Bike only runs on hot start, dies with throttle

Landon

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Aug 4, 2021
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Hi guys!
I've got a bit of head scratcher here (at least for me - I'm no mechanic)

Here's the deal: I am helping a friend trying to get a 2007 KX250f running. He bought the bike used and we have no history on it. He got it with a broken kickstart lever and we were hopeful that was the only problem. Now that the kickstart is fixed, this thing will only run with the hot start lever pulled in and it dies as soon as it's given the smallest amount of throttle. And yes I have read all the related forums I could find 🙂

Here's what's been done and what I am assuming

1) I am assuming there is a rich condition since the hot start creates a lean condition and it only runs with hot start pulled in (creating a leaner mix)
2) The carb 't has been disassembled and thoroughly cleaned twice - no clogged jets
3) spark plug looks ok (I'd say)
4) float height has been checked and is in spec
5) accelerator pump has a nice strong jet of fuel squirting out and the diaphragm appears to be in good shape
6) the needle was 3 clips too high (supposed to be 2nd down from top, was 5th down). I have reset to factory and this made no difference
7) the carb is fitted with an aftermarket fuel screw. This thing is missing the spring and I think an o-ring? It is not leaking fuel, but maybe air into the bowl? I've ordered a new one and it should be here in a week or so from the date of this post.
8.) tank was flushed and fresh fuel put in

Someone had suggested an air intake leak, but my understanding is that this would cause it to run lean. If pulling in the hot start allows more air, and there is already an air leak - that doesn't check out to me.

Another thought was the slow air jet maybe clogged. Is this something that can happen? Seems to me this would cause the bike to run rich at idle, but you would think once you opened the throttle it would get a better air/fuel mix and run fine...

Any ideas? The only running with hot start issue certainly sounds like its running rich, but the dying as soon as it gets throttle could go either way so that throws me off.
Much appreciated in advance!!!
 

darwin

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What is a "hot start lever pulled in"? Let's start with that 1st.
"slow air jet maybe clogged" What be dat also?
 

Wrench

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Hi guys!
I've got a bit of head scratcher here (at least for me - I'm no mechanic)


7) the carb is fitted with an aftermarket fuel screw. This thing is missing the spring and I think an o-ring? It is not leaking fuel, but maybe air into the bowl? I've ordered a new one and it should be here in a week or so from the date of this post.


Someone had suggested an air intake leak,



Any ideas?
I think you found the intake air leak. It don't matter if you don't understand why This is probably your problem
You should of fixed this first before ya even asked about why your bike won't run
Intake leak symptoms makes the bike seem like it's starved for fuel
 

Landon

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Aug 4, 2021
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I think you found the intake air leak. It don't matter if you don't understand why This is probably your problem
You should of fixed this first before ya even asked about why your bike won't run
Intake leak symptoms makes the bike seem like it's starved for fuel
Thanks for your reply. I have not as of yet found any air leak. As my post mentions, I had gone through all of those other items first. Checking for air leak will be my next move. Almost seems like that's all that's left!
 

Landon

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Aug 4, 2021
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What is a "hot start lever pulled in"? Let's start with that 1st.
"slow air jet maybe clogged" What be dat also?
Hi Darwin

Let me start by again saying I am no mechanic, but here is what I understand:

This kx250 ( and a lot of other dirt bikes) have a "hot start lever". This is essentially the opposite of the choke. It allows more air in creating a leaner mixture. I suppose when the bike is hot it doesn't need as much fuel to start (as compared to a cold engine needing a rich mixture)

My understanding of the slow air jet is that it is a small port on the air filter side that allows a small amount of air to mix with the fuel coming from the slow (or pilot) jet. If this air jet was clogged or restricted it seems to me like that could cause some problems. I haven't had a chance to look at the bike again today, but I'll see about getting this air jet out and inspected next time. One more thing I can cross off the list anyhow.
 

darwin

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May 26, 2008
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That is a complicated carb w/81 parts listed for it. Counted at least 5 named jets and an electrical sensor for it. What that sensor is for is anyone's guess, must be important. My best advice is too have a pro rebuilder rebuild it for you and have it dialed in right. It's either needing replacement parts or it's just dirty and needs a good cleaning. Parts are too expensive to be guessing at what MIGHT be wrong. One of those jets listed is an idle jet but is not listed as so, that is where I'd look 1st but which one is it? Dunno by the diagrams parts listings. I'd have it done right once and be enjoying the ride. Went through this a few months ago with an XT225 and it was a simpler carb. No choke on it either it had a fuel enrchment circuit that was controlled by a choke type knob. Same symptoms as your bike, turns out carb was just dirty and needed to be fine tuned for fuel mixture and air. Good luck bro, I feel for ya.
 

Landon

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Aug 4, 2021
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That is a complicated carb w/81 parts listed for it. Counted at least 5 named jets and an electrical sensor for it. What that sensor is for is anyone's guess, must be important. My best advice is too have a pro rebuilder rebuild it for you and have it dialed in right. It's either needing replacement parts or it's just dirty and needs a good cleaning. Parts are too expensive to be guessing at what MIGHT be wrong. One of those jets listed is an idle jet but is not listed as so, that is where I'd look 1st but which one is it? Dunno by the diagrams parts listings. I'd have it done right once and be enjoying the ride. Went through this a few months ago with an XT225 and it was a simpler carb. No choke on it either it had a fuel enrchment circuit that was controlled by a choke type knob. Same symptoms as your bike, turns out carb was just dirty and needed to be fine tuned for fuel mixture and air. Good luck bro, I feel for ya.

Haha yeah maybe best bet to take it in. We only have one shop around here though and they are $$$. I'll try giving her one more good clean and check those jets again and check for air leaks and see what happens! Thanks
 

Chainlube

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Hold it! Before you go any further, check all your electrical connections. Make sure everything is grounded properly, 90% of all carb issues are electrical. This means everything, especially under the coil cover and any frame connections. Look for any rust or green wires.

I've made quite a few $$$ from abandoned projects that the PO thought were carb issues.
 

Wrench

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Thanks for your reply. I have not as of yet found any air leak. As my post mentions, I had gone through all of those other items first. Checking for air leak will be my next move. Almost seems like that's all that's left!
Air Leak is at the Fuel Air screw
7) the carb is fitted with an aftermarket fuel screw. This thing is missing the spring and I think an o-ring?
Your missing the SPRING, WASHER and O RING on the fuel air screw
All 3 pieces of the air fuel screw need to be installed for the correct operation of the pilot circuit.

Tension from the spring keeps the screw from backing or falling out.
The o ring keeps air from being sucked in. (vacuum leak)
A washer keeps the spring from tearing up the o ring.
 
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