Weird problem, bike running too rich and lean...

[bjl4776]

I picked up a 08 250r that was wrecked as a project bike. Bike had been sitting for a long while and didnt run when I purchased it but had good compression. I rebuilt the carbs and adjusted valves to the looser end the specs. Bike had a slip on and factory air box when I got it. The carbs werent too bad but i went through everything, had stock jets, no shims, and correct float height. Got it all back together and it started right up but was running rich at idle and until about 5k, then it would break up a bit uptop. At this point I already knew I was going to have the carbs on and off so many times I removed the stock airbox for a k&n 0990. Put it back together with 3 shims per and found it ran good until bout 9k and it would break up. Checked plugs, too lean. I know I need a jet kit after removing the stock box but I decided to play around in the meantime. Started adding 2 shims at a time until it pulled hard to redline with 7 shims. But with 7 shims it was running way to rich down low and would flood coming down to idle after running it hard. I went back down to three shims and used "tuning" tape to cover the airfilter until it pulled good to redline. At this point I thought everything was good and went for a long ride. Bike ran great while it was cold and always pulled hard to redline. After riding for a bit it was good and hot I got stuck in some stop and go traffic and she was flooded. Wouldnt rev past 2k and if I let off the throttle she would do. After a few engine starts I got her to clear out and rev fine in neutral. When I would goto take off she would bog again. I would have to slip the clutch keeping it at about 5-6k to keep her goin then it would pull hard to redline but problem would repeat when coming to a stop. I tried removing some tape to a point where it was breaking up a little up top but still had the same problem down low At this point I just pulled all the shims out and put it back together but haven't tried it out yet. What could causing it to be running so rich down low even though im leaning out up top.

Also, even when the bike isnt fully warm and running pretty good sometimes it would stall out when coming to a stop if I dont keep the throttle cracked. Idle has been adjusted by opening idle screws till highest idle rpm is achieved. Smoked tested both carbs for vacuum leaks and didnt find any leaks. Secondary air has been removed and capped. Crankcase vent has replaceable filter. Fuel is 89 octane with stabilizer, fuel filter installed.

I am a L1 BMW tech and know fuel injection like the back of my hand but this is my first in depth use of carburetors. I have a wideband 02 sensor laying around. Im about to weld a bung in my exhaust and ride around with a laptop in my backback and datalog it.

Any help is greatly appreciated,
Thanks
Brandyn

[Motofool]

http://www.dansmc.com/carbs2.htm

[DaBlue1]

This info may save you a lot of work and frustration, but to answer your question as simply as possible. Leaving the airbox in with stock jetting and carb setting and a full exhaust, make close to 30 HP. The average of 2 shims under the needles, snorkel removed from the stock airbox and a slip on exhaust can make up to 28.5 HP. Stock airbox removal requires larger main jets. 87 octane works better (non ethanol if you can get it).

Leave the airbox in???
http://www.ninjette.org/forums/showthread.php?t=124144

Wide band A/F Gauge
http://www.ninjette.org/forums/showthread.php?t=67530

How Constant Velocity Carburetors Work
http://www.motorcycleproject.com/mot...bs_work_v3.swf

Motorcycle Carburetor Theory 101
http://www.motorcyclecarbs.com/carbs101.pdf

Pipe Jetting (A Must Read)
http://www.motorcycleproject.com/mot...s-jetting.html

Carburetor Jet Tech
http://www.motorcycleproject.com/mot...et_tech_v3.swf

Resonant Airboxes: Theory and Applications
http://motorcycleinfo.calsci.com/Airboxes.html

Jetting Database
http://www.ninjette.org/forums/showthread.php?t=10208

Stock 2009 Ninja 250R Dyno Chart
http://www.ninja250mods.com/Ninja-20...Ninja-250r.jpg

[Motofool]

Originally Posted by DaBlue1

Motorcycle Carburetor Theory 101
https://docs.google.com/file/d/0B8nS...eDg/edit?pli=1

[bjl4776]

