250F Another 2004 KX250F Project/Build

[Mihylo33]

So I figured I should take another look at the carb and ripped it off.
The float was a little low, not much, but its set at exactly 8mm right now, and I put that plastic doohickey back in that I asked about in Nov.
So next question is the jet needle - according to the PC site that Mike linked to in my YZ250 thread, they say needle position should be set at 4th position, but my manual says 3rd.

Who knows better? (I have stock exhaust). The settings for my YZ worked well, but it does have a PC silencer...

Thoughts?

I would follow PCs guideline. The silencer type won’t matter much but the pipe will.
As far as positions go. The manual may say from the top and PC may say up from the bottom so it could be the same position. I would follow PC though. PC specs are done running Maxima oil at 32:1 with a 50/50 mix of pump and C12 race gas.

 

[Trfsrfr]

@2Wheels - Its a CR carb.
I unplugged the TPS and it didn't seem to make much of a difference. I got it to run for about 3 seconds then it died. It wont take throttle.
I'm about ready to drag it behind my neighbors truck.

Can anyone tell me what this doohickey adjustment thing (red arrow) on the right side of the carb is? Whoever owned the bike before me safety-wired it together and I'm wondering if it could be part of the problem.

 

[Redordead89]

Anything that's been wired together on a carb is a or the problem!

 

[Trfsrfr]

Anything that's been wired together on a carb is a or the problem!

Maybe. But what is that piece? There's no description of it. It looks like it might be part of the accelerator pump squirter rod? And it has an adjustment screw...

Maybe its not an accelerator pump...???

 

[2Wheels]

Sure you don’t have an FCR? Will be stamped on the side to the right of the idle adjuster

 

[Trfsrfr]

Sure you don’t have an FCR? Will be stamped on the side to the right of the idle adjuster

I only saw CR on the carb top, but now looking at the service manual it says its a FCR37. So I'm assuming that is correct.

I know this thing runs, had it running for 90 seconds 2 weeks ago, so I'm sure its a carb issue.
I'm guessing its not getting enough fuel cuz when I try to give it throttle it dies, so I'm thinking I'm leaning it out, and when I kick it over it sometimes backfires out the exhaust. Plus, it doesn't smell like fuel after kicking it awhile.

 

[Trfsrfr]

I'm missing something stupid here. I'm gonna take that carb off again. This shouldn't be this difficult.

Anything that's been wired together on a carb is a or the problem!

I think you're right Red. I'm thinking its that accelerator pump.

Edit/ ---Progress.
I went back out to look at that carb again and started looking at the part that was wired together. I initially assumed it was wired because it was broken (its plastic) but it's not broken at all. The adjusting screw (A) was screwed all the way out & was wired to the pushrod holder (B). This was keeping the pushrod held in a down position, unable to come back to the top of its stroke; so it was stuck in a pushed in position, and wasn't squirting any fuel into the carb. I removed the safety-wire, screwed the adjuster screw all the way in, and now it squirts fuel. I'm thinking I'll have this thing running by noon tomorrow.
My question now though is why would somebody do that?

 

[2Wheels]

The accelerator pump is just a small squirt upon quick throttle openings to prevent the bog down. It wouldn’t keep it from running you’ve got something else going on.

 

[Redordead89]

I'm missing something stupid here. I'm gonna take that carb off again. This shouldn't be this difficult.

I think you're right Red. I'm thinking its that accelerator pump.

Edit/ ---Progress.
I went back out to look at that carb again and started looking at the part that was wired together. I initially assumed it was wired because it was broken (its plastic) but it's not broken at all. The adjusting screw (A) was screwed all the way out & was wired to the pushrod holder (B). This was keeping the pushrod held in a down position, unable to come back to the top of its stroke; so it was stuck in a pushed in position, and wasn't squirting any fuel into the carb. I removed the safety-wire, screwed the adjuster screw all the way in, and now it squirts fuel. I'm thinking I'll have this thing running by noon tomorrow.
My question now though is why would somebody do that?
View attachment 32278

Fix this first!

 

[Trfsrfr]

Fix this first!

I did.
Here is my understanding -
Smaller bikes dont use an AP because of the small diameter of the venturi. The smaller the opening, the higher velocity the air must travel through it, and that high velocity is enough to suck the fuel up the pipe (jet passage). Coincidentally, this is also why there is always congestion in traffic when there is a merge in traffic, people need to speed the F up, but I digress...

