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Air filters

Printed From: guzziriders.org - moto guzzi forum
Category: Technical
Forum Name: Small Blocks
Forum Description: V35 up to V750 including B750, V7 etc.
URL: http://www.guzziriders.org/forum_posts.asp?TID=8647
Printed Date: 26 Apr 2018 at 06:48
Software Version: Web Wiz Forums 12.00 - http://www.webwizforums.com


Topic: Air filters
Posted By: Po51uhd
Subject: Air filters
Date Posted: 13 Apr 2018 at 15:53
hello
I have a v35 ii and wish to replace the airbox / air filter with simple k&n jobs. Should I use the existing manifolds and buy clamp-on filters (space is very limited) or is there a better solution that bolts directly onto the back of the carbs?
Thanks
Stephen



Replies:
Posted By: cyclobutch
Date Posted: 13 Apr 2018 at 17:24
You should be able to get pods that bolt onto the back of the carbs directly.

But my advice is don't go there, stick with OE. On the big twins the carbs are solid mount but on the littluns you've got those inlet rubbers - you'll be hanging a load more weight off those that they won't particularly like.  

Changing the std filter is a bit of a PITA but in the UK you only really need to do this every two or three years. And you can make it easier by switching to a wing nut at the nose of the unit the next time you do the job. 


-------------
Butch


Posted By: GC888
Date Posted: 13 Apr 2018 at 17:55
Will be interested in replies to this as I'm planning the same. 

I haven't yet measured or looked into any of the technical side.... 

But I will have more space to add a K&N directly to the carb, as I have discarded the big battery, toolbox, filter box and rear mud guard! 


My thoughts/concerns are around 

1 -  the breather box that the cam boxies link to...The small Le Mans breather box could be a solution though its expensive for what it is. So was going down the line of fabricating my own from a thick walled alloy tube (that have knocking around the garage) that I could drill and tap spigots and brackets on/into with a mini K&N vent. (though some people just let it vent to air/road and top up the oil more regularly!)

2 - if re jetting will be required - not sure on this as the OEM box isn't restrictive its just big and ugly. 
This may be more of a question if I change the silencers to something more free flowing! 

3 - the carbs point 'in' on the original I was considering trying to reverse the inlet so it pointed 'out' to give the filters more space - but this isn't something I have tried or even seriously looked at yet.    
 
4 - fabricating a Y piece to accomodate a single filter to service both carbs. (the cylinders will be on different strokes)


Lookforward to replies 


Posted By: Brian UK
Date Posted: 13 Apr 2018 at 19:18
I made a temporary breather box from a jam jar, putting all the pipes through the lid. Worked OK.

-------------
Brian.

Better 5 minutes late in this world than years early in the next.


Posted By: GC888
Date Posted: 13 Apr 2018 at 19:23
Originally posted by Brian UK Brian UK wrote:

I made a temporary breather box from a jam jar, putting all the pipes through the lid. Worked OK.

Have you bought a torque wrench yet? Smile


Posted By: Mike H
Date Posted: 13 Apr 2018 at 19:44
Originally posted by cyclobutch cyclobutch wrote:

You should be able to get pods that bolt onto the back of the carbs directly.

But my advice is don't go there, stick with OE. On the big twins the carbs are solid mount but on the littluns you've got those inlet rubbers - you'll be hanging a load more weight off those that they won't particularly like.  

Changing the std filter is a bit of a PITA but in the UK you only really need to do this every two or three years. And you can make it easier by switching to a wing nut at the nose of the unit the next time you do the job. 

Plus, never had a small block but I've read that the stock air filter box is also a still air reservoir which makes the bike perform better than without. (Issues include flat spot in the midrange, apparently.) I also bought into the K&N sales blurb in the past but according to later info on the grapevines seems they can clog up quickly, especially the small ones ("pods"), and can be too small area for wide open throttle and so restrictive (and especially if over-oiled), come to think of it I tried small pods on my 850 Le Mans, but was not overly impressed. The much bigger oval K&N's on the other hand worked and lasted better, but they only fit the big blocks AFAIK.






