Carburettor cleaning and refurb – 3 Sep 2016.

Removal and dismantling.

In my last post I had got as far as removing and partly stripping the left hand carburettor on The Fire Bike. I asked for advice about removing the float pivot on the Yahoo Loopframe Group. As usual, I got sensible advice from someone who has had the same issue.

There was the slightest gap between the float and its mounting exposing about 1.5mm of the pivot rod.

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I pushed the float to the left and used a small pair of side-cutters to grip the pivot rod. I was able to move it a just tiny bit at a time.

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Eventually there was enough of the pivot sticking out for me to pull it the rest of the way. It hadn’t actually been tight but just hard to get a hold of.

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Having released the float I gave it a shake but couldn’t hear any fuel inside. The end of the float needle-valve had an indentation in it but didn’t look too bad.

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Peering into its housing at the seat, I could see it was very dirty and was likely the cause of the flooding.

I put the float in a bag in the freezer for a few hours then put it in some warm water. The idea is that, if the float has a leak bubbles will be produced as the air inside warms up and expands. My float looked to be OK. As a second check I wired a weight to the float then dropped it in a jar of petrol.

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I left it for the best part of a day. I didn’t see any bubbles and, when I retrieved it, no fuel seemed to have got inside (shaking it and listening again). I decided the float wasn’t the issue.

I stripped everything else from the carburettor and dismantled the accelerator pump. I had to spend time getting this apart as everything was gummed up.

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I didn’t take photos as I went along but there is a good guide to rebuilding a Dellorto VHB carb as fitted to both my bikes here on Greg Bender’s This old Tractor site.

Once the accelerator pump is out of the carb you should be able to put your finger inside the carb venturi and push the atomiser out. It’s the part the main needle goes through. On this occasion it was stuck. It’s made of brass so I used a plastic tent peg to drive it out.

Having now reduced the carburettor to a pile of parts I decided to remove the main body from the inlet manifold. I held the manifold in the vice with a couple of bolts and washers.

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Then knocked the carb off the manifold using a bit of plywood. Alternating sides in an attempt to keep it straight.

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You can see that I was careful to only hit against the end of the carb flange. It became easier once everything had started moving.

I went on to remove and dismantle the other (right hand) carb in the same way. This seems to be in much better condition. It doesn’t show any corrosion and came apart properly as well.

Carb Spec.

I have already said that I was surprised to find that the main jets fitted were quite a bit smaller than those quoted in various manuals (including the factory one). They state a 145 jet should be fitted while I have 135s.

Finally I checked the 1971 parts book for the V7 700. The main jet is given there as;

Guzzi part no. 12 93 57 01, “Getto massimo”, Dellorto part no. 1486.135.02.

So that would suggest my 135 jets are correct.

To be certain I found a parts list for a 750 and that shows the main jet as Guzzi part 13 93 57 00, Dellorto 1486.145.02. So they are different.

For the record the following parts are fitted to my carbs.

  • Main jet – 135
  • Pilot jet – 45
  • Enricher (starter jet) – 80
  • Needle – V5 in second notch.
  • Atomiser – 265
  • Slide – 7454-60

Cleaning and reassembly.

I ordered some parts to rebuild the carburettors from Eurocarb.

  • 2 gasket sets
  • 2 float needles
  • 2 choke (enricher) pistons
  • 1 choke piston return spring (as one was deformed)

Then from Gutsibits I ordered 4 inlet manifold gaskets.

While I was waiting for my orders to arrive I started on the cleaning.

I assembled a supply of rag, cotton buds and cocktail sticks and cleaned everything with carb cleaner and compressed air as per the guide on This old Tractor.

Some of the O-rings were so hard I cut them off.

Advice from the Yahoo Loopframe Group was to clean the seat for the float needle valve with a cotton bud plus fine grinding paste. I decided that Solvol metal polish was probably less aggressive and used that. Afterwards, I made sure any residue was removed with carb cleaner and compressed air again.

Today the parts I ordered were here (next day delivery from both suppliers) and I put both carbs back together as per the guide.

New and old enricher (choke) pistons.

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The replacement “choke” return spring is different to the 44 year-old original. Its shorter but stiffer.

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While reassembling the carburettors I checked the float heights as per Dellorto’s instructions. Elsewhere I have seen advice to set this with the carb inverted. However, as the VHB is a carb with connected floats it should be done this way – inlet downward.

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The setting is 24mm +/- 0.5mm and neither carb needed any alteration despite how it looks in my photo!

Testing.

The final job for today was to put some fuel in the left hand carburettor to see if it still leaks fuel from the air vent.

I carefully mounted the carburettor in the bench vice. There’s hardly any pressure on the carb bowl here. I added a short length of fuel line and a syringe body to hold the fuel.

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There was no leak from the vent hole. I added the plunger to the syringe and put the fuel under pressure. I couldn’t force any more fuel in or cause a leak. I think that’s a result! Just got to refit and tune everything now.

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