Intermittent high rev on my '88 kx500

[bigbore]

Hey BigBore,
 Yes it seems the engine gets caught in a lean burn on decel and bucks or surges.  Usually at the least opportune moment, like wanting to slow down to make a turn.  You get a "mmpow mmpow pow" and get pushed farther than you need to go!
  I don't recall anyone claiming to have any engine damage due to the condition. 
The remedies vary from a new or different carb, to fatter jetting to a kips bolt mod.   
   I'm sorry I can't give you a better answer.    The bike seems to have a lean burn condition when the jetting is close to optimum.   
          The engine draws in just enough of an air/fuel mix through the pilot circuit when the throttle is chopped and the engine is decelerating to burn lean instead of Idle down.
    Sorry I can't be of any more help than that.
  Tuck\o/

hi all,

just an update on this, i stripped the carb and here is the spec of it:

the small stumpy jet is 175 (main jet?)
the skinny long jet is 45 (pilot jet?)
Needle has: NR1371 stamped on it and is on the 3rd clip down (or up) basically its in the middle
Havent been able to check the air screw yet.

According to my clymer manual standard spec is:

the 175 jet is correct,
but the 45 jet should be a 62
the needle code doesnt match
but the needle is on the correct cilp /notch

So from what i gather im running lean at the moment?

Is this right?

Oh and the other thing i noticed is that when i start the bike from cold, there is hardly any smoke out the back on it at all? And when you take off in first after just starting it and bring in the power, it doesnt have to clear itself like all other 2 strokes i have. What i mean by clear itself is that is doesnt splutter and blow a lot and smoke and then it comes alive.

i currently mix the fuel at 5 litres of petrol to 125ml of premix oil.

Many thanks for all the replies, much appreciated guys 

[Polar-Bus]

Hey BigBore,
 Yes it seems the engine gets caught in a lean burn on decel and bucks or surges.  Usually at the least opportune moment, like wanting to slow down to make a turn.  You get a "mmpow mmpow pow" and get pushed farther than you need to go!
  I don't recall anyone claiming to have any engine damage due to the condition. 
The remedies vary from a new or different carb, to fatter jetting to a kips bolt mod.   
   I'm sorry I can't give you a better answer.    The bike seems to have a lean burn condition when the jetting is close to optimum.   
          The engine draws in just enough of an air/fuel mix through the pilot circuit when the throttle is chopped and the engine is decelerating to burn lean instead of Idle down.
    Sorry I can't be of any more help than that.
  Tuck\o/

hi all,

just an update on this, i stripped the carb and here is the spec of it:

the small stumpy jet is 175 (main jet?)
the skinny long jet is 45 (pilot jet?)
Needle has: NR1371 stamped on it and is on the 3rd clip down (or up) basically its in the middle
Havent been able to check the air screw yet.

According to my clymer manual standard spec is:

the 175 jet is correct,
but the 45 jet should be a 62
the needle code doesnt match
but the needle is on the correct cilp /notch

So from what i gather im running lean at the moment?

Is this right?

Oh and the other thing i noticed is that when i start the bike from cold, there is hardly any smoke out the back on it at all? And when you take off in first after just starting it and bring in the power, it doesnt have to clear itself like all other 2 strokes i have. What i mean by clear itself is that is doesnt splutter and blow a lot and smoke and then it comes alive.

i currently mix the fuel at 5 litres of petrol to 125ml of premix oil.

Many thanks for all the replies, much appreciated guys 

Your pilot jet seems way too lean. I believe I am running a #52 .  Do some jetting searching  and compare our jetting to yours. Try to replicate the weather and altitude conditions as well to ensure your jet change is going to be accurate.  I believe my summer setup is 165 main, 52 pilot and stock needle in the middle clip position. 

[bigbore]

Your pilot jet seems way too lean. I believe I am running a #52 .  Do some jetting searching  and compare our jetting to yours. Try to replicate the weather and altitude conditions as well to ensure your jet change is going to be accurate.  I believe my summer setup is 165 main, 52 pilot and stock needle in the middle clip position. 

cheers or that, im going to order a 55 and a 62 pilot and try them first. the needle (although it doesnt seem the correct one) i will leave alone and with the main jet.

The air is meant to be 1+1/2 turns out as standard so im going ot set that to incase its wrong.

il keep all of you posted.

thanks again.

[IcemanK5]

I find changing to many settings causes more problems than it solves.
Try working on one problem at a time and see how the bike responds.
It least this way you can allways go right back and change it again.

