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The old O ring is probably permanently distorted from age and use. New ones are round in their cross-section.
An O ring does NOT have to fit tight from side to side to seal. Most O ring glands have some play. They must, however contact the base and the surface of the two materials you are sealing.
In case I am not describing this correctly, let's imagine a wedding ring. It would fit the finger snuggly, but could still slide up and down the finger between the knuckle and the first joint. Of course, the O ring wouldn't move that far in its groove, but it can have a llittle side play and still seal, as long as the top and bottem contact each surface that should be sealed. If you can install 2 o rings side by side, that might be too much side play.
In my experience, the measured cross-section diameter is usually less than the specified nominal diameter. For example, an O ring that was spec'd as 1 1/2 OD and 1 1/4 ID "should" have a cross-section that measures .125 (1/8"), but it might actually measure .118
 

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1986 Honda Rebel 450
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My O-rings came in today. The dimensions I previously gave are correct to a point. The issue I see is that the diameter of the original o-ring is not a circular cross section. It's wider than it is tall, more like an ellipse. The new o-ring fits nicely on the tube and extends above it correctly, it just doesn't fill the horizontal gap completely. I remedied this by using two of the new o-rings. I'm hoping this works with no leaks. I'll try to get to it this weekend.
I found Metric 3.5IDx5.5OD Viton o-rings on McMaster-Carr that fit nicely for the accelerator pump crossover tube. Like you I also found that putting two o-rings on each side fits best since they are only 1mm wide. I will run fuel in the carbs this weekend before installation to make sure nothing leaks.
 

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1985, 86 CMX250C, 81 CM200T, 74 CL360, Invisible to cages, treat them accordingly. Avoids Road RAGE!
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Good to see such old pet posts are still of use. This is the first update this thread has had since 2016.
 
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