KDX200 needs help

[Tom H]

I just got done rebuilding the bottom end of my son's'90 kdx200. I changed the crank bearings and leaky crank seals. While I was at it I changed rings but the piston was OK. Now I have it back together, and it has virtually no power. It starts right up, doesn't miss, and runs smoothly in neutral. When you ride it, it is absolutely gutless. No power. It doesn't miss but just wont rev out.
So I've tried the following so far. I tore the carb apart and cleaned all the jets (it was clean), checked the reed valves. (they look good) and checked the Power valve. It appears to work right it partially blocks the ports at an idle, and the arm moves out, opening the ports more as it revs. If anything, the plug looks a little rich, but the engine doesn't really gurgle or load up.
It feels like the ignition is severely retarded. I marked the plate locations when I tore it down, and put it back in the same spot. The key is in the flywheel properly. I don't see how one could screw the timing up. Any suggestions?

 

[Tom H]

The compression seems OK, although I haven't checked it, but it has pretty good resistance on the compression half of the stroke.

 

[skipro3]

You might want to check the exhaust pipe and silencer to be sure it somehow didn't get plugged with something while sitting around in the garage. Also how is the air filter. I once left a rag in there while tearing down and forgot to remove it. Try running with the choke. If that helps, it is an air/fuel mixture problem. How's the gas? You might want to start with a fresh gas / oil mixture. There is a clue there with your comment that it starts right up but has no power. Hmmmm, check the brakes and make sure there is nothing dragging on the bike in the drive train. You said it doesn't rev out. Is that while riding it? How about just in neutral? Does it rev out then? It's got to be something simple. Keep us up to date when you find it and pass on the knowledge.
One more thing: When you changed the rings, did you pull the piston off the rod? Be sure you have the piston on the correct direction, usually an arrow on the crown. Your bike will start fine with it in backward but run like crap under load. Don't ask how I know. I'm not proud of my mistakes.

 

[Tom H]

Thanks for the ideas Skipro3.
The bike revs out fine in neutral, but bogs while running through the gears. It will eventually rev out in the lower gears, but doesn't in the upper gears.
I am pretty sure the piston is in correct, but I can re-check. It is a Wiseco, and as I remember, the arrow on top the piston was pointed forward.
It has a brand new NoToil air filter. It is oiled with the correct amount with Bel Ray Oil (not too much). I checked the entire intake system for leaks, and I am sure there are no rags in it.
I will check the expansion chamber for free air flow. The silencer clogged last winter after 13 years of riding. I cleaned it thoroughly then. I will check it again, but I doubt it is clogged again already.
I use the same gas that I run in our CR125. Premium w/ 50 to 1 mix of Golden Spectro. It is fresh and works well in the CR
The gas runs freely from the gas tank to the carb.
I don't think it is the brakes as the bike pushes easily, and the brakes work when you apply them.
One thing I thought of to check is to make sure the throttle is opening all the way. I will do that too.

 

[skipro3]

O.K. give me another crack at it: new plug? how about trying a reliable old plug? Is the engine warming up completely? An air bubble in the antifreeze will cause the engine to cool differently. Vent on the gas cap? I've seen 'em clog and the vacuum pulled with an increase in fuel flow will cause leaning out. Float level too low? This is another lack- of- fuel symtom when at higher rpms.

 

[kdxtaz]

Like skipro3 said, check the pipe & silencer for blockage. Mine acted just like that once after taking the rev pipe off to try the torque pipe. After a couple of weeks I put the rev back on (hated the torque pipe) and a mouse had made a nest in it. Took me a while to figure it out. :confused:
Good luck!
'Taz

 

[Tom H]

I just checked the float level. It is slightly lower than Parallel to the seam. I have ran it like that in the past to keep it from spilling fuel excessively, and it has worked better in the past than where it is supposed to be set (which I think is with the seam level). The carb has pleanty of fuel in it when you pull the bowl (which probably doesn't mean much). I pulled the fuel line and gas comes a flying out the hose when you turn the gas on. The cap is not plugged.
I haven't tried a new spark plug, but the plug is fairly new. I will do that.
I just got back in from double checking some of the earlier items. The arrow does point forward on the piston. The pipe & silencer pass air without restriction. I found an old compression guage, it runs slightly over 100 psi with moderate foot cranking. I am not sure that is that good or not. I forgot to check the carb opening all the way. I will do that in the morning.

