A small positive pressure is required on the float needle valve to effect a seal regardless whether it is a top or bottom feed. If a bottom feed then fuel level will be stable as the pressure is already there, however if it is a top feed, (as I suspect), when the float bowl cover is removed, (containing the float needle seat remember), the float will rise and the level will drop giving a significantly false reading.

It would be much more accurate to temporarily introduce an extra tube to show the fuel level along the lines of that shown in Fig.2 this because all components are in their proper place and with nothing open to the air evaporation will not be an issue.

I am very happy - because (at least) someone understood what I am concerned about!

The idea, of trying to find out what the petrol-float-height of Bicarburatores with FIXED floatchambers are, is very good.
I think I remember a letter here in this Forum - some time ago - about crankcases of bicarburatores, beeing filled with gasolene - if the fuel tap was not closed emediately after a drive?
I believe this "problem" to be of great interest - even if not so many members having those "remote float chambers"...

For the moment, I had adjusted my petrol level to be just 1 mm above the calibrated hole in the Main Jet.
But the weather doesnt really allow me to do much testing!
Snow & 6 degrees minus, together with a lot of cold wind from the east.
I attach a picture of my "set-up" - and as you can see, I had "merged" two DellOrto "Trumpets" with the original, old velocity stacks from 1953.
Just to be sure, that small children wont fill gravel or stone into the motor...

Hi Lasse,

I attached several options and some photos to assist in your search on the optimum performance of your "Bicarburatore --

This is only concerning to the fuel level between carburetors and his remote float chamber-

This does not guarantee that your bike work better --

Everything related to the behavior of your carburetors is a very complex and depends on many factors,
very good riders have failed in certain competitions for this reason ..........-

In relation to the proximity of the remote float chamber toward the carburetors, his opinions are correct-

This attenuates the problems occasioned by braking and acceleration very violent-





first option

second option








1- carburetors and his float chamber are fixing on the engine -



2- remove these elements --







3- move up and down the float chamber, --


4- observing the time it begins to leave a little of gasoline inside the carburetor –


5- a one millimeter (or so) before you see out gasoline, is the level of the parts of whole –


6- reassembling the carburettor components-


7- Start the engine


8- In this instance, start playing your good ear


9- begins to accelerate, climbed or lower simultaneously the height of the float- chamber (very soft )


10- when you hear his sound more clear and clean, in differents grade of acceleration, congratulations, you have found your optimum fuel level


Or a combination of all..........







I hope this serves in a minimum form to colaboration-





Greetings

Juan

A very neat and tidy set-up horskjaer!

Very interesting comment about fuel filled crankcases; don’t remember that one but it rather goes to confirm my suspicions that compromises have been made in the quest for more horsepower.
Those intake screens are very good idea but for maximum protection of the engine, airborne dust being more than a little abrasive, I’d be inclined to fit modern air filters unless I was seriously racing and wanted maximum mass flow.

An I.C. engine is capable of creating a sufficient vacuum to lift an excess of fuel through the jet assembly even if there is a considerable negative head. The main jet is a metering device whose function is to limit the quantity of fuel ingested by the engine. To function thus the main jet needs to be submerged in order that it meters only fuel and not an emulsion of fuel and air the composition of which cannot be guaranteed. It is worth noting that upstream from the main jet the fuel may be expected to be at least partially atomised due to the pressure gradient created between the restriction caused by the main jet and the main air passage, indeed solid fuel is not really welcome here and lowering the level can be expected to assist the supply to the engine of a good mixture ratio.


One thing that could be tried, although it would be in the nature of an experiment and require the manufacture of some new components, is to move the main jet to point below the axis of the jet assembly. This would permit the fuel level to be lowered by an equal amount and thereby much reduce the risk of flooding.
I can see no obvious reason why the main jet must be in line with the jet assembly although it is clearly a manufacturing convenience. There may be a case for arguing that an in-line main jet will provide a better fuel flow past the needle jet and needle. Conversely lowering the fuel level could have positive consequences in the needle jet as a result of the reduced risk of excess fuel accumulating upstream of the main jet particularly on overrun. An accumulation at this point could result in an excessively rich mixture during acceleration.

I've a feeling this one is going to run and run!

