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Daimler Carburetors

Technical issues not related to a DLOC car marque, eg tyres, ethanol, other car makes, etc. and legal, political and insurance
Stan Thomas
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Re: Daimler Carburetors

Post by Stan Thomas »

Hi Chris,

Firstly, thank you for your continued interest, but my particular model of Daimler carburettor does not have an air valve for the primer. It works like this (having taken Sydsmith's advice and had some shredded wheat):

The slow-running jet is submerged in the float bowl and drillings through the carb body channel fuel to a supply orifice just in front of the throttle butterfly (inlet manifold side). On the way, the fuel meets up with another drilling through the carb body which feeds in air - adjusted by the usual tapered screw arrangement to get the right mixture for normal "hot engine" tick-over (*) - this and the throttle stop screw being the only adjustments which can be made.

When the primer is used (an enrichment device in modern parlance) a needle valve is lifted off its seat to connect the primer jet to the same drillings serving the slow-running jet, whilst also blocking off the slow-running air supply - the combination of fuel via the slow-running jet plus the primer jet increasing the fuel delivered to the supplying orifice at the throttle butterfly - which with the slow-running air supply also cut off, provides a considerably enriched mixture for cold starting.

It is because the air supply to the slow-running circuit is cut off whilst supplying this rich mixture during primer operation that the hand-throttle on the steering wheel must be open a little to produce the desired venturi effect to draw this rich mixture into the manifold -which is why if the hand throttle is opened too far or the accelerator pedal depressed too much whilst cranking, the ventui effect over the slow-running orifice is lowered, thus compromising the starting ability of the engine.

The throttle butterfly has a small block soldered to it on the inlet manifold side to shield the slow-running fuel supply orifice so as to maintain the required ventui effect over the fuel supply orifiice for small slow running throttle openings - but not when the throttle butterfly is more fully open during normal driving - then the slow-running circuit is automatically de-activated because there is insufficient venturi to draw fuel out of the slow-running supply orifice.

That is why I have reasonable fuel consumption during normal driving, but for whatever the cause, an over-rich mixture on tick over!!

Note for all carburettor setting:
If the slow-running mixture is not set correctly, the tendency is to compensate by increasing the amount by which the throttle butterfly is opened. For that reason, the slow running should be set with a vacuum guage to the highest reading (then for some very technical reason) - the mixture should then be enriched by just a very small amount.

Regards to all,

Stan.

(*) Actually, a sleeve valve cannot "tick" over, as there are no percussion parts in the engine!! (more Shredded wheat)

Christopher Storey
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Re: Daimler Carburetors

Post by Christopher Storey »

Gosh, Stan, it IS complicated ! Just one further thought, which may be useless, but I shall give it nonetheless : the description of the carb suggests that while there are 5 or 6 of those vertical tubes in the " tube block " in the main body , not all of them carry fuel by having a jet in them , but some are permanent conduits of air ( see the explanation in image 4 of 7). It seems to me that if one or more of these got blocked ( or occluded by the wrong "choke plate " being used) the mixture would be richened . Is this a possibility ?

Stan Thomas
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Joined: Sat Feb 13, 2016 2:14 pm
Location: Penkridge. Staffs.

Re: Daimler Carburetors

Post by Stan Thomas »

I'm afraid not Christopher - the holes in the choke plate a quite large and some are infact "redundant"

The carb as the facility to have up to ten jets, dependant upon which engine size it is to be fitted to. Mine has five jets. These jets are brought into play as a heavy internal brass bell is lifted by suction (same as an S.U. piston) . All jets are interchangable by size, and the amount of fuel they dispense is further controlled by the size of the holes in the choke plate. Hence the permutatuion of jets, their size and the variation in the holes in the interchangable choke plate is considerable. It was even said when I was serving my time back in the early 50's that not even the man who designed it knew how it worked.

Coming up to the present day, N.A.S.A said they would offer me a job if they fitted a Daimler carb on the Parker Sun Probe - but in the end they declined to fit one on the grounds that it was too complicated!

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