Big carbs

Has anyone tried larger carbs on their 400-426 bikes?


That would probably be fruitless for many reasons. How would you tie into the TPS with a different carb? Even if you could, a larger carb would hurt the low end power. Some already believe that the 39mm FCR carb is too large for this machine.

.....and besides, if this bike pulled any more air it would probably suck the rider right down in there!!

OOps, :) I thought that the 400/426 was a Race bike, not a Trials bike! Keep slammin' those mid-range pipes on and don't worry about any power gains! <grin>

No, you're right Swiss, I can see that you must have a need to modifiy the big YZ for more power. Please fill us in on what you intend to do with more power. Is it Bonneville, or the Everest hill climb? I'm guessing it isn't just MX cause if you're using all the current power available then, I'm wondering why I haven't seen you on the podium with Ricky! Or are you just getting ready for the outdoor Nationals? :)

One option, in lieu of a larger carb that results in a noticeable gain, is to port the head. A properly prepared head on a 426 can yield about a 1.5 horsepower gain. Since these machines are aimed directly at M/X racing, there is a desired powerband for this purpose. In my opinion, the power my 426 makes is nearly perfect. "Nearly" being the operative word. After careful jetting with C-12 fuel, my engine runs and responds to my standards. I wouldn't want to sacrifice

the abundant lowend-to-mid tractable power for more on top. If a larger carb....or any other modification would give me more power across the range with no loss of throttle response, then, I would go for that. The downside to that would be even quicker chain deterioration and rear tire replacement. A DID ERT lasted only about 10 hours. My 426 is plenty powerful enough......I'm the slow poke in this equation.


I can read through any number of posts to the 400/426 list and the 250 list and the rest of the lists and see numerous questions about more power. One of them is:

"Like I said...We need a revival of the OPEN CLASS!!

Maybe 400cc-550cc two strokes, 450cc-750cc four strokes? That sounds good to me.

Could you imagine a YZ650F or YZ750F? That would be quite the ride! Would the front wheel EVER touch the ground?

Oh and of course, the 500 two strokes must be updated! I think there still is room on the market for them! They could haul azz with updates. I know that if Honda had the 500 motor in the aluminum frame, I'd be on one.

And if you've never ridden a 500cc two stroke, you haven't lived yet.

Just my 2 cents."

I just wanted to know if anyone had tried larger carbs in looking to raise the bar a bit for the Yamaha engines. Is the 39mm carb too large, or the largest that should be used? Probably no to either question. In the '70s there were some 44mm Mikunis put on 400cc XL350 engines with resulting good powerbands. In '90 I talked a friend into putting a 41mm FS Mikuni on his XR350. It pulled hard across the entire rpm range. Neither of these older 350/400cc engines can turn the rpms that the 400/426 Yam can, and neither had the potential for intake flow that the 5-v engine has. You don't have to be a McGrath to want more or better power.


Swiss, Been away for awhile. I apologize if I offended you. I was merely making a point like what boit pointed out in his last sentance. I'm a motor-head from way back. I love extracting every ounce of power from any motor I've ever owned! My point though is that unless you're using this bike in some race or area where you've reached the limit of this bike, then I believe it has plenty of power. The tracks that I've ridden this bike on so far are very tight and I haven't hardly gotten out of third yet!

Now obviously, I'm no Carmichal or Mcgrath but I do alright and this bike rips. I've ridden open class 2-strokes and I don't think this bike has any less power, it's just more user friendly. Like I said in my previous post, if you have a need for extracting more power from the 426 then best of luck and let us know how it goes.


00 YZ426F

01 TT-R125L (my son's)

91 CR125

83 YZ490

74 Hodaka Super Combat(gone but not forgotten!)

I dunno, if any motor can hit the rev limiter like a YZF the last thing I’m looking for is a bigger carb.

I went bigger bore and higher compression and I highly recommend it, but you will need a gearing change. :)

[This message has been edited by Hick (edited 05-16-2001).]

Hi. i have a ported head (40 hours on the bench) and flowed too, and its extremely smooth. mine is a 98 400, and ive riden a 01 426 with a r-series pipe on it, mine felt faster all over accept on top (darn those Ti valves). it was polished and flowed by BBR motorsports, and when i had to rebuild my engine, i got to see it, it was like a mirror, beautiful!!! it makes the bike rev a little faster, but it still gets traction, and it makes it have a little more balls. i recomend it, dunno ho much H.P itll give ya, but oh well, feels good. just my 2 cents.


some of those old carbs sat way way away from the head compared to todays carb. thats hy they were bigger. also, the flatslide is worth 1mm more than an old roundslide.

i can imagine though that a larger carb would help in a desert race, but for me there isn't & never has been a sure sign that when i whack the throttle out of a hairpin that i'm going to get the rear wheel to light up everytime.

someday, somebody is going to take the 37mm carb from their 250 & try it on their 426 & the results will be very interesting.

remember that yamaha must have paper horepower to sell bikes. the top riders need that 39mm carb but the rest of us could do with something that little bit more responsive.

so when is someone going to try it?


So, when is someone going to try the 426 carb on the 250? <grin> Reportedly there are some fairly nice horses available in some louder performance pipes for the 250 and with some good porting and a 2mm bigger carb, it should be nearing its potential. If the 400 was full potential, then why did the 426 happen, and now the planned 450 and the Honda 450 and the more powerful 520 KTM etc. Real people CAN ride bikes with more performance, the performance just needs to be well balanced and correct rather than pipey. Of course, suspension and the rest of the bike needs to be right also.



I think the answer you are looking for may lie with the factory Yamaha team guys. Maybe they have tried a larger carb on their bikes?

"Most" of the guys on this site are happy with the performance of their bikes and can't justify spending $$$$$ on engine mods, when a few trips to the gym would be more beneficial to race results.

It's always great to speculate over what next year will bring and wether you will buy it :D.

BTW if you want more poke from a 426 and are prepared to spend try to get your hands on a Team Rinaldi 450cc kit with exhaust & ignition, raise the compression ratio 15% and then run some methanol through it. If you need more power mix in some nitromethane about 40%!

You may need to see a doctor about your shoulders after the first ride :D


Because golf courses can be MX tracks! :)

[This message has been edited by YZR426 (edited 05-21-2001).]

I too wanted to see what the performance gains would be from taking the carb from my 426 and putting it on my 250F... It made it run funny down low. Almost like someone stuck a sock in my airbox. But when it came on the pipe.... It was strong. As in real strong. Definitly shows a need to upgrade to an o'ring chain. Gold ERT won't last, even on the lil'250. Never did try the smaller carb on the 426. Figured that it was a step backwards. If I find the time, I will make the swap, and keep you guys informed.


Mr. Wizard says: "Go BIG, or go home!"

Well, I guess someone on this list likes more power. Of course, we won't count all of the pipes and airboxes and misc performance stuff that everyone "Else" has on their bikes. What need would the list be once everyone had been told how to adjust their chain and which spark plug works best?

Some good porting to clean up the flow (unless you YZ-xperts figure that the Yamaha Porting can't be improved?) and the right pipe combined with the right jetting, the 250 should run strong everywhere.


Hey, put a larger carb on the thing and tell us the results. Geez.

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