Jump to content
Sign in to follow this  

KX250 Plug Fouling

Recommended Posts

Hi All,

 

I was hoping to get some feedback with regard to a problem I've been having with my 2000 KX250. It fouls plugs after 30-45 minutes of riding time. If I put a new plug in, it starts right up again and will run another 30-45 min. When first started cold, if I use the choke, the engine will rev as though the throttle is being held wide open until the choke is closed, at which time it will run normally. I'm running Yamalube 40:1, and stock jetting. I leaned out the main jet one size and it had no effect on this problem. The bike has an FMF turbinecore silencer and the stock expansion chamber. The bike is almost certainly overdue for a new piston and rings. Any thought on this would be greatly appreciated. Thanks,

 

Ian

Share this post


Link to post
Share on other sites

Its probably running rich on the pilot and needle.  I bet you're getting blue smoke out of the exhaust pretty much all the time right?

 

Are you using the iridium plugs?  I get years out of them, and when I do have to replace them its due to wear on the terminal where the cap attaches.  I have not had a plug foul out in many years.

Share this post


Link to post
Share on other sites

A few things come to mind here:

Yes you should rebuild and get things back to spec. Needle and pilot jet can be leaned (And is most likely the problem) another thing to think about is the mixture. I run Yamaha lube R at 32:1 . This is actually a leaner fuel mixture than 40:1 (remember we are jetting for fuel not lubrication). Also the main jet does have “some” (more so as the slide opens) effect over the entire range so that should looked at as well. I can’t tell you what to jet to as your environment is specific to you, if you get my meaning here.

Another thing I’ve seen happen is the grounding wire (top coil ) comes loose and can cause problems like your experiencing. So pull the tank and check the wiring.

Edited by jim9006

Share this post


Link to post
Share on other sites

Well heres what I think the main jet and needle play a big part on how your bike runs at the bottem end and all the way through the rpm range , and as far as the 2 stroke ratio goes and this is the real deal ,the less oil you have in your gas the less lube your crankbearing and piston rings and piston skirts are going to have that's why a lot of people use 32-1 because its a pretty good mix back in the day Honda was telling every body that had two stroke three wheeler to use 20-1 that's a lot of oil but any ways if u don't belive me goggle it but Heres the other thing if u really want to know what's going on with your bike and want to fix it check your compression , do a leak down on the motor and make sure the crank seals aren't leaking if that parts come back good then u should start by leaning the carb out slowly meaning lower the clip on the niddle one notch and ride your bike to see if it changes if seems to get better go another notch and ride it again if its not fouling and it seem to run really good leave it

Share this post


Link to post
Share on other sites

The other thing I wanted to tell u is the main jet and needle are your most important part of the fuel system that deturmens how rich or how lean your bike should run your polit jet is only for start up and idle you should go on line and tri to look up specs for your bike so you can put the carb back to factory settings

Share this post


Link to post
Share on other sites

I had the same problem with an old RM250 once. I'll bet the plugs look good when you take them out too, right? The intake boot was cracked/split, almost inperceptable, but leaked enough that the plug fould after a short time running. It was like it wasn't properly formed by the manufacturer. A new plug and it ran like new again. Replaced the boot and never had the issue again. Try to check for leaks anywhere between the carb and the engine.

Share this post


Link to post
Share on other sites

Hi All,

 

I was hoping to get some feedback with regard to a problem I've been having with my 2000 KX250. It fouls plugs after 30-45 minutes of riding time. If I put a new plug in, it starts right up again and will run another 30-45 min. When first started cold, if I use the choke, the engine will rev as though the throttle is being held wide open until the choke is closed, at which time it will run normally. I'm running Yamalube 40:1, and stock jetting. I leaned out the main jet one size and it had no effect on this problem. The bike has an FMF turbinecore silencer and the stock expansion chamber. The bike is almost certainly overdue for a new piston and rings. Any thought on this would be greatly appreciated. Thanks,

 

Ian

 

 

Main jets don't typically cause plug fouling, because at larger throttle openings there is enough load on the engine to produce enough heat to burn off deposits.

 

At sea level, the stock jetting on the 2000 KX250 was actually pretty good.

 

I would check the reeds for chips and fraying. Then mix the fuel at 32:1, as this is the ratio the bike is jetted for from the factory.

 

Your screaming idle during a choked cold start-up indicates a rich pilot circuit, so that is where I would start.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×