'02 426 any jetting recommendations and what fuel to use


I just purchased my first 4-stroke and after 2 rides I love it. It seems to be running great but I have a couple of questions.

1) Anybody in the NC or surrounding areas figured changed the stock jetting on the bike. My seems to run fine but it being my first 4 stroke I don't have any experience to go on.

2) I see all these topics on what type of fuel to use. The dealer I purchased the bike from suggested race fuel. (which I have no problem running) His reasoning is to make it run cooler. However, I was reading some info on pipes and the FMF literature specifically stated to run 93 octane in a 4-stroke with their pipe as for that's what it is designed for. Then the first thing a respected friend (Billy Liles) informed me to do is to run only 93 octane in a 4-stroke. (Preferrably Amoco.) His reasoning was the opposite of the dealer, in the fact that he indicated that the 4-strokes were designed to run hotter so you get better performance out of 93 octane.

In my two rides, pump gas seems to do the trick. I would just like any opinions anybody that has experimented with both on an 01 or 02 bike. I'm hoping to get enough material to help make an educated guess without having to "reinvent the wheel" so to speak.


Whether a 4-stroke...or any bike...runs hotter or cooler is not dependent upon fuel, unless the engine pings/knocks. Many people mistakenly believe that race fuel makes a 4-stroke engine runner cooler. Overly rich jetting makes an engine run cooler, not the fuel. If you can get pump gas that is consistent in quality, the 426 will run OK. However, an engine that has a redline of 11,200 RPM as the 426 does, begs for a fuel that will allow it to rev cleanly. Race fuel satisfies this need much better than any pump gas can. To get the benefit of race fuel, jetting accordingly is required. Fuel such as VP C-12 is a good general choice but it requires leaner jetting. Once crisp jetting is attained with race fuel, the greatest benefit is a much improved throttle response. Don't expect anything more than a slight power increase using race fuel. The nice thing about the 426 is that the jets are very easy to change. I can change a main jet on a hot engine in less than 5 minutes.

Why Mr. Liles believes that the 426 is designed to run "hotter" is a mystery to me. Hotter than what? Granted, there is a "normal" or "typical" heat range that these engines are designed to run at. That includes your automobile as well. If these engines overheat, performance suffers. If a 426 is in good mechanical condition, using race fuel and proper jetting will provide you with a machine that runs very well, has excellent throttle response, and operates within its normal temperature.

I've used C-12 for over a year in my 426 and I've just started experimenting with MR-2. While the C-12 performed very well, the MR-2 is even better. The low-end crispness is substantially improved and I get even better high RPM operation. As far as dialing in the jetting for these fuels, it's not that difficult. All you do is make sure that you start at with an overly rich jetting gradually go leaner until the engine comes alive. With C-12, the stock jetting was too rich on top and blubbered at full throttle. I simply went one step leaner on the main jet and the blubbering improved, but still blubbered a little. I went another step leaner and the engine suddenly revved cleanly and strongly until it hit the rev limiter. Now I knew my main jet was correct. From there, all I had to do was lean out the fuel screw a little and had fantastic response and power right off idle. This machine is so much fun when you have the carburetion spot on.

[This message has been edited by Boit (edited September 29, 2001).]

Boit, I have also experianced this blubering at full throttle operation on the C-12 but like you stated going leaner on the main eleminates this problem.

The main reason I run race fuel is to keep the piston, exhaust port , header, and pipe clean.

I had a 4 month old 01 426 that had so much carbon build up in the exhaust port and header that the exhaust valves started hanging up, this was a dirrect result of the dirty pump gas.

The C-12 or any other leaded race fuel requires more maintenance because of lead deposits, you need to change your plug more often and about once a year pull the head to inspect it for deposits built up in the combustion chamber which increases your compression ratio.(not good)

Boit, what is the octane on the MR-2 ?

Later, Jason

Hey Boit when are you going to make that drive from New Orleans Over to Orange,tx and let me put you on some tight Arena cross ?

We have a covered track over here now, ride rain or shine.

[This message has been edited by jaybird67k (edited September 30, 2001).]

Boit, Jaybird67k & any other Houston area riders,

We need to make a trip to Longhorn MX one weekend, Jaybird67k has been wanting me to come up there and ride and now the weather is great. We should plan something in the next 3 wks or so. I live in Houston so it is a 2 hour drive (not bad at all). How far are you away Boit?

