What causes EXCESSIVE smoking of a 2 stroke?


21 replies to this topic
  • dylans503

Posted July 01, 2008 - 05:51 PM

#1

i have a 1994 kx 250, most likely needs a top end,
and ive heard that when rings are bad it pushes oil up and burns the oil and cause bikes to smoke,

ive also heard its cause by bad jetting

and ive also heard its caused by a bad crank seal

whats the dealio?


and my bike is spooging alot to

  • Sinner73

Posted July 01, 2008 - 06:12 PM

#2

from what i remember from back in the day your eather running way to rich or your rings are bad but im sure there are guys on here with a better idea then me

  • Slackkinhard

Posted July 01, 2008 - 06:31 PM

#3

i have a 1994 kx 250, most likely needs a top end,
and ive heard that when rings are bad it pushes oil up and burns the oil and cause bikes to smoke,

ive also heard its cause by bad jetting

and ive also heard its caused by a bad crank seal

whats the dealio?


and my bike is spooging alot to


All of the above are possible...and on a bike that old, probable. Are you sure it's all smoke and not steam? Have you noticed a loss in coolant as well? Does it start hard? Sounds like time to tear it down.
Good luck!

  • dylans503

Posted July 01, 2008 - 06:40 PM

#4

coolant levels fine, its not steaming.
and its EXTREMELY HARD TO START! but when its warm it starts 1-2 kicks.

and i noticed every time we go riding i start it up and go on a trail and im always thinking "goddam i need to downsize!!!!"
but after we ride for a little it seems the powerband isnt so irratic and im thinking
"F*** ya!!!! i love my 250!"

  • Caseus

Posted July 01, 2008 - 06:43 PM

#5

I have found that excessive smoke can be caused by any of those things you mentioned. For instance my 1993 YZ250 smoked like crazy when I bought it. It was stock so i threw out all the jetting related issues and re set it to stock jetting specs as per Yamaha. It helped a bit but still spooged oil like crazy. It felt powerful enough but I did a compression test anyway and found the top end was worn out. It was also a bear to start cold, but fired easily after I warmed it up. I replaced the piston/rings/gaskets and it cleaned up the excessive smoke and spooge.

Coincidentally I had picked up a Suzuki RM 125 for my wife which had the exact same problem (excessive smoke). The only difference was I bought it in non running condition with the knowledge it needed a piston. I went ahead and put one in and It too smoked like crazy, just had a lot less spooge. Of course I could immediately eliminate the top/bottom end as I inspect those when I do the work to the top end. The only logical conclusion was a stuck float in the carb or just plain bad jetting. This bike came with a lot of bolt on mods and I did suspect the previous owner didn't know how to change the jetting to compensate for the new parts (ie: fmf fatty gold and a reed set who's name escapes me at the moment). After tearing the carb down, setting the float and leaning out the jetting a bit I have a clean running RM that has less mid range bog and very little smoke.

These are just two recent examples of two differet causes of the same symptom. I find its pretty necessary to tear down that motor/carb and have a look at what is going on to be sure.

  • T-roy JK

Posted July 01, 2008 - 06:45 PM

#6

If it were a 4-stroke, then bad rings would make it smoke but, a 2-stroke would just loose hp. What will make a 2-stroke smoke is mixing too much oil with gas, or carberation problems like inlet float valve not sealing or float level set too high. maybe even a clogged airway. another thing would be leaking right side crank seal allowing tranny oil to enter the cylinder and get burned. I would first check carb. you also can either pressure test the motor as stated in sticky and that will show if the crank seal is leaking or measure in the tranny oil / ride then remeasure what comes out. if you loose tranny oil and you don't see a leak, then it's either a crank seal or a bad center case gasket. usually gonna be crank seal.

  • CBus660R

Posted July 01, 2008 - 06:48 PM

#7

and its EXTREMELY HARD TO START! but when its warm it starts 1-2 kicks.


This is a pretty good indication that the top-end is worn out and time for a replacement. If it was a jetting issue, then it would be hard to start cold or hot. As to the smoking issue, pretty much everything has been covered.

