Mikuni RS38's - velocity stacks/filters??

[Guest Flatline]

So, what are people using on their RS38's (or any other flat-slide/smooth-bore carb's??

will be used on the road, so needs some sort of filter methinks.... 

This looks a good set-up:

 

 

[Bondy]

Im running RS36’s with stacks. Ive just fitted some ramair foam filters. It runs good but dont know what it will be like in the rain. 

[bluedog59]

For a bellmouth/stack to work well it needs to have air space all around it's end, not just behind it, so that looks like it could be a decent set up as the stack end appears to to sit into the filters internal space. A bit more "roll round" on the end radius of the stack would be nice tho.

[Foz]

50mm bellmouths with ramair filters over them. As long as they are oiled I've had no issues in the rain either

[Suban12]

Same set up as Foz on mine.Had them for a few years and no issues .

[370steve]

just had open stacks on mine

 

[Foz]

As much as I like the look of open bellmouths i’ve heard so many stories of debris getting sucked in and wrecking a motor I thought some sort of filtration would be best 

[Sheep]

I use the same as the pic......all good so far..

I have used FranksMx before but you can get slightly cheaper over here

[MeanBean49]

Velocity stacks and make some round mesh discs to sit over the ends. Ive been running a big bellmouth with mesh over the top with about 2mm holes in it on my turbo for years, never sucked owt in, it sits on the front of the bike pretty close to the floor. Ran my race bike on open velocity stacks with no problems too. Only issue is gravel possibly getting in when I had a tallent fail. But but slide closing always stopped owt getting from the engine.

[dupersunc]

Mesh is very restrictive.  and dornt stop dust grit grinding your slide rollers away.

 

[MeanBean49]

42 minutes ago, dupersunc said:

Mesh is very restrictive.  and dornt stop dust grit grinding your slide rollers away.

 

Not when is big holes lol.

Bit of a myth that theres abrasive dust floating around in the air all the time. Jet engines fly with no filters at all for hundreds of thousands of hours.

Only real issue is debris like stones  etc flicked up by wheel. A simple guard will catch those.

[bluedog59]

So where does the muck that dirties an air filter come from ? Never mind the other stuff you often find that has been caught by the airbox before it gets to the filter.

Any size mesh over a bellmouth will cause some restriction due to disrupting the smooth airflow  nevermind how it ruins the flow around the circumference of the stack.

Run whatever filter arrangement you're happy with but air filters have a service life for a reason.

[MeanBean49]

31 minutes ago, bluedog59 said:

So where does the muck that dirties an air filter come from ? Never mind the other stuff you often find that has been caught by the airbox before it gets to the filter.

Any size mesh over a bellmouth will cause some restriction due to disrupting the smooth airflow  nevermind how it ruins the flow around the circumference of the stack.

Run whatever filter arrangement you're happy with but air filters have a service life for a reason.

Mesh with decent size holes will have much less of an effect than foam. And if you go so far as to make it hemispherical the area of the holes will be the same if not greater than the area of the velocity stack at its biggest so very little effect on airflow at all.

The much in a filter aquires over thousands of miles, pretty much all of it is soft materrial that would easily pass through an engine with no adverse affects at all, as would the odd bit of fluff or even a spider or a fly.

Ive run open bellmouths for whole years and upon strip and check never found any wear at all over and above what would normally be expected.

[bluedog59]

It's not the area, it's the presence of the mesh in the airflow that causes turbulence in the air column, that's why a bellmouth should project into a filtered area with nothing around the stack opening. This is why stock carb to airbox rubbers don't just end at the airbox wall but actually project into the box. In your case with a bloody great turbo it may not cause too much restriction and, given it's angle to the passing airflow, momentum probably carries large/heavier particles past the intake but it will not perform as well as a well designed bellmouth drawing from a chamber of filtered air. 

As I said, run whatever filter style you want but, you know all that muck that dirties your bike when you ride it, it's all airborne (even more so when it's raining) .

 

[MeanBean49]

39 minutes ago, bluedog59 said:

It's not the area, it's the presence of the mesh in the airflow that causes turbulence in the air column, that's why a bellmouth should project into a filtered area with nothing around the stack opening. This is why stock carb to airbox rubbers don't just end at the airbox wall but actually project into the box. In your case with a bloody great turbo it may not cause too much restriction and, given it's angle to the passing airflow, momentum probably carries large/heavier particles past the intake but it will not perform as well as a well designed bellmouth drawing from a chamber of filtered air. 

As I said, run whatever filter style you want but, you know all that muck that dirties your bike when you ride it, it's all airborne (even more so when it's raining) .

