kx80 sputtering topend

[jabbahoo]

My sons 87 kx80 has a problem with sputtering and missing real bad on topend. I checked the carb and its clean. I checked the muffler its fine and I checked the cdi. It ran fine this summer but when the temps changed from upper 90's to low 50's it now runs like crab on topend. Could this be caused just from jetting? I'm new to the 2 strokes and just wondering what I could look for? The bike is all stock with a 200 main jet. Should I try and go leaner or richer on the needle? any advise will be greatly appreciated.

[r1ray]

Definitely sounds like a jetting issue.

[jabbahoo]

So should I lean it out or richin up the needle? Which would cause the sputtering?

[FuriouSly]

The main jets for that stock bike are 170,180,190,200,210.  Would be safe to go down one at a time from 200.  Not knowing the condition of the bike and motor, hard to tell what you got going on.  Elevation, temp, humidity all effect jetting.

Going leaner on the needle is different than going leaner on the main.  Your terminology is a bit off. 

The main is in control of the top-end and overall fule flow off on idle.

The pilot jet runs the idle up to 1/4 throttle and a smaller effect up to 1/2 throttle.

Then the needle height effects the midrange throttle response.  Dropping the needle down into the main jet leans out the fuel (raising the clip up the needle) and crisps up throttle response.

Have to get the float height and main jet dead on to get the needle set.

Sly

[John]

Hi,

Is this the first time the bike is causing you trouble? How long have you had it? Is the bike new? Has it worked fine before?

It would help to understand the problem if you could give a bit of history...

//John

[Johnniespeed]

Just another note, a two stroke will not rev up fully until the pipe is hot. I was hoping your problem was super simple like not allowing enough warm up time. It has been cold here in Michigan and the warm up time has increased for me, if I was new to two stokes, I might think something was wrong.

[jabbahoo]

Hi,

Is this the first time the bike is causing you trouble? How long have you had it? Is the bike new? Has it worked fine before?

It would help to understand the problem if you could give a bit of history...

//John
Yes this is the first time the bike caused me trouble. I got it about 2 months ago and its a 87 kx80. It worked fine all exept the gas was leaking out the overflow tube, which I found out it was the needle and seat. I did adjust the flows and at that time the temps changed alot so I did'nt know if it was caused from the floats. I did put the floats back to where they were in the begining and per spec. but still same result. I understand how the jetting system works as far as what a pilot, needle and main do. but someone on another site said try and dropping the needle a couple clips to see. I'm just new to the 2 strokes and jetting. I ride a YFZ450 and do all the jetting myself. But this bike has me pounding my head on a wall.

[jabbahoo]

Ok, I raised the needle a couple clips from the bottom and it helped out alot. So cooler temps leans out 2 strokes? It did'nt have the zip that it had in the higher temps but it cured the sputter.

[KXcam22]

Jabba,
  The colder air is more dense, hence you get more actual oxygen taken into the engine with each stroke for the same amount of fuel. That makes it leaner. Altitude make it go the other way.  At higher elevations the air is thinner, making the bike richer. Hope this helps. Cam.

[jabbahoo]

ok I must have been mistaken, The problem is still there. I went up and down on the last clip and no differance. I went to the cycle shop and picked up a 210m (it had a 200m) same thing still. 
Then I fouled the plug. Could it be to rich? 2 clip from bottom on needle and 200m? its all stock? I'm around 800 ft elevation bike is all stock. Does the 87 kx80's have a powervalve or do they have that kips? How can I check?

[Matt Fisher]

When you change the float level, that affects jetting.  If it was leaking, try installing a new float needle, and set it to the correct specs.  Go back to stock jetting and start from scratch. 

Also, check to make sure there's enough compression.  Without enough compression, it'll run really rich, foul plugs, ect.

Don't know if it has a powervalve.  Look at the exhaust port area, is there anything that indicates a powervalve in the area?

[jabbahoo]

Thanks, I will keep you guys updated.

[jabbahoo]

Okay, When I took off the exhaust pipe I did'nt see anything that looked like a valve. just the ex. port and piston. I think it has something to do with the carb? I hope