Thanks for the good reading. I didnt remove the stock airbox looking for tons of horsepower, hell its just a 250 isnt it. I mainly removed it for easy access to the carbs. It wasnt running right with the stock airbox in and the carb completely stock so I knew it was gonna take more than a couple R&Rs of the carbs before I was happy. I need to go run it now that there are no shims in it and see if the problem is gone. It is possible that I got one of the few bikes that came rich from the factory so Im working against myself. I got in trouble for riding it the other day with no tag so Im gonna wait till its legal to try again and unfortunately riding it around a parking lot wont do the trick. Salvage inspection is on monday so after that Ill have a better idea and hopefully Ill have my jet kit ordered next week and get my wideband in.

[bjl4776]

So after reading the article on airbox resonance and thinking back to some individual throttle body runner length tunning ive done in the past i got to thinking. I looked at the factory airbox and found that the runners can be removed. I took them out and with slight modification to the k&n put them in. It should be the best of both worlds. I know ill lose the resonating design of the airbox but ill regain the air velocity I lost with the filter alone making the mid range loss much less. Ill have to get my old formulas out to determine what the optimum length for this engine should be and make some custom ones, but this should help out considerably for now.

[alex.s]

you already know the answers. you need different jets. why go past the correct answer?

[DaBlue1]

Originally Posted by Motofool

Sorry. Fixed it

http://www.motorcyclecarbs.com/carbs101.pdf

[Motofool]

Originally Posted by DaBlue1

Sorry. Fixed it

[bjl4776]

Originally Posted by alex.s

you already know the answers. you need different jets. why go past the correct answer?

Well i know i need new jets cause of the air filter and leaning out up top. I am just confused on why it is also running so rich coming to idle that it is almost flooding. It seems like it is 2 different issues but it may not. While im waiting on the jets I figured id see if I could get any input from people with similar problems.

[alex.s]

there are two jets, a small and a big. they are called pilot and main jet. the needle transitions the big jet on and off depending on how much air is going through the carb. so with two jets and a needle, you have two points and a curve between them. this represents your fuel map... i'll draw it in text:

fuel map:

vacuum gets higher --->
^ ...................................................
^ .........................................******
^ ..............................******...........
F .....................*****.....................
U .............****...............................
E *******.......................................
L ....................................................

... ^-- pilot jet
... needle transition --^
.............................................. ^---- main jet all the way open


adding shims simply adjusts how quickly the needle moves in relation to the vacuum curve. so for example adding shims to the above would make it look like this:

vacuum gets higher --->
^ ........................................................
^ .................................*************
^ ......................******.......................
F ..............*****.................................
U ......****...........................................
E ***..................................................
L ........................................................


you can see the levels for the pilot jet and the main jet are the same, but it transitions much lower in the vacuum range. this is usually NOT what you want. you can see in this graph the fuel stays the same for about the last 1/3rd of the graph. but the air is still climbing at that point, so in this fuel graph, you would probably be super lean up top but nice and rich in the middle. maybe still lean on bottom since the pilot jet hasn't been changed and the needle doesn't effect it at idle.


your fuel curve should match the flow of air as the throttle opens and the rpms climb. if for example at low rpm, there is much less vacuum because the intake is wide open, it pulls less fuel and so the air/fuel ratio becomes lean. that would mean either the airbox needs to change, or if you are set on that air filter setup, you would need to adjust the pilot jet to correct the fuel ratio.


the problem with shims that nobody seems to understand is that it DOES NOT CHANGE your fuel ratio at idle or at the top of the vacuum range. it only effects mid range. so when you change your air flow from exhaust or airbox changes, you need to change the JETS. not the needle. JETS.

[LoD575]

alex.s is spot on.

Here is some reading from Factory Pro on CV carb tuning

Only way to adjust A/F ratio for idle circuit and main circuit is through changing jets.

[Lychee]

Great post Alex. I was thinking the same, the OP never mentioned adjustment of the idle mixture screw.

