User avatar
By Slappy
Good morning all

I am using a couple of parts from our host
1 is the Bellmouth filter : Bellwouth44
2 is the tailpipe 91045
so I will be looking at re jetting the mikcarb, I have copied the hosts list of jets and I am guessing it would be close to S & B filter with open silencer, would I be somewhere close with this setup
Thanks Dave.

Standard 110 O8 25 3
S & B Air Filter with standard silencer 117.5 P0 27.5 3
Standard Air Filter with open silencer 117.5 P0 27.5 3
S & B Air Filter with open silencer 125 P2 30 2.5
User avatar
By stinkwheel
Has to be said, I doubt the negligeable potential improvement in airflow/performance justifies the dramatic decrease in partical removal over a "proper" air filter of some sort. Unless you are on the raggedy edge of what is possible by tuning or are looking at pressure waves (and I think I recall from past discussions, you'd be looking at a stupidly long bellmouth on a bullet to start getting a benefit from pressure waves).

I know I tried my 350 bullet with both a conical filter and an open bellmouth on a proper dragstrip and noticed no discernable or measurable change in performance. I'll add that that bike had a gasflowed head and big carb fitted.

So in answer to your question, I'd go with the jetting for an S&B filter because beyond that point, the airflow through the filter is almost certainly not the limiting factor with regard to performance.

Not such an issue with a mikcarb because they don't have a tickler but a mate of mine had his BSA A7 self-immolate after it spat back through the bellmouth and ignited the fuel he'd tickled out of it.
User avatar
By Slappy
Thanks Stinkwheel
very in depth answer, I am not doing any mods to the engine at all just making it cleaner and decluttering a little , I know I will upset the purists but it's just how I like things, just want to make sure I have got the right setup from the off with the tailpipe and bellmouth there is nothing wourse than spending weeks playing with different jets when there is a known formula
Slappy :D
User avatar
By Presto
The S and B filters are sufficiently ‘free-flowing’ to be jetted according to the ‘book’ as an open bellmouth or bellmouth with gauze. Having said that, it’s often necessary to fine tune the ‘book’ settings to suit any particular engine.

As for bellmouth length for tuning, stinkwheel is right – the bellmouth would be long!!

Just for fun. Tuning to the 1st harmonic will give the greatest effect, after which each subsequent harmonic has reduced effect. Using the 1st harmonic will often give an impossibly long intake tract. Using short bellmouths will have no effect unless intake tract is extended. For example on a Bullet 350/500, tuned to 4th harmonic – the least effective but shortest option – the length from valve seat to carb mouth will be approx 170mm. (Calculation: 343 [speed of sound] x 4150 [4th harmonic] / 5000 rpm = 284mm from bellmouth to valve seat, supposing 4 inch bellmouth.) Tuning a Bullet to the 2nd harmonic would require a bellmouth of approx 282mm or 11 inches! And so it goes on...

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