"Normal" Spitback?

[tippo88]

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!
Am running an open carb and am getting some spitback , as per attached photos taken after a 10 kilometer town ride. Is this normal and the solution maybe just to fit a remote filter? Its a bit annoying to be constantly cleaning the floor board and side panel.
Its a piston ported engine, I understand adding a reedvalve would help but would prefer not going to that extent.

Thanks in advance.


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[cgt75b]

Hi

I run a the rubber hose but with no filter, it seems to work.

Hope that helps

Cgt75b

[tippo88]

Hi

I run a the rubber hose but with no filter, it seems to work.

Hope that helps

Cgt75b

Thanks! Sounds like a good idea.

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[Warkton Tornado No.1]

It must be something that I do with an engine that doesn't enable piston ported to run with a standard bellows or, as I have tried, an elbow of below 40 mm.

Mind you, engines that I have fettled will run extremely well with suitable air filters, it's just that the size needs to be adequate.

In fact, if I had an engine with a bigger carburettor that I had worked on, & it did work with the standard bellows, I'd be rather worried. To produce reasonable power, a modified Lambretta will need to increase it's air consumption as part of the optimisation process.

The area immediately 'updraught' from the carburettor can be considered as the plenum. It doesn't necessarily need to be smooth, but does need volume. DIY versions can be made from the silicone bends available for cars, the best being those that will fit the carburettor then have a greater diameter @ the other end to which a filter &/or connecting pipe can be fitted.

A remote filter gives the advantage of being virtually 'unfloodable' thereby staying cleaner for longer.

A piston ported engine with a good, usefully sized filter will produce virtually the same power as unfiltered but will also not waste fuel & remain cleaner with a significant decrease in wear.

However. The bad news is that the Dell'Orto PH range of carburettors so beloved by many (obviously not by me!) will tend to flood if a warm/hot engine is left for a while, say whilst refuelling. Getarounds include drilling holes in the connection/elbow which rather defeats the object of filtering the air.......

[tippo88]

So today I tried putting on an "Atomic" elbow designed for a remote filter, but with no filter. Normally starts first kick once warm, with the elbow on it was very difficult to start. Elbow is off again and still wiping the petrol off the floorboards after every ride

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[Warkton Tornado No.1]

So today I tried putting on an "Atomic" elbow designed for a remote filter, but with no filter. Normally starts first kick once warm, with the elbow on it was very difficult to start. Elbow is off again and still wiping the petrol off the floorboards after every ride

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Although I advocate filtration, I stress that the feed must be adequate. I did a lot of experimentation to prove that it could be done & that inevitably also proved to me the carburettors most filter friendly.

Not only do some carburettors take to filtration & require minimal re-jetting as a consequence, but they also appear to cope with any necessary convoluted intake route to their inlet.

I am uncertain what carburettor is fitted (Mikuni round slide?) or the volume/cross sectional area of the elbow that you tried, but, given the available space beneath the sidepanel, filtration is often possible.

For space, the carburettors on TS engines are notoriously tight to a panel, but a DIY 'sock' fashioned out of filter foam proved beyond doubt that a 34 mm PWK works incredibly well. That being proven, the next test will be the VHSB 34 that arrived with the engine. I fully expect it to work as well as it is a 'proper' two stroke Dell'Orto & is equipped with anti-flooding vents.

[tippo88]

So today I tried putting on an "Atomic" elbow designed for a remote filter, but with no filter. Normally starts first kick once warm, with the elbow on it was very difficult to start. Elbow is off again and still wiping the petrol off the floorboards after every ride

Sent from my SM-G985F using Tapatalk

Although I advocate filtration, I stress that the feed must be adequate. I did a lot of experimentation to prove that it could be done & that inevitably also proved to me the carburettors most filter friendly.

Not only do some carburettors take to filtration & require minimal re-jetting as a consequence, but they also appear to cope with any necessary convoluted intake route to their inlet.

I am uncertain what carburettor is fitted (Mikuni round slide?) or the volume/cross sectional area of the elbow that you tried, but, given the available space beneath the sidepanel, filtration is often possible.

For space, the carburettors on TS engines are notoriously tight to a panel, but a DIY 'sock' fashioned out of filter foam proved beyond doubt that a 34 mm PWK works incredibly well. That being proven, the next test will be the VHSB 34 that arrived with the engine. I fully expect it to work as well as it is a 'proper' two stroke Dell'Orto & is equipped with anti-flooding vents.

Hi, carb is a Mikuni VM26, elbow as per attached photos, I guess it does provide some restriction. Amazing that it can make a difference to carb performance!

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[Warkton Tornado No.1]

So today I tried putting on an "Atomic" elbow designed for a remote filter, but with no filter. Normally starts first kick once warm, with the elbow on it was very difficult to start. Elbow is off again and still wiping the petrol off the floorboards after every ride

Sent from my SM-G985F using Tapatalk

Although I advocate filtration, I stress that the feed must be adequate. I did a lot of experimentation to prove that it could be done & that inevitably also proved to me the carburettors most filter friendly.

Not only do some carburettors take to filtration & require minimal re-jetting as a consequence, but they also appear to cope with any necessary convoluted intake route to their inlet.

I am uncertain what carburettor is fitted (Mikuni round slide?) or the volume/cross sectional area of the elbow that you tried, but, given the available space beneath the sidepanel, filtration is often possible.

For space, the carburettors on TS engines are notoriously tight to a panel, but a DIY 'sock' fashioned out of filter foam proved beyond doubt that a 34 mm PWK works incredibly well. That being proven, the next test will be the VHSB 34 that arrived with the engine. I fully expect it to work as well as it is a 'proper' two stroke Dell'Orto & is equipped with anti-flooding vents.

