Quote:
Originally Posted by willk
Well, that's the thing - I've really only used the engine (many times) to travel a couple of hundred yards from a cold start. So I'd naturally start with a bit of choke and then back off - but I'd expect to do that as the engine warmed anyway - I may simply have missed the issue through having a touch of choke on.
|
Aha! So now we've got a whole lot of doubt to deal with! It's best that you know exactly what's going on in the operation of the carb - I'd like to give you the benefit of the doubt but you said something about lifting the slide and that is not what you have to do so I'm gonna go ahead and explain how a slide carb works then you can choose what to do.
Firstly, the needle is a taper and though it looks even it normally isn't, it tapers in short steps. Different needles for different applications have different diameters down their length to match the engine requirement for fuel for that engine.
The needle fits into the jet, the jet is a fixed diameter. When at idle and the mixture is correct and the needle taper is correct, when the slide is lifted it lets air into the engine and the correct amount of fuel is let into the carb venturi through the annular space between the jet and the needle.
Now, when something in the engine changes from the exact manufacturing tolerances, the fuel requirement will change and this appears to be where you are with your engine. If the fuel requirement changes then the needle you have in the carb will not be an exact fit within the jet to provide the required mixture.
Given your circumstances of use for this engine, the fact that it's plainly short on fuel, you've checked it over and it appears to be ok and that there is an option to richen the mixture built in to the carb, then, imho, you should use this option and see how the engine performs before starting to mess around with it in other ways.
Your option is to lift the needle in the slide so that it provides a bigger annular space around the part of the needle in the jet so letting through a little more fuel. Since the slide will be in the same position as previously then the air flow will be as before so with the lifted needle the mixture will be a little richer. Note though that the mixture will be richer right through the rev range not just at idle. That's why you have to give it a try and see if you're happy with the way it performs generally.
I suspect that since the air and fuel requirement for such a small engine is so little and there is manufacturing tolerance and inevitable useage wear, keeping that fine balance between air and fuel will be difficult throughout the engine's life, especially so when there in no variable mixture adjustment - it may even have been marginal when the engine was new and you've just tipped the balance too far with engine useage.
The fact that there is three mixture settings possible suggests that it is known that there is the likelihood for some adjustment to be necessary in some circumstances.
Lift the needle and give it a go.
If lifting the needle proves to correct the idle mixture but provides an overly rich mixture from the rest of the throttle opening then there is a few things to be looking at... but leave that until you've tried it.
BLACK PIG
Piglet
Sula
