Ask The Editors: Honda Carb Identification Issue
What carb did I get?
I ask because I just got a 400EX and the main jet is only 127. Stock on the 300EX is 122 so I was wondering if this means someone installed a 300EX carb on my 400.
A fairly common mod back in the day was to run the larger 400EX carburetor on the 300EX to allow for greater fuel intake but the opposite swap you suggest would have been rare indeed.
You aren’t incorrect that a 127 main jet is quite a bit smaller than the stock 400EX’s 148 but that doesn’t necessarily mean the entire carb has been swapped.
The 400’s carb is larger overall, coming in at 35.5mm compared to the 300EX’s 32mm unit. While you may not notice such a difference at a glance, it is enough to require replacement of the intake boot to compensate as well.
Is the machine running the stock exhaust? A very common modification to the 400EX with a freer flowing exhaust was to run a smaller main jet and a slightly larger pilot jet (stock is 38).
Finally QB 10A refers to 400EX carbs (QB 02B is how a 300EX carb would be stamped).
Based on all of this and your above photo, we can state that you absolutely have a 400EX stock carb, seemingly with aftermarket jetting (which can use numbering systems that vary wildly from stock).
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