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Carb bog problem, 04 250excf- FCR 37- rebuilt, going crazy!


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from the beginning. FCR 37. bike starts great. bike idles great. bike throttle response great. bike rides great for 40 min or so then slowly develops a bog at 1/8-1/4 throttle. usually this is riding slower technical trails. then if i ride on pavement (example) in 6th gear for 10 minutes or so the bog is gone. just rebuilt carb, new o-rings, float valve, jets etc. float height is approx 9mm. I thought it was the float valve o-ring leaking but replaced and still developing bog. HELP!!

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Kahrannee you are the man. Just checked my gas cap line and is seriously clogged. Will check tomorrow but makes sense. Bike was used when i bought so needed carb rebuild anyway. Thanks so much!

Also how to check stator for overheat and why would it overheat?

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The older RFS stators are notorious for going bad mostly from overheating the solder connections. Stators live in an inhospitable environment where there is little if any cooling. Stators in later bikes have proven to be more reliable.

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The older RFS stators are notorious for going bad mostly from overheating the solder connections. Stators live in an inhospitable environment where there is little if any cooling. Stators in later bikes have proven to be more reliable.

Understand. But if the stator was bad the bike would be running poorly all the time, yes?

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Understand. But if the stator was bad the bike would be running poorly all the time, yes?

 

No

 

The electrical connections when cold (contracted) are good, and when hot (expanded) become open and arc voltage (intermitant spark) or no voltage (dead motor).

When the stator cools down, it resumes near normal operation. 

In addition, Transistors become non-linear once they have overheated, and go to clipping must earlier than normal.

This is a common problem with hot running motors.

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Jetting requirements do not change over time.

 

You certainly are full of misinformation among other things. The jetting should be changed in a worn motor. When the compression drops so will the intake signal requiring a larger main jet to keep the af ratio where it should be. 

Edited by dirteta
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You certainly are full of misinformation among other things. The jetting should be changed in a worn motor. When the compression drops so will the intake signal requiring a larger main jet to keep the af ratio where it should be. 

 

I think he meant time during the ride, since the original poster indicated his motor started bogging during the same ride.  In regards to jetting changes as the motor wears; 2-strokes definitely lean out as they wear.  In my personal experience, I haven't had to change jetting on any 4-stroke due to wear, they don't seem to be affected nearly as much as 2-strokes; but 2-strokes do definitely lean out as they get worn. 

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You certainly are full of misinformation among other things. The jetting should be changed in a worn motor. When the compression drops so will the intake signal requiring a smaller main jet to keep the af ratio where it should 

 

That is true.....on a two stroke.  He has a four stroke. You do not change jetting on a four stroke no matter how worn the top end is.

 

That is true....if you choose to ride a two stroke with a worn out top end, instead of just fixing it.

 

That is still NOT the problem he was having, which was poor running in 40 min..........which cannot be jetting, because jetting requirements do not change, with time.

 

Nice attitude, by the way.

Edited by Kah Ran Nee
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That is true.....on a two stroke.  He has a four stroke. You do not change jetting on a four stroke no matter how worn the top end is.

 

That is true....if you choose to ride a two stroke with a worn out top end, instead of just fixing it.

 

That is still NOT the problem he was having, which was poor running in 40 min..........which cannot be jetting, because jetting requirements do not change, with time.

 

Nice attitude, by the way.

Well a follow up to my post. I went for a ride after clearing the vent hose, better but still bogging a little but not near as bad. I did change the jetting from stock (160/42) to (152/40, clip at midway #4).This was prior to clearing vent, rebuild etc to try and fix the problem.  I will go back to stock jetting (and clean out carb again) as soon as they arrive and let ya know how it works. I really appreciate all your input. Also on a note there was no over heating on my last hard ride so thats a plus!

Edited by kma
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Well a follow up to my post. I went for a ride after clearing the vent hose, better but still bogging a little but not near as bad. I did change the jetting from stock (160/42) to (152/40, clip at midway #4).This was prior to clearing vent, rebuild etc to try and fix the problem.  I will go back to stock jetting (and clean out carb again) as soon as they arrive and let ya know how it works. I really appreciate all your input. Also on a note there was no over heating on my last hard ride so thats a plus!

My bike use to do the same thing until I installed an adjustable leak jet. The squirt is too long for tight riding where you are on and off the gas a lot. It's a fairly common thing with KTM's due to the non adjustable leak jet.

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Dude, I am an engine builder. I am not a professional.

Done over 20 motors in the last 35 years

I run my motors to the piston spec limit, and I don't need to change the jetting.

When the motor is worn out, you repair the motor, not change the jetting. That is just ridiculous. 

 

Yes, you absolutey could improve the AF ratio as the top end wears out......same with the carb wearing out, the change in altitude, the specific gravity of the fuel, etc etc.

 

You just want to argue about someone else post. Good luck with that.

 

If your theory was so accurate, why don't racers change jetting/mapping between motos in supercross?  Those motors only last about 4 hours....

No one is changing maps mid-race, that is good way to ruin a run.

 

You are confused that I am runnning a techical shop here. This is a forum.

 

Please change your attitude.

No-one questioned what you do with your motors. I stated a simple fact about worn top ends and jetting after you made an inaccurate statement in relation to jetting. YOU are running nothing but the KTM haters club of North America. I'll change my attitude when you stop posting inaccurate information. I believe the last one I corrected you on was when you stated that an 07xcw head couldn't be used on an 05 exc. And there have been others. If you aren't sure of what the factual answer is to a question, don't try answer it and mislead people.

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My bike use to do the same thing until I installed an adjustable leak jet. The squirt is too long for tight riding where you are on and off the gas a lot. It's a fairly common thing with KTM's due to the non adjustable leak jet.

Yeah I have heard this a few times now. Which brand/maker did you use?

Edited by kma
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Yeah I have heard this a few times now. Which brand/maker did you use?

I used a merge racing one. I like the low profile design which still clears the starter when you turn the carb. I not sure but I think the boysen ALJ is bulky and makes changing jets a pita. It was a few years ago I fitted mine but I recall  something along those lines. They do work though. I ride a lot of single track and found even with the honda diaphragm it was still too much and would cause a hit in the power as it cleared itself which isn't good in tight tracks. I've reduced my squirt to about half a second and it's great now. Let us know how you go with it.

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you can fix the squirt by changing the diaphragm or adding a button to it, some have even used a model airplane collar for $2 to fix the diaphragm button size. Ive got two KTM 4st and have fixed a few others as well and the diap change works. Also do the o-ring mod or some version of it as well.

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There was a mod for fcr carburetors to add a set screw to limit the output of the accelerator pump. Search you tube to see if you can find it, it became popular with the first yz's. I made the mod to my yz426. Basically you drill and add a set screw, and then the accelerator pump output is adjustable. Not the same effect as a leak jet, but does allow you to limit the amount of fuel delivered by the pump.

Side note; I was thinking to myself, I wonder if you're boiling the fuel in the carburetor.. But if this is a known problem on this bike with a known fix then the must not be the case.

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