1993 KDX250 idling woes...

[kmx125r]

Hello All,

My 1993 KDX250 currently has the following carb set-up...
160 main
40 pilot
modified #5 slide (1mm removed)
1368 needle 2nd from top
1.5 air screw
B7ES plug

Bike has a DG pipe and an FMF ISDE spark arrestor. I don't know if there have been any engine mods (porting, etc.).

When I first start up the bike it runs well for a few minutes. Then (when temp comes up?) the bike begins "surging" on idle. It oscillates between "normal" idle speed and the "surging" idle speed.

It continues to run well at 1/3 throttle to WOT, but surges on idle or minimal throttle opening. It does not foul the plug even after 20 minutes of slow trail riding, but the surging persists.

Any thoughts? Similar experiences? Thanks.

 

[G. Gearloose]

I had that same symtom when I had a loose lower hose clamp and lost coolant slowly and didn't notice it leaking.

Head temp increased and it surged at idle to 1/8 throttle only.

Perhaps the coolant is low, or there is air in the head that needs to get purged out with the vent-bolt, or both.

 

[Matt90GT]

Check your gas flow from the petcock to the carb. The Kawasaki petcocks after some years have issues with leaking and not sealing. When I turn mine off, it still will pour slowly.

Last ride I took my boy with me and he turned off the fuel. The idle would lope really bad. started back and sounded like I was out of gas. Looked down and the fuel was to off (but still leaking through). Turned it on, fixed the issue.

The other thing is you want to go down 1 on the pilot and 1-2 on the main. You are still rich there depending on the oil mixture. This is my setup:

ported head and boost ports
Vforce II reeds
DynoPort pipe
FMF Turbine2 core silencer/sa
new top end with KX .26 gasket, new kips installed and cylinder cleaned

Carb:

Pilot: 38
Main: 155
Slide #6
Air screw = 2 turns out
Needle: stock, 2nd from top clip

Gas/oil: Premium (need to run race gas 25/75 now) and 50:1 Amsoil series 2000

Plug: BR8EIX
Coolant: stock radiator cap, 1/2 bottle water wetter with the rest straight DexCool (the red GM stuff)

My bike flat out rips as is! It will lift the front end from idle in 1st thru 3rd. It is a fast bike to handle in the tight trails as extreme throttle control is needed.

My suggestion is to go back to the stock needle, drop it down. Drop the jets down and put in a 9 spark plug. I am going back to the 9 series plug also. The 8 works good, but I get a little detonation in 4th and 5th on the top end at times. The colder plug with better fuel should help that. Also by changing jets you can check that the car is not clogged. Clean the air filter and make sure the airbox to carb boot is sealed (ie take it apart and seal it with gasketmaker silicone).

 

[kmx125r]

Matt & Gearloose,

Thanks for the replies...

Coolant level is good. Doesn't appear to be consuming coolant at all.

The petcock works fine. If I pull the gas line from the carb and turn off the petcock it doesn't even hint at dripping...

I run Maxima at 40:1 using premium (92 octane) gas.

I had a 38 pilot in it and the surging "seemed" to be lessened when I went to the 40. But nevertheless, it still surged.

The 1368 needle is the stock needle.

I thoroughly cleaned the carb last month... it is spic and span inside.

I will get some smaller main jets... the previous owner gave me a few extra mains with the bike, but 160 was the smallest... I will get a 155 and 158 and give them a try.

I'll try the 9 plug, too. I had an 8 in it and the surging seemed worse and the plug actually fouled quite easily. I'll try the jets and a cooler plug and give it a go...

I have some more ammo to work with... any other ideas? I only have 56 miles of experience with the bike, so I'm still learning...

Thanks again guys. I'm always open for more suggestions...

 

[dirt bike dave]

Does the bike have a lot of miles on it? I have heard on some bikes the slide and the carb's bore for the slide will wear enough that the engine's vacuum can cause the slide to rock a little bit, effectively altering the throttle opening.

 

[kmx125r]

Hey DBDave,

I don't know how many miles are on it, but I would suspect quite a few...

But why the surging only after a few minutes of riding?

I have another carb body I could try...

Thanks...

 

[dirt bike dave]

As to why the surging when warm, it is possible that when cold the bike is running a little ratty and not burning all its fuel. After getting up to temp, the rings might be sealing better and the bike could be pulling more vacuum as it is generating more power at idle.

If you have a spare carb body, it is worth a try. I have heard of the slide sticking on high mileage 4 strokes, but 4 strokes tend to pull a lot higher vacuum at idle than two strokes.

BTW, my KDX250 used to surge when trying to maintain a constant rpm on the highway, but I attributed it to the powervalve 'hunting' between closed an partly open. The PV should not be impacting how the bike idles, but if the carb swap doesnt' help, you might want to make sure your KIPS valves are closing all the way.

 

[dirt bike dave]

Forgot to add if the carb isn't it, you might be developing an air leak, possibly at the carb boot, reed valve or left side crank seal.

 

[kmx125r]

Thanks DBD... I'll check it out.