Hi, carb is a Mikuni VM26, elbow as per attached photos, I guess it does provide some restriction. Amazing that it can make a difference to carb performance!

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It would be wonderfully convenient if the OEM bellows could cope with larger carburettors, let alone more efficient carburettors requiring a greater volume of air.

Truth is, no such luck! The 26 mm carburettor you have needs 40% more air even if it is only of the same efficiency as an SH22. If the bellows depicted are for the smaller size SH's then that makes the scenario even worse!

Believe you me, if there were a suitable elbow/bellows 'off the shelf' for even a moderately sized carburettor (coming from somebody that has run 39 mm venturis......) such as 30 mm, I would have bought one.

That is the only reason I have been compelled to resort to making my own which, in turn, has gained me some knowledge about filter manufacture & the inadequacy of certain carburettors leading to hot start problems when filtered.

What size is the spigot on your carburettor? That is, the part to which any bellows or elbow can be affixed? What distance is between it & the side panel? Is it difficult to get the carburettor closer to the engine, as is the ideal for two strokes? Do you have any objections to the inlet manifold being downdraught? It may be that I can suggest an available elbow that can be utilised.

I am aware of the belief that a modified OEM air filter box can be made to function, but I have my doubts that even they can function when a certain size of Venturi is reached. That size will be something less than 30 mm so it would be useful (& brave!) if the sellers of such devices put their b@lls on the line & guaranteed a definitive size.

[tippo88]

Although I advocate filtration, I stress that the feed must be adequate. I did a lot of experimentation to prove that it could be done & that inevitably also proved to me the carburettors most filter friendly.

Not only do some carburettors take to filtration & require minimal re-jetting as a consequence, but they also appear to cope with any necessary convoluted intake route to their inlet.

I am uncertain what carburettor is fitted (Mikuni round slide?) or the volume/cross sectional area of the elbow that you tried, but, given the available space beneath the sidepanel, filtration is often possible.

For space, the carburettors on TS engines are notoriously tight to a panel, but a DIY 'sock' fashioned out of filter foam proved beyond doubt that a 34 mm PWK works incredibly well. That being proven, the next test will be the VHSB 34 that arrived with the engine. I fully expect it to work as well as it is a 'proper' two stroke Dell'Orto & is equipped with anti-flooding vents.

Hi, carb is a Mikuni VM26, elbow as per attached photos, I guess it does provide some restriction. Amazing that it can make a difference to carb performance!

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It would be wonderfully convenient if the OEM bellows could cope with larger carburettors, let alone more efficient carburettors requiring a greater volume of air.

Truth is, no such luck! The 26 mm carburettor you have needs 40% more air even if it is only of the same efficiency as an SH22. If the bellows depicted are for the smaller size SH's then that makes the scenario even worse!

Believe you me, if there were a suitable elbow/bellows 'off the shelf' for even a moderately sized carburettor (coming from somebody that has run 39 mm venturis......) such as 30 mm, I would have bought one.

That is the only reason I have been compelled to resort to making my own which, in turn, has gained me some knowledge about filter manufacture & the inadequacy of certain carburettors leading to hot start problems when filtered.

What size is the spigot on your carburettor? That is, the part to which any bellows or elbow can be affixed? What distance is between it & the side panel? Is it difficult to get the carburettor closer to the engine, as is the ideal for two strokes? Do you have any objections to the inlet manifold being downdraught? It may be that I can suggest an available elbow that can be utilised.

I am aware of the belief that a modified OEM air filter box can be made to function, but I have my doubts that even they can function when a certain size of Venturi is reached. That size will be something less than 30 mm so it would be useful (& brave!) if the sellers of such devices put their b@lls on the line & guaranteed a definitive size.

Thanks for your help! Tbe spigot on the carb is 44 mm. Need to check gap to side panel tomorrow. Sorry don't really understand how I would move the carb inwards or what a downdraft setup would look like ?

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[tippo88]

Hi, carb is a Mikuni VM26, elbow as per attached photos, I guess it does provide some restriction. Amazing that it can make a difference to carb performance!

Sent from my SM-G985F using Tapatalk

It would be wonderfully convenient if the OEM bellows could cope with larger carburettors, let alone more efficient carburettors requiring a greater volume of air.

Truth is, no such luck! The 26 mm carburettor you have needs 40% more air even if it is only of the same efficiency as an SH22. If the bellows depicted are for the smaller size SH's then that makes the scenario even worse!

Believe you me, if there were a suitable elbow/bellows 'off the shelf' for even a moderately sized carburettor (coming from somebody that has run 39 mm venturis......) such as 30 mm, I would have bought one.

That is the only reason I have been compelled to resort to making my own which, in turn, has gained me some knowledge about filter manufacture & the inadequacy of certain carburettors leading to hot start problems when filtered.

What size is the spigot on your carburettor? That is, the part to which any bellows or elbow can be affixed? What distance is between it & the side panel? Is it difficult to get the carburettor closer to the engine, as is the ideal for two strokes? Do you have any objections to the inlet manifold being downdraught? It may be that I can suggest an available elbow that can be utilised.

I am aware of the belief that a modified OEM air filter box can be made to function, but I have my doubts that even they can function when a certain size of Venturi is reached. That size will be something less than 30 mm so it would be useful (& brave!) if the sellers of such devices put their b@lls on the line & guaranteed a definitive size.

Thanks for your help! Tbe spigot on the carb is 44 mm. Need to check gap to side panel tomorrow. Sorry don't really understand how I would move the carb inwards or what a downdraft setup would look like ?

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I checked and there seems to be space between carb and side panel. 4 to 5 cm I think.

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