5 THINGS YOU NEED TO KNOW…
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.
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