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FZ1 2nd generation Fuel Snatch Problem

From Bike magazine (article since deleted from their web site)

Reports from the Fazer launch at the end of 2005 said the bike had a snatchy power delivery. It’s an ECU programming flaw, said some, or heavy throttle springs, crude fuel injectors or even the crank (40% heavier for ‘better traction’ than the snatch-free R1, with which the Fazer shares most engine and fuel-system parts).

When Bike’s long-term Fazer arrived, it too had a snatchy throttle. I learned to ride round the problem so it didn’t bother me, but plenty of owners were less accommodating (while many had no problem at all).

And it’s worth mentioning that the Fazer isn’t the only one – many fuel-injected bikes suffer. Suzuki’s GSR600, BMW’s latest K and F series, and even Kawasaki’s new Versys are all tricky to ride smoothly.

Anyway, we promised to find a cure for the jerkiness that sometimes troubles Yamaha’s otherwise magnificent FZ1 all-rounder. So here it is.

What’s the problem?

It’s a hesitation opening the throttle from fully closed, at its worst mid-corner when bringing the power back in. No matter how smoothly or gently the throttle is opened, the power comes in suddenly, jerking the transmission. It’s unlikely to cause a crash, but for some riders it’s deeply irritating.

Not all riders feel it, because the snatchiness only happens when the throttle is opened (from closed) above 4500rpm,

so it only bothers riders revving the engine down through the gears. An example would be braking into a turn, going down the gears on a closed throttle, with revs dropping but above 4500rpm. Open the throttle at the corner apex and power comes in with a lurch/snatch/jerk. But if the revs drop below 4500rpm beforehand, there’s no problem.

What’s the cause?

The snatch only comes at the transition from closed to open throttle, and only above 4500rpm. And it’s a dramatic change, not gradual. So this rules out a heavy crank or stiff throttle springs – their effect would be gradual, not instant.

Next, an examination of 2006 R1 injectors shows they’re identical to the Fazer’s – four holes per injector. No snatch on that bike, so injectors are ruled out. Which means the problem is something else.

A brief history of fuel injection

Fuel injection systems (efi) replaced carbs because they’re cheaper, take up less space, are lighter and offer more control (maintaining high engine performance while regulating emissions). But carbs are very good at smoothing out (or interpreting) ‘clumsy’ rider input and letting the engine respond in a natural manner.

This turned out to be efi’s biggest problem. Early injected Hondas (VFR800s, SP-1s, FireBlades) had crude throttles, while others, like Suzuki’s GSX-R750, moved seamlessly from old technology to new. Some manufacturers concentrated on increasing the fidelity of injector ‘mapping’ (the set of instructions given to the injectors) to match the response resolution of carbs, and some even developed efi systems to ‘imitate’ carbs (Yamaha used a cross between carbs and efi to smooth out throttle response on the first injected R1).

Frantic development meant that by 2003 manufacturers had all but eliminated throttle snatch from their efi systems. So why has it suddenly come back now, to blight the Fazer (among others)?

So what’s the cause? (Part II)

Fuel injected motors are getting snatchy again because emissions legislation is toughening (see p22). That means engineers have to make engines cleaner, and an easy way is to reduce the fuel swilling around inside… like when the throttle is shut (because you’d think engines would only need fuel when they’re working – when the throttle is open).

Yes. But no. Obviously an engine needs fuel on a closed throttle at tickover, otherwise it’d stall. But it’s also good to fuel an engine on a shut throttle from speed, ‘coasting’ down through the rev range.

At tickover, carbs trickle fuel into the engine on a closed throttle. But they also add fuel when the engine is coasting down through the gears on a closed throttle. You might think this is just an environmentally unsound waste of fuel, but it’s crucial to smooth throttle response.

On an efi engine, tickover fuelling is handled by injectors, but the engineers turn them off on closed-throttle coasting. Nobody should notice because the throttle is shut – so the rider won’t care – and there’s less fuel, unburned or otherwise, in the exhaust mixture.

But if you cut off fuel to an engine spinning at, say, 5000rpm (with pistons whizzing up and down 40 times per half second), the motor purges itself of fuel (in fact an efi motor purges itself of everything – with the throttle closed there’s no air either, which creates a vacuum, a problem in itself).

