Atomisers ?

[Scooterdude]

Was wondering if anyone could explain the role of the atomiser within the carb, obviously we change the main jet that screws into it to suit the given situation but why the atomiser?

[garry inglis]

an atomiser is there to disperse fuel and atomise it before it enters the manifold and into the combustion chamber to go bang in the head

[Adam_Winstone]

Atomisers are the drinking straw that the engine sucks petrol up and out of the float bowl with. If you imagine the factors that control how hard you have to suck to get liquid up a straw, the same applies to atomisers...

If the straw is thin then you do not have to suck very hard to lift the column of fluid. If you had a straw that was 3 inches across then you would have to suck damned hard to get the fluid up the straw.

Think the same with a long straw and a short straw, both of the same diameter, and the suck required changes.

Then imagine that the straw has holes in the side, the more holes or the bigger the holes, the more suck required to lift the liquid. The height of these holes also has an influence.

The imagine that suck is not a problem and you have plenty of suck to raise the liquid with ease... then the size of the hole becomes the limiting factor. The bigger the hole, the more fluid you can get through it.

Now imagine all of these factors!!!

As far as the function in a Lambretta engine, the suck is controlled by the cc of the engine and the speed that it spins (RPM). The atomiser needs to have the correct profile of each of the above elements to make sure that it sucks up, and into the engine, the correct amount of fuel for that engine size and at the right revs. Thankfully, the atomiser does not have to do all of this on its own, with other factors having influence too; float height, pilot jet, slide, atomiser, needle profile and main jet.

When a carb has a needle, this allows for additional adjustment for many of the throttle settings, however, the standard SIII Lambretta carb does not have a needle, which makes it incredibly important to get use the right atomiser for the demands of any one specific engine. If not, you can be rich or weak at various stages of throttle progression. Get it wrong and you'll have a rich bike that is hard to ride or a weak motor that may seize, hole a piston, etc.

If you're thinking about the atomiser role in a needle type carb, especially a modern carb (PHBH, VHSB, VHSH, TMX, etc.) then you can try a very informative test where you actually remove the main jet altogether and still have the ability to ride around normally from tickover to 3/4 throttle with no main jet fitted! However, look to go to 3/4 throttle or above and the motor will (should) instantly flood and try to cut out, unless you are quick and close the throttle to below 3/4, in which case it should return to normal running. Funnily enough, I did hear a tale about someone riding a newly built/finished kitted smallframe Vespa to IOW who found that the bike tried to cut out every time they opened the bike to 3/4 throttle. Not wanting to miss the rally, and assuming that this was an issue with the newly built motor, they kept below 3/4 and rode the distance at low throttle positions/speeds. Whilst not having twigged to this being a carb issue, thinking it might be an electrical issue or issue with the kit, they stripped the carb to check that it didn't have water in there or some other simple carb/jetting related issue... only to find that a main jet had never been fitted (lol)! Quite, whether this is an urban legend or not, I cannot say, but having ridden my bike around normally with no main jet fitted, during testing of atomiser and needle position, I have no doubt that it could be true.

Atomisers... very important as they control the biggest range of throttle progression (influencing 1/4 throttle to 3/4 if your motor is jetted correctly and 1/4 to full throttle if the atomiser is a restriction to main jet flow)!

Sorry if the above reads like the ramblings of a madman but I hope that it helps a little to understand the function.

Adam

[garry inglis]

That's what I said Adam you just copied my statment

[Adam_Winstone]

... pretty much, I just like to waffle... but that's probably not news

[Scooterdude]

Wow, great reply Adam thanks for taking the time to write all of that you've explained things well. So in essence if i took the main jet and atomiser out I could start the engine and Rev it to a point at which it would cut out then make a mark on the throttle, then do the same again but with the atomiser fitted but without the main jet... And again make a mark then repeat with the main jet. So now I have two definitive marks were each process takes over from the former so based on these settings I can now do plug chops to correctly jet at each given rev range??

Or in other words if I had say a 120 main jet I would be getting exactly the same reading flat out weather I was using the smallest or largest atomiser Dellorto produced?

But one thing still confuses me: we're does the mixture screw fit in with the grand scheme of things??

[garry inglis]

come on adam i can t wait for this

[Adam_Winstone]

Unfortunately, progression stages do not follow one after the other, rather, they overlap and have influence on each other, meaning that you don't get a clear cut position where one has sole influence. See link:

http://www.iwt.com.au/images/effects3.gif

It would help me to know what model of carb you are adjusting so that I can better tailor my responses.

With regard to the main jet - Too rich an atomiser can cause a motor to bog down before reaching the full throttle rev range that it would reach with a weaker atomiser, meaning that you can never fully assess how it would run on main jet alone (full throttle). Likewise, a restrictive atomiser can limit the fuel flow so much that it doesn't matter how big a main jet you fit (even removing it completely) as no more fuel can flow through the atomiser.

Hopefully, it is becoming clear that you cannot consider any one progression stage in isolation as each has an influence on the next. As such, carburation is about harmony of all stages, from tickover to WOT (wide open throttle), rather than thinking addressing any one stage can resolve all problems.

Mixture screw simply fine tunes flow from the pilot jet for tickover and progression from tickover up. I actually believe that the Mikuni link overplays the range of influence of the air screw and mixture screw adjustment. Yes, it may have some influence but by the time you are starting to open the throttle more you should be looking more towards the carb parts that have more influence on that progression stage; pilot + adjustment screws, combo of previous plus slide value, slide value with atomiser, atomiser and needle shaft (if a needle carb), atomiser and needle profile + needle clip position, needle profile/atomiser/main jet, main jet alone. Don't forget though that one bad stage can stop your motor from progressing past that stage and give you a false appreciation at the next, e.g. previous comment about bogging a motor at 3/4 throttle so that you cannot correctly evaluate main jet size.

Adam

[Adam_Winstone]

For a better understanding of throttle position and testing to see how you are jetted, do a search on here (or LCGB) for 'choke tests' which is the only way that I ever evaluate my jetting, for each and every marked throttle position.

That's not to say that I don't have bikes dynoed every now and again though (wonderful tool in the right hands)!