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Carbs run­ning poorly? Here’s what you can do...

Is the fuel get­ting in?

One of the big­gest stum­bling blocks with carbs is com­mon sense and logic. Peo­ple of­ten don’t en­sure a clean, un­in­ter­rupted fuel sup­ply. They might rely on the fil­ter in the tank, or put sealant in to bind up the rust. They’re not stop­ping to think how things work, and track down prob­lems by a process of elim­i­na­tion. It’s best to as­sume that all fuel tanks – no mat­ter how new, or how much time has been spent on them – are full of filthy stuff that’s go­ing to get into your carburettors and block them. So put an in­line fil­ter in. That’s prob­a­bly the most im­por­tant piece of ad­vice.

Can I sort the tank?

As I don’t know of a good tank sealer I would rather have a tank full of rust, and stop the rust get­ting through with a fil­ter. Ob­vi­ously do your best to get the rust out, and try to stop it get­ting worse, but as­sume you haven’t re­moved it all. I think sealants of­ten make things worse. Many’s the time we’ve sold a carb and it’s come back not work­ing, and it’s got sealant block­ing the pi­lot jets.

Don’t leave it stand­ing

Modern fu­els cause prob­lems I man­age to avoid trou­ble in my own bikes by us­ing them ev­ery so of­ten. The prob­lems come from leav­ing a bike stand­ing: the ethanol in petrol at­tracts wa­ter. Some­times the cor­ro­sion at­tacks the casting: I’ve seen the pil­lars for the float pivots dis­ap­pear! Brass jets can cor­rode too, or go green, or get seized in.

How to trou­bleshoot

In gen­eral you’re look­ing for a small hole that’s got blocked up. That will usu­ally be a gallery on the pi­lot cir­cuit, or the pi­lot jet, or some­times the starter jets or ac­cel­er­a­tor pump jets. Or you are look­ing for wear in the mov­ing parts. The favourite is prob­a­bly float valves. If they’ve been stand­ing they might be cor­roded, or if they’re rub­ber the tips can go hard. At high mileage they wear out: the float arm moves in an arc and the valve goes up and down, so the tip wears oval, and won’t lo­cate quite ver­ti­cally. Then there’s the nee­dle in the nee­dle jet get­ting pushed back and forth by the pulses in the in­let tract. That wears the jet oval, which makes the en­gine rich and woolly.

Get­ting des­per­ate?

Bike man­u­fac­tur­ers don’t like us to of­fer spares for orig­i­nal carbs, but some parts we sell may fit. To re­place pressed-in parts you need to make a draw bar to get them out, and have a friend in a shed to make new ones. Or even just find a nee­dle that’s a bit thicker! We can al­ways sup­ply a bank of Kei­hin or Mikuni flat slides, which you can adapt to the air­box or use with pod fil­ters. You can even build your own air­box – it de­pends how much ef­fort you want to put in.

In­spect the nee­dle for wear. You could get it re­placed

Modern fuel gums float bowls

Check for a blocked fuel fil­ter

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