ET: Engine (Twin) Running Weak on Part Throttle

roy the mechanic

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Try polishing the needles with fine scotchbrite or a scouring pad. They tend to corrode and weaken it out. Also put fresh fuel in.
 

Bill Thomas

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I was thinking more about "Specific Gravity ", Just wish I could Spell it !.
I had a good idea when I was 16, To Tune my 250 BSA, I rigged up a lever to hold the tickler down, To let more Petrol into the Engine, When I was Flat Out, 65/70 mph.
No it didn't work !!, But good Fun. Cheers Bill.
 

Peter Holmes

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It might have worked if you had devised a way of getting more air into the engine at the same time, like some sort of fan that worked of the exhaust gas flow maybe!
 

Sakura

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Now that makes sense. If the specific gravity of the fuel is higher or lower then the float will sit higher or lower accordingly. Thank you.
I don't get this? Surely the float level is influenced by the amount (volume) of fuel in the float chamber, not the sg of the liquid? Isn't the top level constrained by the needle seat?
 

Bill Thomas

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It might have worked if you had devised a way of getting more air into the engine at the same time, like some sort of fan that worked of the exhaust gas flow maybe!
I gave up and bought a 1000cc Vincent Outfit, When I was 17, With L plates !. Cheers Bill.
 

Chris Launders

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Sakura, yes it is set by the needle seat, but you have to set the float height to match with this (valve closes when the float is level with the chamber top). You do this to set it, but in use the petrol is lifting the float and how high the float sits in the petrol depends on it's specific gravity. So if it's SG is higher the float sits higher and the valve closes with less petrol in the chamber. I know the floats can be different as I had a new one that was fouling the float chamber before it lifted the needle off it's seat, this is why I had contacted Amal and was told about the float height and alloy float needle scenario.
 

greg brillus

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On both the 276 and 289 carb's the lower brass cap nut that holds the jet block into the main body come in various heights as well. On the 276's I have 3 different types all differing in height......... This will have a large impact on the float level............. More than meets the eye here.
 

mercurycrest

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(The carbs were refurbed by Martin Bratby who made the slides, the cutaway measures somewhere between 3.1/2 and 4.)
Why not do the obvious and try 3.1/2 slides? With the engine running and riding down the road, the specific gravity of different petrols isn't going to be any kind of issue at all.
 
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