 

Pretty much all that muck is soft particles that wont do any harm at all passing through the motor in the air. You may notice the area under the tank is generally free from muck when you take the tank off even after you have been out in the rain.

Mesh of decent hole size does not cause and real turbulence at all. We put debris guards on jet engines at work that are mesh when we ground run them. Makes no difference to the performance at all. Im not talking about mega fine stuff here,  (2.5mm ish square holes made of 0.2mm ish wire). I know that can have a big effect because the area of hole to material is significantly decreased, some stuff effectively halfs the area of the velocity stack

Made no difference at all on the dyno on my old 1216 race bike with or without it.

Edited April 3, 2018 by MeanBean49

[bluedog59]

Rotor tip clearances are somewhat different to those inside an internal combustion petrol engines and rubbing road dirt ( including that that isn't "soft particles") into your paint and polished ali will not mark it. Jet engines also run a good radius around the circumference of the intake nozzle to aid flow and fitting anything in this area disrupts flow.

However, we will conclude that that the whole area of research into airflow and filtration is in fact smoke and mirrors and that the motorcycle, car and all other relevant industries have wasted countless time and money developing  filters, bellmouths and airboxes when they could have just stuck a tea strainer over the carb.

Even before intake noise became an issue due to noise regs.    

[Guest Flatline]

19 hours ago, Foz said:

50mm bellmouths with ramair filters over them. As long as they are oiled I've had no issues in the rain either

Like these?? The twin or single filter??

 

[Sheep]

I'd go twin....

 

[370steve]

twins worked well on my Keihin's when i had them fitted

 

[Madb]

twins.

[Simbec1863]

+1 for twins

[MeanBean49]

15 hours ago, bluedog59 said:

Rotor tip clearances are somewhat different to those inside an internal combustion petrol engines and rubbing road dirt ( including that that isn't "soft particles") into your paint and polished ali will not mark it. Jet engines also run a good radius around the circumference of the intake nozzle to aid flow and fitting anything in this area disrupts flow.

However, we will conclude that that the whole area of research into airflow and filtration is in fact smoke and mirrors and that the motorcycle, car and all other relevant industries have wasted countless time and money developing  filters, bellmouths and airboxes when they could have just stuck a tea strainer over the carb.

Even before intake noise became an issue due to noise regs.    

Who said anything about wasted time and money into research? What I said was a simple mesh over a bellmouth wont affect the performance of a velocity stack, especially if its domed.

Ive tried it on N/a and turbo and found it as expected to be exactly the case. In much the same way F1 cars did exactlty the same back in the day when running on velocity stacks.

 

[dupersunc]

That's Charlie Nearburg's Brabham Bt33. He runs than filters on that these days.

Sticky throttle slides aren't fun.

[bluedog59]

8 hours ago, MeanBean49 said:

Who said anything about wasted time and money into research? What I said was a simple mesh over a bellmouth wont affect the performance of a velocity stack, especially if its domed.

Ive tried it on N/a and turbo and found it as expected to be exactly the case. In much the same way F1 cars did exactlty the same back in the day when running on velocity stacks.

 

I'll make this my last comment as this is hijacking this thread.

There has been a lot of research into bellmouth design by the industry, tuners and numerous academic institutions regarding length, taper, surface finish and the radius required at the entry point and is backed up by what we see around us in production. They all conclude that the entry point should have a smooth radius and that it should project into an area of "free air".  If your tea strainer attaches to the bellmouth at this radius it will have an affect on flow (fact based on research). The reason you've not seen this on the dyno ? Simple, your air intake capability is greater than required by the engine and so can tolerate losses .

 EG, Your engine can only use 90% of what your air intake will flow so if you compromise the intakes flow by 5% it's not going to notice.

If your mesh attaches at a point other than the entry radius and is clear of the entry at all points (which yours does not) it will have less or no effect (but will still not filter much apart from small rocks and passing children).

Why did old F1 cars run mesh ? Well, technology has moved on a little since then regarding filtration, airbox design etc etc and the engine were pretty much a service item designed to last to the finish line before a rebuild/refresh . Do modern F1, MotoGP, Superbike engines run open stacks with mesh ?  

Hands up those who think it's a good idea to build an engine with lots of nice expensive parts and run it on cheap oil.                                                                                                                Hands up those that think it's a good idea to run said engine with just a bit of mesh to protect it from ingesting abrasive material.

If the dirt found in a dirty air filter is only "soft particles that wouldn't do any harm"  why do the factories both fitting filters ? 

 

[kokolis]

I ve run pods, bellmouths, for years, raced on street circuits.

If you keep the airbox on what so ever bike, you ll have her for life.