Great thanks to both of you - Juan & UHJ200!
This is the way the forum should work - real troubleshooting - share of ideas & experiences...

To UHJ: Tthanks for your opinion of my "set-up" - I always try to have so "clean" as possible fixings, everywhere.
(The round box, by the way, is my "toolbox" - nice to have - also for extra oil)
The reason for "just" having screened "trumpets" is, that I mostly drive in fairly clean conditions, and also just think that the bike will do approx. 2000 miles pr. year - so, at the costs of durability, I prefer "free breathing" the most!

To Juan: Beautifull pictures - lucky man to have such goodies as an "Gobetto"!
You have done a lot to entlighting me - many thanks for that - I really appreciate that.
As you might know, my bike, many years ago, did some racing at "Hedemora" in Sweden.
"Someone" used a lot of money to uprate the motor: Special roller conrods - big carburettors - remote floatchamber -racing exhausts - "Z" cylinders, overbored - high compression 10,8:1 - etc.
That might explain the fact, that the float-niveau of gasolene were 9 mm ABOVE the centerline of the Main Jets whwn I got the bike?
At that time, I couldnt understand why (because it was before I started really to THINK about, what was going on inside a carb. "lying down")
When I opened the fuel taps, the gasolene started to "bleed" out of the small vent. openings in the carbs!
I thought it wrong - now I know better!
I still believe, that the "trick" is to have the gasolene niveau JUST OVER the centerline (by say, 1 mm) of the Main Jets - and NEVER forget to close the fuel taps after a drive.
Otherwise the bike will be near to impossible to start next time, because the crankcase then will be filled with oil & gasolene...

I am so lucky to be in contact with an old Swedish Racer - one that raced "hot" Rumis in anger in the fifties & sixties.
He will also guide me in this question!
In the end - I will come back to this Forum - to share my findings - hopefully succesfully - for all to learn from.

Best Regards,
Lasse,
Denmark.

Hello Lasse,



Well, I'm happy for you,

You now have a good reason for a see one winter better and shorter.

Your own experimentation, based on successes and failures, will make you find of the optimal performance point for your racer kit. ( 22 mm SSI carburetors & SS1 remote float chamber)

The perfect experimentation and more desirable, its is a returning the engine to his original condition – ( 18 mm carbs and common float chamber).

Under these conditions, you take his optimum performance parameters , and from this moment, then, you take a test and measure the performance changes with a senior race kit –

This way you will know the truth, if you expect to know by comments from other veterans riders ....... you know ....... their motorcycles were always the best and fastest. and if can not finish winner a race, is because have a problems with the other riders ....

In these attitudes are very similar in some respects to the fishermen in their comments.

Is advised for all us, subtract several kilograms and kilometers per hour a their anecdotal stories......


attached a couple of old photos --

one of an engine (Gobbetto second series) and the other shows a Rumi with remote float chamber placed far from the carburetor (The picture is not good, but I think that is correct)














Best Regards


Juan



-

Latests news - in this "never ending story"!
As I already wrote some time ago - I am in contact with some genuine old Swedish "Racers".
One of them even drove together with Gianni Zonca at Hedemora - and became friends with him.
Zonca even raced this Swedish Gentlemans "Junior Gentleman" (with remote float chamber...)
Therefore I hold this Gentlemans opinions in high regard!

He told me: "Keep the petrol level just UNDER the center-hole in the Main Jet"
I wondered why (and I guess that UJH200 also wonder).
The explanation is the following: The carburettor create a vacuum - this vacuum, at the very beginning offcourse, draw air - as the petrol is just under the hole.
BUT - one have to remember, that the very small cavity around the Main Jet is 100% airtight - so, the vacum around the jet will at once expel this tiny "air-pocket" - and start to draw PETROL.
So here is the explanation - at last!

Now I expect - naturally - a lot of comments from you - doubting this (new) theory.
I am looking forward to it!

B.R. Lasse.

I for one find little to disagree with in that; the partial vacuum formed by a turning engine is capable of lifting the fuel a considerable height.

The volume around the main jet is, I think, not too important, the suction will fill it, certainly to a point somewhere above the jet, it will just take a little longer.
Set as described it will tend to delay starting until sufficient air is drawn through. Slow starting probably matters little in the paddock but is of more importance on the road.

