KDX 250 known for lack of top end ?

[GhostofSilverider]

My KDX 250 (SR model recently bought used) feels to me like it lacks a bit of top end power . The restricted stock pipe was opened, and it has an aftermarket silencer, the packing sounds good to me.
This 91 model is in this setup probably similar to the enduro model of that year.

First gear in particular tops out very quickly (gearing is stock) , in that gear it sounds like the valve is not opening or as though I have water in the airfilter, in other gears the bike runs well, but I am disappointed with the power when the throttle is wide open, it just doesn't keep going like a good motocross bike, the engine sounds like it's about to have a heart attack and I don't like to stay wide open for very long for fear of seizing. This is with a jetting that was way too rich (piston black and wet, spooge leaking) but I have recently removed the airbox cover and installed a Twin air filter (which btw is slightly smaller) , it now feels a bit more responsive but no improvement on top end. I am not too keen on playing with the jets as I heard this bike is finicky , next step for me is moving the needle and playing with the airscrew. But that won't improve the top end in first gear I suppose.

The bike has 1300 miles and is otherwise in exceptional condition, a valve issue is probably not on top of the list I would say. My last 250 2 stroke was a new Yamaha YZ back in the 90's, and I recall the engine was very smooth yet ripped at the top end, it never sounded like it was about to seize. Is this behaviour normal for a KDX 250 ?

 

[dirt bike dave]

Not typical of the non-SR model, and certainly not typical of one that is jetted right.

The fact that it is worst in first gear points to overly rich jetting. Basically, when the motor is under heavier load in the higher gears, it needs more fuel and it sounds like the jetting is closer.

If the SR is anything like the non-SR, you really can't get the bike to run properly without addressing the jetting. Kawasaki really missed the boat.

On my old '91 non-SR, the powervalves always functioned properly, BUT when fully opened the bottom of the opened powervalve was still at least 2mm above the exhaust port. I used a dremel to take the excess material away from the bottom of the power valve and gained some top end power. But if you are not willing to jet the bike first, I'd leave the dremel alone, lol.

 

[GhostofSilverider]

I have a jetting problem because despite opening the airbox,
the bike still fouls plugs and I haven't been riding at excessively low rpm . The plug is still black only less wet.
I don't get it, removing the lid should be the equivalent of dropping 10 points on the main if I take the numbers I read in an article about this very model.
I am refering to a March 1992 issue of Moto Verte , the French dirtbike magazine. I have the same model, according to the then # 1 Kawie dealership involved in racing, the removal of the airbox cover along with derestricting the pipe calls for a change of the mainjet from 152 (stock they say) to 165.

I don't know what 's in the carb and will look at it but the previous owner must have put an even bigger mainjet. And even then what I don't understand is that with all that the engine doesn't sputter and works well except for what is for me a rather minor lack of top end power. In my mind a jetting that's too rich leads to an engine with a noticeable poor performance.

So my question is : Is there something unrelated to jetting that could foul plugs (i.e. wet combustion chamber) ?

 

[dirt bike dave]

Well, no way removing the lid on the airbox is the equivalent of leaning out the main jet by 10. Opening up the exhaust likely had a bigger impact.

Does your bike have the side barrels and top KIPS valve like the '91 US model, or only the top KIPS valve? Maybe the SR is noted for no top end... never seen one in the USA.

Besides jetting, things that contribute to fouled plugs are:

Weak spark
Bad plug
Too cold a heat range on the spark plug
Restricted intake or exhaust
Air filter oiled too heavy
Fuel leaking into the cylinder from carb

 

[Rich Rohrich]

You can add leaking right side crank seal to Dave's list of possible plug fouling factors.

 

[GhostofSilverider]

thanks
fuel is leaking a little from the petcock , otherwise I wouldn't know if the carb leaked or the crank seal was bad (although no apparent drop in oil level) . I do not know what the side barrel you are referring to is .

I guess I am in for a carburetor removal and inspection.
I have read on other forums that this model is really a bitch to jet properly and many owners just keep having problems with plugs or cold starts.

 

[dirt bike dave]

The cylinder for the US model '91 KDX250 is shown below. The view is from the intake side. The KIPS valve on this model has two side openings. A barrel with a groove opens and exposes an additional exhaust port on each side of the main exhaust port. The barrels are 12005 and 12005A.

FWIW, the US model came with a 165 main jet and a 42 pilot. Most riders found big improvements with a 158 at sea level or a 155 at altitude, and a 38 pilot. If you want to get the most out of it, you also replaced your slide for one with 1mm more cutaway.

Also, if the petc*ock is leaking and the bike is leaned over alot on the side stand, you can fill up the cylinder with fuel overnight. That'll foul plugs, and could even cause worse problems.

