How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Doug E. Your Own Question
Doug E.
Doug E., Kawasaki Master Technician
Category: Motorcycle
Satisfied Customers: 2886
Experience:  Professional motorcycle mechanic since 1978. Vintage motorcycle restorer.
Type Your Motorcycle Question Here...
Doug E. is online now
A new question is answered every 9 seconds

Why does my two stroke bog down at half throttle It revs good

Customer Question

Why does my two stroke bog down at half throttle? It revs good to halfthen holds and bogs. 1985 two stroke Kawasaki 200 kdx. Idles good, I can hear that there's barely a difference in the way the engine sounds, it sounds like a slight hesitation or miss. New plug, cleaned tank and petcock, cleaned carb and measured float. All is working, I believe... Right now I just dot know. Good compression even rebuilt top end just for the heck of it. Looked brand new inside like someone recently did it. Still a little crosshatch
Submitted: 7 years ago.
Category: Motorcycle
Expert:  JonRK replied 7 years ago.

This may sound weird, but I bet it will help you out of your situation. I want you to remove your muffler. Once off check the exhaust port and make sure that there is no carbon built up inside it, if there is, clean it out. Now get some hearing protection and start your mower. Does it run up to full speed? If it does then your muffler is most likely carboned up, you may be able to clean it out or you may have to replace the muffler, either way you will be up to full speed again.

Customer: replied 7 years ago.
Just ro verify, remove my exhaust pipe and clean the exhaust port, it's a two stroke dirt bike, 1985, u can remove the exhaust and see the piston fine. I'm going to clean up what's there like you said and try it, I haverepacked the silencer on it, the bike ran great, I been riding it fr months, I rode out to a fiends, didn't feel like riding home and put it in my roomates truck, as i've done quite a few times before, and came home. Next day this situation strted immediately. Justwondering if any of this changes your diagnosis
Expert:  JonRK replied 7 years ago.

It does, I read this as being a 2 stroke lawnmower.

You can still check this, but I doubt that it is your problem as dirtbikes run wide open most of the time. I will opt out and let someone better with dirtbikes help you out.

Expert:  Doug E. replied 7 years ago.

When did this problem first start happening?
If you hold the bike at full throttle then shut it off and pull the plug, what color is the plug burning?
Did you notice any oil behind the ignition cover when you worked on it?
Does it smoke more than before?

Customer: replied 7 years ago.
No oil in ignition cover, had sprocket seal leak, replaced it, at wide open it looks clean and wet.
I have been ridin it for a while this spring, it had a loose ignition wire on the coil, I replaced end and bike tan great for three months. I rode it an average of three miles a day for fun, not hard on it, and every other day about fifteen miles. Trails on the side of the roads for riding on here. I rode it 7 miles one day, didn't want to ride home, I did have a small low speed fall over on the trail, bike didn't seem to care. Rode to my location, parked it for an hour, didn't want to ride home, put it in a truck, not laying all the way down, but the handle bars sticking over the bed. Went to start the next day and it started and I took off and it immediately started acting this way. It idles good, I can hear there's barely a slight skip in the engine, almost unoticeable. Revs happy except for that slight skip all the way tobhalf then bogs and smokes. If I hold the gas wide open it will run down and die like it ran out if gas. Plug is barely wet and occasionally dry. If it sits for a while and I start it then for a few seconds it sounds like it should when it's cold then starts this. I would think electrical but it has great spark. I rode it, then it started tuis after being in a truck. Icantfigure it out. Thanks for your help
Expert:  Doug E. replied 7 years ago.
I would check a couple of things. First, remove the kill switch wire from the circuit and run the bike. (Pull choke to kill or put in gear with brakes on) The wiring or switch might have been damaged in the truck.

I would air test the motor. If the RH crankseal is leaking, it will draw in oil from the tranny under load. This will cause extra smoke that isn't normal. Block the intake and exhaust. Replace the spark plug with a fitting from a compression tester with the valve removed. Attach a hose to a low pressure gauge and hand pump. Pump to 8-10psi and shut off the air. The motor should hold pressure. If not, track down the leak and repair. Common areas for leaks are the crankshaft seals, but a 2-stroke can leak at any gasket or even where the case halves meet.