The bigger bikes venturis are larger and so the velocity is not there at start-up, so they use an accelerator pump to give a squirt of fuel until the rev's come up enough to have the added air velocity start using the jet system.
The reason the wire was there (many people use an o-ring) was to combat the "lean bog" that happens when you quickly crack the throttle open; the throttle valve goes up quickly, the pressure in the venturi drops, but the air velocity needs a second to come up as RPM's rise, and the system goes lean. This is where the AP comes in again, it gives an added blast of fuel to keep the system from going lean, until the RPMS come up enough to start using the jet system again.
In my case, the AP arm was being held in a half-open, half-closed state, and not allowing any fuel to be squirted, especially at start up.
If too much fuel is squirted, you get a rich condition, and this is where the leak jet comes in. The leak jet determines how much fuel goes into the squirt, by leaking a smaller or larger portion of the squirt back into the float bowl. The smaller the leak jet, the larger the squirt, and vice-versa. A leak jet is like a faucet in your house. You stand at the kitchen sink and run the water and it comes out full blast; until your kid flushes the toilet, or uses the shower, or another faucet (leak jet) gets turned on, and the pressure at the sink drops. Same concept.

Yesterday was an awesome learning day for me, and I expect to have this thing running before noon. I might even take it to the track with me on Saturday before we go to Supercross .

 

[CDA]

Yesterday was an awesome learning day for me, and I expect to have this thing running before noon. I might even take it to the track with me on Saturday before we go to Supercross .

Supercross!!! I'll be there as well.

 

[2Wheels]

You stand at the kitchen sink and run the water and it comes out full blast; until your kid flushes the toilet, or uses the shower, or another faucet (leak jet) gets turned on, and the pressure at the sink drops.

Pressure is equal, what you have there is a volume reduction due to an undersized extended branch.
Just sayin’ M-39404

 

[Trfsrfr]

Pressure, volume, same difference!

More progress on the kx today, bike pops over a lot more than it did before, still not running yet though. I didn't mess with it a whole bunch cuz my back is about 2 steps from being out, and I was working on the kids yz125 trying to get it ready for this weekend, which may not happen now.... Replaced the shift shaft and lever only to find that the kick starter wouldn't return, just like my 250, took it apart to find that its broken...might just bring a rope to the track, lol!

 

[Redordead89]

I did.
Here is my understanding -
Smaller bikes dont use an AP because of the small diameter of the venturi. The smaller the opening, the higher velocity the air must travel through it, and that high velocity is enough to suck the fuel up the pipe (jet passage). Coincidentally, this is also why there is always congestion in traffic when there is a merge in traffic, people need to speed the F up, but I digress...

The bigger bikes venturis are larger and so the velocity is not there at start-up, so they use pussy pump to give a squirt of fuel until the rev's come up enough to have the added air velocity start using the jet system.
The reason the wire was there (many people use a lubed o-ring) was to combat the "lean bog" that happens when you quickly crack the throttle open; the throttle valve goes up quickly, the pressure in the venturi drops, but the air velocity needs a second to come up as BPM's rise, and the system goes lean. This is where the chick comes in again, it gives an added blast of fuel to keep the system from going lean, until the BPMS come up enough to start using the jet system again.
In my case, the chicks arm was being held in a half-open, half-closed state, and not allowing any fuel to be squirted, especially at start up.
If too much fuel is squirted, you get a rich condition, and this is where the leak jet comes in. The leak jet determines how much fuel goes into the squirt, by leaking a smaller or larger portion of the squirt back into the float bowl. The smaller the leak jet, the larger the squirt, and vice-versa. A leak jet is like a faucet in your house. You stand at the kitchen sink and run the woman and it comes out full blast; until your kid flushes the toilet, or uses the shower, or another faucet (leak jet) gets turned on, and the pressure at the sink drops. Same concept.

Yesterday was an awesome learning day for me, and I expect to have this thing running before noon. I might even take it to the track with me on Saturday before we go to Supercross .

Bwaaaaahaaahhhaa it almost reads like your talking about your women

Supercross!!! I'll be there as well.

Hi @CDA

Pressure is equal, what you have there is a volume reduction due to an undersized extended branch.
Just sayin’ M-39404

 

[CDA]

Bwaaaaahaaahhhaa it almost reads like your talking about your women

Hi @CDA
.

Sup