"Normal" Spitback?

[tippo88]

To clarify my opinions & experience, I have continuously owned Lambrettas since 1973, the majority which have been tuned. My history includes a fair amount of racing.

I mention the above as it has not been practicable to run to a dyno operator. Also, like many of my contemporaries, we learnt 'work-arounds' as in the case of the Dell"Orto VHB which only nowadays seems to have other slides than the 50 they all came equipped with!

Nowadays, I know how to fit a two-stroke atomiser to a VHB. Also, I know how to convert slides to 40 or 45.

I only say the above because it truly makes for a cracking, responsive carburettor (with it's flat guillotine type slide) & would have been a far better evolution than the PH range. Opinionated Me?

Anyway, even better than either of the above are the current two stroke specific Dell'Orto range VHBC''s etc. Sorry folks, but I don't think you'll find PH's being used on the track Lambrettas....

However, a PWK carburettor made under license from the originators Keihin is my current best choice for the road tunes.

The KOSO's, OKO's, STAGE6 etc are all good but should be checked over like ANY carburettor. Blue-printed if necessary. Information is available on t'internet.....

They work well with filters & have anti-flood. Easy & incredibly cheap to set up.

They work well, fuss-free across quite a range of capacities/states of tune. For example, a 30 mm venturi might be considered oversize for a 185 by other readers, but it would work well.

Conversely, a smaller Venturi would also work well & be just as happy on a 240 plus.

For your current engine, I'd look @ what best fits the spigot & orifice sizes of what you have then make a decision based on that.

Good Luck!

Thanks! Funny when I built it I put a PWK on , maybe I chose the wrong copy as it was a cheap one from Scootrs. Drove me completely crazy, no matter what I did it was an absolute b'stard to start from cold and just couldn't get it to behave consistently at all. The local shop recommended the Mikuni which are readily available and cheap here in Malaysia and is used a lot by the local tuners. If I didn't have to wipe petrol off the floorboards every ride I'd be quite happy with it.
Might look at a better quality PWK but probably will just enjoy riding it for a while first .

Sent from my SM-G985F using Tapatalk

[Warkton Tornado No.1]

To clarify my opinions & experience, I have continuously owned Lambrettas since 1973, the majority which have been tuned. My history includes a fair amount of racing.

I mention the above as it has not been practicable to run to a dyno operator. Also, like many of my contemporaries, we learnt 'work-arounds' as in the case of the Dell"Orto VHB which only nowadays seems to have other slides than the 50 they all came equipped with!

Nowadays, I know how to fit a two-stroke atomiser to a VHB. Also, I know how to convert slides to 40 or 45.

I only say the above because it truly makes for a cracking, responsive carburettor (with it's flat guillotine type slide) & would have been a far better evolution than the PH range. Opinionated Me?

Anyway, even better than either of the above are the current two stroke specific Dell'Orto range VHBC''s etc. Sorry folks, but I don't think you'll find PH's being used on the track Lambrettas....

However, a PWK carburettor made under license from the originators Keihin is my current best choice for the road tunes.

The KOSO's, OKO's, STAGE6 etc are all good but should be checked over like ANY carburettor. Blue-printed if necessary. Information is available on t'internet.....

They work well with filters & have anti-flood. Easy & incredibly cheap to set up.

They work well, fuss-free across quite a range of capacities/states of tune. For example, a 30 mm venturi might be considered oversize for a 185 by other readers, but it would work well.

Conversely, a smaller Venturi would also work well & be just as happy on a 240 plus.

For your current engine, I'd look @ what best fits the spigot & orifice sizes of what you have then make a decision based on that.

Good Luck!

Thanks! Funny when I built it I put a PWK on , maybe I chose the wrong copy as it was a cheap one from Scootrs. Drove me completely crazy, no matter what I did it was an absolute b'stard to start from cold and just couldn't get it to behave consistently at all. The local shop recommended the Mikuni which are readily available and cheap here in Malaysia and is used a lot by the local tuners. If I didn't have to wipe petrol off the floorboards every ride I'd be quite happy with it.
Might look at a better quality PWK but probably will just enjoy riding it for a while first .

