Part throttle Issue

[dudehog]

I'm assuming that it's now drawing the air through the filter and making it more breathable which would explain why the coughing and spluttering less frequent.

[tony.mon]

No, the clogged filter isn't allowing enough air in maker the fuel/air mix too rich to combust properly.
To prove the point, just remove the filter and ride it round the block- it'll be too lean that way but will run lots better than it does now.

[dudehog]

When I took it apart I saw that that two crank breakers put the air back into airbox under the filter panel. Has anyone done a mod to have a tube leave the airbox so it only draws air from above the filter?

[tony.mon]

The purpose of having it inside the filtered section is so that any oil vapour leaving the engine is returned and burnt through normal intake combustion. Also any air drawn into the engine is filtered.

There's no reason to aim the vents at unfiltered air- unless I'm missing something?
If you did, any oil exiting the engine would be drawn into the filter, clogging it.
Also any air drawn in would be likely to contain particulate contaminants and debris.

You can route the breathers to an external (to the airbox) catch tank, this is normally done when enlarging the airbox volume.
Search on here for the many experiments on airbox mods.

[dudehog]

I'll check that out. I thought it would maybe add a bit more performance to have the crank breathers leaving the airbox via a tube and then the only air is fresh air filtered from above the filter. I'm sure Honda know what they're doing (or done) but to have have hot oily air can't add to performance surely?

[tony.mon]

It's tiny amounts compared to the fresh air being drawn in, and the engine is a sealed unit- it doesn't pump air in our out, the vents just allow pressure to equalise as each piston rises and falls, effectively changing the interior volume of the engine as a whole.
Try putting your hand over one of them with the engine running- nearly no air movement at all.

There are some neat theories regarding reducing the crankcase pressure with valved crank vents, assuming that the rotating bits of engine will have lower losses if moving through a less dense that atmospheric air mass inside, but these disregard the much larger losses from thrashing crank webs through hot oil.

I have rerouted the breather pipes to the original catch tank but sited it outside the airbox, and added a little air filter. It's a sealed box, but there's never anything more than a drop of oil inside, so it never needs emptying, even though i check it every time the airbox is off for more tuning work.
It's probably worth doing as part of airbox improvements, but like all tuning and mods, leads to other areas.
If it breathes in better, it'll need to breathe out better....

[tony.mon]

I've just had another thought- take the petcock off and check the diaphragm for holes and splits....if all ok, check the carb diaphragms.

[dudehog]

What do you mean by breath out better?
I replaced the petcock rubbers so they're all ok.

[MacV2]

What do you mean by breath out better?
I replaced the petcock rubbers so they're all ok.

What the rubber pipes from the petcock to the carbs ?

Tony is referring to the Diaphragm in side the petcock...

A recent fault with another members bike was traced to this...eventually.

[dudehog]

Yeah I changed the diaphragm inside the petcock.

[tony.mon]

Actually I meant airflow through the engine and exhaust in general.
Gasflowing heads, larger inlet valves, full systems all enable gas flow out of the engine to match increased gas flow into it.

Everything helps. of course, individual things help in isolation, but ideally you increase intakes and outlets together.