1990 KDX 200 Running Issues

[Dasher]

I will try to keep a long story short and only give relevant info to finding the issue. Essentially, the bike is running poor but also inconsistently while progressively getting worse.

When I first bought it, it was running decently, not perfectly, but well enough that I thought it could be cleaned up with jetting. No part of the rpm or load scenarios indicated that there was a problem. We took it for a ride at about 4000 ft and it lacked some power to pull away at lower rpms. Such as starting out on a semi-steep grade. Leaning out he air screw helped the situation some, but did not fix it. As such, I believed that some jetting would fix this however, I didn't have jets with me on that trip. BTW, the plug looked a little rich, but it was not a fresh plug so not a good tuning aid.

The next time we had it out, it was similar elevation, but this time with a new plug and a leaner pilot (40 instead of 45). Now it ran like absolute crap, it would bog cut out quite bad and it was difficult to get it to rev up, but once it was on the pipe it ran fine. Well I ended up putting a 45 back in and continued to have the same issue, no change even though the pilot was back to how it was originally. I had also put a new plug in and after ripping in up and down the road to get a feel for how it was running, it had almost no color on it, indicating it was running pretty lean. Mind you this was the same with the 40 and 45 pilot.

After this I took off the flywheel cover and there was some fluid inside with some smut that was coming from the bottom of the crank seal. Naturally I figured the crank seals may be starting to leak. So I pulled apart the motor and replaced the crank bearings and seals. The mag side bearing was great, spun smooth with minimal play, such as a new bearing would have. The clutch side bearing, however, had more play, did not spin as smooth, and had carbon built up on the balls and races.

With it put back together, I took it out at around 3000 ft and initially it seemed a better but not perfect, so I let it warm up and set the idle speed and air screw and it idled wonderfully. I hopped back on it and rode it around and it not ran worse, same cutting out and bogging at low rpm. Mind you this is not with the same getting as when I received the bike, 45 pilot, 50 main, factory needle in middle position, factory slide. Also, now it would not idle even though it idled perfectly just before I hopped back on. So, then just for giggles I put a 155 main jet in and set the idle. It idled fine so I went to ride it again. Now it ran decently on the bottom, interesting I thought, then I opened up the throttle to get on the pipe and it absolutely hit a wall just when the power should have hit. Engine sounded like a rev limiter and stopped accelerating, but the bottom end no longer had issues. At this point it was getting late so we went to load up the bike. This area had a dirt bank that I could back the truck up to and drive the bike onto the bed. I started the bike up to ride it over there and now it ripped on the low end. I could wack the throttle and the front end would instantly come off the ground. This bike has never been able to do that, however, as soon as it should have hit the pipe, it smacked a brick wall again. As I went around the dirt bank I kept slowing down to idle speed, wacking the throttle, lofting the wheel, then smacking a wall and having not power with a rev limiter type sensation. When checking the plug, it still indicated a very lean burn. As I understand it, for this elevation a 45 pilot would be on the rich side, with the 150 main being about right, however it just wont run right.

After getting it home I decided to pressurize the crankcase and search for leaks. I made up a plug for the carb boot and end of the pipe and pressurized the case to about 4 psi. After a little over 10 min is was just under that. My setup just used a schrader valve and I used a tire gauge to measure it. Not the best setup, but its what I cant do. So the small drop in pressure could be accounted for by the air loss by using the tire gauge. I then pressurized 5 psi with a similar result. Then, I decided to go to 7 psi, after about 5 min I checked it and it had dropped to in between 5 and 6 psi, definately more drop than before, but also might be too high of pressure. I then left it for about 30 min and came back and it was still in between 5 and 6 psi. Not bad I though, maybe 7 psi was just too high. I came back after work the next day and it still indicated somewhere around 4 psi. So the crankcase does hold pressure.

That's the end of the saga, this short version was still too long but idk how else to get the info written down. I also put a new carb boot on just to be safe, which didn't help anything. I have been using 91 octane non-ethonol with a 40:1 premix. BTW, carb is most definately clean and timing has been set at factory, middle, position. Any help would be much appreciated as I would still like to get some riding in before the season is over. If not, this bike is getting a complete rebuild over the winter.


TLDR: Bike runs inconsistent and very poorly even when nothing has been changed and with normal jetting. Crankcase bearings and seals have been replaced and new carb boot installed. Carb is clean and timing is set in middle, factory, position.

Thank you all for any help.

[KDXGarage]

reed condition?

plug in use?

did you soak the carb in Berrymans or other dip?

Have you read up on the jet block gasket?

[Dasher]

reed condition?

plug in use?

did you soak the carb in Berrymans or other dip?

Have you read up on the jet block gasket?

The reeds are in perfect condition and as far as I know a bad jet block gasket will cause an overly rich condition and I am having the opposite.

I haven't soaked the carb, but have otherwise given it a thorough cleaning.

Originally it had a factory br9es but on the last outing i switched to br8es.

[2fastlx]

Have you rebuilt the carb? Are you using quality parts in the carb. I had a similar issue after rebuilding the carb with an ebay parts kits. It had a terrible lean bog off idle. If i snapped the throttle it would bog but if I slowly rolled into it, it got better. I changed out the idle air screw and spring to a factory one and it solved the problem. I changed the pilot jet 3 sizes with no change in performace which led me to the screw. If that doesn't help I would suspect a large air leak somewhere like crank seal or the gasket on the reed cage if you replaced the boot already.

FWIW have you checked the KIPS?