Air pressure affecting fuelling?

[Iain.]

PWK 30mm carb, no air filter mounted 90 degrees pointing out the side of the engine. No panel work to cover the carb. Set up the jetting last night, running in a tunnel I can get the jetting to within a few sizes (132 or 135, I need to buy jets inbetween these sizes)

Caught a crosswind coming out of the tunnel blowing at the side of the bike where the carb is mounted, I got a massive bog, splutter and lost all power till I shut the throttle completely and wound it back on again. After around 3-5 seconds of bad fuelling it sorted itself out and ran fine again. Diddnt have this issue with the 21mm PHBG i'd run before with a foam pod filter and that had been jetted with the same method as before. Float bowls vented on two tubes around 10cm long pointed directly downwards, again as per the PHBG setup.

Whats the best way of resolving this? I was told that facing the carb any way other than backwards could cause carburetion issues due to weird air pressures at certain speeds. Was mentioned that some ram air systems actually need to pressurise the float bowls slightly or the fuelling goes to pot at speed? Recall Pete mentioning bandit fuelling could be screwed by incorrect float vent routing?

[WD Forte]

Sounds about right
the filter as well as stopping particles helps smooth the airflow
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bikers smell of wee

[Commuter_Tim]

Genuinely interested, why would one run a bike without an air filter mounted to the carb?
Is it part of the diagnostic process of testing new jets or some other reason?
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The above post is most likely nonsensical.

I ride a Bandit 600... badly.

[temeluchus]

Carburettors work entirely on air pressure......

What engine are you running?

Did you set the float heights correctly?

What you might want to do is run a length of pipe on the intake, this effectively makes behaves like an airbox, and smoothes out airflow a bit.

Doesnt have to be long, maybe 15-20cm. You can slap a pod filter on the end.

If you have over carb'd the engine it will also not be terribly happy.
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Some shite cruiser. Now with guns and FREEDOM!

[temeluchus]

[Commuter_Tim]

[temeluchus]

[Commuter_Tim]

[stevo as b4]

Iain as said above, have you got a bellmouth on the carb yet?

It's possibly at certain engine speeds and thus airflow rates through the carb, that your getting a pressure wave reflacting back from the end of the carb inlet that could be interfering with the fueling/vacuum through the carb.

It's a big carb, so will probably need very fussy setting up and large jets due to the lowish air speed through it. I bet if you experiment with a bellmouth on the carb intake, and try different lengths it will either move the problem to different engine speeds or eliminate it all together.

Like people have said, a lot of damping of pressure waves goes on in an airbox, and they are usually big enough to provide a reservoir of still air for the carb to draw on. A big enough clamp on filter but one without a flat plate end cap might also fix the issue.

[talkToTheHat]

Agree with stevo.

The effect you are getting may be resonant like blowing across the top a beer bottle and getting a musical note, or just straight up venturi effect of airflow across your carb changing internal pressures. Flared bellmouth or a big filter might help.
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Bandit. does. everything.