250-500cc fuel fouling plugs

[schrode]

i have an '08 wr250 that i just recently put a gnarly pipe on. i rode it for 1 day with the new pipe in the desert and it ran like a raped ape. next day i rode it in tight woods and it started to foul plugs in short order. the plugs were wet with fuel but not with any oily residue. the bike ran well prior to the pipe going on. a touch on the rich side w/ a bit of spooge out of the tail pipe after each ride. i'm really kind of stumped as to why it would start fouling plugs all of a sudden. intake leak maybe? idk, any suggestions would be appreciated. btw, i'm running the stock mikuni carb. can't recall the jetting specs but it did run well for quite a while prior to last friday.

 

[Dirtdame]

Sounds like you need slightly leaner jetting for woods riding with the new pipe. Maybe it's tuned more for top end.

 

[schrode]

Sounds like you need slightly leaner jetting for woods riding with the new pipe. Maybe it's tuned more for top end.

that's what i'm hoping for. i was just wondering if there could be something amiss with the intake that i'm not aware of that this could be indicative of.
thanx for the quick response

 

[Dirtdame]

Well gosh, from what I've heard, those Mikuni carbs are fussy anyway even with the stock pipe, so I would imagine that any change in exhaust dynamics would have an effect on the jetting.

 

[Johnnymannen]

I would lower the needle and adjust the airscrew to slightly lean.

 

[rockdancer]

My mikuni was running rich and I lowered the needled and this helped . But there was a lean bog occasionally. Plug was slightly brown not wet though and no spooge now its very light . Maybe look at the fuel mixture as well.

 

[Old Geezer]

If most of your woods riding is done in the mid power range (less than 3/4 throttle) then I would suggest you drop down one size on the pilot jet and/or open the air screw a bit. Dropping the jet needle one click to lean it out won't really have much affect unless you're above 3/4 throttle.
Try the pilot jet change first, then perhaps drop the needle a notch so you're not too rich when running wide open.

 

[sean]

my'n did the same open areas, fine.... tight and twistie, no good, i didnt fook about with jets though, the problem is you aint getting it up to temp, the radiators are capable of keeping it cool at race speed in high 30c

if it fine when you are running in open areas your jetting will be fine you need to keep the 250 hot, it doesnt like the cold, i could tell you a simple way to keep it fine in the wood but last time i helped lots of people shopt me down even though my tip comes from some husky race lads........

ok.. get a piece of card or some duct tape and block off one radiator the card it better as its a quick to remove, here in the uk in winter i run one blocked off all the time and it dont foul, soon as the pace slows down and i havent blocked one off it fouls

but what do know

 

[schrode]

it was running just ok in the tight stuff but when it would open up and i'd get on it wot it would then hesitate, bog, and foul. it seemed like it was loading up in the tight stuff but when i tried to clear it out on the more open terrain is when the problem really happened. i'm thinking it may be a needle issue due to this, thoughts?

 

[sean]

it was running just ok in the tight stuff but when it would open up and i'd get on it wot it would then hesitate, bog, and foul. it seemed like it was loading up in the tight stuff but when i tried to clear it out on the more open terrain is when the problem really happened. i'm thinking it may be a needle issue due to this, thoughts?

if you have access to a dyno take it there and have it done, you cant dyno them properly as they are 2 stroke but you can see the power out put and when you get it nice and smooth, you'v got it

what you running in the carb at the moment?
if you can be bothered stick the origanal pipe on and see if it still does it, if not your in the jetting situation

 

[schrode]

not sure of the current jetting specs. i need to tear into it and find out what it's at so i have a baseline to go from. i picked the bike up used about 3 months ago from a local racer who had the jetting,"perfect" in his words, and it seemed ok until the new pipe went on. i just need to get to work on it. it's just surprising that simply putting the pipe on would change it this much. i tried to open the air screw a half turn after fouling the first plug and it actually fouled the second one in a matter of minutes. i then reduced the air screw to a half turn more closed that before and it ran for about 45 minutes before fouling the second. then it was just a matter of swapping and drying plugs all the way back to the truck.

 

[Old Geezer]

it was running just ok in the tight stuff but when it would open up and i'd get on it wot it would then hesitate, bog, and foul. it seemed like it was loading up in the tight stuff but when i tried to clear it out on the more open terrain is when the problem really happened. i'm thinking it may be a needle issue due to this, thoughts?

You need to do some plug burn diagnostics the basic way....get on an open road with a relatively new plug and run WOT for a good long stretch, pull in the clutch and kill the engine with the kill switch, stop and check the plug color. That will tell you where you're at with the main jet/jet needle/needle jet combination. Depending on how much adjustment room you have left on the needle, you may need to change the main jet to dial in the correct mixture.

You can use the same approach to tuning the midrange, running at at around half throttle then shutting down to check the plug color, and adjust the pilot jet as needed. Swapping out pipes can have a significant impact on carburation, so it just goes hand-in-glove to check/verify jetting with any such change.