Sent from my SM-G985F using Tapatalk

There are checks that must be done prior to using the PWK's but that information is available & the companies that specialise in not only supplying them for sale, but tune them to fit the bikes they sell, are very open about the changes that should be performed. A great example of such a pro-active approach can be found here:

http://www.mid-atlantictrials.com/Sales.html

They are not in denial that Chinese copies exist & to an extent their website helps with even setting them up.

Critical is the float setting, but easily achievable.

The correct feed hole size to the venturi//atomiser is detailed & how to safely make any necessary modification.

Personally, I grind out any float stops from the bowl & I also slot the float bowl allowing easy access to the pilot jet without bowl removal.

Set up is not complicated, helped by the fact that there are not zillions of slides to buy & Lambrettas tend to be happy with one type of needle.

As soon as you can get your head around the fact that piston ported pilots are likely to be low 30's, the rest of the setting up is fairly intuitive.

The only shortcoming I can think of is the lack of inbuilt fuel filter, but the addition of an in-line filter is easily done & arguably more obvious of catching any cr@p as well as easy to clean.

[B-Race Tuning]

Tippo, please clarify your opening sentence please, "Just back to my scooter after a couple of years in a different country and finally getting to enjoy riding it AFTER ITS COMPLETE REBUILD!"
Did it spit back as previously? Was the top end "interfered" with in ANY way during the rebuild if so?
If port timing is of a sensible duration then spitback should be minimal, but will occur.
Carb type does not influence spitback! Induction method/type/ timing does. And reed valves still spit back.
My finding's with many types of filters, (or none) is that you can massively affect airflow but a lot of it can be traced back to sound wave/ harmonics (got to love em) in the inlet tract/ carb/ manifold. (think of affects a good expansion chamber can have on engine performance, then reverse to how BAD the harmonics in the inlet tract "can/ could" randomly be. Some filters have proven to increase airflow/ performance over an open carb on SOME engines (Train Driver can confirm this after his visit to me YEARS ago).
I currently use the Casa filter on lower spec engines and has proven a good product regarding power delivery and fits a scooter very well.
I'd hold any judgement until you can state the spit back is same/ worse than before, and what if any work has been done to the top end/ pipe swap.

[Warkton Tornado No.1]

Tippo, please clarify your opening sentence please, "Just back to my scooter after a couple of years in a different country and finally getting to enjoy riding it AFTER ITS COMPLETE REBUILD!"
Did it spit back as previously? Was the top end "interfered" with in ANY way during the rebuild if so?
If port timing is of a sensible duration then spitback should be minimal, but will occur.
Carb type does not influence spitback! Induction method/type/ timing does. And reed valves still spit back.
My finding's with many types of filters, (or none) is that you can massively affect airflow but a lot of it can be traced back to sound wave/ harmonics (got to love em) in the inlet tract/ carb/ manifold. (think of affects a good expansion chamber can have on engine performance, then reverse to how BAD the harmonics in the inlet tract "can/ could" randomly be. Some filters have proven to increase airflow/ performance over an open carb on SOME engines (Train Driver can confirm this after his visit to me YEARS ago).
I currently use the Casa filter on lower spec engines and has proven a good product regarding power delivery and fits a scooter very well.
I'd hold any judgement until you can state the spit back is same/ worse than before, and what if any work has been done to the top end/ pipe swap.

Excellent comments, but like many people that ask advice on these Forums, the person asking generally do not know all of the information relative to the engine's state of tune.

Consequently, my advice taken in context, is how spitback may be not eliminated, but managed. That said, some versions of "two stroke" carburettors are "more two stroke" than others! Even pukka Dell'Orto two stroke (guillotine) carburettors are available with a choice of needle shrouds that intrude the venturi, which is yet another variable to contend with.

In addressing the question asked about my own, personal recomendation, I would still advocate the PWK, were a substitution desired.

I'm sure that many are aware that 'common-or-garden' PWK's function on both two & four strokes yet they possess no significant intruding shrouding to the needle. .That's not to say marginal improvements may not be had by researching the Keihin catalogues & making further modifications.

However, to qualify & reiterate my reasoning, my experience is that PWK's are filter friendly. Besides their built in anti-flood, they appear much more tolerable of attachments up stream to add filtration.

During my experimentation concerning adding filtration, from optimised 175 to 240+ engines, with venturis above 28 mm, none fitted with Dell''Orto PH or VHB were tolerable of even a 40 mm bore elbow, let alone the standard OEM bellows, even with no filter. How some shop tuned engines run to any satisfaction with the bellows fitted is a mystery to me.

