yz400f Jetting Questions?

Hi everyone I'm new here and Just bought a 98 yz400f and it has the biggun header and muffler.The problem is backfireing when reving and allowing the engine braking.Just cleaned the card and tank and installed new gas.Big gun suggested a 178 main and 48 pilot but this guy had the 185 main and 45 pilot so why is he going so big?He has the bike set up real nice so I could only assume he new what he was doing with the jetting.Could I get some help from you guys on jetting recomendations,does 178 and 48 sound right?Your help would be appreciated.Jeff

The main jet is on the large side. You might want to compare it to a 180.

Either 45 or 48 pilot will work. Try turning the pilot screw out 1/4 turn at a time (richens) and ride to see if the popping is reduced.

Thanks for the info.I do have a 182 that he gave me perhaps I should try that also.You said pilot screw is that the air fuel mix screw on the bottom of the carb? Also what oil do you recommend for this bike?Thanks alot for your help.Jeff

I think the manual refers to it as the pilot screw, I call it the fuel screw because, unlike a two-stroke it meters fuel and not air, and important distinction since out is richer (more fuel) instead of leaner like an air screw (more air).

I don’t suppose it matters what you call it when you adjust it though, yes you have the right screw.

Suggestions for oil always create controversy, a lot of people say you can’t use any automotive synthetic. They usually say this because most automotive oils have a heavy dose of additives that will make your clutch slip.

I don’t know first hand that Mobil 1 15W-50 does NOT have them, but am convinced nonetheless and that is what I use. Yamalube, like many other factory parts, is overpriced and probably not made by Yamaha anyway. Having said that I did use it for about 50 hours of use on my 426. I switched to Mobil 1 about six months ago and haven’t noticed any difference in the clutch. I think there are quite a few other guys here that would say the same, but that is my two cents.

BTW here’s is an article that helped convince me: The Great Oil Debate.

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