Kurt

[sandblaster]

Depending on the temperature I run a 58-62 pilot jet at sea level to 2000 ft.
The hotter it is outside the more mine tends to surge after riding for a bit and then idling.
Since my jetting is close to optimal when it does surge I just hit my kill switch for a second or two until the engine winds down, just enough not to kill it...
Also, depending on the temperature I run a  168-175 main jet and a stock needle at the 2nd position at the same altitude.
It's important to remember that surging is not pinging.
If it's pinging, it's time to worry...

[Polar-Bus]

KX500's all experience a small amount of rpm "surging" no matter weather your jetted ideal or slightly rich, so don't beat yourself up to try and rid the engine of surging. Inspect your plug after a long WOT plug chop run and that's about it.

[bigbore]

Depending on the temperature I run a 58-62 pilot jet at sea level to 2000 ft.
The hotter it is outside the more mine tends to surge after riding for a bit and then idling.
Since my jetting is close to optimal when it does surge I just hit my kill switch for a second or two until the engine winds down, just enough not to kill it...
Also, depending on the temperature I run a  168-175 main jet and a stock needle at the 3rd position at the same altitude.
It's important to remember that surging is not pinging.
If it's pinging, it's time to worry...

im over in ireland and the temperature is always really low e.g. between 5-10 degrees celsius. i plan to get it as close to stock setting as possible. i understand it might not fully go away so im going to try the 55 pilot first and see how it goes with that.

i presume the reed spacer and boyesen power reeds i have wont require a different setup for the jets? since i fitted them the bike still runs the exact same just a little more low power but no affect on power delivery.

[Polar-Bus]

Depending on the temperature I run a 58-62 pilot jet at sea level to 2000 ft.
The hotter it is outside the more mine tends to surge after riding for a bit and then idling.
Since my jetting is close to optimal when it does surge I just hit my kill switch for a second or two until the engine winds down, just enough not to kill it...
Also, depending on the temperature I run a  168-175 main jet and a stock needle at the 3rd position at the same altitude.
It's important to remember that surging is not pinging.
If it's pinging, it's time to worry...

im over in ireland and the temperature is always really low e.g. between 5-10 degrees celsius. i plan to get it as close to stock setting as possible. i understand it might not fully go away so im going to try the 55 pilot first and see how it goes with that.

i presume the reed spacer and boyesen power reeds i have wont require a different setup for the jets? since i fitted them the bike still runs the exact same just a little more low power but no affect on power delivery.

If your running in cooler temps then your 45 pilot is DEFINATELY too lean. Even my #52 is a tad lean in the cooler months...

[bigbore]

If your running in cooler temps then your 45 pilot is DEFINATELY too lean. Even my #52 is a tad lean in the cooler months...

ah ok, i didnt realise the temperature affected it that much. yea the weather over here is crap always cold and raining!

[Polar-Bus]

If your running in cooler temps then your 45 pilot is DEFINATELY too lean. Even my #52 is a tad lean in the cooler months...

ah ok, i didnt realise the temperature affected it that much. yea the weather over here is crap always cold and raining!

Generally speaking KX500's are low revving bikes, and if you're just a casuall weekend rider your low to mid range correct jetting selection is more critical than a desert rider who spends far more time 3/4 throttle and above (where the desert riders need to have proper main jet selection. Precise jetting takes some time to learn. It's the #1 question asked on the internet. It's very difficult to give a good answer. You eventually learn to use your "ear" as well as your "seat of the pants" when determining ideal jetting. The last tip I will give you is  jet for a good running bike in all conditions that doesn't foul plugs. Don't attempt to jet lean for every last bit of hp, or there is a high probability that you will end up with a melted piston... I don'r claim to be a jetting "guru" but I do clearly understand jetting basics.  My KX5 has a great jetting selection, but it took me severall tries to get there. I have a hot summer jetting setup, cooler "fall" weather jetting, and also a "winter" cold weather setup. I also make a practice to log all my jetting data so I can swap jets at a later time easily.    I have awesome acceleration, and I never foul a plug even in extended riding in the woods. Do I have ideal peak hp ? I don't know and I don't care. I do know I have "safe" jetting and that more important to me (and my wallet      )   Good luck hope this all helps !

[Friar-Tuck]

Hey Big B,
 How goes the fight?  have you had any time to work on the bike? 
    Tuck\o/

[bigbore]

hi all

sorry for the delay on updating this, was sick for a while so delayed me getting out for a test run.

I tried the new 55 pilot and it was better but still a little bit of surging. Have ordered a 60 pilot and will see if that cures it or even if its a little better id settle for that.

thanks again for all the info