[potboiler]

I was thinking of buying a bellmouth (with gauze covering the opening) for the Mikuni carb on my Bullet 'sixty 5' (page 15 of Hitchcocks catalogue). I don't know what the effects of unfiltered air will be over the long term on the carb and engine. Any opinions? Will the carb need re-jetting do you think?

[ace.cafe]

It will let some extra dirt and grit onto the engine. Depending on the environment, it might have mild or severe consequences. Most dirt will probably just blow thru, but some will remain in the engine, and some that blows thru can cause increased wear on valves and seats.
However, I have seen some circumstances where the engine lasted quite a while with an open bellmouth. It really is situation dependent, but it will always increase wear to some degree.

Jetting might be affected, so you should always check it after making any changes like that.

[Mr.Mazza]

Possibility of the bellmouth with a good filter hidden inside?

[ace.cafe]

Possibility of the bellmouth with a good filter hidden inside?

No real possibility of that.
Either the filter would be way too small, or the bellmouth would be way too big.

[Ice]

I was thinking of buying a bellmouth (with gauze covering the opening) for the Mikuni carb on my Bullet 'sixty 5' (page 15 of Hitchcocks catalogue). I don't know what the effects of unfiltered air will be over the long term on the carb and engine. Any opinions? Will the carb need re-jetting do you think?

 Cool on race and show bikes, bad idea otherwise. Dirt and dust make fine grinding and lapping compound.

[cafeman]

There are ways to make things work if you really want. I wanted a velocity stack on my Mikuni, problem is, finding one is next to impossible. I found a pair on EBay by someone in Vietnam! Quality was to be expected, I just finessed it into something not so bad, IMO. The Uni coarse foam is simply ultra black siliconed around the rubber ring (there is a brass screen covering the opening held in place by the rubber) Air flow is fine, no difference in how it runs, and I got what I wanted with a little improvising. And it is better than running with nothing.

[ROVERMAN]

Nice! especially because it works for you, good job Cafe.
Roverman.

[Adrian]

The standard carb will be jetted to run with an air filter and will need some richer jetting for the freer-flowing bellmouth. The severity of any additional wear effects of running filter-less will depend on your riding environment, the UK is usually not too bad, and my project bike is running with a bellmouth on a Dell'Orto PHF36, but in dry and dustier climates I would not risk running without one.

Regards,

A.