[alex.s]

note the pilot mix screws are more for fine-adjustment. if you are more than one turn away from 2 turns out, just change the pilot jet.

[LoD575]

I will be doing the entire process again later this summer. Moving from 4,300 ft elevation to sea level and putting the air box back in.

[Motofool]

Originally Posted by alex.s

.........the problem with shims that nobody seems to understand is that it DOES NOT CHANGE your fuel ratio at idle or at the top of the vacuum range. it only effects mid range. so when you change your air flow from exhaust or airbox changes, you need to change the JETS. not the needle. JETS.

[choneofakind]

Originally Posted by alex.s

the problem with shims that nobody seems to understand is that it DOES NOT CHANGE your fuel ratio at idle or at the top of the vacuum range. it only effects mid range. so when you change your air flow from exhaust or airbox changes, you need to change the JETS. not the needle. JETS.

Quoted for truth.

Wonderful post Mr Shank.

[krolinked]

Adjust your valves to the middle of the tolerances for both exhaust and intake. Remove all washers and clean out carbs/set them back to their default configuration. Make ABSOLUTELY sure your cam timing is correct.
Clean your filter and button it all back up.


I was messing with my carbs for weeks until later i found out the problem was either my timing or valve settings.

[bjl4776]

Originally Posted by alex.s

there are two jets, a small and a big. they are called pilot and main jet. the needle transitions the big jet on and off depending on how much air is going through the carb. so with two jets and a needle, you have two points and a curve between them. this represents your fuel map... i'll draw it in text:

fuel map:

vacuum gets higher --->
^ ...................................................
^ .........................................******
^ ..............................******...........
F .....................*****.....................
U .............****...............................
E *******.......................................
L ....................................................

... ^-- pilot jet
... needle transition --^
.............................................. ^---- main jet all the way open


adding shims simply adjusts how quickly the needle moves in relation to the vacuum curve. so for example adding shims to the above would make it look like this:

vacuum gets higher --->
^ ........................................................
^ .................................*************
^ ......................******.......................
F ..............*****.................................
U ......****...........................................
E ***..................................................
L ........................................................


you can see the levels for the pilot jet and the main jet are the same, but it transitions much lower in the vacuum range. this is usually NOT what you want. you can see in this graph the fuel stays the same for about the last 1/3rd of the graph. but the air is still climbing at that point, so in this fuel graph, you would probably be super lean up top but nice and rich in the middle. maybe still lean on bottom since the pilot jet hasn't been changed and the needle doesn't effect it at idle.


your fuel curve should match the flow of air as the throttle opens and the rpms climb. if for example at low rpm, there is much less vacuum because the intake is wide open, it pulls less fuel and so the air/fuel ratio becomes lean. that would mean either the airbox needs to change, or if you are set on that air filter setup, you would need to adjust the pilot jet to correct the fuel ratio.


the problem with shims that nobody seems to understand is that it DOES NOT CHANGE your fuel ratio at idle or at the top of the vacuum range. it only effects mid range. so when you change your air flow from exhaust or airbox changes, you need to change the JETS. not the needle. JETS.

Awesome, that was exactly what I was looking for. Like I said I know fuel injection, not carburetion, so this whole process is a learning experience to me. Im ordering me a jet kit as soon as I get paid and am gonna do some wideband datalogging to get it tuned right.

krolinked- I know my valve adjustment and cam timing is spot on, that is something iv'e been doing for awhile and on much more complicated engines. I like the looser end of the spec because it allows more seat time for the valves, cooling them off more and making them last longer as well as putting less pressure on the seats causing the lash to stay in adjustment longer. Not optimal for top output, but optimal for longevity.

Thanks for the great feedback everyone, glad to be a new member.

[choneofakind]

Carburetors are much easier to wrap your head around than fuel injection. Fewer variables to think about.

[cuong-nutz]

Sticky worthy @Alex

[Alex]

Where would this fit as a sticky? I'm not convinced yet, but it's also late in the workday and I'm cranky.

[csmith12]