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Idle and poor running conditions

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Alright bare with me guys, I need some help with getting my bike to run so i can ride it.

Back story, I have an 04 KX250F i have owned for a few years and the bike has never ran very well since i have owned it. By that i mean it has never wanted to idle and has been a pain to start hot or cold. So week or so ago it finally quit starting all together. Got to the point i had to pop start it every time and finally wouldn't pop start. Tore the motor down (I am an automotive technician am very mechanically inclined) how ever this is just getting frustrating. Pulled head and found intake valves had the seats worn, strange but i cleaned the head up and the seats look ok there didn't appear to me to have any damage to the seats on the head. So I installed new intake valves and resembled the motor, it will start now but im having issues getting it to idle. Piston and cylinder all look good.

I have cleaned the carb and have the following in carb. Main jet is a 180 (pulled a 168 out), I have a 40 and 42 pilot jet. I have tried the air screw at both 2 and 2.5 turns did not seem to make much difference.

Right now the bike will start when kicked with the choke out, but revs very high. If i throw the choke off it comes down and dies.. the header begins to glow with in 30 to 45 sec of it idleing so high... It seems to me that i am running far to lean. Im not a huge carb guy so thats were i am falling short on what direction i need to go. I will have the head redone this winter but as right now summing isnt getting any longer and i just want to ride. I feel like this is a carb issue that im having as the only thing that has really changed after replacing valves, is that it will kick start and actually start. When I was able to get it to idle for brief periods before, when i did blip the throttle it would shoot off idle like an on off switch with no rpm variation and take too long in my opinion to come back down to an idle condition. I feel like im am fighting a few different variables/ issues and would really appreciate the help. I am also seaching and working on this while waiting for replys and help. Thank you guys.

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Hey,

1. check the cam sprockets for slippage. Symptoms sound alot like what I experienced with one of our bikes. Intake cam gear had slipped 2 teeth out of time. Retarding the intake valve closing. Pull it off, reset the time, press it back on. Ensure clearances are shimmed to spec, go high, larger gap end of spec

2. Put that 42 pilot in. Fuel screw at 2.5

3. Inspect hot start. That it functions smoothly, plunger moves freely and returns to closed easily. Not sticking open.

4. New spark plug

5. Fresh new fuel in a cleaned/rinsed gas tank, cleaned petcock, and new fuel line.

 

 

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Cam gear had spun i have put it back and have tack welded both. Have had it running after putting the cam back (It might not be degreed back to 0.0 deg but close to the point it runs just wont idle. Edit I may just have to buy a new set of cams but had planned on doing that this winter when I tear it down and do a complete re-fresh and new seats pressed in and cut for the head. I had it down and piston, head, seats, cylinder all look fine. 

1 through 5 there are all checks. The hot start i think is working smoothly i don't usually use it, but when i had the carb apart i made sure it was clean & free. Is there any adjustment to the hot start?

Had a small oil leak, found it coming from the filter housing o-ring pinched, so now im waiting on a new o-ring kit for the boyesen water-pump/ oil filter housing.

Ill tear the carb back down and put the 42 pilot back in and see if i can play with the pilot air screw some more.

Edited by kx250f484

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That is kinda what i thought, i might have to step up another size, but didnt want to if its something not common in these bikes. But with how lean it is running i kinda figured i would end up going up a size and adjusting the air screw to fit that jetting. Thanks for the help i will watch that vid here shortly. Appreciate the input gentlemen.

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1 minute ago, kx250f484 said:

That is kinda what i thought, i might have to step up another size, but didnt want to if its something not common in these bikes. But with how lean it is running i kinda figured i would end up going up a size and adjusting the air screw to fit that jetting. Thanks for the help i will watch that vid here shortly. Appreciate the input gentlemen.

Here are some jetting specs that will work well.

https://www.howtomotorcyclerepair.com/how-to-kx250f-jetting-specs/

I'm assuming the carb is clean and no worn parts? No vacuum leaks? No exhaust leaks? Did you have the valve seats cut?

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Thank you for the jetting specs that is very helpful. I didnt have the seats cut they visually looked alright, i did not have any major grooving or abnormal shape to the seats on the head. I could still see both angles to the seats. The carb is clean the only thing i am unsure of is vacuum leaks, it might be the only other thing i am fighting but unsure how to properly test or find vacuum leaks on a carb like this. In the automotive world i would just hook up a smoke machine and away i go... but i don't want to do that and think i have vacuum leaks from something that is supposed to breath or something. This is were my lack of experience with a carb comes to play. But i went through the carb following the svc manual and everything is clean, lubed and seems to function properly. I am going to try to make sure the accelerator pump spray is timed right with the leak jet and throttle slide tonight but I don't feel that is affecting my poor idle condition.

