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Lumberjack

Yz500f jetting help

7 posts in this topic

My bike (2007 YZ450f) has a stroker crank and big bore kit that bring it up to 500cc and other mods include a full hindle exhaust and a hot cams intake cam. What jetting changes do you guys think need to be made over stock? I am going to be breaking in the bike this weekend before bringing it to the dyno to get it properly tuned and just wanted to make sure that I have somewhat decent jetting while I break it in. I don't really have any jetting experience, so I need help.

Also, before my bike blew up it had a really bad bog when I wicked the throttle wide open at low RPM, it would basically stall the bike, what do I need to do to fix this as well?

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In general, an increase in displacement, and especially an increase in stroke, with the same carb as the original engine, will increase the strength of the vacuum signal over the discharge ports in the carb. That increases the pressure with which fuel is brought through the jets and into the airstream, and can cause the overall jetting to be slightly, sometimes significantly, richer than it originally was. As long as the engine isn't so rich as to wash oil out of the cylinder (really pretty damn rich), you'll most likely be OK.

Remember when breaking in the engine not to baby it. Run it in a fairly normal manner, about 80-90% as hard as it will, and avoid the lower 30% and the top 15% of the rev range. Break in should take no more than an hour.

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Thanks Grayracer, you are the man for knowledge on Thumpertalk eh. Currently, my bike has a 160 main and I am at roughly see level, so I would assume that it would be a little lean, so to be safe I might go to a 168 or 170 for break in and then see what the dyno says for my air/fuel ratio. Do you agree? and thanks for your help.

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Currently, my bike has a 160 main and I am at roughly see level, so I would assume that it would be a little lean, so to be safe I might go to a 168 or 170 for break in and then see what the dyno says for my air/fuel ratio. Do you agree?
No. leave it like that and see how it acts. At the most, go to a 165.

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Do you know what the ideal air/fuel ratio should be for a motocross bike? I have been hearing 12 to 1 and 13 to 1. What do you think?

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It depends a little on the fuel used, but with normal gasoline (whatever that is these days), chemically equivalent to a straight octane/heptane blend, stoichiometeric AF ratio (where there is exactly enough oxygen for the fuel present) is 14.7:1. With such a fuel, maximum power, not economy or efficiency, is achieved at an AFR ranging from 12.5 - 13.3:1.

Oxygenated fuels, or fuels with a low energy content because of inert additives, will need to be a little richer, in the former case, in order to compensate for the addition oxygen released into the reaction.

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have you finished the project bike? im about to do the exact same thing to my 08 yz450f and i was wondering what you thought of your bike once it was all said and done and if its worth it. 

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