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How to "tune" choke adjustment on YZ250 carb - 38mm Keihin with TPS


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Apparently the rotation of the choke on this carb makes a big difference in the idle and overall running character.  I'm troubleshooting some other issues but came across this and did not really expect it.  Full in can kill the bike at idle.  Full out can cause a slightly rising idle speed.

 

I've never seen anything about turning the choke knob (not just moving it up and down) for tuning a carb, but it seems to have a big effect?  Is there a procedure or a standard number of turns for the choke knob??

 

YZ250 carb with ESR 325 kit.  The bike was bogging and dying under quick throttle whacks.  Idled great.  Tried all sorts of different brass and verified fuel flow through the line and through the main jet/needle port.  Needle and slide cycle normally.  Looking for ideas here as well.

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I think you have the Carb bike choke and the FI bike enrichment knob's confused.

 

The Carb choke is just a plunger that includes the Starter jet, or bypasses it. No tuning, accept by changing the starter jet.

 

The FI enrichement knob is a idle adjustment (pushing in, counterclockwise for higher ) and the enrichment is for starting (pull out and lock/push in to disengage, like a carb choke).

 

When you raise the idle, you put the TPS in a new position which changes the FI and IGN map.

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If you bike dies with the choke in, you need to replace your pilot jet with a new one, and clear the passage above it with fishing line and spray cleaner

 

You will always have a 'bog' with fast WOT if you are sitting still.

You can't actually use the throttle that way when riding

 

All WOT issues are a function of the accelerator pump and have nearly nothing to do with jetting.

 

Read this

 

https://thumpertalk.com/topic/1053317-06-crf450r-still-bogs/?hl=%2Baccelerator+%2Bpump#entry11143305

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Pilot jet is clean and clear, first thing I replaced.

 

This is a two stroke, no accelerator pump.

 

The choke pulls up and down to add fuel (I assume) on startup.  When it's DOWN, it can be turned a few revolutions and makes the idle climb or lower.  What's it doing?

 

I've always had throttle response from WOT throttle whacks on 2-strokes.  I need to get the front wheel in the air over whoops, logs, rocks, etc.  If it doesn't respond to that input, and fast, something is wrong.

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On older rm 2 sks ,that's how u adjust air mix,the choke knob is the adjuster[like efi],but i've never heard that on a yz,idle is probably changing not because of knob,u are supposed to turn air screw 1/4 turn,wait20-30 secs,,that small air screw is for air not gas,counter clock more air [lean],  clockwise/rich less air,,,,,when u hit throttle u go from pilot to needle diameter,,i'm still playing with needles on my esr325,getting way better,linear,,40 pilot,,went with sudco needles easier[ebay],needle charts on web,,,been playing with  cgh,dgj,egj,next just ordered cek,cej    dgj runs great crispy long over rev ,torq,but then a little flat...testing goes on

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Apparently the rotation of the choke on this carb makes a big difference in the idle and overall running character.  I'm troubleshooting some other issues but came across this and did not really expect it.  Full in can kill the bike at idle.  Full out can cause a slightly rising idle speed.

 

I've never seen anything about turning the choke knob (not just moving it up and down) for tuning a carb, but it seems to have a big effect?  Is there a procedure or a standard number of turns for the choke knob??

 

YZ250 carb with ESR 325 kit.  The bike was bogging and dying under quick throttle whacks.  Idled great.  Tried all sorts of different brass and verified fuel flow through the line and through the main jet/needle port.  Needle and slide cycle normally.  Looking for ideas here as well.

 

Your headline reads YZ 250 but your sig says 05' RM/YZ 325, do you know which bike the carb came from and what year?

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2006 YZ250 carb. Replaced choke assembly, still has the same symptom, but slightly improved. Tried jetting combos from 35 to 45, 155 to 172, 8 slide, CGJ needle 1st and 3rd clips, 4k elevation, castor 927 40:1 pump 91. Gnarly/q. Esr 325 kit. Vf3 reeds.

New choke assy, new float needle, new coil, new air filter, new silencer packing......new gas..

Edited by GHILL28
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my mid 80's cr500 had a pj carb on it -from seated the choke knob liked to be about 12 turns out and it used a stupid big pilot like a 60 and I think a 1368 needle, I think the point of me saying this is to this day I still run this motor but with a pwk air striker carb in a crf frame. getrid of that piece of crap carb

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Yup, no difference with the TPS disconnected.

 

I'll check out the timing plate.  I was wondering if a clogged bowl vent line could cause it, but I just replaced those real recently.  I guess it could temporarily vapor-lock the float bowl?

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Made a small discovery.  Apparently the pipe had melted one of the right side vent hoses shut.  It's the one that comes out sideways about mid-body.  I take it this vents directly from the float bowl somehow?  Local-ized vapor locking would make a lot of sense.

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Made a small discovery.  Apparently the pipe had melted one of the right side vent hoses shut.  It's the one that comes out sideways about mid-body.  I take it this vents directly from the float bowl somehow?  Local-ized vapor locking would make a lot of sense.

 

 

You found your problem!

Funny I was just going to suggest this. Had the same thing happen to my RM. Now I keep those vent lines well zapped strapped away from the pipe.

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