I have had it running and can maintain it running but have to keep it under throttle and it ran well just very snappy, kinda on/off throttle wasn't very smooth and at lower speeds and when stopped header would glow. But erratic idle/ no idle condition is my main focus. Like i had stated I have plans to tear it down this winter and refresh the whole top possibly the bottom end. I just feel with the way the internal components look and the way it is running almost identical to the way it ran before that i have a carb issue. 

I will see if i can find a 45 pilot jet in town for it and go from there, thank you guys again for your help and patients.

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To check for vacuum leaks, just spray carb clean all over the carb boot and note and idle changes. Blow dry with compressed air right after test.

Also make sure hot start plunger is clean and operating properly. Junk tends to accumulate in that bore.

Make sure you crank up the idle to 1700+ 

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Ok so thinking i had this backwards, with the Air/Fuel screw, turning it CW or in leans the mixture? and turning it CCW or out adds fuel for a richer condition? 

I had that backwards in my thought process, i was thinking that it was rich going in and lean coming out. I will through the 42 and go from there but still thinking i may have to jump up a size like the bike if your first video an go with a 45 Pilot jet. 

Thanks again really appreciate the videos Matt, the valve adjustment video was a big help for me as i had not done one before on a dirt bike and helped out a bunch.

I will try to update post so i can finish with a fix and help others out that im sure have run into a similar condition.

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5 minutes ago, kx250f484 said:

Ok so thinking i had this backwards, with the Air/Fuel screw, turning it CW or in leans the mixture? and turning it CCW or out adds fuel for a richer condition? 

Yes. It is a fuel screw. In for leaner, out for richer. It's also upside down, so make sure you factor that in LOL.

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This may or may not be helpful .... Here is my carb tuning method, described in a separate thread.  Cut and pasted here with some revision for here:

""""

So, here is my advice, if you really want to get this bike working properly and smoothly.  Back out and take a fresh look and start a do-over.  

First, ensure your valves are setup properly.  Triple check the timing.  Look for the cam lobes to be at 10 and 2 when all the dots are set in the right place.  Check the valve clearances and shim as necessary to be in spec.

Next, fuel system checkout.  <snip> check and service the hot start cable and plunger.  Service the stock accelerator pump ensuring it is clean and working properly, check for grit or torn diaphragm, fix it, adjust the linkages so it is working properly.  Now, go through the rest of the carb, clean it well, blow through all ports and passages.  Any jets that are discolored, replace them, do not try to clean them.  Put a size 42 or even 45 pilot jet in, 40 is too small, and make sure it is a NEW one  Set the fuel scew at 2 1/2 turns out from bottomed.  Remove and inspect the rubber carb to head boot.  Look for age cracks or whatever; you are looking for source of an air leak.  Repair/fill cracks with a good rtv or replace the boot.  When reinstalling the boot and the carb, put a light grease smear on inner of boot.  Ensure the boot is seated properly,  line up clamps properly and tighten down snug but not gripripped.

Put a brand new spark plug in.  New, brand new.  Not barely used or looks new or almost new.  New means NEW, as in just bought and in the crisp new box.

Now set the bike on a stand.  Start the bike and set a high idle speed with the idle throttle stop screw.  Let the bike warm up.  When the bike is warm, back off the idle screw and set to where you like it.  

Only after all of the above is done, tune your bike.  With the bike warm and idling on the stand.  Pull the hot start and listen/observe what it does.  When the hot start is pulled...

A - if the bike does nothing.  The pilot is too rich.  Turn the fuel screw IN 1/2 turn.  Try again.

B - if the bike fades off and dies.  The pilot is too lean.  Turn the fuel screw OUT 1/2 turn.  Try again.

C - if the bike picks up 200-300-500 rpm and holds.  The pilot is close to perfect.  Fiddle a bit more at 1/8 turns if you want.  The the best setting is when the idle picks up 200-300 quickly.  If there is a lag to the idle up, it is still too rich, turn IN 1/8.  If it picks up but then falls off, it is still too lean, turn OUT 1/8.  

D - When you think you have it, blip the throttle.  There should be no hesitation and it should return to idle quickly.  If it hangs after blipping - it is still bit too rich, you can try fuel screw turn in 1/8, check hot start pull idle up response .... repeat, tweak, repeat.  Until you have what you want.  You can also try dropping the needle 1 slot to lean.  If it fades off and dies or has hard to recover when blipping the throttle, it is still too lean.  Fuel screw out 1/8, or raise the needle one clip slot to richen.

There.  All of your problems are solved and your bike is properly and perfectly tuned.   Now Go ride!

