big bore kit question

once upon a time I had a yz426f loved the power but got tired of wrestling that heavy thing around all the time. ended up getting an 03 yz250f about 2 years ago, the guy I bought it from said that he bought a athena big bore kit and put in it (290 I think) I rode it and it wasn't that impressive but I figured that it was just because I was coming off the 426 and wasn't used to the lack of power. then riding it next to my brothers 04 crf250 its didn't seem any faster than a stock 250f. I missed the hp of the 426 so I got a 250 2-stroke and sold the yzf to my friend. I told him I would try to figure out what might be wrong if it indeed does have the big bore kit in it (it does have an athena cylinder). it still has the stock pipe on and someone told me that is the problem. they said that the stock pipe wouldn't let it breath like it needed to and that it wouldn't be any faster until it got an aftermarket pipe. it makes since but I was wanting to get some input before I told my friend to go ahead and put the money into a new pipe, another thing that crossed my mind was jetting, I have no idea what the jetting is like on it, it seems to run good just no extra power like you would think it would have, so what do you guys think I should tell my friend he needs?

thanks

hmmm i put a 290 kit in my 06 and although stronger somewhat not what i expected for 40 cc's more...to be honest it was almost like an 07 stock or a lil less...so im rather curious too...could also b ethe same reasonin u were sayin cause i still have a 426 and have a bad habit of riding my friends 450s and a buddies 525, so who knows

290 Kits run too rich due to the higher manifold vaccum sucking in more fuel, you need to lean them out heaps, and have a decent pipe. Then they stomp allover 250F's.

Head porting and hotcams increase the head flow to the requirements of the BBK and make them hammer even harder.

I have a 290 kit with head porting, hotcams, FMF powerbomb and GYT-R pipe and it will eat a 426 no problems.

290 Kits run too rich due to the higher manifold vaccum sucking in more fuel, you need to lean them out heaps, and have a decent pipe. Then they stomp allover 250F's.

Head porting and hotcams increase the head flow to the requirements of the BBK and make them hammer even harder.

I have a 290 kit with head porting, hotcams, FMF powerbomb and GYT-R pipe and it will eat a 426 no problems.

X2 on that. I'm down on the pilot and the main and still get a bit too much buildup on the piston after 40 hours. I'm on the bubble for a big bore after riding a well jetted stock '09 KX250F. All the work I've done on mine and it is just a tad weaker in 3rd gear roll-ons as I have to do a quick fan of the clutch to get the same acceleration as just twisting the throttle on the KXF. :p

X2 on that. I'm down on the pilot and the main and still get a bit too much buildup on the piston after 40 hours. I'm on the bubble for a big bore after riding a well jetted stock '09 KX250F. All the work I've done on mine and it is just a tad weaker in 3rd gear roll-ons as I have to do a quick fan of the clutch to get the same acceleration as just twisting the throttle on the KXF. :p

which makes me wonder if i should say eff it and buy a new bike lol

When I installed my 290 kit is was pissed at the power. My jetting was pretty close to the same as the 250, but i changed the gearing and it ran like a raped ape. Go up to a 14 tooth in the front and also go up one in the back. You will get great low end without losing much top end. I loved it when I did that.

alright I'll tell him to get a 14 tooth sprocket for the front he is about to buy some anyway, how many teeth are on the rear stock? I didn't have the bike long enough to go through sprockets so I have no clue. and any recommendations on a pipe, prefferably an older one he won't want to put more then a couple hundred $ max into a pipe.

I wouldn't be so quick to rule out the fact that the dillhole that sold you the bike had installed an Athena kit with the stock bore....

I just picked up and 04 a week ago, pulled it down to replace the piston and rings (never know the condition). The guy said it had a 290 kit in it that he installed a couple years ago. It said Athena on the side of the jug. I ordered a 290 piston and gasket set. I got the thing tore down and whent to drop a ring in to check ring end gap and guess what....the ring was not even close.

He had installed a 250 cylinder.......

yeah the thought of a stock bore athena cylinder did cross my mind I'd say we will rebuild it before the summer is over so I'll know for sure then. they should stamp the piston size on the outside of the cylinder then there would be no questions.

When I installed my 290 kit is was pissed at the power. My jetting was pretty close to the same as the 250, but i changed the gearing and it ran like a raped ape. Go up to a 14 tooth in the front and also go up one in the back. You will get great low end without losing much top end. I loved it when I did that.

hold on a sec I got to thinking. wouldn't going up a tooth up front take away from bottom end and be like taking 3 from the back plus one tooth you said to add to the back. that would be the same as just taking 2 teeth off the back right? I may be wrong but wouldn't you lose bottom and gain top with that combo? More low end is more what I'm looking for.

I'd focus on the jetting 1st as a well jetted 290 has much more bottom end than stock,

Everyone so far has had great luck leaning the MJ 2-3 sizes. and the PJ stays either stock or leaned out 1 size.

I've had saddletime on 2 290's. both were jetted well IMO. 1 had 13/48 stock gearing and the geariing was off in that it went thru the gears too quickly. I'm a firm believer in "the 3rd gear rule" which is 3rd is the gear you want to use the most, so depending on your track/trail, gear the bike so it will pull 3rd in all but the slowest corners. I blew thru 3rd like it was 2nd. It wheelied too easily also as i was having a hard time keeping the front planted while cornering with the throttle on. The 2nd bike had a 13/46 and it had none of the bad traits as previous and it still pulled 3rd well in slow corners on the track. Owner switched between 13/46 for track and 13/47 for trails.

