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Acc. Pump Tuning?

Fark

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#1
I picked up an 02 250F.  I checked valve clearances, installed a clean filter, but the bike has a bog off idle when you stab the throttle.  The only engine mod is a Dr D pipe.  How can I determine if it's a lean or rich bog?

Do most of you guys leave your AP alone and try ajdusting the needle first?
 

jmics19067

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#2
Good question Fark,

I have been trying to help a buddy of mine tune up his 450quad and have been running into the same question of jetting or acc. pump.

what I have learned is the airscrew does make a big difference and what I tried to do is run the bike at a steady throttle,in the different carb circuits, and load the bike down by dragging the brakes.Then seeing how well it picks back up while maintaining the steady throttle to try and eliminate the acc. pump. sorta Mickey Mouse but I have been able to succesively tell him that he is drowning out the engine by stabbing the throttle open with acc. pump and to start rolling the throttle open instead of just whacking it open.


ooops sorry fuel screw!!! :eek:
 

Fark

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#3
Fuel screw, this is the one where the kit exists to make it adjustable on the fly correct? And this affects the mixture over the entire range?

I borrowed a freinds service manual, I suppose I have to bite the bullet and get my own...

I don't want to ride around the machines shortcomings. I will be ruthless until it obeys my every whim!!!!! Basically, I want to be able to stand it up off idle and ride away on one wheel :)
 

Hick

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#4
Originally posted by Fark
...the bike has a bog off idle when you stab the throttle.
I don't own a 250f and have only ridden one, but I do know that leaner leak jets for the AP are a popular mod over at Thumpertalk.

Also, if the bog is instantaneous it is probably more on the pilot/idle/pump than the clip. If you are lean on the clip you will get what I would describe as a hiccup, the bike falls flat when you come on the taper (probably about 1/3 throttle or thereabouts).

As for determining rich vs. lean on pilot, I like to find a smooth road, and pull the tallest gear you can at a low throttle setting, like 1/4. If the bike surges you are lean, if it sputters you are rich. Also, a lean pilot circuit will produce more popping on decel (all else being equal) vs. a rich condition...

Hope this helps.
 

jmics19067

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#5
sounds like good advice Hick

when I was talking about the fuel screw I was talking about the adjustment screw under the carb towards the front. It's similar to the two stroke air screw in that it fine tunes the pilot jet but opposite that you turn it out for more fuel compared to the two strokes out for more air. That is your idle/off idle adjustment provided the acc. pump isn't drowning out the motor when you whack it open.

Since it is difficult for me to tell whether it is jetting or acc. pump problems maintaining a steady throttle eliminates the acc. pump. Once that is figured out you have a better chance understanding what needs to be done with the pump shot if you choose to chase it closer
 

BSWIFT

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#6
Check into the BK mod. It is cheap and easy.
 
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#7
Check the YZ250F FAQ at Thumpertalk.com or the Garrett Berg Site for the BK mod. This eliminates the bog when you hit the throttle. It's a known issue on the older 250F and 426F.
 

BadgerMan

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#8
Originally posted by jmics19067

when I was talking about the fuel screw I was talking about the adjustment screw under the carb towards the front.
This is a real neat item for quick adjustments of the fuel screw. It replaces the existing screw.

http://www.ziptyracing.com/display/eCommerce/productView.asp?ProductID=600&CategoryID=110

However, I am wondering if it is possible to entirely rid a YZF of the bog when you whack the throttle. I have been tweaking mine for three years and the bog is still there. I have measured the squirt from my AP and it is less than a second (too short to measure with a stopwatch) so I don't think that BK mod is needed in my case. I am back to running stock jetting after all the experimentation since it seams to work best. I have not tried tweaking the AP's timing.
 

DEANSFASTWAY

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#9
Fark those stock idle mixture screws tend to fall out also. New ones are @ $10. We like to knacker up the last few threads on the screw to help it stay in .I know some wiseass will say it sounds butcherous but thats what helps. The zipty screw is trick though.Also the carbs can tend to pop out of the intake boot on hard jump landings and so we tastefully safety wire them to the top motor mount.Sometimes on dirty old YZFs the accel pump parts can actually get all corroded in the pump body and not work but Ive only seen that on 2 or3 older 400/426s. If it bogs off idle in neutral and not while riding there may not be anything wrong and you might be searching for a needle in a haystack . When these bikes are put in gear the ign timing changes.. I have done the accel pump mod on these bikes but only on woods riders where theyre constantly blipping on and off throttle.Does the hot start work correctly if aftermarket , isit stuck ? If its a Dubach and its stuck Doug will send you out a new one when he receives your old one ,there was a tolerance problem on some of his early production causing them to stick. He makes good . Hes a good dude . If no D/hotstart then you should buy one even if youre not racing.That and by all means get that 03 ex cam in there , It makes all the difference in the world when starting the older YZF .Ive got a couple of them let me know if you need one. Did you go to the jump show at the Spectrum? It was too cold to party in the parking lot thats for sure! Good Luck
 

Fark

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#10
Hey Dean,

I tried to PM you but your box is full.

Yes, I wouldn't mind taking one of those cams off your hands. I need to get a skinny set of feeler guages before I pop off the cam caps. I bet you have a crap load of extra shims don't you...

And BTW, it doesn't bog on the stand, only when loaded.

I went Friday night! Almost didn't make it. I was working in NY and really pissed my boss off by taking off at 3:30. I still got there a half hour late! I've been going for 3 years running. Good time. Finally blessed us with the back flip! I really like the tandem competition too.

Thanks brother! I'll talk to you soon!
 

DEANSFASTWAY

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#11
Fark , Yeah man I emptied my box(sounds like a bodily function doesnt it?) Shoot me a PM I guess. I didnt make it to the jump show but some of my local friends did I heard it was good . Hey I was over there by the PPA auctions and went to the Penns Port Pub one day and I saw the HOOTERS boat was sinking? Whats up with that?
 

whyz

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#12
Originally posted by Fark


And BTW, it doesn't bog on the stand, only when loaded.

Mine does the opposite
Never when loaded>> That could be hazordous :silly:

FS and LJ adjustment.
 
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#13
First off the bog will happen when whacked really fast in idle since there is no load on the bike. It is normal and even stated in the manuals.

now if you have a problem riding, that is a problem! Be sure the carb is cleaned out and the plug is good in the bike before doing anything else!

First off do the blue wire mod:

http://motoman393.thumpertalk.com/tech/bluewire.html

Then do the BK Carb Mod ( i have reposted all the info on a website ) here:

http://m.bobbitt.home.comcast.net/yz426fm/bkmod.html

After that get the ZipTy or similar fuel screw! It is a lifesaver there!

Once you get all 3 things working there, the bog should go away for you while riding.

if not, get a James Dean jetting kit:


http://shop.thumpertalk.com/SearchResults.asp?Cat=278

And lastly you could install a P-38 accel pump cover.


off topic here you should also have a 03 YZ exhaust cam installed in that bike and the VibeGuy (thumpertalk member) $5 plug to remove the decompression system.
 

Fark

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#14
A follow up. I'm a huge fan of KISS (keep it simple stupid). All I did was make certain the carb settings were stock and raise the clip in the needle. This was my plan of attack before I went nuts with the above mods. Bog was eliminated. It really cleaned up the low and midrange response which is exactly what I was looking for (wheelie time!).

Dean, I still want that cam but I think I need an RC51 more.... I'll keep you updated.
 
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#15
Also FYI, if you disconnect the blue wire, there is no difference in the timing anymore in nuetral vs in gear.