Help - miss at 80% throttle.

My mates 05 YZ450 has this miss at high speed only when the throttle is opened 80% & more. We have tried taking the carb off stripping & cleaning everything & replacing it, checked all the breathers, fuel lines, cap etc changed the plug as well. Still no success in solving it.

Anyone have any ideas? Could it be the coil?

You might want to try changing main jet cause this jet is responsible for throttle openings from closed to WOT.

this might be a stupid answer...

are you sure you're not hitting the rev limiter?

edit.

whoops... didn't see how many posts you had... guessing its not something stupid then.

You might want to try changing main jet cause this jet is responsible for throttle openings from closed to WOT.
¿What? Your homework for today is to read from 7-1 through 7-10 in your service manual and be prepared to explain the chart on 7-3.

My first question is (even though you said you cleaned everything), what does the air filter look like?

If it's carb related, it may be the main jet, IF it or the weather has changed a lot lately, or if it's clogged, or it may be the needle position.

If the miss is regular, and accompanied by black smoke, it could be rich (jets, float level, or air filter). If it's irregular, it could be lean, or it could be an ignition problem, even as simple as a bad spark plug, or a cracked or leaking coil.

If it only does it in one gear, it isn't missing, it's jumping out and back into gear, and it needs trans work.

It could be a slightly too small main jet.

I ordered a Jetting kit from J/D jetting. The recommended jet was the 170, at the temperature/altitude I'm at. A local Mechanic told me to install the 168 instead. When I ran the bike along an open area, she would start to stumble at higher RPM/big throttle openings. I installed the 170, and all was back to normal.

normal???? meaning......the bike is fine now or it still has a miss at 80% throttle?

We run at sea level, nothing has changed, the bike was running perfectly a couple weeks back & the plug was changed.

I think next step is to chnge the coil, if that doesn't do the trick may be renew the main jet.

What about a faulty ht lead to the plug?

What about a faulty ht lead to the plug?

There isn't one. The coil is built into the plug boot.
:thumbsup: of course, thanks:thumbsup:
normal???? meaning......the bike is fine now or it still has a miss at 80% throttle?

The bike ran perfect after the 170 was installed..

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