99 YZ400 carb question

4 replies to this topic
  • JamesF

Posted August 08, 2004 - 11:04 AM



I was getting a hesitation when going off idle and was told the accelerator pump probably wasn't giving the initial shot of gas it needed. I took it apart today to clean the AP rod (as I've read they need polished here & there) and found the diaphram was trashed along with a fair amount of dirt in there. I was looking at yamahopper's parts site and found there are several diaphram's to buy. Is there any reason to diviate from standard? I bought the bike used last year and this is the 1st time I had the carb off. Hoping this cures the hour of kicking it needs to start it too... I don't know if the jetting as been changed either.

The other question is if there is a rebuild kit for these carbs. Couldn't locate one searching google.


  • PumpkinHumper

Posted August 08, 2004 - 03:46 PM


hey James replace the stock diaphram and see what that gets you. If you still have a bog then go to one of the differant ones. Chances are that you will need a shorter duration squirt as the earlier carbs had a long squirt.

I have never heard of people beeing able to get the differant diaphrams aside from the stock one. I had seen that they had them listed but you could never get them.

A diaphram with a longer center pin will give you a shorter squirt.

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  • spy

Posted August 09, 2004 - 12:02 AM


Saw carb rebuild kits at www.sudco.com
Might wanna email them for more info though.

  • JamesF

Posted August 09, 2004 - 04:44 AM


Thanks for the info y'all. I'll see about ordering parts now.

  • Fly-nBri

Posted August 09, 2004 - 08:14 AM


Also be aware that you may want to silicone the top of the rubber piece that slides onto the ac pump arm. It will help keep some of that dirt outa there in the future. If your squirt duration is still to long then you may want to consider the James Now Mod, which is all over this web site. This is a simple mod by where you install a small collar on the pump arm and set it by means of a set screw on the collar, to where your pump duration is best. You can actually see it squirt. Around a 1 sec to 1.5 sec. duration has worked for me. Mine was set to about 4 secs.
Good Luck!! :thumbsup:

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