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Simmoto View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Simmoto Quote  Post ReplyReply Direct Link To This Post 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....
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Brian UK View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Brian UK Quote  Post ReplyReply Direct Link To This Post 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.
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GC888 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GC888 Quote  Post ReplyReply Direct Link To This Post 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?    
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GC888 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GC888 Quote  Post ReplyReply Direct Link To This Post 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! 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BobV7 Quote  Post ReplyReply Direct Link To This Post 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.
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Brian UK View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Brian UK Quote  Post ReplyReply Direct Link To This Post 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.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Vegas Pete Quote  Post ReplyReply Direct Link To This Post 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. 
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GC888 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GC888 Quote  Post ReplyReply Direct Link To This Post 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.


 
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Simmoto View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Simmoto Quote  Post ReplyReply Direct Link To This Post 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. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote iansoady Quote  Post ReplyReply Direct Link To This Post 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
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Post Options Post Options   Thanks (0) Thanks(0)   Quote c13pep Quote  Post ReplyReply Direct Link To This Post 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

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GC888 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GC888 Quote  Post ReplyReply Direct Link To This Post 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 ....  
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Brian UK Quote  Post ReplyReply Direct Link To This Post 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.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote GC888 Quote  Post ReplyReply Direct Link To This Post 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!
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Simmoto View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Simmoto Quote  Post ReplyReply Direct Link To This Post 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. 
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