2005 YZ450F with 480 bore kit.

My 2005 yz450 is super hard to start. It has a 480 bore kit. When I bought it, I changed the spark plug, air filter, drained the gas, cleaned the carb. And nothing helps. There is times when I can't even get to start. Incredibly hardd to kick, usually have to push start it. Once I get it started, it runs great, until I get to the top of a gear. At the higher end of all 4 gears, the bike will sputter, and pop. I live in Utah, my elevation is around 5000 feet. Changed the jets to bigger sizes to see. Currently have a 195 and a 48 in it. Thanks in advance!!

At 5000 feet, the air is less dense, so you shouod have gone leaner, not richer, on your jetting.  Take it back down to a 165 main and a 42-45 pilot for starters, and be sure you can get the pilot adjusted for a small RPM drop at around 1 1/4- 2 turns out. 

 

If it's really that hard to kick through, you should have a look at your cam timing.  If the exhaust cam has slipped forward, the auto decompression will also be timied wrong, and you'll have more cranking compression than you should.

When I bought the bike, it had a 165 and a 45 in it. Ran the same way. Sometimes it fires right up, first 3 kicks and other times, it won't even start. Usually it starts cold better than when it's warm. Someone told me to change my accelerator pump, do you think that would cause this type of behavior?

Accelerator pump will not affect starting in that way.

 

GR is usually spot on on this stuff, and a hard to kick and sputtering top end symtom would point to cam timing for sure.

I had my buddy look at all that, when I first purchased the bike. He said the timing was good. Buhh I'll take it him this weekend and try again!!

Would the fact that it has the 480 bore kit, change the jets I need?

Would the fact that it has the 480 bore kit, change the jets I need?

 

Yes, you need to go down on your main jet at least one size.

That could be your problem (gurgling at wide open thottle) if you were already too rich.

Okay so you think I should try maybe a 160? And if that doesn't help. Check my cam timing??

At 5000 feet, the air is less dense, so you shouod have gone leaner, not richer, on your jetting. Take it back down to a 165 main and a 42-45 pilot for starters, and be sure you can get the pilot adjusted for a small RPM drop at around 1 1/4- 2 turns out.

If it's really that hard to kick through, you should have a look at your cam timing. If the exhaust cam has slipped forward, the auto decompression will also be timied wrong, and you'll have more cranking compression than you should.

And by the pilot be adjusted are you talking about the little knob just below my carb? The one to adjust the idling??

Okay, I know what you're talking about. So turn that about 1 1/4 -2 turns out? With the 165 and 45 jets. And that doesn't help, I'll check my cam timing again!! Thank you!

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