Motorcross Action recommended jetting?

6 replies to this topic

Posted July 17, 2008 - 06:12 PM


I just got the August issue, there is a big article on the 08yz 450. I have an 07. The magazine recommends as follows= main 160 / pilot 45 / nflr needle 3rd from top / leak jet 55. I did the o ring mod and added a Sunline fuel screw. All other specs were the same as the article, exept I did not move the needle from the fourth position because I read some threads that said when moved to third from top it ran like crap. I ride so-cal high desert, the pipe and filter are stock. What do you think?

  • bigred455

Posted July 17, 2008 - 06:40 PM


I have a 07 also ,I am at sea level here in Jersey. 85 DEGREES and up a 155 main is the ticket. You probably wouldnt know you were running rich on a 450,but the 155 pulls stronger than the 160.I am running 93 pump and AVGAS100LL.Running AVGAS will richen it up some,I am 50/50 PUMP AND AV. The needle is good where it is at,I moved both directions 4th pos is dead on. Pilot is pretty much on with the 45,flirting with a 42 hot weather. Do not pay attention to jetting information from mags,just move them and see what you get. If I were you I would go to a 155 main and see what you get,pull 4th or 5th gear in loam and see if it pulls stronger with the 155. I am thinking it is pretty hot where you are at and if you are in a high elevation definetly try the 155. You can move your needle 1 clip leaner if you like,for me in jersey 4th pos is the ticket,but it may like the 3rd pos where you are at. Give it a shot,but do the main by itself test and needle by itself test.Also go to a 165 main to confirm you are rich,move your main jets in 2's 160 -165 or from a 160 to a 155 etc. You are wasting your time going up or down one on the main.

  • grayracer513

Posted July 17, 2008 - 08:00 PM


The '08 carb is subtly different from the '07, but enough so that jetting recommendations for the new bike are not transferable. Check the jetting specs sticky to see if any other '07 owners have posted.

As far as the needle goes, you won't know if it's better for yours in 3 or 4 until you try it, but ask yourself whether it's running good now or not. The needle contributes only to part throttle operation (from about 1/4 to 3/4), not full throttle power.

Also remember that the recommendation for the #55 leak jet was made for a bike without the O-ring mod. You might want to go to a 60 to reduce the pump shot if you have a stumble on throttle opening.

As for AV gas, notwithstanding any opinions to the contrary, it is simply the wrong fuel for any motorcycle engine under any circumstance. The vapor rates are entirely wrong, the lead content is way too high, and the octane is totally unnecessary. For recreational riders and most racers in California, pump gas is all you need.

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Posted July 17, 2008 - 08:11 PM


I just went for a ride on the street real quick, no bog, but the bike seems to surge compared to my wr. Is that the flywheel difference. The surge is at about 15-20 mph, but when on the throttle it hauls ass, no surge. I have never run AV gas but I will be running 50/50 91 octane and u4.2 just to use the last gallon or so that I have left. Maybe this will help to clean the carbon, if any off of the valves.

  • grayracer513

Posted July 17, 2008 - 08:37 PM


U4.2 is highly oxygenated, and as such, requires richer jetting to offset the oxygen content. Don't expect your pump gas jetting to work exactly right with even 50% U4.2.

If the the "surge" is erratic, irregular, and accompanied by the occasional pop out the exhaust at a steady throttle, that indicates lean. Raise the needle. If it is a regular misfire, try dropping it.

Either way, it will make it better or worse if it's a mixture related problem.

  • bigred455

Posted July 17, 2008 - 10:09 PM


As for AV gas, notwithstanding any opinions to the contrary, it is simply the wrong fuel for any motorcycle engine under any circumstance. The vapor rates are entirely wrong, the lead content is way too high, and the octane is totally unnecessary. For recreational riders and most racers in California, pump gas is all you need.[/QUOTE]

I did not tell him to run AVGAS,Have you ever run it and be honest? First off I do want the lead content and please do not tell me the lead is going to carbon up the top end or cause harm,.2nd you are not losing any power with avgas,if you jet for it this is a must .There is no lost of throttle response,anyone who says that did not jet correctly,or they have never even used it.I do not care what the numbers read for AVGAS it works,I have been running it for 7 years. Sure I do agree I do not need the xtra octane,but avgas is 102.5 straight up . yes that is what it is. Distallation curve this vapor rate that, it works I should know I use it.I have torn apart motors 2 stroke using avgas straight and the motors that are run on pump. AVGAS motors are cleaner .I have no reason to make this up.You can tell me all day long AVGAS just is not right for my application,but running it for 7 years and KNOWING it works with no ill effects. What are you kidding me the benefits of AVGAS outweigh pump ,there is no contest. Today with gas prices the way they are I feel even better using it. I wonder how long a delivery load of super unleaded sits underground at stations before it is pumped out. Give me the xtra lubrication from lead,give me the safe octane(antiknock index) give me the cleanliness,give me the long storage,and give me the consistency of AVGAS 100LL.

I have switched back to straight pump and then to avgas straight and back to pump and switched and switched. Just for testing purposes and the outcome is AVGAS is the winner. The only reason I cut it with pump is to bring the octane down a little. Avgas will always make a motor run rich,being you have correct jetting for pump. On the average you have to move 2-3 sizes down on the main. You can throw all the numbers at me,I still stand on my belief. If I was losing power or throttle response do you think I would be running it? You see I do not run it for the novelty,I run it because it works.

  • grayracer513

Posted July 18, 2008 - 06:32 AM


I did not tell him to run AVGAS,Have you ever run it and be honest?

I didn't say you did. But you raised the subject, and I believe any reasonable person would see your post as recommending its use. I recommend otherwise.

And yes, we ran it for quite a while in our team bikes until we ran them on the dyno back to back with pump gas and found that it accomplished nothing. The need to jet richer comes from the low vapor rate, which exists because the fuel is blended for use at extremely high altitudes in engines with long intake tracts. Motorcycles have neither of these features, and a good deal of the combustion energy of such fuels is wasted because it is released too late in the power stroke, or in the exhaust. The result is that you have to use more fuel to accomplish the same result.

And lead will not extend the life of your valves, or serve as a lubricant in any practical way. In an engine designed to run without lead, it can even be detrimental.

What I have not used is any recently blended gasoline from Jersey. With some of the gas available in some parts of the country, it's possible that your local pump gas is so bad that almost anything would improve it. But the OP is from California, and that is not the case here. There are better choices than aviation fuel in any case.

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