I have heard from people I ride with that Longhorn MX is awesome. Me and my Dad will for sure head up there within 3wks. Roost ya later LOL,



I get my kicks on a 426!

Motoman393's MX Site

Orange is not all that far from me. About 3 hours or so. Houston is a 6 hour drive. I would love to come ride with you guys. I met YZThumpa last year and we practice together when time permits. He and his brother both have 426's so maybe a bunch of us could hook and get in a full day of riding. Some time in the next few weeks sounds perfect.

MR2 is oxygenated and leaded rated at 102 motor octane compared to 108 for C-12. This particular fuel has an excellent distillation curve that is a great match for the 426. Phillips B37 is another oxygenated fuel that is an excellent choice.

Thanks Boit, I really appreciate your response. I got the ride the 426 yesterday and it ran perfectly fine on pump gas. I appreciate all your detail, however one thing I'm curious to know. I don't plan on purchasing an FMF pipe. However, the FMF is the one that I saw posting literature specifically stating not to run race fuel using their pipe on a 426 as for they are designed to run with 93 octane gas? If race fuel is better why would a performance manufacturer of pipes specifically recommend against race fuel?

Oh well, like I said I appreciate your response. As I mentioned I gathering all the opinions I can and I'll use a little of all of them for tuning if I run into problems. Again, many thanks!!!!!!

Kfrosty: why FMF recommends pump gas is something I would like an answer to as well. Pump gas is not a good choice for an engine that has a redline of 11,200. Pump gas is distilled for use in the typical automobile,SUV, pickup truck, etc. How many of these vehicles come close to that RPM range? NONE! As an engine's RPM's climb, there is less and less time for the fuel/air mixture to ignite and produce the high pressure needed to push the piston down. Since this is a 4-stroke, that means that at, say 10,500 RPM, there are 5,250 ignition cycles per minute and to break it down even further, 87.5 ignition cycles per second. And thats not even at redline for the 426. To accomplish this feat, a fuel is required that has sufficient flame speed that matches this RPM range. I would like to know how anyone can design an exhaust system for the 426 that runs better on pump gas especially when the RPM redline of this machine is taken into account. Perhaps their system makes the engine anemic on top. I think I will try to email them and get their opinion on this.

Thank you again Boit for your response. Seems you are passionate about your advocation of racing fuel and I understand and don't disagree with what you are saying.

I was trying to find the link where I found the info and couldn't find the exact one. However, I found FMF's racing website and the info was on it.

So you can go to fmfracing.com. Click on "Jetting Guides". And after jetting info for every bike, you'll see this disclaimer under the info for Yamaha 4-strokes.

"FMF HI-FLO header will need a 48 pilot. Do not use race fuel, ignition is designed for 92 octane pump fuel "

In closing, I'm not trying to be argumentative with you but I've heard advice from both sides of the spectrum here. However, yours has been more detailed than any and I really appreciate it!

not to mention that the 426 has 12:1 comp.and we all know what happens to pump fuel in a high comp.motor.....ping pong ping ping,ping....



kfrosty, I live in Charlotte also, and too just bought a 426. Where do you ride?

Hey there,

I ride here and there. I ride at Dogwood, the Performance Track in Waxhaw. (Have to purchase a membership.) On weekends I don't race, I go to Mike Andrew's track. Then finally sometimes I travel to Morganton ride with a friend. Course I'm using getting lessons when I go over there.

Where do you ride? I have about 3 or 4 guys I ride/race with. We're always looking for good tracks to practice on.

BTW. Where'd you get you're bike from?

I rode a '96 XR 600 (now for sale) before my YZ, so I did alot more trails than tracks. I've been to Whitmire SC (trail system), TNT Motorsports in Chester SC, and the mountains of West Virginia (absolutly awesome hill climbs and trails). The guy a ride with is moving to Texas, so I'm trying to find new places and people to ride with.

I got my bike two weeks ago from a guy in Columbus GA. It's a 2001 that was only 5 months old and looks like it came off the showroom floor.

Cool. We can take this off-line if you want to talk more. I don't do trail riding anymore but I can get you info on a couple of tracks if you are interested. Also, recommend a good Yamaha shop if you aren't dealing with anybody.

Email me at kris@n-sv.com if you are interested.

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