-------------
"Chicken nuggets don't dance on a Tuesday."


Posted By: GC888
Date Posted: 13 Apr 2018 at 19:48
Originally posted by Mike H Mike H wrote:


Plus, never had a small block but I've read that the stock air filter box is also a still air reservoir which makes the bike perform better than without. (Issues include flat spot in the midrange, apparently.) I also bought into the K&N sales blurb in the past but according to later info on the grapevines seems they can clog up quickly, especially the small ones ("pods"), and can be too small area for wide open throttle and so restrictive (and especially if over-oiled), come to think of it I tried small pods on my 850 Le Mans, but was not overly impressed. The much bigger oval K&N's on the other hand worked and lasted better, but they only fit the big blocks AFAIK.


interesting more food for the Y section and a bigger filter..


Posted By: Brian UK
Date Posted: 13 Apr 2018 at 20:50
Originally posted by GC888 GC888 wrote:

Originally posted by Brian UK Brian UK wrote:

I made a temporary breather box from a jam jar, putting all the pipes through the lid. Worked OK.


Have you bought a torque wrench yet? Smile

I've owned a torque wrench for more years than I can remember.

-------------
Brian.

Better 5 minutes late in this world than years early in the next.


Posted By: Brian UK
Date Posted: 13 Apr 2018 at 20:53
Originally posted by Mike H Mike H wrote:

never had a small block but I've read that the stock air filter box is also a still air reservoir which makes the bike perform better than without.

Certainly the air boxes fitted to the new range do incorporate a large volume for this reason. This also applies to the CARC models.

-------------
Brian.

Better 5 minutes late in this world than years early in the next.


Posted By: Motty
Date Posted: 13 Apr 2018 at 23:11
The rear rubber section of the original airbox fell to pieces on my bike (there was a photo on here at the time)
I have fitted individual pod filters, whilst doing so I increased both pilot jets and main jets. Since then I have only done a couple of hundred miles, I still need to fine tune the jetting.
I vented the breather tubes to fresh air, if this actually becomes an issue I will sort out a catch bottle for the oil



-------------
It’s more fun to try to ride a slow bike fast .....


Posted By: GC888
Date Posted: 16 Apr 2018 at 07:44
Originally posted by cyclobutch cyclobutch wrote:

You should be able to get pods that bolt onto the back of the carbs directly.

But my advice is don't go there, stick with OE. On the big twins the carbs are solid mount but on the littluns you've got those inlet rubbers - you'll be hanging a load more weight off those that they won't particularly like.  

Changing the std filter is a bit of a PITA but in the UK you only really need to do this every two or three years. And you can make it easier by switching to a wing nut at the nose of the unit the next time you do the job. 

There are foam pods available that are lighter and obviously more squishy to fit in the space?


Posted By: c13pep
Date Posted: 16 Apr 2018 at 10:11

There are foam pods available that are lighter and obviously more squishy to fit in the space?
[/QUOTE]

I fitted foam pods to my V50 for the above reason but also because they were less restrictive in the inlet tract than the stainless mesh type.
A point to make is that on my PHBH28 carbs there is a drillway from the intake flange to the `choke` plunger chamber (not sure if V35 are same) which needs to be plugged if pods are fitted directly to carb. (Eurocarb advice)
CHRIS

-------------
you can`t have any fun in a straight line

V50/3 x2


Posted By: Brian UK
Date Posted: 16 Apr 2018 at 15:27
The shape of the air intake for the carb is important. Look at the original parts, you will see a trumpet shape. Doing away with this by fitting pod type filters can lose some bhp.

-------------
Brian.

Better 5 minutes late in this world than years early in the next.


Posted By: GC888
Date Posted: 16 Apr 2018 at 17:39
Originally posted by Brian UK Brian UK wrote:

The shape of the air intake for the carb is important. Look at the original parts, you will see a trumpet shape. Doing away with this by fitting pod type filters can lose some bhp.

Hi Brian 

Do you have any more information on this as I am trying to understand the engineering behind it.

The existing filter is a long folded paper cone filter fills a good size air box. 

