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Haynes’ World: fuel leaks cause a headache

Fuel system problems

Haynes' World is a regular feature that takes a look at what the people at Haynes are doing with their cars, bikes and other vehicles. This time, fuel system issues are affecting Martynn's bikes.

MZ Baghira motorcycle

Motorcycles: MZ Baghira 660, Yamaha V-Max 1200

Owner: Martynn Randall

We’re all probably aware that the increase in ethanol in our fuel probably isn’t good news for older vehicles, but I think this is the first time that I’ve come across fuel system issues that could be caused by it – or it could be just old age (not mine – the parts!).

I’m always on the lookout for new projects, and am lucky enough to be able to indulge my appetite for older motorbikes. 

But internal combustion engines don’t like to be left standing for extended periods without being run.

So when the first project (an MZ Baghira 660 from the late 1990s) didn’t tick over properly, and was hesitant under acceleration, I immediately pulled apart the carbs and expected to find the usual blocked jets/air holes/etc.

MZ Baghira spray

However, everything inside the Baghira's carbs looked good. So I put them back on the bike and started to look at the ignition system, checked the valve clearances and checked the compression.

Everything seemed to be fine. 

So, I started it up again, and thinking there might be an air leak between the carburettors and the cylinder head, squirted GT85 maintenance spray around the inlet manifold. 

Sure enough, spraying the area between the manifold and the cylinder head on the right-hand side, the engine speed immediately increased - definitely a leak!

MZ Baghira seal

Back off with the carbs and then the manifold. The problem was obvious: an O-ring seal between the manifold and head had split, and a bit was missing.

New O-ring fitted, and the bike was transformed. Sweet tick over, and no hesitation when the throttle was cracked open. Result.

Fuel leak on Yamaha VMax 1200

Yamaha V-Max 1200

The second project is my fifth Yamaha VMax 1200. I've always had a soft spot for these (obviously!), but they tend to be low-mileage, and locked away for long periods not being used. Consequently, it’s common for the carburettors (all four of them) to need attention.

This one is about 25 years old, and has done fewer than 1000 miles in the past 12 years. So, I was surprised when upon firing it up, it ran great!  Tick over was good and a short test ride showed no sign of hesitation or misfire. 

However, back at base, there was a strong smell of fuel. On closer inspection, there was fuel dripping from the carbs.

Being a V4 engine, removing the carbs takes a little time, and to disassemble them takes an evening. The carbs are of downdraught design, so they must be separated from each other before the float chambers can be removed.

Yamaha V-Max carburettors
Yamaha V-Max chambers

Upon removing the float chambers, the problem was obvious. The rubber O-ring seals had hardened so much, I could literally snap them in half!

Everything else in there looked good, so I suspect that when the bike went into storage, someone had done it properly, and drained all the fuel from the system. 

So was the cause of these rubber-related problems ethanol in the fuel or just age? I suspect it’s a bit of both.

Yamaha V-Max seals