 

[GhostofSilverider]

thanks I shut off the petcock as I indeed heard about gas ruining the seals in the engine. The gas leaks outside, I find it on the engine cover, it's not much and comes from the screws on the petcock. The bike is on its side stand and fouled plugs tend to happen after the bike was garaged overnight and once after a city ride (lots of idling ) .

 

[dirt bike dave]

Are you using a BR9ES? I think that was what kawi recommended for the US model, which was probably too cold a plug. The guys I knew would use a BR8ES for dual sport rides, or an even a hotter BR7ES for tight single track trail riding.

 

[GhostofSilverider]

yeah I think that's what I use. However the plug is still black and once fouled is wet. Previous owner had a BR8ES on it if I am right and it didn't last long either.

So according to you, given a piston covered with spooge how many sizes should I go down on the main ?
And can you give an estimate for the drop in size for the pilot, knowing the reduction on the main ? (I suppose an X point drop on the main should be matched roughly by a Y point drop on the pilot ) . For example if I put a 165 main jet , what pilot jet size would be appropriate ? Between 42 and 40 ?

I think I may try the 165 as recommended in the article, however it would be a bummer if that's what I already have or if I have an even smaller size.

 

[GhostofSilverider]

btw I do have the same cylinder as on the PDF you posted,
I wonder if there isn't a spooge leak from that screw on the left (#92066) , said screw I would like to replace or thoroughly clean up as it got very rusty. What is its purpose and can you safelyy replace it without removing the cylinder ?

 

[GhostofSilverider]

I opened the carb today, I have got a 152 mainjet (as stock), I could not see what th epilot jet was . I tried to remove it with a screw driver but it would not budge.

So I have a stock jetting whereas I should be having a 165 main to accomodate the pipe and airbox mod. However the bike is too rich and fouls plugs.

My petcock leaks , that would be the explanation for why plugs are fouled overnight. However would the spooge be caused by a leaking petcock as well ? It does not make a whole lot of sense to me, and I fouled one plug while riding (10 minutes after a 3 hour stop granted) .

There are 2 little screws on the petcock, one is loose and can't be tightened. It leaks from that screw, and it leaks into the carb as well (one drop every 3 seconds) as the shut position doesn't stop the stream. Is there a known issue with that petcock and a way to fix it ?

 

[Tom68]

Search my sr posts, aus delivered KDX, running with oil injection near standard jetting and a BR9eix for tight technical single trail, never fouled a plug.

These bikes have poor power below the power band as there is no port height timing valve, just the side barrel valves they only have a 34mm carby so you won't get much top end.

Mid range power is good and you need to be shifting all the time to keep it there.

1st gear is absurdly low and the gap to second enormous. The gear box is a wider ratio unit than the E model and with 15/52 I can walk beside mine idling in 1st yet top speed is potentially over 80mph.

Love my nearly 300lb KDX, currently leading fortnightly rides at Kinglake Victoria in tight tech' terrain.

 

[EYY]

Kinglake mate? I'm not far away. Good riding up there.

 

[Tom68]

Kinglake mate? I'm not far away. Good riding up there.

I like it, we ride from Extons Rd carpark Kinglake West.
http://www.dirtbikeworld.net/forum/showthread.php?t=77848

 

[jgas]

I helped buddy sort out one of these back in 92. I think it's been mentioned, but I remember getting the most added power with a pipe and silencer. The stock ones were very restrictive. My stock kdx 200 was faster. His may have been built on a hangover Monday, because it was a constant problem bike. It ate waterpump seals, blew fork seals, had erratic jetting issues, just weird little things. Other guys that had the same bike had no problems. I think these bikes had either a heavy crank, and/or flywheel, so if you are used to an MX bike it will feel like it revs slow.

 

[jaguar]

My KDX200SR had an extra perforated pipe inside the header to reduce pipe noise but it also reduced performance. I replaced it with a ProCircuit pipe and was so glad I did. I plan on building my own pipe for an SR I am planning on buying. Mine had the "port height timing valve" (top of exhaust port). It was an '89. see http://www.dragonfly75.com/moto/KDXpowerband.html

 

[EYY]

My KDX200SR had an extra perforated pipe inside the header to reduce pipe noise but it also reduced performance. I replaced it with a ProCircuit pipe and was so glad I did. I plan on building my own pipe for an SR I am planning on buying. Mine had the "port height timing valve" (top of exhaust port). It was an '89.

I couldn't justify spending money on something that was going to get wrecked anyway, so I just cut mine open, removed all the insides of the pipe then welded it back together. After jetting, there was a massive increase in power everywhere. Easily as good as an aftermarket pipe

 

[jaguar]

Thats good to know. I can do that and then weld on a different diffuser/belly/baffle section of my own design. I will also make the stinger start at the belly to reduce noise (especially needed since this bike dont have the good original silencer, just a KX one).