T3 Carburation

Hi Peeps, a little assistance required regarding the above.

The bike has been returning 32mpg and showing a rich-running rh spark plug since I bought it a couple of months ago. I am fairly happy that the cables are set up well enough and that the chokes are seating properly.

My next place to look is the air filters and floats, so I whipped the carb’s off this afternoon. It has K&N’s so I’ve given them a soak in Fairy, rinsed them off and left them to dry overnight. In fairness, they both looked okay to me.

It has 10g floats (see pic), which the manual says should have 18.5mm height from the carb edge. They are both set at around 26.5 and I’m not sure there is enough room in the bowl to reduce them to the correct height! Also, uncertain whether to bend the tang to each float or the fork holding the jet. Help!

Finally (and apologies for the long post), I had a Quick Look at the observable jet sizes. The idle is a 50 as per the manual. But the main is a 135 (over and against 120 per manual). Could this be the reason why I am returning such low mileage?

I am confused because all of the above is true of BOTH carbs, but the lh side appears to work well.

All helpful suggestions are gratefully received. Thanks in advance.


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I had this problem and it turned out to be a worn accelerator pump in the side that was running rich.

The pump itself is very crude - but it was the root of my problem.

IIRC - it was 40 years ago - I replaced the whole assembly - tube, spring and plunger.

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Many thanks for the reply.

What you have said rings true.

Is this part also referred to as an ‘atomizer’? I’m new to this kind of thing and need now to find a source for these old parts.


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source: Eurocarb - pretty much everything for the ‘old’, square VHB30 carbs remains available. Not 100% everything, but close enough.


And, crucially, “the little ball” (o;

@Scruffysteve - there are useful VHB30 diagrams, descriptions, tables and ‘rebuilding’ resources available on line, not least on ‘this old tractor’, eg. Carburetor - drawings, charts, and a how-to guide. - Technical - guzzitech.dk archive - courtesy of Jens Lyck

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I would also recommend Eurocarb for spares, if they don’t have what you need, then Ducati paddy is another place to try. Needles and needle jets can also wear over time and may be worth renewing if your bike has a high mileage.
This is the assembly.

Ooh. I think there’s more to it than illustrated, @Don-Spada !

That is the parts that get blocked up and cause bad running. There is a small plastic ball that lives inside the centre part, but mine does not come apart. I was blocked up some years ago and I cleaned it by working through the small holes in the side with some fine wire and plenty of carb cleaner.

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That’s brilliant - thanks! I understand there is also a steel ball in there too.

I’m hoping to dismantle mine later today to see if there is anything obviously missing/sticking before biting the bullet and ordering from Eurocarb.

Thank you all for the input.



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The ball inside the little barrel shaped part is plastic. It is easily removed & cleaned.

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…and easily lost, too (but the ball, alone, is available from Eurocarb!)

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I had a similar problem - turned out to be a worn needle seat - these aren’t replaceable on the T3 carb

Hi Steve

The picture should give you the info you need . Both taken from a genuine Moto Guzzi T3. The increase in Main Jet will be to try and cover up the weakness in the midrange due to the K&N’s. Two suggestions. Change the Atomiser to enrichen the midrange and bring the main Jet closer to std, maybe 1 size up from std OR ditch the K&N’s and put the std airbox back on and put back to std jetting etc. This is of course assuming no other issues like pencils in the combustion chamber. Sorry, couldnt resist!!!

If you need dellorto spares and Eurocarb are out of stock i have a pretty good range There should be a little plastic ball in the picture from Don.

Regards, Nik

Hi Nik, thanks for the input. I’ve checked for the pencil and can confirm that I am now definitely running on unleaded :rofl:.

I dismantled the carbs yesterday. The little balls appear to be fixed inside the piston assembly on my bike. I’ve cleaned them and ensured that they rattle freely.

Time to re-assemble the bike today. Yesterday’s debacle involved fuel p*55ing everywhere after I installed inline fuel filters whilst it was in bits!

Thanks for the diagram. I’m going to leave the floats where they are for now as the levels are the same and my problem is only on one side. Trying to make one change at a time.


Playing cards or greeting cards? :rofl:

Oops! Well spotted ……

Not sure if this has been mentioned but on my old Spada, sometimes a choke plunger would stick open slightly, leading to the symptoms you describe.

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Good shout @rogershuff - those things might almost be considered a “service item”. Some say: if the rubber insert on the choke is significantly indented, they can be gently prised out and turned…

I’ve decided to bite the bullet and ordered a couple of carb overhaul kits from Eurocarb. It might be a few weeks before I get round to doing the work, but I will report back when I’ve resolved this issue. Cheers, Steve

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Please do. Learning what worked and what didn’t is essential to providing ongoing support to other Guzzisti.