07 YZ250f Big bore/stroker

I have a 2007 YZ285F that I am trying to get the jetting right on. The motor was just built by me not too long ago and has less than 5 hours on it so everything is mint. I was going to run stock jetting because it seemed to work a little better during the colder months, but now that it is warm outside it blubbers and bogs on the bottom end. Once it gets past that into the top end it just flat out hauls ass.

Heres whats done to the motor...

Balanced/trued/welded Hot Rods stroker crank +3mm

MXtime recut my valve seats and rebuilt the head

New Titanium OEM valves/springs/seals

New Hot Rods Stage 2 cams (both intake and exhaust)

New +3mm Cylinder Works cylinder

New +3mm Vertex piston

Stock exhaust

Not sure what jetting is in there right now but lets assume its 07 YZ250f jetting. If its anything like jetting a 2 stroke, right now to me it personally feels rich on the PJ but its not hard to start after a fall.

Too rich. You need to lean things out some.

Pilot is for idle only. Set the fuel screw for a hot engine. If you have to be less than 3/4 of a turn from closed, try a smaller pilot.

You noticed issue is the needle and probably the main. The needle probably needs to be thinner on the root and a longer needle. Main will need to go down a size or two.

Too rich. You need to lean things out some.

Pilot is for idle only. Set the fuel screw for a hot engine. If you have to be less than 3/4 of a turn from closed, try a smaller pilot.

You noticed issue is the needle and probably the main. The needle probably needs to be thinner on the root and a longer needle. Main will need to go down a size or two.

If the pilot is only for the idle, then its probably right or pretty close. Bike starts great.

I will try a size or two leaner MJ and see if it improves. Think its safe to go 2 MJ's leaner right out the gate?

Main on its' own will only make a difference at WOT. You need to get a needle leaner not only in the straight diameter (thicker needle) but in the taper (longer needle).

Do not worry about hurting anything if you ever are too lean. All that will happen is the bike will not 'pull' and will just 'dry bog'. Not like a 2S where you can burn up a piston.

Main on its' own will only make a difference at WOT. You need to get a needle leaner not only in the straight diameter (thicker needle) but in the taper (longer needle).

Do not worry about hurting anything if you ever are too lean. All that will happen is the bike will not 'pull' and will just 'dry bog'. Not like a 2S where you can burn up a piston.

OK sounds like a needle would be best to try first then. It seems to run fine WOT as far as I can tell.. thats when it runs best actually is when your ripping the grip off and just clicking through gears. It just has an annoying bog/stumble/blubber down in the lower revs. For instance, if you stab the gas to get the front to come up to wheelie, it will bog badly, then pull hard.

What needle would you suggest?

What

What needle is in your bike/. 'Stock' is not an answer. The actual code from the needle is needed.

I would have to consider upgrading the exhaust with all them mods a aftermarket exhaust would really wake it up

Just got my suspension revalved on the bike and Im pumped to ride it but still cant due to jetting and a bunch of other issues! Built this thing like 6 months ago and still havent ridden it at the track and have put less than 5 hours on it in that time. Just sad at this point how much $$$$ is in the bike..

Tomorrow or Friday I will go pull the carb and check the main jet, pilot jet, needle, and leak jet and see what is in it. The bike now when you start it has a bit of white smoke (I assume too much fuel) and if you stab the gas it will stall.

Ok

Pilot jet - 42

Main jet - 178

Needle - OBEP (Q?) 3rd clip

What needle should I go to to make it run leaner ? Its so rich somewhere that at idle if you stab it and hold it, it will stall.

Any bike, if you nail the throttle at idle, will stall. You have to ride the bike and see what it does during normal riding.

Needle is steady state (meaning holding the throttle and riding in 4th gear up an incline)

Acceleration is the AP. Have you confirm,ed you get a good, strong squirt? What leak jet? Sounds like you should be a 40 or a #50 and probably have a much larger one. Oring mod/stiffer AP spring?

White smoke if the bike is cold, is probably just condensation in the exhaust. If the bike has been run 15 minutes and still smokes, it is probably coolant (head gasket)

Any bike, if you nail the throttle at idle, will stall. You have to ride the bike and see what it does during normal riding.

Needle is steady state (meaning holding the throttle and riding in 4th gear up an incline)

Acceleration is the AP. Have you confirm,ed you get a good, strong squirt? What leak jet? Sounds like you should be a 40 or a #50 and probably have a much larger one. Oring mod/stiffer AP spring?

White smoke if the bike is cold, is probably just condensation in the exhaust. If the bike has been run 15 minutes and still smokes, it is probably coolant (head gasket)

We have an 08 YZ450f that is completely bone stock and when the bike is cold, you are right, it will stall. If its hot, it will not, it revs up as it should. Im not talking about no gas to 100% within a nanosecond, but your average "hey, rev up your bike for me" kind of rev if that makes any sense. It should be able to respond to it and it does it just as badly when riding except in much worse situations, like the face of a jump.

So today I swapped out the 42 pilot jet for a 38, and the 178 main jet for a 170. Also raised the clip on the needle one notch , so now its in the 2nd clip. Bike did NOT like this!! It was spitting and popping and the bog on the stand was much worse. I pulled the 38 pilot jet and replaced it with a 48, and replaced the 170 main jet with a 180. This actually helped the bog, but made starting more difficult and the bike wouldnt idle very smooth, and it actually knocked off on me once when I pulled in the clutch rolling to a stop.

I also noticed if I turned the fuel screw way in with the 48 pilot, the idle got alot higher and much smoother, but the bog would get worse again.

