03 YZ450F Accelerator Pump Adj Screw?

6 replies to this topic
  • joshuathomas

Posted April 20, 2013 - 12:37 PM


Hey Grey or anybody, I just bought this 03 YZ450F off of craigslist a few weeks ago. I got all the jets dialed in but i have some bogging / hesitation when i blip the throttle and i read everywhere that says to check the AP timing and adjust with AP screw. Well, i cannot seem to find my AP Adj Screw? Its really frustrating. I even downloaded the 03 manual and even in the manual it tells me how to adjust the AP screw BUT i cannot seem to see /find this on my bike. I am beginning to wonder if 1. I got sold something different than an 03 or 2. Is it possible to be missing parts or 3. Is it possible that there isn't an AP Adj screw?? Help is needed please.

The tenth digit in the VIN is a "w" - is this bike an 03?

Pics of what i see (or really, dont see) and a pic of my bike:

Attached Thumbnails

  • aft looking forward.jpg
  • Rt Side of Carb.jpg
  • Right side of Carb2.jpg
  • photo (17).JPG

  • grayracer513

Posted April 20, 2013 - 07:26 PM


Well, I was afraid of this. That's a 1998 YZ400. Clearly not, from the pictures, a YZ450.

  • joshuathomas

Posted April 22, 2013 - 12:02 PM


Wow, i am speachless, that is not cool! No wonder things were not matching up.

Grey, how can you tell the difference between a 1998 YZ400 vs YZ450?

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  • joshuathomas

Posted April 22, 2013 - 01:27 PM



So now that i know i have a 1998, i found the Factory manual- it's not that good. (Ordered an actual Clymer repair manual today)

But searching the forums and trying to find possible solutions for my problem:
" some bogging / hesitation when i blip the throttle"

i am really only seeing a couple remedies:

1. Make sure my pilot circuit is good- Check, this is good now. I used this link to adjust: http://www.thumperfaq.com/jetting.htm

2. That there is no adjustment to be made on the accelerator pump

3. that this year carb is a piece of crap and i should upgrade to a YZ450 05' or higher

Is this it? Nothing else to try and get the bog / hesitation out?

  • grayracer513

Posted April 22, 2013 - 01:32 PM


All of the plastics are obviously '02 or earlier. The 450 came out in '03.

The VIN code is '98, and that was the 400

Looking at the front of the cylinder base, the size of the engine will be stamped there. Yours will likely say 399cc, although it could have a later engine or a 426 cylinder on it.

The rear master cylinder has an external reservoir. This was done away with on the '03 model (the first 450)

The hubs will have a thick rib running across them. This was eliminated in '02

It has a 400 header

Last but not least, the carb itself is an FCR. The 426 and 450 had an FCR-MX.

  • grayracer513

Posted April 22, 2013 - 01:36 PM


Is this it? Nothing else to try and get the bog / hesitation out?

The Taffy Mod: http://docta-zaius.f.../whats_new.html

I can't believe that you would even consider spending any time or money on this when you should, by rights, be calling law enforcement regarding a fraud complaint.

  • joshuathomas

Posted April 22, 2013 - 03:57 PM


Thanks Grey,

Yeah, i know, and am still thinking about this. I dont think the kid i bought it from Intentionally did this because the bike was his dad's and he was just given it. I found an old OHC registration in the handlebar pad and the guy who registered it back then thought i was an 03 as well since that is the Year he put in the registration. The bike is really clean, not really any dings, dents or scratches and i bought it for $1500- i think that is still a good deal for a 4stroke

But then i worry about it being 15 yrs old now vs 10 yrs old (when i thought it was an 03') and if its going to turn into a big maintenance hog.

i dont know.

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