Best Online offroad parts site

I've been told that there is a website which shows how to tell the difference between genuine Keihin carbs and fake ones.  Does anyone know of the site - or how to spot a fake?  

Motorcycle Superstore is my favorite.  
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When the power of love overcomes the love of power; the world will know peace.
*North Carolina*

Dennis Kirk has always done me rite, had one back order in the past but they emailed me and let me know. Plus over 100 buck's they ship free.  

I always use Motosport because of the fast/low cost shipping, the free catalog, and the amount of items that they have.  

I've used bikebandit.com they have good prices but the shipping takes for ever for me.  But that's because I live on the other side of the country.  I also use Bob's Cycle Supply.  They have faster shipping.  
03 YZ 125

Dennis Kirk is okay, but they need to work on the way you find the right part for your bike. I lke Rocky Mountain, and as YZ_Ripper mentioned, Gnarly Parts is also good.  
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Adam
B.C, Canada
2007 CRF150RB

I used to love bikebandit,  but one time it took for ever too get some gear almost 3 weeks.  Also i have a 2000 ktm 520sx,  and i was looking for a part well i found a 2000 but it was a 525.  for one the 525 wasnt around in 2000, and i know525 parts are different from the older 520's so why risk a couple hundred dollars and insane wait on a part that may or may not be right,  i would not recomend bike bandit.  I prefer motorsport i love that site.  I also get alot of part from ktm world thats a great site if you own a ktm.  They ship fast and free and unlike most ktm sites they can actually get ktm parts in just a few days not weeks,  plus they are on top of notifying you about your order.  And i love rocky mountain  
IBEW LOCAL 697 1st year apprentice,  fuels my passion of riding.

Try bikebandit.....I use them and they are really reliable. (www.bikebandit.com)  

It really depends on what I am buying here are some of the gems I've found lately,
Motorcycle superstore 2009 MSR gear on sale around $80.00 for NXT jersey and pant they also still carry the Dunlop K490 front tire, IMO the best Int front made for a 2 stroke.
Rocky Mountain for 09 One Ind gear and 756 tires they also tend to have the best price on Maxxis Tires.  Also the best tool selection for dirt bike repair and maintenance.
I use Spy Goggles and Bobscycle always has at least one style of the Alloy on sale I also get my helmets from Bobs.
For pistons and oem Midwestactioncycle is as good a price as you can get and they stock a lot of parts.  
Motosports outlet and Dennis Kirk are always worth looking at their sales and clearance aren't as good as some but they tend to do a good job with shipping and have items you won't find at some of the others.

Don't get fooled into the free shipping frenzy, yes it can make a difference in price but some non-free shipping site can actually be less expensive.  As always I make a list and look around.  At times I will pay the shipping to save more in the long run.  
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Colorado

2006 RM-250

I have to agree, it depends on what I am buying and sometimes who has the best deal at the time. I have found big price differances in prices on some things so check around.  
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I love this dirt stuff.