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99 YZ 400f backfiring!

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Hello long time reader…

So just got a 99 YZ400f and as mentioned in the title it backfires fairy bad through the exhaust when backing off and the header gets super-hot I assume from the popping?

Background- guy I got it from had it from new, he crashed messed himself up somewhat after about two years riding (he said no more than 100 rides) since then its sat in his shed. It starts no worries has a ton of compression and doesn’t seem to have any cracks in the header (all stock pipes).

I have not checked the valves yet.

Now I have read all the topics and know these are great for it and have pulled the carb apart and found that the jets have turned kind of green? What would cause this?

Is it just from sitting around for the past 8 years?

I’m planning on replacing all the jets and cleaning the carb completely but just thought I’d find out a few things first...

How hard is it to do? I’ve never played with these bits before...

I’m fairly mechanical minded but have a terrible history of starting a job while doing others and not finishing it straight away then forgetting what’s going on...

I just finished a quad a few months ago and have a road bike on the go too (nearly there) but this is what I want to be on :-)

All advice welcomed!


Oh I am in Melbourne Australia and its hot hot summer here :ride:

Edited by number0six

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The backfiring is due either to a leaking exhaust system at either end of the head pipe, and/or a lean pilot.

When brass turns green, it's usually water that did it. You most likely have a varnished pilot jet. Read:


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my 99 400f backfired like a beast when i bought it. it had NO exhaust gaskets at all. threw some in, and no more backfires! easy and fairly cheap fix.

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