Bellmouth - which one?

[Felters]

While I was cleaning the carb recently, it seemed to me that all the schematics I was looking at on the Amal site etc. showed the carbs wearing a much shorter bellmouth than mine has.

The bike is now running fine - but I'm curious as to which length bellmouth is best for a road going A7 - and why?

Cheers
Mike

[Rocket Racer]

If its running fine, I'd suggest leaving it alone although I personally run air filters on all my road bikes, but my race bike does run small bellmouths but lacks room for much more.

Conceptually the bellmouth or velocity stack assists with accelerating air into the carb and reduces the mixed air turbulance that exists as you go faster. I recall Paul Dunstall's tuning book talking about negligible benefits from using larger velocity stacks and my own view is they are largely for style.
The ideal solution is a large capacity still air box but thats not going to happen.

A fine gauze over the outside of the bellmouth is useful to reduce larger items getting into the motor! (tea strainers!  )
Running without an air filter does increase upper engine wear rates due to the general air pollution/snot being ingested into the valve guides/bore.

[beezermacc]

Original spec was for A7 iron heads to run with a filter, the A7SS swinging arm (alloy head models) had a short bellmouth about 1.25" long. The original filters weren't very good, basically a gauze held in by wire, capable of sifting out bricks and spanners but incapable of sifting out fine dust! If you've got a big bellmouth it probably won't make a lot of difference (unless it's like a bucket!) as the standard iron head A7 is fairly tolerant of tuning variations but fitting a slightly larger main jet may offset the bellmouth's tendency to weaken the mixture at high revs - a sensible precaution against holing pistons. If occasionally you experience spitting back in the carb, even when the bike is warm, you might try taking the bellmouth off or jetting up a couple of sizes on the main jet and/or one size on the pilot jet. Lifting the needle one notch will also richen your mixture at mid range r.p.m. It's worth experimenting a bit but don't forget what your settings are now, just in case experimentation makes things worse. Don't jet up too much as rich mixture will wash the oil off your bores. Keep an eye on your spark plugs to indicate whether your bike is running richer or weaker as a consequence of any changes you make.
Regards P-o-D!