This should give a relatively slow air velocity through the filter without hindering the induction flow.
I can see why MG would decide to go this route as the after all the bike is a basic commuter and would be expected to contend with Mediterranean dust day in day out and potentially poor maintenance.

I guess the cone shape is to aid airflow...but don't know?

Directly replacing the OEM filter with a pod folded paper and mesh (unusually) may restrict the flow more than the OEM as the area of a folded pod is about half the size of the OEM and has no air box. 

It would reduce the volume of the 'air dam' in the box. Though in theory that would only effect initial acceleration of the engine as the pressure reduced in the air box over a second or so before maximum flow through the filter was attained.

This would make the bike run rich - unusual as pods general do the opposite!

Therefore there is the potential that running the carbs without filters open to air may be relatively close to putting a new clean filter on...I feel a sudden liking for mesh trumpets....

So if using pods a bigger longer profile will be as closer to original flows than the short stubby ones.
(easy to work out the comparative surface areas of original and replacement)

Or the reticulated foam pods that provide very little restriction (some of these have an angle inlet) 

But it would mean trumpets mesh or otherwise are an option.

Have people switching to pods gone up down or same on jetting with what results?  










   


Posted By: Mike H
Date Posted: 16 Apr 2018 at 19:28
Pods can make the engine run rich if they can't pass enough volume of air for large throttle openings. Especially when getting clogged up with dirt. Because of the small surface area that could be a short time.

I saw a graph recently when someone actually tested all these filters for volume of flow, using a special machine for the purpose, I think with dust added also to show drop off of performance with use. IIRC all the pods did poorly in terms of working lifetime.




-------------
"Chicken nuggets don't dance on a Tuesday."


Posted By: Simmoto
Date Posted: 16 Apr 2018 at 19:47
I've changed both of my small blocks back to standard airbox. Pods seem to screw up the fuelling in many cases and it's hell to put right. Unless you are a carb expert with a dyno etc....


Posted By: Brian UK
Date Posted: 16 Apr 2018 at 22:15
I wasn't talking about the size of the filter or restriction at all. I was referring to the profile of the air intake into the carb. This provides a smooth airflow through the carb, mainly at full throttle of course, and will give that little bit of extra power compared to a square butt joint which you usually get with pod filters.
Look at the profile of the OEM carb inlet plastic moulding. The way it reduces smoothly in diameter.


-------------
Brian.

Better 5 minutes late in this world than years early in the next.


Posted By: GC888
Date Posted: 17 Apr 2018 at 08:38
Originally posted by Simmoto Simmoto wrote:

I've changed both of my small blocks back to standard airbox. Pods seem to screw up the fuelling in many cases and it's hell to put right. Unless you are a carb expert with a dyno etc....

Can you take an educated guess on how it was screwing it up - rich or lean?

K&N type oiled cotton has a better flow characteristics than the original folder paper so while the surface area of is smaller the initial air flow is similar. I say initially as dirt builds up both paper and reticulated foam loose flow at a higher rate than oiled cotton.

Generally I'm an advocate of keeping it original, however in this case it looks like MG took a big compromise on the filter box keeping it cheap but functional where there is potential to bring it into the 21st century.

Still interested on any lean/rich effects and jetting changes (if any) that people made to compensate?    


Posted By: GC888
Date Posted: 17 Apr 2018 at 08:48
Originally posted by Brian UK Brian UK wrote:

I wasn't talking about the size of the filter or restriction at all. I was referring to the profile of the air intake into the carb. This provides a smooth airflow through the carb, mainly at full throttle of course, and will give that little bit of extra power compared to a square butt joint which you usually get with pod filters.
Look at the profile of the OEM carb inlet plastic moulding. The way it reduces smoothly in diameter.

 
With a pod this smooth profile would be maintained - just with a different filter on the outside.
It could be improved as the flow would not need the various tubes and elbows...? 

Just on the smooth transition there has been some interesting discussion on deliberately causing turbulence in intakes to enhance the quality of the petrol air mix - which is contrary to most thinking! 


Posted By: BobV7
Date Posted: 17 Apr 2018 at 09:19
Does anyone remember how we used to use the venturi effect on intakes to speed up the flow of air into a carb? Seems to have been forgotten lately so perhaps it's not as important with FI.