  • dylans503

Posted July 01, 2008 - 06:59 PM

#8

thanks all for the replies

i noticed somewhere near the top end its rattling
most likely the piston moving from side to side.

and my bikes leaks DOES leak oil, BUT, from what i can see its not loosing cary much oil out the tranny. about just enough for the leaking. if i were burning it up the oil would significantly drop. but it hasnt.

and evary time we go riding the snoke just keeps getting worse and worse and worse.

i think ive summed it down to the piston.

  • T-roy JK

Posted July 01, 2008 - 07:03 PM

#9

Top end is easy to check, if you don't have one, pickup a compression tester from most auto parts stores and screw it in and kick about 10 times. I don't know what a yz should be but, I've had 2 crs that were 230lbs. with a fresh piston/rings/hone.

I doubt it would be bad rings. the only oil in a 2-stroke above or below the piston is in the premix. (No motor oil to burn). although it may change jetting a little.

  • dylans503

Posted July 01, 2008 - 07:15 PM

#10

but the engine is pressurized. so it pushes tranny oil

  • T-roy JK

Posted July 01, 2008 - 07:19 PM

#11

Tranny oil should never enter the crank area or the cylinder. if it does, then you have a bad crank seal or bad case gasket. crank and cylinder are lubricated by oil in premix only.

  • makKX250

Posted July 01, 2008 - 07:20 PM

#12

When in doubt tear it down. Look for blow-by on the piston. Also check the reeds and make sure they are in good condition. I had a bike that started smoking bad - replaced the top end and it didn't help. It had a chiped reed and the bike would load up and fog for mosquitos until it was reved up. Replaced the reeds and it was back to normal.

  • dylans503

Posted July 01, 2008 - 07:21 PM

#13

the guy who had it before me put the pilot jet from
165 to
162 to
160
im at 1,000 feet elevation, does that sound right?
and it doesnt smoke at idle, only when i rev it. and i just moved the needle up a notch (1 from the top) so could it be the main jet?
and im running 40:1

  • T-roy JK

Posted July 01, 2008 - 07:26 PM

#14

Bad reeds will definately make it hard to start. I had one with a cracked reed that would only start if you pulled it behind something. changed reeds and would kick right over. leaking reeds will allow the mixture to just flutter back and forth and not pass through the engine like it should.

  • dylans503

Posted July 01, 2008 - 07:29 PM

#15

for SURE i need a new top end. but does it affect smoking, i keep hearing people say it does and people say it doesnt.
and since im leaking oil you know that means the seals are bad.
so it could be the crank seal.

how hard is it to change a crank seal?

  • dylans503

Posted July 01, 2008 - 07:33 PM

#16

Bad reeds will definately make it hard to start. I had one with a cracked reed that would only start if you pulled it behind something. changed reeds and would kick right over. leaking reeds will allow the mixture to just flutter back and forth and not pass through the engine like it should.


but after its worm it fine. its not evary time i start it, just when it's cold

  • T-roy JK

Posted July 01, 2008 - 08:00 PM

#17

Where is the oil leaking from? You cannot see a leaking crank seal, it leaks oil from the transmission through the seal and into the crank and cylinder. Remember, there is no oil in the crank shaft area of a 2-stroke motor. there isn't an oil pump either. the only oil that is stored in a 2-stroke motor is in the tranny and it should stay there and not enter the cylinder or crank. you may have a right side cover leak or a counter shaft (drive gear) shifter seal, or clutch shaft seal leaking. as far as starting goes, all engines are easier to start warm than cold. Reeds are easy to check also, just pull the carb and reeds and look at them. if you see cracks or gaps to where air can pass from the cylinder to the carb then replace them.

  • todthebod

Posted July 01, 2008 - 08:50 PM

#18

My cr250 was doing the exact same thing as yours. I replaced the right side crank seal and it still smoked. I rebuilt the top end and it still did it. I jetted it perfect (JD) and that was it. Hardly smokes at all now. Plus I had white plastics and they would be covered after about 5 minutes. Now just a little tint on there.

  • dylans503

Posted July 01, 2008 - 09:11 PM

#19

the right and left crankcase is leaking

and im not talking about 5 kicks when its cold im talking like.....20

  • dylans503

Posted July 01, 2008 - 09:18 PM

#20

how hard is it to change out crank seals?





Related Content

 
x

Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.