 

[89kdx200rdr]

did you stagger the ring gaps? i ringed my bike in 92 after 3 yrs riding, put it up till last year and i have 165 psi compression on a cold engine. put the gauge back in hold the throttle open and kick it till it wont go any higher and see what u get.

 

[skipro3]

O.K. Now for checking the ugly stuff.....Did you say the plug looks like it loads up but doesn't foul out or sputter? Just oily? Measure your crank case oil carefully. Ride around for a while, half hour or so. (might not need that long. Check crank case oil again. A bad seal sucks crank oil into the bottom of the topend to cause black oily plug, unexplained crank oil drop, no fuel foul. I hope that isn't it.

 

[Tom H]

I went out this morning and took it off the work stand so I could kick it hard without killing myself. With the throttle full open, it will only muster about 102 psi. If it is 'sposed to be 165, sounds like I have no compression.
The rings are new. The piston looked acceptable, & only has 1 year of light riding. I did have to take the old ring and scrape out the top ring gap to get the new ring to seat fully. The bore looks good. The rings had about a .012 gap, and the ring ends are in the little notch that holds them in place. What to do next?
Will leaky crankseals affect top end compression? If so, sounds like I need to do a leakdown test to be sure I didn't somehow screw up the seal installation. But I dont have the resources to do the bottom end test... (I'm positive I didn't put a seal in backwards, which would be a reasonable question)
One thing I dont understand; the reason I changed the crank seals in the first place is that the bottom end blew gas/air into the crankcase. It would blow all of the transmission oil out of the transmission vent. I also hear that it will suck transmission oil into the crankcase. I know the crankcase cycles from a positive to negative pressure. Can the seals fail either just under pressure or under vacuum as opposed to both?

 

[skipro3]

Thats what mine did on a 84 KDX. It had poor horsepower and leaked oil into the combustion chamber, smoking somewhat more than usual. I was not willing to split cases, so I had a cycle shop do it for me for about $100 total.

 

[mcoker]

Also, you might try replacing the top end gasket. When mine get worn, I lose compression just like you're describing. I replace it (cause it only takes a couple of minutes, and they're pretty cheap) and power is back to normal. I typically leak a little oil from my cylinder head when my gasket's worn though. Just a little bit...

 

[Tom H]

Here is what I did this weekend. First I read the old posts concerning leak down tests. I've never done one cause it appears quite hard to do. My strongest fear was a leaky crank seal, even though I just changed them. So I figured I better due it anyhow.
With the help of posts from KDXQuebec and JDbrush, it was surprisingly pretty easy to do. I knocked the porcelean from an old plug, brazed a 1/8" X 2 1/2 brass pipe nipple in it and installed a pressure guage. I got a 1-3/4" expandable pipe plug and corked up the exhaust port. I made a plastic plug out of 1/2 " plastic sheet and put a chrome wheel air valve in it and stuffed it in the inlet port. With the piston at BDC, I pressurized it 10 psi. (Any more and the plugs would blow out. It held pressure what I suspect is pretty good. It took probably 2 minutes to go to zero. I would guess it leaked out the plugs.
I am sure relieved the seals are OK. I question now weather or not the seals were ever bad in the first place. But it was probably due for a new set of crank bearings anyhow (I have to justify my efforts somehow, I guess)
So I pulled the barrel and re-checked the ring gap. It was around .018". Probably a little loose for rings with maybe 1 hour on them, but not excessive.
It had a new head gasket installed when I put it together after splitting the cases.
I haven't got it back together yet, but I still suspect it has low compression. I have no idea why. Maybe my compression checking guage is bad. I inherited it from my dad, and hasn't been used in over 10 years. (It feels like it has fairly decent compression when I crank it.
In any case, I am still nowhere. I will put it back together and see if something fixed itself (??) for lack of anything else to try. One thing I didn't do was hone the cylinder. I did this because I didn't think you did that to plated bores. I have read that some people do. Could that be my problem?
I will post anything more that happens, & accept any more ideas. thank for the help to date.

 

[canyncarvr]

You can hone plated cylinders. EG has quite a write-up on that process on his site.

Granted, your gauge may be off. In any case, 102psi is low.

Wouldn't you generally assume the last thing done to be suspect? As opposed to looking for new stuff?

How was the piston determined to be 'ok'? If put-together ring gap measurement was .012" and it changed to .018", that's a lot of ring wear in not much time! How do you account for that?

