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More on Airboxes - Now Filtration

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More on Airboxes - Now Filtration

Postby nsaints » Tue Aug 16, 2022 12:34 pm

As a follow-on to messing around with standard airboxes, documented in this thread http://www.ilambretta.co.uk/forum/viewtopic.php?f=7&t=14074&p=128613&hilit=tale#p128613
I have recently bought another large volume air box from SR, Nottingham to retire the holes drilled in the airbox mod. This means the TV is now breathing fresh, cooler air rather than very hot air under the panels
This worked well to and from Arundel at the week-end with no jetting mods required

This means both my TV and SX are running large volume air boxes with the standard shape BGM stainless mesh filters

I'm now interested in improved filtration a standard paper filter would/could offer versus the stainless mesh fitted to the large volume airbox

Aside from the marketing talk on performance improvements the BGM mesh filter offers, has anyone got any evidence (dyno or otherwise) what air flow impact a standard paper filter compared to the mesh style?
If the impact is minor, improved filtration, reduced top end wear and tear could be a compromise worth considering
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Re: More on Airboxes - Now Filtration

Postby Warkton Tornado No.1 » Tue Aug 16, 2022 7:24 pm

Unless somebody deliberately commissioned dyno time with the object of comparing various air filters, I doubt there are results to be seen. Given the costs involved it's only likely that a manufacturer such as BGM would want to pay for such an exercise, but you could always ask them if any tests were carried out.

I think that there is definitely a requirement for an updated version of the airbox but apart from the logic of drawing cooler air via the frame hole (50 mm) my thoughts are that to follow the OEM box style & oval element is not going to overcome the restrictions required for a carburettor with a venturi of 30 mm or more. My thoughts are that a successful design will only be similar to the OEM concept in using the frame hole to draw air, be fully enclosed yet serviceable & have an outlet to which a flexible connection to the carburettor can be made. Some carburettors are not so filter friendly IMHO & I think that any new design should be capable of supplying adequate filtered air to a venturi of up to 34 mm to either side of the engine, given the fact that many kits operate well with that size. That's not to say that all carburettors of a size larger than 30 mm will work equally on an engine, filtered or not, but I don't want to feel that I'm browbeating others stating my preferences, but suffice to say that the considered opinions of two stroke tuners in general is that a flatter guillotine slide is the better choice. Notably, virtually all such products have built in anti-flood breathers to the float bowl because the designers take the hot restart issue seriously & I mention that as adding air filtration can exasperate the problem immensely.

For reference, in Sticky's Lambretta Kit Book, page 209 demonstrates 'Big Carb vs Little Carb' applied to an RB25 244 but I stress that the test must have been carried out with no attempt @ air filters. As that additional test was beyond the remit of the book I think it amounts to a very interesting observation & it makes me wonder that had time & resources been available whether a LH manifold to allow for adequate under panel air filtration would have have affected the results significantly. Almost certainly fuel economy will increase with a good adequate air filter designed in such a manner as to prevent saturation of the element.

Having been very pleased with my own under panel air filter developments, the next logical step is to evolve it so that the air is drawn from the outside, preferably via the frame hole specifically for the purpose. Despite the likes of the Targa Twin & others that persevere with air drawn from under the panels, I don't consider that the best option......
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Re: More on Airboxes - Now Filtration

Postby vegansydney » Tue Aug 16, 2022 8:07 pm

Just to add confusion to the mix there were a few suppliers to Innocenti for paper air filters and some '80s SIL production came with a factory fitted K&N filter. I'd love to find a NOS stash of these. Tutto Lambretta/Stratos also made a high-flow paper filter for years. Scootopia now make them too.

As per the (excellent) thread linked above, my personal preference for mildly tuned scooters is a S1/early TV/2 wide neck airbox or a modified S3 with an Ø50mm inlet and a BGM stainless filter. Alternatively, a K&N RU-0200 filter horizontally mounted to the rear floorboard strut and connected to the carb with a GP air hose. While still under the panels the movement from the rear hub fins is theoretically better than upright and directly above the hottest part of the engine.

