Cabot

Member
Sep 3, 2007
158
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First off: 2005 CRF250, 800ft at around 70ish degrees.

Just wondering what you guys are running for leak jets, and what mods your doing.

Had my bike on the dyno yesterday. After we got the jetting sorted out it was putting up 33.89 hp, with 18.8 foot pounds of torque. Horse power seemed a bit low? I've heard Honda claimed stock it had 34, but couldn't really find an actual claim from anyone who really knows anything.

Bike has pro circuit T-4 complete system, Wiseco 13.5:1 piston (about 10ish hours ago), and has engine ice for coolant, cus it was all I had laying around. Everything else enginewise is stock.

Anyways the bike had a real bog right from acceleration. Too lean. Has a size 55 leak jet. Smaller the leak jet, the more gas. We didn't have any other size leak jets, but we did have another 55er. Sottered one of them shut, then drilled out the hole smaller. This helped a little bit, but nothing to where it should be. Just for kicks we then drilled the hole bigger, made it worse. Next we put the other 55 in, and tried the O-ring mod on the AP, helped a little bit as well, but not close to perfect.

Well we ordered some more leak jets of differing sizes, and a different stiffer spring for the AP which is supposed to eliminate the need for the O-ring mod. If this doesn't work, I am probly looking at a Boysen quick shot cover, or adjustable leak jet. Which one?

I'm wondering if others are having this problem, and how they solved it?

Also getting the head ported and polished next week, and if this doesn't get me over 35 I'll probly do the ever popular 08 cam swap.
 
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whenfoxforks-ruled

Old MX Racer
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Oct 19, 2006
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What is the differance between, a lean bog, rich bog or an ap bog? If the stock rings call for a 15 hour service in the manual, how much shorter is it with the high compression piston?
 

_JOE_

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May 10, 2007
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I hear everyone saying the AP is too lean. Mine runs great with no bog using a 70 leak with the diaphragm shaved and a thick o-ring. The bike had an aweful stand bog stock. I never rode it with the stock jetting. I also run the JD kit with a 190ish main depending on conditions. I have the needle all the way up(bottom groove), and may try the cold temp needle in the near future. If I dropped the needle the bike developed a lean bog coming off idle and was reluctant to pull through the mid range, this happened as soon as 2 clips higher and improved all the way to the bottom groove. I haven't heard of any crf250s putting out over 40hp no matter how built they are. I think 35 on a stock head would be great.

What is the differance between, a lean bog, rich bog or an ap bog? If the stock rings call for a 15 hour service in the manual, how much shorter is it with the high compression piston?
I'll let you know they wear. I'll be pulling it down for valves soon, so if there's around 15hrs on it by then I'll yank the cylinder too and measure everything out.
 

whenfoxforks-ruled

Old MX Racer
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Oct 19, 2006
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70 is what we were running also. Lean bog no, rich bog YES! Being able to distinguish without a wideband oxygen sensor, or looking at the plug is difficult, if not impossible. And yes, plug checks on a 4st sucks!
 

_JOE_

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May 10, 2007
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Lol, don't have the time to go through plug chops on mine. I get very limited free time to ride as it is. I use the good ol butt dyno. The nice thing about the 4t is you can just swap in a couple size smaller or larger jet and not have to worry about siezing it up or fouling a plug. I have even heard of people comletely blocking the leak jet. :coocoo: Have you ever watched how much fuel comes out of there when you hit the throttle? It's a good amount. I heard a story about a guy who did it cause some guy on the internet said it was the hot setup. The bog got so bad the bike would barely run. I would like to get one of those adjustable leak jets to try.
 

Cabot

Member
Sep 3, 2007
158
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Thats what doesn't make sense to me. The guy that was helping me with it knows more about it then me, Im a noob when it comes to jetting.

Yeah, I have seen the stream that pumps out of there, seems like alot of gas to be running too lean. But the sensor showed that on throttle wack it started lean (same time engine cut out), then after about 2 seconds the mixture was right where it should be all the way through the range. Mixture was good at idle as well.

Unless There was too much gas to be burned, and the sensor was not picking this up?

I don't know....
 

whenfoxforks-ruled

Old MX Racer
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Oct 19, 2006
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Merrillville,Indiana
What kind of sensor? If it is not a wideband O2 sensor, placed in a bong in the header pipe, exactly 20 inches from the exhaust flange, you will not get an accurate reading. Just as important is the timing of the ap squirt. If it hits the slide, or too late, bog! My son took jets and blocked them off also, and ignorantly drilled his own holes. These are machined orifices, their sizes do correspond to drill sizes, but are machined! The best bet is go a couple sizes each way, you can feel the difference! The ap adjuster mod and the adjustable leak jet, sure seem like the hot set up for an 06 or older fcr carb for sure! I have never understood the purpose of the quick shot, other than the weep hole to prevent a vacuum lock, more fuel no way! The correct amount, at the correct time is the ticket. 1 time JD sent me some 35 and 40 leak jets, I tried them and had a good laugh! Accidents happen, 1 time does not justify anything, and laying your trust that friends know more than you, be very careful!
 

_JOE_

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May 10, 2007
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That's very true about the timing! There's also people saying to just "crank that screw in two turns to change the duration". Lol, then the fuels sraying all over the slide as it goes up. Great idea. I used a flashlight and took the subframe off so I could see it squirt. I kinda wish I would have ridden it a bit before setting up the carb. It has always run great for me.
 

Cabot

Member
Sep 3, 2007
158
0
Not exactly sure what kind of sensor it is..

But when I put the piston in it, I had the subframe off and airbox off, and the squirt was not hitting the slide at all. Could be too late though? But when the different size leak jets come in, we will figure it out.

Rode it today and yesterday, and it was already a big improvement from before.

This guy has been through all the training etc, and does dyno tuning/mechanic work for a living. I'm 17 years old, and learned everything I know from hands on experience, Clymer shop manuels, and the internet.
 

_JOE_

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May 10, 2007
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Correct. The AP system is designed to deliver a charge of fuel into the combustion chamber prior to the carb drawing enough air to pull fuel up through the main jet.
 
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