2003
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#4
@Twiztid one

Running standard N.G.K. BR9ES. Have tried N.G.K BR8ES (hotter). for slightly better results

@ericlachance

Have ordered 152 main I will let you know once i have tried it (i will reply in this forum) Thanks for the help

-- Mickmax
 

boyo

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May 12, 2003
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#6
150 is too rich? As I know stock is 165 or so. As I could see lot of 250KDX are running with 155-165-er main jets.
I'm confused, I own a 93'er KDX250 and I tryed 165-158 jets, but I had the same result, I tryed the !122! jet from my 89'er KDX200 and it was much better, but tinkling ( jingling? ) at WOT on high rev. ( I think pure mix. ) and boiled the coolant out immediately.
It is very difficoult to get jets here in Hungary.
Can somebody explane me why there are so big differences between jettings in the same bikes. ( I know, I know... but I read all "jetting" stuff here in the forum )

I have stock pipe
pilot jet:42 ( too rich )
stock slide and needle
 
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#7
Altitude, temperature, humidity, air density all have their affect on jetting. Someone like Rich Rohrich, who have their bike jetted spot on, would have to change their jetting often, sometimes a few times a day just because of different weather changes. As for a 122 main, what you were experiencing was probably detonation from a too lean main. There's a reason they have small increment jets. going from 150 to 122 in one step is very dangerous(it could very easily seize the engine), not to mention dumb(no offense as you might not of known better.) But you did say that aside from detonating, your bike was running better no? You'd have to find the right jet, somewhere inbetween 122 and 150. But allways start from a rich main and go leaner in small increments.
 

boyo

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May 12, 2003
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#8
Thank you for the replay. Because of my bad english I couldn't explain well, it wasn't only one step. It was a quick stupid idea to use the 122 er jet ( I did only for 5 minutes, or less ) but It had a sense.
We have a bad Kawasaki dealerchain here, I ordered the jets 4 month ago - but still nothing. Jets I have are from different keihin karbs I have found in wrecks.
But anyway, my bike is running great now, Justkdx was/is a big help also for us here in Hungary. We are a few ( 8 guys ) with KDX and since we explored your side the handling and motorcaracteristic of our bikes is much better than before. Great Forum!!!!! Big help!!!!