Image
Last edited by vegansydney on Tue Aug 16, 2022 10:50 pm, edited 1 time in total.
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Re: More on Airboxes - Now Filtration

Postby Warkton Tornado No.1 » Tue Aug 16, 2022 9:26 pm

I meant to mention this earlier:

When the SIII air filter system was conceived, the nominal choke size was 18 mm with 20 mm being used subsequently as other models were introduced. During the life of the air filter box, it went through a number of changes to improve it's capability, culminating with use for the 22 mm choke sizes. As Innocenti were renowned for their reluctance to spend money on the components hidden away, you could assume that the changes were necessary for any costs involved to be sanctioned. I'd suggest that 22 mm, with a safety margin, is about the limit of the standard OEM air filter system capability.

By the time a bigger carburettor is added to an engine that may well rev comfortably beyond the, say, 6900 RPM of a standard motor, it is easy to imagine that there may well be inadequate air flowing. Even if an engine had identical characteristics but was optimised to use a larger carburettor, a choke size of 31 mm would need DOUBLE the volume of air.

No wonder that the 30 mm Amal MK 1 running through the OEM air box fitted to my TV200 when I bought it kept heat seizing......
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Re: More on Airboxes - Now Filtration

Postby nsaints » Wed Aug 17, 2022 12:20 pm

Thanks for the feedback

I'd forgotten Scootopia claim their paper standard filter to be a greater efficiency

yes, I suppose WT1, you're correct - unlikely anything exists, I had already tried an internet search before asking the question
Perhaps the next instalment of Stickies guides this topic could be considered - wishful thinking or maybe no one else cares (LOL)?

I'm currently changing the configuration (top-end/carb) of my SX, inline with Stu Owen's recommendations. Although going 'off script' with the large volume air box
I think for jetting I'll start with the BGM mesh filter, and at a later date may try high flow paper as an experiment - if it bogs down with a paper filter I'll at least get some instant feedback
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Re: More on Airboxes - Now Filtration

Postby Leotech » Wed Aug 17, 2022 2:38 pm

I have just constructed a higher flow box for my GP200, I made a 54mm neck hole in the box and removed the baffle, I am also using the mesh BGM filter and the larger BGM bellows. SH22 carb as I want to retain a standard ish look, The cylinder is standard cast iron at about 205 cc but tidied up a little to my port timings.


I am at the initial stages of testing so not really got any good feedback yet, I have jetted it slightly up from standard but it feels far too rich. 50 rather than 45 pilot and 128 main as opposed to 122. I would have thought with a Avanti exhaust and a less restrictive intake that would have been a good start point but it seems I am going to have to go leaner.


I am beginning to think maybe I have wasted my time with the air box and a standard GP200 one would be fine.
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Re: More on Airboxes - Now Filtration

Postby nsaints » Wed Aug 17, 2022 2:54 pm

I have jetted it slightly up from standard but it feels far too rich


you may want to consider Adam's SH2/22 jetting advice on this post SH2/22 Jetting
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Re: More on Airboxes - Now Filtration

Postby Leotech » Wed Aug 17, 2022 7:03 pm

nsaints wrote:
I have jetted it slightly up from standard but it feels far too rich


you may want to consider Adam's SH2/22 jetting advice on this post SH2/22 Jetting



I've not got as far as that yet, I have used the -6 atomiser in the past. But yes I may need to swap the atomiser and start from scratch.
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Re: More on Airboxes - Now Filtration

Postby Warkton Tornado No.1 » Wed Aug 17, 2022 7:44 pm

Leotech wrote:
nsaints wrote:
I have jetted it slightly up from standard but it feels far too rich


you may want to consider Adam's SH2/22 jetting advice on this post SH2/22 Jetting



I've not got as far as that yet, I have used the -6 atomiser in the past. But yes I may need to swap the atomiser and start from scratch.


You may have retarded your ignition a little too far. Personally, apart from race engines, I set @ 19 DBTDC even on a tuned engine, but always use a Super Unleaded fuel. Unfortunately, retarding ignition is 'swings & roundabouts' & one person's rule of thumb is likely different to another....

That said, I'd guess @ a 47 pilot & 125 ballpark jetting. Has the Avanti got a larger tailpipe than OEM 'boxes?
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