When you had the top end off I'm sure you checked the reeds. If not, check them for wear or damage.

It wouldn't hurt to pull the muffler and try it. Sometimes a bike will flood laying on its side and the gas will gum things up as it blows back through the exhaust. If it runs worse, that would be an indication it's too lean. If it runs better, that would indicate it's too rich.

I would suspect float damage but you mentioned you looked at this. I would double check the float valve and replace if there are any signs of wear. On a bike that old, it should be replaced anyhow. I would also check the needle in the slide. The needle itself can wear where the clip attaches and cause problems.

Last up would be the ignition. It's possible for electrical parts to break down at higher RPMs. If you have another coil, I would swap it out. The CDI is hard to test, but about a year ago I found a site where a guy made his own KDX CDI box. It's fascinating stuff and the tests and mods he did, might help you out. Go to

Let me know what of that you have checked and get back to me if you can check the rest. We'll work forward from there.

Customer: replied 7 years ago.
I'll try this tomorrow and get back to u, thanks alot. I've done the exhaust part, I measured the floats but is there anything else I need t check for?
Expert:  Doug E. replied 7 years ago.
The float valve itself can be worn or the pin that holds the float can be worn. If either are worn, it can stick the float either too high or too low or both.

The only things that would be related to hauling the bike on its side would be the kill switch wiring or if the motor flooded with gas. When started it could have blown out a crankshaft seal or filled the muffler with gas that might have loosened some carbon and plugged up the muffler a little. It could be a total coincidence of course and the problem could be one of the other things I mentioned.

Check those things and get back to me when you can .

Customer: replied 7 years ago.
The air test was goin both ways, it held the first time not the second, on the third attempt it held at and below six, but anything over would drop to six. Another me hanic said ro test that by also leaning the bike on it's sides while it running and letting the oil go to the side of the case, and if it runs good or revs high then those seals r bad. Do you believe this theory? I haven't tried that theory with success, it runs the same on it sides. I'm gointo swap the coil with one I have on my kx 420. I'll let you know what happens. Would a failing cdi operate this way? Like you said it all could be coincidence, I pulled and rewrapped the silencer, the muffler was full of oil or gas or both after I laid it over in the truck too. And itsat there overnight in the truck. Thanks for all your help, I will definitely accept once we get this figured out
Expert:  Doug E. replied 7 years ago.
Sometimes a crankshaft seal will leak when the bike is over on its side. This is a common thing for chainsaw motors. It runs fine until you flip it to cut a tree and it dies. It's tougher to run a bike on its side because the carb will flood gas into the motor so it isn't as practical.

If you have it apart you can also try a vacuum test. Hook a vacuum tester to your adapter and see if the motor holds vacuum. That way you are testing the seals under pressure and under vacuum. If they pass both then they are ok.

You might want to try checking the timing with a timing light. If the ignition isn't advancing and retarding, then the CDI is bad. It might show an ignition miss as well.

KX420? I bought one of those new. It was the worst bike I've ever owned and I've owned about 75 bikes.

Customer: replied 7 years ago.
I got the kx 420 for three hundred bucks. With a full ehaust sytem and the ports matched it really smoothed out the vibration and gave it power in more than the top end. Really fun bik to ride but I can only turn corners off the throttle. It's has way too much power and Hilda thebfront tire off the ground. Where do I check with my timing light, under the stator cover?
Expert:  Doug E. replied 7 years ago.
Yes, remove the stator cover and put a dab of yellow or white paint on the little pointer on the engine case. Put a mark on the rotor on the F mark. You would need to hook up a tach to see if the timing advance is at the correct RPM, but sometimes a CDI will fail and won't advance at all so it's easy to spot.

Also check for oil under the cover. There should be no oil in that area. If there is, your seal is bad.