400ex carb on a 250R?

[knobbyboy]

Anybody ever tried this?? Results??

[wilkin250r]

Two strokes and 4-strokes have different airflow/fuel requirements.

Because the 4-strokes have a larger displacement, the intake velocity is much higher. Sure, they only have half as many intake strokes, so the total average airflow is about the same, but your carb doesn't go by "average".

So because the air velocity is lower in a 2-stroke carb, they are designed a little different to get fuel up from the bowl better. Plus, 4-stroke carbs generally have an accelerator pump, but that's not usually necessary on a 2-stroke carb. I could go into more detail, but your eyes are probably glazed over right about now.


If they were able to swap back and forth, and work the same, don't you think the manufacturers would just MAKE them the same in order to save money? Just the simple fact that they are different should tell you that they aren't swappable.

[wilkin250r]

I don't know if it would bolt up, I've never even tried it. But honestly, they are so different in their requirements, I wouldn't suggest trying it, regardless of price.

If your carb is flooding and fouling, you might have a problem with the floats. Sure, most of the time a float problem will dump fuel from the overflow tubes, but it might be dumping into your intake path instead.

Before running off and attempting a radical carburetor swap with a 4-stroke, I would highly suggest cleaning your stock carb again (paying special attention to the float valve), and adding an inline fuel filter before you start it up again.

If you STILL feel like you need a new carb after that, I would suggest just spending the necessary dollars and get a proper 2-stroke carb. I can't tell you for certain how a 4-stroke carb would perform, but I imagine it wouldn't work at all.

[knobbyboy]

...found a broken a reed pedal. So strange because I had just inspected all that a few weeks back when I rebuilt the carb.

[2muchquad]

While the engines have different needs,sometimes carbs can be changed or made to work.For expample,a few years back White Brothers had a mikuni 38mm tmx carb for the Honda xr600 which is a 4 stroke,the mikuni tmx carb is original equipment on the 125/250 mx bikes,2 strokers.I have recently seen a mikuni tmx kit for the bombadier 650 and polaris machines as well.So in some cases different carbs can be used,but not a 400ex carb on a 250r,shame on you for thinking about disgracing such a legendary machine.:)