2004 crf450 running wierd

I just bought a 2004 crf450 recently. You can tell by looking at the bike that it has very few hours. It's 100% bone stock.

But it's backfiring and crackling like the jetting is way off. The jetting has never been tampered with. It has all stock sizes in it, including the stock 165 main, but it still runs like the jetting is off. The carb is clean as can be and the shims are within spec. The head pipe also gets cherry red within a couple minutes almost like it's running way to lean.

Why would I have to adjust the jetting if nothing was changed and it's 100% stock? Anything else it could be? Thanks

From what I've heard the head pipe is normal. Make sure fuel mixture screw is adjusted correctly, install new pilot jet as the old one may be varnished just a tad smaller and make sure you have no air leaks in the exhaust.

I just bought a bike that is in the exact same condition as yours and mine does the exact same thing. Im sorting out the more used one I got for $900 first before I work on my garage queen bike.

Lube up your suspension and head bearings as both of mine were very lightly lubed!


If you had the carb apart, make sure that the flat plate on the back of the carb slide, AKA "floating valve set" is not installed upside down. It is very easy to do, makes the bike run lean and will back fire and run hot quick. I did it once and took me a while to figure it out.

How long has it been between starting? If its been a while your pilot jet is probably clogged. This will cause low/unstable idle and backfiring. I would check that first. It is usually the cause. They have a very tiny opening and tough to clean. At about $5 each, I would just buy a new one. Or, like Sky said, your slide plate is upside down.

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