 

[schrode]

You need to do some plug burn diagnostics the basic way....get on an open road with a relatively new plug and run WOT for a good long stretch, pull in the clutch and kill the engine with the kill switch, stop and check the plug color. That will tell you where you're at with the main jet/jet needle/needle jet combination. Depending on how much adjustment room you have left on the needle, you may need to change the main jet to dial in the correct mixture.

You can use the same approach to tuning the midrange, running at at around half throttle then shutting down to check the plug color, and adjust the pilot jet as needed. Swapping out pipes can have a significant impact on carburation, so it just goes hand-in-glove to check/verify jetting with any such change.

yep, i think this will be my approach. pull the carb apart to get the specs. then pick up a few sizes of main jets and spare plugs and start doing chop tests and start tweaking main and/or main/needle depending on results. i normally have good results with this approach i just hate wasting my riding days with diagnosis and wrenching.


thanx for all the responses guys. i'll post my jetting specs once i get to it

 

[Johnnymannen]

If most of your woods riding is done in the mid power range (less than 3/4 throttle) then I would suggest you drop down one size on the pilot jet and/or open the air screw a bit. Dropping the jet needle one click to lean it out won't really have much affect unless you're above 3/4 throttle.
Try the pilot jet change first, then perhaps drop the needle a notch so you're not too rich when running wide open.

I thought the needle was controlling the fuel between 1/4 and 3/4 throttle? Above 3/4 throttle the mainjet mostly controls the amount of fuel. Up to 1/8 throttle it´s mostly the pilot, right? My bike fouled plugs too, and i had the needle in 3rd clip, and i lowered it to first and the problems is gone. Woods riding on low revs all the time! Many bikes are way too rich on the needle. I´ve leaned out all my bikes quite much on pilot and needle and have them slightly rich on the main to be safe.

Johnny

 

[Motosportz]

You are to rich on the bottom. Go down one on the pilot jet and play with the air screw. Old Geezer is right on.

 

[Old Geezer]

JohnnyMannen, you are a wise person to never fully trust an Old Geezer like me (too many years of inhaling pre-mix can waste brain cells and affect memory).
Good question on where the various Mikuni jets come into play (throttle position, throttle slide, needle jet, jet needle, main jet, and pilot jet). They all work in conjunction with each other for sure, but at different stages. Changing the height of the jet needle can have a small impact on the low to mid range performance, but only a small one (there just isn't enough airflow to draw much fuel up through the main jet network until the throttle is opened up), so the pilot jet does most of the work down low. The needle jet/jet needle combination does most of the mid to upper work to around 3/4 throttle, but even it is dependent upon the size of main jet installed. Once past 3/4, it's pretty much all main jet.
Here's a diagram that I've always felt does a great job of pictorially describing how mikuni's work http://www.mikuni.com/pdf/vmmanual.pdf Go to the very last page of the document for the diagram.

 

[schrode]

main is a 410. haven't checked the needle or pilot yet. soon

 

[Johnnymannen]

You old geezer man!

Dropping the jet needle one click to lean it out won't really have much affect unless you're above 3/4 throttle.

It was this i thought about! I thought that was mainly the area where the mainjet was operating!

Maybe i misunderstand you in some way! Maybe i´m reading it wrong in some way!

Johnny

 

[Old Geezer]

You old geezer man!

Dropping the jet needle one click to lean it out won't really have much affect unless you're above 3/4 throttle.

It was this i thought about! I thought that was mainly the area where the mainjet was operating!

Maybe i misunderstand you in some way! Maybe i´m reading it wrong in some way!

Johnny

Actually, I think we're probably on the same page here, just saying it differently (so much for the written word!). I could be all wet, and there's probably a huge sticky on carb tuning somewhere on this forum.

It's as much a science as it is an art. I spend too much time messing with jet combinations until I get the result I want...and quickly forget all the faulty logic I used. As long as she runs strong through the entire power band and the plug looks good under all riding conditions, that's all that matters. When I'm happy with it in the end, I stop and crack open a beer or two. Then, at night I dream that I'm a carburetor and work through my aging mind just exactly what's really going on to meter the fuel. In the end, it's all magic

 

[Johnnymannen]

Actually, I think we're probably on the same page here, just saying it differently (so much for the written word!). I could be all wet, and there's probably a huge sticky on carb tuning somewhere on this forum.

It's as much a science as it is an art. I spend too much time messing with jet combinations until I get the result I want...and quickly forget all the faulty logic I used. As long as she runs strong through the entire power band and the plug looks good under all riding conditions, that's all that matters. When I'm happy with it in the end, I stop and crack open a beer or two. Then, at night I dream that I'm a carburetor and work through my aging mind just exactly what's really going on to meter the fuel. In the end, it's all magic

Haha! Yes, i think we´re on the same page! Maybe i should explain myself with throttleopenings in percent instead or something! Or maybe i have worked with too much thinner and glue through the years!

Johnny