What's the problem?

Bike not running right? Our experts have the answers to the toughest questions

vHonda NC30

Q My Honda NC30 stopped running which I traced to a low compression issue. I have undertaken a full engine rebuild, but I just cannot get the bike started easily and often not at all. When I do get it running, the front two pipes get hot very quickly but the back two (especially on cylinder three) take a while to get to temp or just don’t run at all on idle but heat up straight away when I rev the engine. When I pull the choke the engine bogs down rather than revs up.

I have checked the timing, I have 150psi of compression on all four cylinders, checked for a spark on all four plugs, replaced both coils, set all pilot screws at two turns out, vapour blasted the carbs and ultrasonically cleaned them and made sure the passageways/jets are clear, and set the float height as per the manual.

A Gary Hurd says: Sounds like issues with the pilot jets, particularly given that the bike behaves better at higher revs. Look at those, then check the carb balance using vacuum gauges. If none of this yields any joy, it might be worth double checking the cam timing, and also the valve clearances.

Honda CBF500

Q The cylinder head on my Honda CBF500 has been newly rebuilt. Now the bike starts easily, revs to the redline, and doesn’t seem down on power. Spark plugs are brand new.

However, particularly when cold there’s a puff through the intake on cylinder number two. I’m told this suggests a sticking valve. It is intermittent, and largely disappears when warm and higher in the revs.

Any ideas? I’ll admit the fuel was at least three months old. Could one valve clearance be too tight? Dirty valve stem?

A Gary Hurd says: I’m thinking it’s lean on that cylinder. Check for an air leak. If that isn’t the issue, try turning the pilot screw out a quarter turn. You could try fresh fuel but that doesn’t explain why the problem would only be on one cylinder. There’s also the chance that you have a suspect ignition coil.

Kawasaki GPX750

Q My Kawasaki GPX750 has a 16-inch front wheel. The tyres usually available are normally 110 or 120 section. However, I’ve found a 130 section sports tyre for a good price.

Will it be worth fitting? Or is a 130 no good on a bike that age?

A Paul Fairclough says: I’m guessing you’re talking about a 130/70 16 – the early Honda FireBlade fitment. Your Kawasaki’s rim really isn’t wide enough to take the 130. When it’s squeezed onto the rim, you might also find you have clearance issues with the mudguard.

Q I kept my battery on my bike connected to a brand-name ‘smart’ charger but it still packed up. Am I missing something?

A Ferret says: It could be the battery rather than the charger. That said, here’