-------------
V7 Classic Black and gold is the best colour


Posted By: Brian UK
Date Posted: 17 Apr 2018 at 10:00
Originally posted by GC888 GC888 wrote:

With a pod this smooth profile would be maintained - just with a different filter on the outside.It could be improved as the flow would not need the various tubes and elbows...? 

Not so on the pods I had.

-------------
Brian.

Better 5 minutes late in this world than years early in the next.


Posted By: Vegas Pete
Date Posted: 17 Apr 2018 at 17:19
Here is my unsolicited take on filters and intake systems, which is worth everything you paid for it.

Due to cam timing overlap, there is some reverse flow out of the carburetor at lower speeds.  The designers take this into consideration, expecting some of the air to pass through the venturi more than once, collecting fuel each time it passes, and design the carb accordingly.  This is why removing the filter screws up the low speed mixture- this "back puke" air gets blown away and replaced by clean, un-fueled air.  You can see this by running the engine with the filters removed and observing the fuel spray coming out of the intake side of the carb when the throttle is opened.  Without the airbox and filter, this "back puked" air doesn't get sucked through and fuel added to it again, resulting in a lean mixture.  This effect is minimized at higher RPMs, so adjusting the mixture for low speed will mean an improper fuel/air mix at high speed and vice versa, which is to say removing the airbox or intake tubes makes it damn hard to get it to work right at both ends.

Adding a tube to the intake side of the carb keeps the fuel/air charge from getting away, and allows the carb to be properly adjusted across it's operating range.

As for pod filters, the oiled foam or oiled paper types tend to attract dust whether the engine is running or not, adding to the headache of keeping them clean.  Living in the desert as I do, I've found dry paper filters are the way to go.

OK, I'm off for three days of camping on a dry lake bed, where the winds are only expected to be 40-60 mph.  No oiled filters for me!

Vegas Pete. 


-------------
2010 V7 Classic, 2005 Ural Gearup, 1980 Yamaha DT175, 1973 Rokon RT-140, 1969 Rokon MkIII


Posted By: GC888
Date Posted: 17 Apr 2018 at 17:21
Originally posted by BobV7 BobV7 wrote:

Does anyone remember how we used to use the venturi effect on intakes to speed up the flow of air into a carb? Seems to have been forgotten lately so perhaps it's not as important with FI.

Yes its venturi effect - put the same volume of air through a venturi/constriction and it speeds up but you also importantly get a pressure drop. 

You can make a two lovely profiled reducing and expanding cones or just use a Venturi 'plate'. (a restriction)

In a carb its used to pull fuel into the airflow (also why its not so important with FI which just pumps it in!)

Air enters the mouth of the carb which reduces in size under the throttle slide in the carb which induces a lower pressure that sucks in the fuel then expands at the other side of the slide mixed with the petrol. 

I think what you are referring to is a velocity stack, where you tune the length of the 'trumpet' to the resonance of the engines intake - on most modern bikes this isn't considered a good thing as its quite noisy which gets jumped on by the environmental elves.

But the idea is that you have a resonating higher pressure in the trumpet that increases the charge (a bit) going into the engine at full throttle (before then it does very little) so if done right you have a high pressure wave in the inlet corresponding to when its valve opens... to increase the charge.

The trumpet does also smooth airflow but don't think it does much +/- for the venturi effect in the carb, as the various bores of the inlet stay the same. 

So while I am happily suggesting I stick a set of trumpets on my carbs... this would do no harm but unless the length was correct it would not have any effect either, even if correct it would do nowt until it was at full throttle! - but they look nice and are good to fit a grill on.


 


Posted By: Simmoto
Date Posted: 17 Apr 2018 at 20:03
I think they ran lean, I tried various sizes of main jets and other carb internals but gave up as it was getting expensive. Jumps in the iginition curve for the Bosch ignition bike seemed exacerbated; points bike starts better on standard airbox. Not everyone shares this view but many do. 


