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14C Carb upgrade Kit

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jensen:
Hi Nigel,

Yes, torque is flat and reaches it's (first) maximum around 2700 rpm second maximum around 7500 rpm, this is a run of my CB400F for comparison

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From standstill, the CB450 is much faster at the CB400F, the CB400F get's better while the RPM increases, and the top speed is a little higher, but then the CB450 is already out of sight.....

K2-K6:
Thankyou for posting the traces.

A question about how they are run,  is it in one gear and roll the throttle open from low speed to max to record the output and if so how soon is the throttle wide open.

I was interested in the 400 trace when I looked at it on the net forum, particularly that weak mixture trace leading up to the 4000rpm level, I think that may help answer something I was thinking about for another thread on here.

Nigel.

jensen:
Hi Nigel,

One gear only (4th), just rolling the throttle open. The mixture issue you see is due to opening the throttle a little too quick, which explains the lean mixture. If you look at the trace of the CB450 this effect is much smaller, because the CB450 has vacuum carbs.

This is just an impression of a run, unfortunately, I don't have a video of the CB400F, the link will lead you to a flickr video, I don't know how to implement this smoother.....

https://ci-27594733506.http.atlas.cdn.yimg.com/flickr5/40215250@N04/27594733506/27594733506_360p.mp4?dt=flickr&x=1512417878&m=video%2Fmp4&fn=27594733506_360p&bt=0&a=flickr&d=cp_d%3Dwww.flickr.com%26cp_t%3Ds%26cp%3D792600246%26mid%3D27594733506%26ufn%3D27594733506_360p&s=aa6a5c0262c66484d1c8c2dbb75f9bdd

Just before the riding season starts, I run the CB450 on a dyno, because the original 14C carbs are difficult to adjust. The dual cable setup is due the issue's with these carbs, and the tolerances between the vacuum piston and it's cylinder.

K2-K6:
It's a subject that fascinates me,  about what really happens to get mixture correct but also so full of misinformation generally when discussed.

Real traces show so much of how some things can be the inverse of what is thought to happen.

Yes as you say on the 400 mixture trace, you can see the throttle slide too far open and ahead of the volume of air the engine can pull at those revs. This resulting is lower vacuum that simply draws less fuel out of the main jet. As the engine revs increase you can see it going back to parity so the main jet starts to receive more vacuum and deliver more fuel, bringing the overall mixture back to more normal range.

The CV carbs obviously try to avoid this by,  as you point out,  keeping that vacuum constant as the slide is independent of the throttle control plate.
This area I've worked alot with on cars fitted with SU carbs ( older British vehicles,  CV type but have oil damped piston control as opposed to air chamber) finding that I could, by changing the damper oil viscosity,  alter the rate of transient change in jetting. Thicker oil delays and causes increased venturi vacuum,  thinner oil the opposite. Without changing the overall jetting I can alter the mixture curve during operation.

Think they do this on bike carbs with air blead valves to chshge piston rise and fall. Not readily adjustable though.

jensen:
Since these are the first vacuum carbs on a production bike ever, Honda was experimenting with the vacuum, using an air bleed jet. Only on the early carbs, btw. The later 14C's have a fixed air bleed jet.

Here a view pictures to show the differences, the first picture shows the later version (press fit), the two lower pictures show the older version with a bleed jet one could change for another (threaded).

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