Carburettor Sizing

Technical Small Block V Twins (not new V7s)
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Hi All,

I have an interesting (to me at least!) question and thoguhts about why the V50 has such small (24 mm) standard carburettors. I’d be interested to know if anyone has put larger Dellortos on, with the standard airbox and got a great result?

The background:

When the Moto Guzzi V50 was introduced, it was used as an Italian Police motorcycle, and there is a school of thought that it was detuned to cope with the rigours of high usage. It has a relatively high (for the time) compression ration of 10.8:1, has an over-square engine (74 x 57 mm) - and therefore should be able to take higher revs and consequently produce more power, as Power = Torque x rpm. But it only produced 45 to 48 bhp (depending which source of information you use).

Dellorto’s Guide to the choice, setting and use of tapered-needle motorcycle carburettors gives a graph for determining choke/carb size. It states that for a two-cylinder engine, take the bhp, divide by 2 (the number of cylinders) and read off the graph to get the range of ideal choke size. In the case of the V50, if we say it produces 45 bhp, the graph suggests the user should be looking at a carb size of between 28 and 34 mm. Being towards the lower end of this should give better pick-up at the expense of outright power (and vice-versa).

What are your thoughts on the above? Is there a reason for continuing with the 24 mm carbs, over say, the 28mm carbs?

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Interesting. Both of my early V50IIs came with VHB 26 carbs. Somewhere recently I read that later V50IIs and V50IIIs were fitted with VHB 28s and the Monza (of course!) had VHB 30s.

Ah! Remembered where. Here we go:

V50 carb sizes

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DaiG, thanks for that. It’s a bit of a minefield - there seems to be examples from history of 24, 26 and 28 mm. Maybe it doesn’t really matter if you get the jetting right. But I suspect there is an optimal set-up which will be affected by inlet port size, airbox set up and many other variables. Just trying to gain a concensus from those with real world experience before I commit my own money and time to a particular solution. I find it really interesting that Dellorto’s own tech manual suggests the smallest optimal size is 28 mm, yet so many were supplied with 24 or 26mm. I guess that is because of the other variables at play.

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