Impulse Hose

[bmp01]

I think it's good people take the time to explain how stuff works. I think how it's presented makes a huge difference to how easy it is to understand. I'm inquisitive enough to have messed with old SU carbs in the past and more recently 2 stroke carbs. SU's were about as simple as it comes so it was an easy introduction, adapting to 2 stroke carbs was then a relatively small step.

But here's the thing - I understand a 2 stroke carb and therefore I can read what other members write (or 'cocks' if you prefer - ref posts above) and decide if its correct or not. To a newbie, a carb might be a thing that mixes fuel and air ... it will be a lesson just to appreciate it has a minature pump in it too. Talk of 'metering diaphragms',  'needle valves' or any other important widget isnt necessarily going to cause a light bulb moment.

I like the YouTube video posted by Peatff, it assumes little or no knowledge but does go into some detail. There's no way anyone can match that with words alone..... Ought to be a sticky IMHO.

 

 

Edited November 16, 2018 by bmp01
Correction

[bmp01]

I understand a carb needs compensation to cope with clogged air filter. That's because a clogged filter causes  a depression in the airbox, which adds to the depression in the carb making a bigger depression. And a carb supplies fuel in proportion to that depression. So less airflow through the engine (due to clogged filter) but a carb which is being asked to supply more fuel..... so too much fuel in the air - fuel mixture.

This is improved upon by changIng the reference pressure seen by the metering diaphragm. Normally the reference pressure is ambient pressure (that's why the metering diaphram cover has a hole through it). On a compensated carb the reference pressure is taken from the airbox (or filter housing) where the small depression exists. That in turn reduces the pressure difference across the fuel circuit in the carb so less fuel is supplied to the engine. 

Worth remembering the engine is still derated with a dirty filter because the engine can't pull the same air (and fuel) through it compared to a clean filter situation.

 

I don't understand why altitude affects engine fueling as it should affect air flow AND the reference pressure on the carb, so less air and less fuel.

 

And I really don't understand why you need to adjust the carb (add fuel) for a tuned engine. Ok, so tuned engIne needs more air, but if the carb worked properly, it should add in more fuel with out manual intervention.

 

Are carbs just operating in the constraints of size, weight and cost and are therefore not perfect devices? Did some one mention Fuel Injection....

 

Food for thought. 

Edited November 16, 2018 by bmp01