Blowback into airbox

[NM_KDX200]

Went riding in the mountains yesterday at 8,000-10,000' elevation. I'm jetted for 6,000' and expected the bike to run rich, but it seemed to run fine all day. No bogging, no plug fouling, no problems at all, so I left my current jetting (148 main, 38 pilot, needle all the way down). However, when I was loading the bike on the trailer, I noticed some oil on the swingarm. At first I thought I'd blown the shock but after tracking the oil, I found that it was dripping from the drain in the airbox. Popped the seat and sure 'nuff, the airbox is coated with oil, plus I can see where small pieces of foam are sticking out from the air filter. Up front, there's some spooge dripping from the exhaust- not normal on this bike as I have gasket sealer up there.

My riding alternated between putting along in front of an ATV and XR80, and hammering it down open roads and up steep hills and such. I also did some major uphill climbs followed by- you guessed it!- major downhill grades. We're talking miles of uphill, turn around, miles of downhill. On the downhill, I tried to keep the engine buzzing, but there was no real load on it. My guess is that the bike was running rich, building up oil, and then blowing this back a little bit when I had opportunity to hammer it, or the long downhills let it blow back.

When I get a chance today or tomorrow, I'll check the reed
valves, and remove the air filter to look at it. I'm also going to order some leaner main jets for next time (148 is as lean as I've got and the charts say 145 might be better).

Any other suggestions?

 

[strider80]

Could it be coming from the tranny breather line that hooks into the airbox (at least I think there is one on KDXs)?

Maybe you have a bad reed allowing some blowback through the carb?

 

[bikepilot]

It shouldn't be possible for the bike to spooge into the air box. Perhaps your filter was a bit over-oiled and some of the oil driped off it? Of course check that it didn't come from the transmission vent or leak down from the joint between the pipe and silencer. I'd replace the air filter as you don't want to take a chance of dirt entering the engine.

Lastly, I'd check the carb vent lines, often they are routed into the air box, and perhaps they spewed a little fuel into the air box.

good luck

 

[NM_KDX200]

I think the problem was that the drain vent at the bottom of the air box is worn. When I went to check things out, I found water at the bottom of the box and was able to squeeze some water from the filter itself. I was wheelying thru numerous puddles, some of them over the axles, and I bet some water got splashed up thru the drain vent. The air filter was clean on top, but muddy brown in a little area right above the vent. I didn't find anything else amiss, so I bet that enough water splashed in there to pick up some oil from the filter and mess it around. This, of course, drains right onto the swingarm.

 

[adam728]

If your reeds are getting worn out they will hang open a very slight amount. This will cause positive pressure out through the carb, causing some spit-back. It's not uncommon at all. It's possible even with good reeds or on a 4 stoke, as the intake shuts off the air moving though the carb will "bounce" off the reeds or valve and cause some small spit-back.