I only briefly looked in the past, but did not find a piston distance measurement for timing the kdx. You should be able to determine what the distance in mm from TDC the piston is from firing time. I don't know the method for that procedure on the kdx. It would for sure tell you what the timing was. Using a mark on the case that isn't actually related to where the piston is isn't of use..unless it's a known quantity having used it before (and the engine has NOT been disassembled).

At BDC, what part of the crankcase is exposed to pressure applied from the intake port? What am I missing?

The crankcase is vented. What sort of pressure is at that vent when the bike is running? Or, if at BDC, what pressure should the tranny hold (pressurized thru said vent)?

Does the tranny fluid smell like fuel?

 

[Tom H]

I can't account for the growth between .012 to .018 after such a short run time. Everthing is clean. Dirt is definately not the problem. My original measurement was not quite as precise as my last measurement. so some of it may have been that. I would assume that new rings wear faster as they seat than they do after break-in, but I don't know that.
The reason I say that I think the piston is OK is because it isn't scored, doesn't seem excessively loose in the bore (compared to the one I changed out last fall, which had a LOT more miles). I don't have a mike, but I don't think of it being the problem.
You probably are right. I quite possibly didn't even pressurize the bottom end during my leak down test. I mistakenly thought that hole in the piston would always expose the bottom end, and the reed valve is what prevents back flow out the intake. Now that I think about how it looked, that is probably incorrect.
I put the valve in the intake because I couldn't find a 1/8" NPT tee and other required hardware at the local hardware stores to put both the air and the guage in at the plug. Guess that was kind of dumb on my part.
If I do it at top dead center, I wont get pressure to the guage, and that wont work either. Maybe either I can do it with the piston half way up(??), or I'll have to take the piston out, or I'll have to find the right hardware.
What did you mean by the crankcase being vented? I am unaware of any vents.
I do suspect that I may have somehow messed up the timing. As I noticed originally, it feels like the timing is severely retarded (I have messed with old car timing and seen the effect). But I have no idea on how to check it or set it. I simply marked the position of the ignition components when I pulled them, and put them back in the exact same place, for lack of knowing any better method.
I haven't ever smelled fuel in the trans oil. The oil seemed to dis-color really quick after a oil change before I tore it down. (what hadn't blown out the transmission vent. I'd be low in fluid after every ride, and what was in there seemed darker than normal, so I'd quite often change it rather than just add oil.) But it never appeared to be sucking tranny fluid in to the crankcases. At least I never fouled plugs.

 

[RocketJ]

Tom,Did the ring(s) fit tight, or were they loose in their grooves?

 

[Tom H]

I re-did the leak down test. This time I turned the engine slowly by hand after I pressurized it to 10 psi. The pressure came up some as I went past TDC. It came back down to 10 as it continued through the revolution. It stayed at 10 psi for a long time (several minutes). I feel I can finally put crank seals to rest.
I then thought of something we used to do with cars. I rechecked the compression, still about 100 psi. I put about a half a teaspoon of oil down the plug hole with the piston near tdc., then quickly re-checked compression. On cars, they used to say if your low compression comes up after adding oil, your rings leak. If it didn't, your valves leak. When I re-checked, the compression went up to about 130 psi.
If that logic hold true for two cycle rings, sounds like I better find that EG article on honing a plated bore, and buy another set of rings.

 

[Tom H]

Originally, the top ring was too tight in it's groove and wouldn't seat. I brike one of the old rings and cleaned the carbon out, which wasn't too hard to do, and it seated fine and was free to move. I didn't check the side clearance in the grooves, but they sure didn't seem too loose.

 

[canyncarvr]

Start HERE for the honing info EG has posted. Edit the address line to get to his main page if you drop in at the above point.

Putting oil in a two stroke will effect piston/cylinder seal also. The fact that the piston 'looked' ok doesn't mean a whole lot. I don't see a piston/cylinder clearance number. What was it? Did the piston have a dull gray color? That's a sign of done worn out. If you didn't check the clearance...and didn't mic the piston and the bore, it's kinda hard to say what you've got.

Well, you've got 102psi. Something isn't sealing.

You need to know what the piston/cyl clearance is.

re: crankcase vent

There's a tube from the case to the airbox (most likely).

The timing is a big question. As stated, I am unaware of the method used to check the piston position against spark time. Easy enough on a point triggered ignition..don't know about a mag.

I'm sure someone does. I'll try a post in 'advanced'...see if RR spanks me back to kdx just cuz I'm green (and therefore stoopid. ...just kidding...sorta).