Again, both were well jetted and pulled seemlessly from off idle until high rpms. 1 just pulled too quickly thru the gears.

alright I'll try the jetting first. as far as I know its got stock jetting still

alright I'll try the jetting first. as far as I know its got stock jetting still

That's your problem, if true. Go down at least 2 full sizes on the main and work from there. My 290 was a DOG until I leaned it out. I was amazed how bad it could run just a few jets too rich (although mine ran well at low RPM's but totally gave up past about 7k)

The 290 kit makes a HUGE improvement. There is something wrong for sure if regular 250's are keeping up.

you wouldn't by chance know what the stock jet sizes are would you? I'm going to go into the carb tonight or tomorow and see what sizes are in it and tell him to pick some jets up at the dealer next week

assumin u have a us/cdn version unlike myself ur MJ 178, Pilot 42, leak 80 wit the needle in the 3rd positon

with my 290 i believe i put in a 170 MJ with the needle in the 2nd clip...and its much better now as it warmed up and wouldnt ya know it..a clean air filter lol

i ordered a 50 leak jet and the '09 needle as williams reccommends those and i fig what hte hell ill giver a shot

ok so I got a chance to pull the MJ tonight and its the stock 178, thats the only one I pulled because thats the only one I know where to find lol. don't know much about carbs but I did change the mj in my 426 when i had it so I knew where to find that one. Any suggestions on what size MJ to go with for about 2000-2500ft elevation and from 70-90F? Do I need to change all the jets or just the main, my 426 was running lean or rich and I went up or down one size I can't remember but just changing that seemed to fix the problem. and about hoe much do jets run at local dealers? thanks for all the help getting this running better, its not even my bike anymore but my friend knows nothing about fixing these things, but I like to learn all this new stuff so when my bike needs it I'll be good to go! truthfully, he doesn't really need more power then a stock 250f makes, is coming off of a 4 wheeler (Z400) only had it for a little over a year, so still kinda new to riding in general, he's got about 5-6 hours on the bike and hasn't quite got his wits about him on 2 wheels yet. but his bike stays at my house and I ride it every once in a while so it would be cool to get it running right so my brother will shut up about his crf250 keeping up with a 290yzf lol. he might even want to riding it once the power is there since his crf450 is out of commision at the time. but thats a whole other barrel of monkeys lol honda pos! but again I appreciate the help and maybe I'll know something that one of you guy's don't know one day(doubtfull) and can return the favor!

hold on a sec I got to thinking. wouldn't going up a tooth up front take away from bottom end and be like taking 3 from the back plus one tooth you said to add to the back. that would be the same as just taking 2 teeth off the back right? I may be wrong but wouldn't you lose bottom and gain top with that combo? More low end is more what I'm looking for.

When I installed my 290 kit I asked the same question on here about not noticing a difference in power and someone told me to go to the 14 tooth in the front and up 1 in the rear also. I did it and it changed the bike completely. You can change all the jets you want, but until you change that gearing you are going in circles.

Head down to the dealer and get the next 3 sizes smaller main jet. Try each one out with the bike fully warmed up (ride it around for about 10 min), and then do a full throttle test. It's best to run it across a muddy or sandy field, or find a long uphill stretch. The main jet affects mostly full throttle settings, so that's where you'll want to test. You'll find the power will drop off a bit if you get too lean. If you feel like you went too far, go back up one step at a time, and once you get too far, the power will feel muddy instead of crisp. You'll see what I mean

Once you get that dialed, try moving the needle clip up and down one slot at a time, and test the throttle response at mid-throttle positions. You'll be stunned how much of a difference proper jetting will make.

Your not going to get the maximum benefit of the larger displacement by just adding the big bore kit and changing the jetting. You will get a noticeable power increase, but to take full advantage of the larger bore other parts of the engine should be addressed. When I put my 290 kit on I had the intake and exhaust ports opened up a little and had the combustion chamber massaged a little. Where I really noticed the power increase is when I had the carb worked on. The shop that did the work did some porting on the carb and rejetted it. They also put the bike on a dyno and checked the carbon output to make sure it was tuned properly and had the correct jets. the earlier four strokes have a slight bog off the line and need to have the carb massaged a little to correct the problem. Now, my bike has been cured of the bogging issue and is able to pull on a stock 450 off the line. It also helps to be running VP C12 fuel as well. I know this is alot to do to get the max power out of the machine and may be more time and money than your friend wants to spend, but to get the engine tuned just right it is worth it to get it tuned on a dyno.

I don't really think that my friend wants to or really needs to do all the other engine mods to really get all the bike has he doesn't race or anything, if it were still my bike I would consider all that work but I got my 2-stroke going good and love that so I'm on the 2-strokes for now at least. My buddy is getting ready to buy a new chain and sprockets I'll tell him to get the 14/49 combo on sprockets and he's keeping his eye out for a used pipe for it so I would say with the jetting and gearing a pipe that should make a good difference. thanks again guys

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