Hence I felt that with the restricted space in which to fit an air filter required some kind of research, which I have spoken of before within this & 'the other' Forum.

An upstream, nominally vertical angled connection is my preference. Whilst 'panel filters' may appear to be the solution, I have witnessed them being drenched @ the bottom which unquestionably affects performance & does little for economy or wear. IMHO

Because of the restrictions in space that a side panel creates with a larger carburettor of even reasonably small size, my belief is that a plenum chamber is what is best to replicate. Routing from the carburettor as vertical as possible is the optimum means by which saturation of the filter is best avoided.

I do not think that any currently available modified OEM air filter boxes will be un-restrictive, which is a shame, not least for convenience.

[tippo88]

Tippo, please clarify your opening sentence please, "Just back to my scooter after a couple of years in a different country and finally getting to enjoy riding it AFTER ITS COMPLETE REBUILD!"
Did it spit back as previously? Was the top end "interfered" with in ANY way during the rebuild if so?
If port timing is of a sensible duration then spitback should be minimal, but will occur.
Carb type does not influence spitback! Induction method/type/ timing does. And reed valves still spit back.
My finding's with many types of filters, (or none) is that you can massively affect airflow but a lot of it can be traced back to sound wave/ harmonics (got to love em) in the inlet tract/ carb/ manifold. (think of affects a good expansion chamber can have on engine performance, then reverse to how BAD the harmonics in the inlet tract "can/ could" randomly be. Some filters have proven to increase airflow/ performance over an open carb on SOME engines (Train Driver can confirm this after his visit to me YEARS ago).
I currently use the Casa filter on lower spec engines and has proven a good product regarding power delivery and fits a scooter very well.
I'd hold any judgement until you can state the spit back is same/ worse than before, and what if any work has been done to the top end/ pipe swap.

Sorry I missed this post!
The scooter was in a mess when i got it and barely ran.
I rebuilt the whole thing, for the engine I used a gp crank, yamaha rod and I probaby made a mistake in going with an SR top end because cheap and available locally.
I had a problem getting decent port timing and ended up with this:
exhaust 171
transfer 135
blowdown 18.24
inlet 140
( to be honest this is my first stroker build so dont know much about port timing)
I had all kinds of problems getting it to run decently with a pwk copy and now its functional with the mikuni but dont have anything to compare it with regarding spit back.

Cheers

Sent from my SM-G985F using Tapatalk

[tippo88]

Tippo, please clarify your opening sentence please, "Just back to my scooter after a couple of years in a different country and finally getting to enjoy riding it AFTER ITS COMPLETE REBUILD!"
Did it spit back as previously? Was the top end "interfered" with in ANY way during the rebuild if so?
If port timing is of a sensible duration then spitback should be minimal, but will occur.
Carb type does not influence spitback! Induction method/type/ timing does. And reed valves still spit back.
My finding's with many types of filters, (or none) is that you can massively affect airflow but a lot of it can be traced back to sound wave/ harmonics (got to love em) in the inlet tract/ carb/ manifold. (think of affects a good expansion chamber can have on engine performance, then reverse to how BAD the harmonics in the inlet tract "can/ could" randomly be. Some filters have proven to increase airflow/ performance over an open carb on SOME engines (Train Driver can confirm this after his visit to me YEARS ago).
I currently use the Casa filter on lower spec engines and has proven a good product regarding power delivery and fits a scooter very well.
I'd hold any judgement until you can state the spit back is same/ worse than before, and what if any work has been done to the top end/ pipe swap.

Sorry I missed this post!
The scooter was in a mess when i got it and barely ran.
I rebuilt the whole thing, for the engine I used a gp crank, yamaha rod and I probaby made a mistake in going with an SR top end because cheap and available locally.
I had a problem getting decent port timing and ended up with this:
exhaust 171
transfer 135
blowdown 18.24
inlet 140
( to be honest this is my first stroker build so dont know much about port timing)
I had all kinds of problems getting it to run decently with a pwk copy and now its functional with the mikuni but dont have anything to compare it with regarding spit back.

Cheers

Sent from my SM-G985F using Tapatalk

Pipe is a scootopia clubman btw

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