Posted By: iansoady
Date Posted: 18 Apr 2018 at 10:19
I have cheap no-name mesh type filters on my V50 and it seems to carburate very well - although I don't have many miles on it as yet.

WRT velocity stacks, they tend to work over a restricted rev range, much like tuned exhausts, and both actually cause performance problems outside that area. Fine for racing where you can keep the engine working in that range - less good for road bikes.


-------------
Ian
1982 V50
1952 Norton ES2


Posted By: c13pep
Date Posted: 18 Apr 2018 at 12:13
As a general rule jets are increased by 10% when fitting pods, to answer the original question, however this would be subject to experimentation and, as has been mentioned, could get expensive. I was lucky in that 10% increase in jet sizes seemed to work straight away, but then again I`m easily pleased as my riding style is more steady away these days. I also retained the original plastic intakes on the filter side to give me the correct angle to fit straight pods instead of the angled ones.
As an after thought with the general advice to increase jet sizes for pods seems to suggest greater air intake than original!!!
CHRIS

-------------
you can`t have any fun in a straight line

V50/3 x2


Posted By: GC888
Date Posted: 18 Apr 2018 at 12:48
Originally posted by c13pep c13pep wrote:

 the general advice to increase jet sizes for pods seems to suggest greater air intake than original!!!
CHRIS

It does , which suggests the paper cone is a lot more restrictive than the K&N thats half the surface area but oiled cotton.

Guess as an experiment you could blank off part of the K&N filter with sticky tape until it came into spec. Then make it permanent ....  


Posted By: Brian UK
Date Posted: 18 Apr 2018 at 13:50
But that rule is generic, not just for one model and one original filter.
You have to remember that the paper OEM filter is many times larger in surface area than any pod filter.
Most people ditch the OEM one because it's such a difficult one to change, not for performance. Those that do it for expected performance gain have never actually proved any advantage.

-------------
Brian.

Better 5 minutes late in this world than years early in the next.


Posted By: GC888
Date Posted: 18 Apr 2018 at 19:23
Originally posted by Brian UK Brian UK wrote:

But that rule is generic, not just for one model and one original filter.
You have to remember that the paper OEM filter is many times larger in surface area than any pod filter.
Most people ditch the OEM one because it's such a difficult one to change, not for performance. Those that do it for expected performance gain have never actually proved any advantage.

I don't think we are expecting any performance improvement, the best we can hope for is no loss! 

However the OEM filter has the ethics of a black pudding is difficult to change and expensive to replace if any component busts.

I think the hope here is just to figure out what can you expect and how to mitigate any problems.

The OEM filter as every design is a compromise and will change its airflow characteristics as it gets older and full of the dust its extracted. So long as you can guess what filter sits between these limits you will not need to change anything.  

Time for hitching a vacuum cleaner to the OEM with a manometer!


Posted By: Simmoto
Date Posted: 18 Apr 2018 at 20:47
I agree with Brian that rule is generic and quoted in Guzziology for AFAICR big twins. Big twins seem to take to pods more easily than small. 


Posted By: Brian UK
Date Posted: 18 Apr 2018 at 22:56
You are unlikely to get better performance out of a small bock heron head, no matter what air filter you fit.

-------------
Brian.

Better 5 minutes late in this world than years early in the next.


Posted By: GC888
Date Posted: 19 Apr 2018 at 17:29
Originally posted by Simmoto Simmoto wrote:

I agree with Brian that rule is generic and quoted in Guzziology for AFAICR big twins. Big twins seem to take to pods more easily than small. 

True there generally is always more leeway with a bigger engine 


Posted By: GC888
Date Posted: 19 Apr 2018 at 17:30
Originally posted by Brian UK Brian UK wrote:

You are unlikely to get better performance out of a small bock heron head, no matter what air filter you fit.

This is true; but performance isn't everything otherwise all V35s would have been scrapped long ago :-) 


Posted By: Po51uhd
Date Posted: 22 Apr 2018 at 08:26
thanks for all the (differing) opinions, it’s great to get so many ideas. I’ve opted for two short conical jobs which fit neatly on the existing plastic induction pipes. We’ll see how it runs when I finally get it going!
Stephen 



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