The fact remains however that these carburettors were made to operate with the main jet submerged and this requires the jet assembly as built to be vertical. Anything else is a compromise.

I am still of the opinion that a modified version of the banjo fitting designed so as to keep the main jet submerged and thereby work as intended would be a worthwhile project. That is as far as I can go because I do not have access to such a carburettor or the dimensions from which to produce a design.

Post subject: Three or four cylinder Rumi's

Interesting looking at these pictures in the light of the recent discussion on the subject of float bowl positioning, some are placed well above the mean, some just about where I'd have expected -- interesting!

I would be interested to hear about the starting technique on these machines and about how the engines behaved, might be illuminating.

UHJ200 - I just tried (for the first time, after many alterations/adjustments) to start the motor, with the float level a shade UNDER the Main Jet hole.
One must remember, that the Main Jet in reality, is a "metering device" - meant to restrict the petrol flow, when the throttle is fully open, and the needle is retracted!
So, IF the float level is adjusted to be OVER the Main Jet hole - then gravity-pressure will tend to force fuel through the Main Jet & Idle Jet, even if the needle is in the down position, where the motor ideally should idle...
So in my view, you have a somewhat "unstable" situation, if you adjusts the floatbowl petrol level too high.

Back to the (very exciting) first start procedure.
I opened the petcocks - pressed the tickler a short moment (to let petrol reach the hole in the Main Jet).
Then I depressed the kick-start pedal vigorously three times, with gas-handle slightly opened, - and, PRESTO, the engine fired!
The "low level" of petrol then were of no importance, as the running motor easily could maintain "raw fuel" to the Main Jets.
Also the idle system worked as it should, and reacted at turns of the air screws!

The starting procedure luckily seems to be unproblematic - also the idle adjustments sems to be OK.
So my theory about having the petrol just a shade UNDER the Main Jets - had proved its worth.
Or so I guess - because driving in anger had not been possible - because of lots of snow...

Best Regards,
Lasse.

I believe that most RUMI Club members have the book: "Moto Rumi" by Riccardo Crippa?
On the front of this book, there is a fantastic "Gobbetto".
Its owned by the renowned Swedish Racer, Gösta Karlsson (you can find a photograph of him on page 228).

He had been helping me for some while with my "remote Float Chamber" - and "Ignition" problems.
Some days ago, he was so kind as, to make a drawing, in order to explain how he always had his float level, in all of his racers - approved by Zoncas RUMI racing mechanic - so I guess it cant be better.
The advice from Gösta is, so to speak, "race proven"!

I attach this drawing - because it makes it very clear, WHERE the level should be

Important information - to all of you - who have "racing carbs"!
I learned even more from "Racing Gösta" from Sweden...

As you might see from his recent sketch - he mentioned something (in Swedish) saying: "Not all of the SS carbs have both of those idle channels".
I contacted him for further information - and then I learned the following: The Racing carburettors, even if looking just the same, came in two "kind of types" - one type with the idle system on just one side - containing air/mixture screw, together with a (removeable) idle jet.
The other type has an extra set of idle channels - the same as on the "top side" - but without airscrew and idle jet!
So, superficially looking just the same, one SS type has just the "normal" idle system with idle jet and airscrew - and its normally the kind of carb. thats used in its upright position.
My SSI carburettors - as you know - are mounted as "downdrought" carburettors - so my idle system is "on top" of the carbs. when they are "lying down"...
Which means, that the idle system, more or less, are deprived of petrol - because the system is positioned OVER the surface of the petrol!
Gösta told me to look UNDER the carb. - and he was right - under the original idle system, lurked a SECOND idle channel system - to serve the carbs. when they - as with racing Rumis, were "lying down"!
I further learned, that idle adjustment of carbs. with the double idle system channels - would be different to the "normal" types!
The throttle valve must be completely closed - and then the idle mixture screw should be opened from 1/4 turn to 1 1/2 turn, dependant of temperature, ignition, compression, fuel etc.
This procedure is a little different to the "normal" single idle channel system -where the throttle valve must be opened approx. 1 mm to obtain 12-1400 revs. - and then be fine adjusted by working the idle mixture screw.

I hope this is new to (some of) you.
I didnt know, before I heard from Sweden!