'89 KX250 tuning tips... Where to start?

[keithluneau]

Ok, I've just got my '89 250 going. It's got a steel sleeve, new Wiseco stock bore piston and rings, new wrist pin and top bearing. It's got a Pro Circuit pipe and silencer, Boyesen power reeds on the stock cage, a TwinAir filter.

I've got more than enough power at the top end, and the bottom end is strong enough. I will not be running this on a track, mostly gravel roads and woods/trails. I could live with loosing some power on top to gain some in the bottom.

I know I need to mess with the jets, the bike runs great in the top, but seems rich in the bottom. If I ride it slow it'll load up and bog until I clean it out. It will eventually foul the plug, which comes out black. I also don't know how old the reeds are in there, but if I hold them up and look through them, I can see light, there is a tiny gap.

So, I want to start with new reeds. Should I go with the same power reeds that are in it? Are there any better, reasonably priced ones to look for? (VForce are a little more than I want to spend) Once the reeds are in it, should I keep the PC pipe and silencer? I would like a FMF Gnarly for the low end power, but I don't see one for the '89s. Any other good pipes? What about silencers? Will any brand silencer fit any brand pipe for the same bike? Any good ones better than the others?

Once I'm settled on the reeds and pipe, what about the jets in the carb? What's a good starting point? I live in Louisiana, which is mostly hot and humid, and less than 100ft above sea level.

So basically, I'm looking for a good setup for this old bike, and a starting point for fine tuning it. This is the first two stroke I've owned and worked on myself, and don't know much about this part.

Thanks for any ideas!
-Keith

[Hillclimb#42]

Great to hear its running now. What was the issue that was holding you up?
Looks like a kehin for your bike originally had 172 main, 55 pilot, and clip on the third (center). Sounds like you are having a float problem. Be sure they are set right, and that the overflow tube on bottom of carb, is not clogged. Kam had a good trick on that. He puts a piece of fuel line on the carb, blows through it and makes sure the floats shut off the air-flow when they are level. Either that or maybe a super rich pilot. Cleaning up the jetting will give it lots of power increase. Sometimes, if there are no broken corners, you can flip the reeds over, to get them to seal. Lots of people think that the internal pressures would cause them to close tight when they need to, and they may be right. You just can't compensate for broken petals. Then I always worry about any broken pieces going through the motor.
 Pro circuit recommended a clip adjustment of one step leaner for my bikes. I like the pro circuit pipes, but FMF is also a good pipe. I think you can mix brands from pipe to silencer, but are risking a little power. There's a better chance that the manufacturer designs their pipes and silencers to work together. But I have a FMF silencer on my 500's PC II pipe and also used to run a FMF shorty on my PC works pipe on my 250. Good Luck

[keithluneau]

Thanks, this is great info! I'll check the carb out first, the floats, jets, needle, and go from there.

The original problem was many, but mostly two stripped holes in the cylinder which was stopping the head from sealing. I put helicoils in there, and a new head gasket. Then there was the rusted flywheel and stator, bad water pump seal and bearings, etc. It's running ok now, except for the rich low end.

I'll keep you posted on what I find out!

[keithluneau]

Ok, so I finally got around to pulling the carb off my bike, and checking things out. Here's what I found...

The pilot was tight, and is a #55. The main was also tight, and is a #175. The slide has a #7 on top, don't know if that matters. The needle clip was on the second from the top, which would make it one notch leaner than stock for the Pro-Circuit pipe. The float level looks good, stop air flow through the inlet when the float is almost perfectly level with the bottom of the carb.

I fixed the overflow tube while I was in there too. It was broken off at the bottom of the bowl, and plugged to stop fuel coming out. I drilled the brass tube out of the bowl and pushed in a little length of carbon fiber tubing I had, the only thing I could find that fit. I don't know how the gas is going to effect it yet though, but so far so good.

So, the only thing out of place is the 175 main jet, as opposed to the 172 stock. I did go ahead and drop the needle down to the last notch to lean it a bit more, and it seems to run better now. It still needs the idle screw bottomed out on the carb body to idle though, and still loads up when it runs slow, but not nearly as bad as before.

I also adjusted the air gap for the pickup next to the flywheel. I don't know what the '89 is supposed to be (I really need a manual ) but I read where a guy was having trouble with a 2006 running bad and pinging, and they told him to widen the gap to 0.039", and that seemed to work well for him. So I set mine to the same thing, and ran the bike again, that helped as much as dropping the needle did. It idles SO much smoother now, and seems to run better with more power. I think I'll leave it alone as it is now, unless someone can tell me more about the main jet, and the air gap for the pickup. Any idea why someone went with a larger main jet, and why the bottom end is still so rich if it's got the stock pilot jet in there? The air screw on the carb doesn't seem to help, only makes the transition better or worse, I ended up with it between 1 1/3 and 1 3/4 turns out.

[Jopiz]

Boyesen Power Reeds could possibly need a little smaller pilot jet?

Here is what Boyesen recommeds for '89 KX250:

STOCK: 172 main, 52 pilot, needle 3rd notch, air screw 1 1/2 turn
Power reeds: 164-170 main, 52 pilot, needle 3rd notch, air screw 1 turn
Rad Valve: 165-170 main, 55 pilot, needle 3rd notch, air screw 1 1/2 turn

EDIT:

Here is a pretty good write up about adjusting fuel level. http://www.allthingsmoto.com/forums/f-15/how-set-your-motorcycle-carburetor-float-level-more-commentary-13608/
And some useful stuff: http://www.duncanracing.com/techfaq/Tech_keihin-carburetion-jetting.phtml

[keithluneau]

Thanks! That's all great info!

Ironically I wrecked this bike yesterday, and now have a broken collar bone and torn ligaments in my shoulder (hit a log laying in the weeds, didn't see it until it was too late). So I guess I've got plenty of time to read up on tuning my carb right while I heal...

The bike is fine though, not even a scratch!