Tell me if Im wrong as Im no jetting guru, but heres what I think. Main jet is fine where it is at as I couldnt notice any significant difference with the 180 vs the 178 (stock). Pilot is way lean on the 38 and way rich on the 48 so I will probably end up with a 45, or back with the stock 42 as it seemed to actually like that. I think the bog is better with the massive 48 pilot is because its acting as a band aid. I think this because typically with jetting if your way off and you make a drastic change , it will either get drastically better, or drastically worse.. like when I went leaner on the jets, I knew it right away. Neither happened to me as I actually created another issue (rough/no idle) but mended another. I think its actually so rich now on the pilot circuit that it is covering up a very lean (not enough gas) AP squirt. I have a very strong squirt (carb is rebuilt as well) but Im thinking I need to change the leak jet size , and pretty drastically. I dont have any, so Im going to pull the carb again, ,see whats in there and order a bunch of different sizes.

Does that make sense or am I crazy? Also, with leak jets, which way (bigger number, smaller number) means MORE gas into the motor and less gas into the float bowl and vice versa? I would take a wild guess that a bigger number means it will leak more into the float bowl and shoot LESS into the motor, is that right?

You are going about it all wrong.

You cannot jet on the stand

A 250, 250 Big bore is not the same as a 450. Carbs on 250's are huge in comparison. You are comparing oranges to melons.

Pilot and fuel screw are for idle only. Set with a hot engine. You will end up with a 42 pilot, fuel screw between 1.5 and 2.25 turns.

Needle is for off idle and below WOT, tested steady state, under load.

Main is for WOT

AP is for acceleration (bog). A leak jet 'leaks' fuel back into the float bowl. Yes, a smaller leak jet is a more sensitive and greater volume AP squirt.

You need smaller number leak jet. I had bog issue also, I changed leak jet and bog was gone. Leak jet does fix only bog issue.

Just learned that the stock leak jet size is an 80, and Im going to assume thats what mine is. I WILL CHECK THOUGH. lol

For giggles Im gonna try messing with the AP timing in the meanwhile. I also read that ideally an AP squirt is only .5 - 1 second, and Im no stopwatch, but Im fairly sure mine is at least that long if not a bit longer. If I put a smaller # leak jet in, this will squirt more gas into the motor (less wasted into bowl) BUT will it also reduce the squirt time?

Sorry page hadnt refreshed, didnt see either of your posts before I made mine.

Gonna go back to the stock pilot and main. Gonna leave the needle position where it is, and going to go to a smaller # leak jet. Gotta order them first since the dealer doesnt have crap in stock.

Do not 'mess with the AP timing' Methodically test AFTER you have the pilot/fuel screw/needle/main correct.

The timing screw does not change the duration it changes WHEN it squirts. Modern FCR's all squirt about .9 to 1.4 seconds and the duration is fine. It is the sensitivity, total volume and when the event occurs.

If you feel complled to time it, use a video camera that shoots at 30 fps. Then you can replay frame by frame, counting them. 15 frames = 1/2 second, 7 frames is about 1/4 second. You reaction time pressing a stopwatch is about 1/2 second. So 1/2 at the start of the event and 1/2 after means you will be adding a second. If you 'mess with the timeing'; and get it out of wack, you are going to have a very hard time resetting it. All I can tell you is keep good notes.

You can wing jetting if you must, I can help only so much.

one part of jetting at a time make a note of every thing you do and take the time to study your notes william1 is very smart at jetting .

if you read through the jetting section i think you will find that the big bore yzf's tend to run leaner than stock jetting i personaly run a atheena 290 kit with web cams an fmf pipe open air box ap mod and if i remember right 42 pilot needle position second or first from top and 182 main jet and i know for a fact the ap squirt just misses the slide, the bike is run in the pacific northwest.

clarification i should have said read through the jetting sticky in the yz250f section -first topic i think it is

Do not 'mess with the AP timing' Methodically test AFTER you have the pilot/fuel screw/needle/main correct.

The timing screw does not change the duration it changes WHEN it squirts. Modern FCR's all squirt about .9 to 1.4 seconds and the duration is fine. It is the sensitivity, total volume and when the event occurs.

If you feel complled to time it, use a video camera that shoots at 30 fps. Then you can replay frame by frame, counting them. 15 frames = 1/2 second, 7 frames is about 1/4 second. You reaction time pressing a stopwatch is about 1/2 second. So 1/2 at the start of the event and 1/2 after means you will be adding a second. If you 'mess with the timeing'; and get it out of wack, you are going to have a very hard time resetting it. All I can tell you is keep good notes.

You can wing jetting if you must, I can help only so much.

I understand it only changes the timing, I was asking if changing the leak jet to a smaller or bigger size somehow affects the duration.

Ive messed with the AP timing screw before, I think it was last year when I was having carb problems with the 250f.. I think I put it back where it should be but I might do what you said and record it with a camera and set it that way.

I think our 450F has a 55 leak jet in it. They have the same carb so I assume I could swap it over to my carb.

Is it possible to pull the leak jet with the carb still on the bike, and is it possible to get out without pulling the float bowl? I cant remember.

EDIT- Answered my own question. Gonna go outside now and see what screws are on the float bowl. If its allen heads on both bike carbs, I may try to do it with the carbs on both bikes. If they are phillips.. guess I have to pull both carbs. Sucks

Edited by Ktmrida4life

Your 450 should not have a leak jet though some do. Read post http://www.thumpertalk.com/topic/1020196-07-yz250f-big-borestroker/#entry10874015 where I answered your Q about leak jets.

Setting with a video camera is the least prefered method and was only really done with the old style FCR. I mentioned it because of you concern (that does not apply here) of squirt duration. Please, please read your owners manual and follow the steps in it, do not wing it or try shortcut methods.

If you do pull the bowl on the bike, place a white towel on top of the engine under tha carb to catch anything you drop.

If they are 'phillips' (actually JIS), use an impact driver to loosen them.

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