Hi Everyone,
Just in case you don't visit the other place, here is a duplicate post of mine:
Well, I’ve been doing some research into carburettors fitted to the NF’s and have found some interesting snippets (I must have too much free time!) particularly involving the differences between the Civilian and Military models.
I came across a web site (
http://www.thisoldtractor.com/topics.html ) which has various Guzzi handbooks for both models and in several editions, and also Dell ‘Orto data.
Both models use Dell ‘Orto 29mm carburettors with 265 Atomizer and 80 cold start jets.
The Military model has a VHB (A) carb, with a #60 slide, a 132 main jet, a 50 idle jet, and a V10 needle.
The Civilian model has a VHB (CD) carb, with a #50 slide, a 122 main jet, a 55 idle jet and a V5 needle.
So, what are the differences? The main one seems to be that the VHB (CD) carb has a piston type accelerator pump, the VHB (A) does not.
Comparing the two carburettors up to about a quarter throttle, although the Military carb has effectively a larger opening because the V10 needle is slightly smaller in diameter, the #60 slide has more cut-up which will result in a lower vacuum and so draw in less fuel. In the case of the Civilian bike, although the needle is slightly larger in diameter and so restricts the atomizer jet more, the idle jet is also larger, and as the #50 slide has less cut-up, the increased vacuum will suck through more fuel. I am inclined to think that one set-up balances pretty evenly with the other at these small throttle openings.
As the throttle is opened up between a quarter and three quarters, the tapered needle takes over the fuel metering. The V10 needle has a shorter taper than the V5 and at both extremes, the figures imply that the Civilian bike runs slightly leaner than the Military at constant speed. The Civilian will get enrichment during the actual turning of the twist grip because of the accelerator pump.
Above three quarters opening the main jet is responsible, again the Civilian running leaner than the Military.
It could be that as the Military bike would generally be used on slow patrol and escort work with a restricted flow of cooling air, it was decided to run it with a richer mixture to avoid over heating, while the Civilian model could run leaner as it would generally be out on the open road. In this case the pumper carb would also be an advantage.
So, why the interest and calculating? Well, as some of you know, when I bought my Military NF, it came fitted with a 30mm VHB(CD) carb. The throttle slide cut-away (a #40) implies that it came off a Sparda. Following suggestions given in a printed download that came with the bike, the carb had been fitted with a 55 idle jet and although the suggestion was for a 140 main jet it was actually fitted with a huge 145; the print-out having been amended in pencil. There was no reasoning given for these recommendations – they were just stated as starting points for experimentation. The standard jets for the Sparda is a 125 main, a 50 idle and a V9 needle, together with a 265 atomiser, one carb filling a 500cc cylinder. Bearing in mind the low cut-up of the slide, the 50 idle jet should be perfectly good.
Where to now? I did 250 miles last weekend going to the MGCGB AGM, and was slightly disappointed that getting above 60mph was pretty impossible, even though it was delightful pottering along at 45 to 55. I think I will take the original carb off to inspect it, clean it and check wear on the atomiser and needle. If I need to order new parts I will also get a couple of new jets for the 30mm carb and do some experimentation with that, particularly as I intend to use it on the open road rather than patrol work! I don't think the extra millimetre will make much difference!
Just as an aside, in all the manuals I have recently seen the tappet clearances have been given as 0.1 and 0.2mm rather than 0.05 and 0.1. Perhaps the experience of valve burning prompted the change?
Best wishes,
Susie
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