<VV> [fastvair] Carburetor Mystery

[Hugo Miller]

I didn't say compromise, I said compensate! By which I mean that the 
essential problem with all carbs are that
1) when you want to increase the quantity of fuel/air mixture the 
engine breathes in, you open a butterfly. And when you do this, the air 
will speed up much more quickly than the (heavier) gasoline. So we have 
accelerator pumps to compensate.
2) A main jet that is big enough to allow sufficient fuel for 
full-throttle operation is going to be too big to atomise the fuel at 
idle. So we have half a dozen different jets which all do different 
things. And they all have to transition smoothly.
The beauty of the SU is that it completely eliminates both of these 
problems at a stroke by making both the jet and the venturi variable in 
size, so the mixture is stable throughout the range. Such a simple 
solution to a complex problem.
When I used to play with old motorbikes, we used carbs made by Amal. 
The idle was controlled by an adjustable idle jet. Then for the next 1/4 
throttle, the mixture was dictated by the throttle slide cut-away. From 
1/4 to 3/4 throttle it was dictated by the needle position (in the 
slide) and the last 1/4 was dictated by the size of the main jet alone. 
The trick was to et a smooth transition between all four, and that is 
the problem that haunts all carbs in some way or other. Except the SU of 
course!
The SU is also the nearest thing to a programmable carburettor. You 
don't need a lap-top either. They make millions of different needles 
with all possible variations of taper, so you can tweak the mixture 
strength at any point in the throttle range by swapping needles on a 
trial and error basis. There is an interesting graph of how each needle 
varies and where, on the bizarrely named Minty Lamb website; 
http://www.mintylamb.co.uk/suneedle/


On 2020-03-21 00:38, FrankDuVal via VirtualVairs wrote:
> Then there has to be a compromise somewhere, as the SU carburetor  
> is not electronic fuel injection, running a software closed loop for 
> a
> perfect mixture at all possible operating conditions.
>
> Just saying, all carburetors are compromises. The more they match all
> operating conditions, the more passages, etc, they have.
>
> Frank DuVal
>
> On 3/20/2020 8:22 PM, Hugo Miller via VirtualVairs wrote:
>> No, because they don't have millions of passages and jets all over 
>> the place that you can't unscrew, all just asking to get blocked or 
>> obstructed! Only one jet in the entire carb, and that is kept clear 
>> because it has a big needle poking up & down through it all the time. 
>> Engineering elegance! Most carbs have all these complicated add-ons to 
>> compensate for this or for that. Because of the SU's design, there is 
>> nothing to compensate for in the first place!
>>
>> On 2020-03-21 00:16, FrankDuVal via VirtualVairs wrote:
>>> Because they never get dirty?????
>>>
>>> That can't be true..... Ha!
>>>
>>> Frank DuVal
>>>
>>> On 3/19/2020 6:54 PM, Hugo Miller via VirtualVairs wrote:
>>>> Now you know why I like SU carbs so much ;)
>>>>
>>>> On 2020-03-19 22:41, Bryan Blackwell via VirtualVairs wrote:
>>>>> Hi Brian,
>>>>>
>>>>> I'm going to go with the 'something is dirty' answer.  I had a 
>>>>> number
>>>>> of mystery issues and finally sorted out how to clean the idle 
>>>>> circuit
>>>>> and that cleared up a lot of them.  My trick is to have the 
>>>>> mixture
>>>>> needle installed but backed out an extra turn or two, then spray
>>>>> cleaner into the cluster hole above the idle siphon tube, and 
>>>>> from the
>>>>> other end back up the discharge port.  Solved a lot of issues for 
>>>>> me.
>>>>> Good luck!
>>>>>
>>>>> --Bryan
>>>>>
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