""""

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Good write up, good descriptions gives me a good direction to head in withe the hot start and what to do with the fuel screw... only one question there when you say drop or raise the needle one clip to richen can we clarify just for argument sake.

You say drop one slot to lean it, does that mean from where the clip is, put the clip one slot higher to drop the needle. As well as vice versa take the clip and drop the clip effectively raising the needle to richen?

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3 minutes ago, kx250f484 said:

Good write up, good descriptions gives me a good direction to head in withe the hot start and what to do with the fuel screw... only one question there when you say drop or raise the needle one clip to richen can we clarify just for argument sake.

You say drop one slot to lean it, does that mean from where the clip is, put the clip one slot higher to drop the needle. As well as vice versa take the clip and drop the clip effectively raising the needle to richen?

wrt clip moves ... Yes exactly as you state 484.  Hope that all helps! Good luck, let us know how goes.

Edited by FaceDeAce
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Just now, FaceDeAce said:

wrt clip moves ... Yes exactly as you state 484.  

Excellent, will try all of the above post tonight after work. I stoped on lunch to a local parts shop and garbed a moose finger adjustable screw and a new 42 pilot jet. He did not have a 45 on hand. But now that i have better knowledge that turning out richens and in leans i have a better shot at what i am trying to accomplish. Like you stated in the above post everything else is new and the carb is clean clean. I have gone through it twice now and this will be the third time into the bottom and will also look for vacuum leaks as well.

Thank you guys for the help and will post back tonight with results, so hopefully others and search and find results like i am looking for.

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For reference, the stock clip position on the needle is 2nd from the top ..... I often do goto 3rd from the top.

As another useful reference.  Here is a typical tuning chart, which gives the idea of how the various parameters in the carb work together and different stages of throttle and rpm.  You can see where there are overlaps that you can manipulate to tweak your machine.

In this chart, the "air screw, slow jet" correspond to the terms we've used above on the KX as the:   (IMS) = fuel screw, and (AirScrew Slow Jet) = Pilot Jet

Further to this, not shown in chart is the hot start.  When you pull the hot start, it opens a small port in the carb that leans out the mix between the carb and the engine.  It opens and acts like an air leak. What that does is it works like a momentary air bleed / air screw. That is why it is so important to ensure the hot start is working properly, smoothly, and sealing properly when it is closed.  That is also why I use the hot start to figure out what is going on and why I find it so useful for fine tuning of the pilot and the 0 to 1/4 throttle mix.

Hope that all helps!

tuning.gif

Edited by FaceDeAce
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1 hour ago, FaceDeAce said:

D - When you think you have it, blip the throttle.  There should be no hesitation and it should return to idle quickly.  If it hangs after blipping - it is still bit too rich, you can try fuel screw turn in 1/8, check hot start pull idle up response .... repeat, tweak, repeat.  Until you have what you want.  You can also try dropping the needle 1 slot to lean.  If it fades off and dies or has hard to recover when blipping the throttle, it is still too lean.  Fuel screw out 1/8, or raise the needle one clip slot to richen.

There.  All of your problems are solved and your bike is properly and perfectly tuned.   Now Go ride!

""""

Hanging idle is lean. If RPM's dip below idle speed after a blip, it is rich.

I never use hot start as a means to set fuel screw. Adjust fuel screw to max idle RPM and smoothness.

All other info is good.

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Ok question here guys. On the carb pic thought there be a ling on the T fitting there. Its never had a line on it but wasnt sure.

On a side not those were my valves!

IMG_20170510_212622_323.jpg

20170523_193921.jpg

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Alright, so after much pissing with it i found that i need to go up a jet size to the 45..... Im not sure where i am at but i started at 2.5 turns out and believe i am somewere near 4 or so now jus to get it to idle and not die and also not need the throttle stop screw turned up.... header quit glowing and less back firing out of the pipe also seeing some black poofs from unburnt fuel. I am going to have to wait for a 45 pilot to come in so i can fine tune a little more... 

I will say i did use the hot start as a reference after fight for a bit with it.... I was letting it idle at a very high rpm to me because any lower it would die. But i pulled the hot start and like stated above it died.... turn the screw out some more and started to do better the more i turned it out.

Still having an issue with it coming back up or having a high idle when riding, also noticed a small bog issue. Pulled it back apart and looked at the spray of the accel pump. Timing and duration seem to be good i have no wet spot on the slide so.

Waiting on the pilot jet and will have to tune more later. Got in trouble last night riding it at about 11pm forgot the fire station down the street has 24hr manning and not just on call so they were a little grumpy about me ripping up and down the street at the shop at that late... but its a large industrial park other then the fire station there is not much around. 

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