SH carburettors main jet bleed

[Storkfoot]

Am I reading this correctly in terms of the influence of the main jet? The main jet will only influence running over, say, 3/4 throttle and not before?

[Adam_Winstone]

Isn't that the same for other carbs too, with the main only having influence below 3/4 if that influence is that the main is too small and poses a restriction when it shouldn't be?

SH carbs are slightly odd though because the atomisers do not function in the same way as needle carbs as an atomiser with a bigger diameter central drilling can be weaker for most of the range than one with a smaller drilling, which takes far less air-flow over the end to suck up the column of fuel (easy to suck through a thin straw than a big diameter one).

SH carbs are quite different to get your head around their function. When others ask 'which is richer?' that show that they have yet to grasp their function as the question should be 'which is richer at what revs or air-flow?' as that's the function of the size, number and vertical position of side drillings... all being influenced by diameter of central drilling.

NB: it is very common to find that 200 (5899-2) atomisers provide too much too soon, meaning that the correctly sized big main for big motor will flood out at 1/2 to 3/4 and never get to WOT. In which case you often find that a weaker atomiser keeps it weak enough through the rev range, without begging, to then run correctly at WOT with the bigger main. A common example is 225s blogging out with any main over 123'ish and -2 atomiser but will happily run the weaker GP150 -4 with much bigger main 125-135, running clean throughout.

SH... not as simple as rich vs weak. Rather, rich vs weak where?

[Storkfoot]

Thanks for the reply, Adam. I just wanted to clarify this point before I do a couple of plug chops. It is a well run in TV175 S3. I do not anticipate altering it from the standard jetting of 50 pilot, 5914-1 slide, 5899-2 atomiser, 106 main. Although, revving it on the stand yesterday for 15 minutes, or so, it was still pretty smoky in the mid range.

[Adam_Winstone]

I wouldn't be surprised to learn that you have to down-jet the pilot, which I and others have found even with standard 175s. You only have to look at Innocenti's own progression to see the drop to 48 and 45 pilot on the 200 models.

Likewise, the SX200 is basically the big brother of the TV175 S3, yet main jet dropped, whilst atomiser and slide remains the same. However, whilst I would be quick to drop the pilot, I'd hesitate before dropping the main until higher speed testing can be undertaken.

Good luck getting it sorted.

[Storkfoot]

Yes, as you know it is the only Lambretta running an SH with a 50 pilot. I suppose it was an early Series 3 and the factory took a cautious approach with their fastest model, at the time. I’ll try a 50 pilot again and then a 48. I’ll do a couple of plug chops at WOT on the standard main 106 too.

Thanks again.