Help diagnosing bogging problem...

[Micahdawg]

I rebuilt my 1989 KX250 and it runs good except after each my spark plug is usually toast. I can ride all day long, but when I get the bike out next weekend it'll fire up and while running (choke on or off) it will start to go "Burb burb burb.." and die out only to have an oil fouled plug.

I have standard bore, fresh rings, crank seals, bottom + top end, but have some old reed valves, unknown carb jetting, and an old, but clean air filter. Oh, and I run Yamalube premix at 32:1.

The bike has always run good, but won't keep a plug in it.

Last week I changed to Klotz premix at 40:1 with a fresh B8ES and it ran good for about an hour before developing a stumble. When I get out on the straights I would nail it and the bike would drag like the plug was fouling.....blip the throttle a couple times and it clears out and pulls good. I rode like this for a couple hours with it wanting to foul (or so it felt) but it would clean back up?!?! Before...when it would start to act fouled...it would die soon with no restarting.

Does this sound like a familiar problem? I don't want to replace anything unnecessary.

The bike also had a weird surging problem where the idle would slowly raise up and if you blip the throttle it would go back down. I was told it was an airleak, so I tightened the carb boots and it seemed to take care of it only temporarily before acting up again. Could this be contributing to my plug issue?

Any suggestions would be welcomed.

Micah

 

[sodflyer]

If it's fouling plugs it does not sound like an airleak to me... have you tried rejetting the bike? Tell you what, I run 32:1 but the other day I had someone fill up the gas can and I told them 5 gallons... I think he put 5 bucks in there as filling one bike up it felt pretty light after... My bike has never ran better! point is... my bike fouled plugs like yours is doing untill I got it jetted close... good luck.

 

[sodflyer]

Might be the float level as well....

 

[UK KXER]

That sounds like a jetting problem to me is it just boging from a standing start if it is try turning your air screw out half a turn out this will lean out your bike and it should run a bit better

 

[Micahdawg]

I popped the plug out last night and it looked dark/black deep into the threads. The electrode and tip where light colored and immediately around the electrode was dark with only a small section showing some brown. Sounds rich to me.

I also took out the throttle slide and my needle is set on the 3rd clip position. Guess this weekend I'll pop the carb off and check to see what jets are in there.

Does anyone have a clue what size jets I should be using. I'm only about 500 ft over sea level and I ride usually in 60-90 degree weather at each extreme (Fall/Summer). Bike is at standard bore with FMF pipe/silencer and filter but thats it.

I'll also be checking the float level. I hear that the bottom of the floats should be roughly parallel to the mating surface (or 16mm) is this right?

Thanks for the tips guys.

Micah

P.S. the bog does not seem to be directly from a standing start. Seems like it will immediatly respond ok but then bogg throughout the midrange and into WOT if I keep the throttle pinned. Only when I let off the throttle momentarily and get back on will it pull good. As it drags down it does not make a popping noise though...but rather a "booooooooggg" like the kill switch is activated and it's cutting spark. This only started happening when I changed from 32:! Yamalube to 40:1 Klotz. Guess the richer fuel mixture is accentuating an already rich jetting issue?

 

[sick 96 250]

go to fmfracing.com and look at the jetting section and type in your bike and year and it will tell you what jets you should be running. These settings seem to be pretty acurate, as i changed my setting to fmf's standards and my bike runs great now.

 

[Micahdawg]

Awesome site...thanks for the link. I printed out their recommendations and will check to see what I have.

I do have a stupid question though....the air screw is different from my idle adjustment screw right? And I assume 1.5 means 1.5 turns out from bottomed.

Also, most idle screws that I've seen have a flathead screw with a spring to keep tension on it. This Keihin carb has a flathead screw with a nut on it. Is this a factory set up or did someone "rig" this?

Micah

 

[sick 96 250]

i'm not sure exactly what u mean. If you are talking about a nut being on the flatthead part that you have to tighten to keep the air screw from turning out, that sounds like a rig job, but i don't know about that year carb. Every air screw i've seen has had a spring on it that keeps the tension while it is a flathead. And yes it does mean 1.5 turns out from the bottom. And on your carb i'm pretty sure you have 2 seperate screws. One for air and one for idle, they are two different screws. Your air screw should be the smaller of the two. On my bike i have one for the air screw and my idle is my choke knob.