So now the inlet tracts and combustion chamber of the efi engine are ‘dry’. Which is okay if you keep the throttle closed and run the engine down to tickover – the injectors just switch back on again to keep the motor running. But what happens if the rider opens the throttle again before that point?

Engineers call this process ‘fuel reinstatement’. The rider twists the throttle and the bike’s ECU reads this, telling the injectors to deliver fuel into the air-stream in the throttle bodies.

At the same time, a butterfly valve in the throttle bodies opens, allowing air into the inlet tracts (the ECU also opens secondary butterfly valves – slowly – using a servo, to control the transition from closed throttle to open throttle).

So fuel is squirted from the injectors and the butterfly valves are partly open to let air into the engine.

But because the motor is completely fuel-less, the new fuel has to travel the length of the inlet tracts and into the combustion chamber before it can burn. It’s not a big distance, but it’s far enough to introduce a delay between opening the throttle and getting a response.

Also, some fuel from the initial injection is drawn out of the fuel/air mixture and re-lines, or ‘wets’, the walls of the inlet tracts, adding further delay in the petrol reaching the cylinder.

The cumulative effect is called the ‘fuel transport time’ – and the longer that is, the worse the throttle snatch.

There are various ways of minimising transport time, but if we assume the manufacturers have already explored the hardware solutions – optimum injector-nozzle angle, rate of delivery, number of nozzles, number of nozzle holes etc – the next step is to make further improvements through a) the resolution of the injection mapping software (how quickly and accurately the ECU monitors, selects and deploys the best fuel map for a given situation); and b) old-fashioned bench testing – going backwards and forwards over the motor, adjusting injection rates and times until it’s as right as it can be.

Of course, the best way of avoiding the problem of fuel-injection snatch altogether would be to add fuel on a closed throttle, like carbs do and like early bike efi systems used to (before the emissions bods had their misguided way). That way there’d be no fuel transport time, and no snatch.

The cure: Ivan’s Fuel Cutoff Eliminator

Back to the FZ1. Our theory is this: to meet new emissions targets Yamaha cut the FZ’s fuel on a closed throttle. This leads to ‘dry’ inlet tracts, which increases fuel transport time, which means when you open the throttle there’s a delay before the engine responds – and when it does, it responds too strongly… hence the throttle snatch.

To prove this, we strap the FZ1 to an air/ fuel meter. A sensor measures unburned fuel levels in the exhaust, which tells us how much fuel is added to the engine. We run the Yamaha up to the redline in top gear, then shut the throttle. As the revs drop, we plot the fuelling levels.

The graph (scroll down) shows when the throttle is closed, the Yam cuts fuel and adds none until 4500rpm (an arbitrary figure Yamaha have chosen), when, with a backfire, fuel comes back in and the air/fuel ratio returns to normal. This agrees precisely with the description of the problem on the road.

So what can we do? We need to override the ECU’s command to switch the injectors off on a closed throttle at descending revs.

This was also the conclusion reached by US bike tuner and Fazer expert Ivan Rovinsky of Rockland County Motorcycles. Ivan’s air corrector kits are legendary within the old, carburetted Fazer 1000 community, adding horsepower and torque, and improving throttle response. His device for eliminating throttle snatch on the FZ1 is a simple plug-in; a small grey cylinder with wires poking out. He calls it a Fuel Cut-off Eliminator, or FCE. It costs £150 and is brought into the UK by SG Motorsport.

The FCE takes an hour to wire into the FZ1, but comes with a manual and colour photos (plus crimping pliers and bullet connectors) to help. It’s invasive – you have to cut wiring, not just splice into it – but the FCE needs no other modifications or dyno set-up. We just wire it in and go.

Ivan won’t reveal what’s inside his FCE or what it does (we reckon it fools the ECU into thinking the throttle is never shut, so it never tells the injectors to switch off), but the important thing is to find out if it works.

The Result

The graph shows the FCE’s effect. When the throttle is closed at 11,000rpm, instead of cutting fuel the injectors keep firing all the way down the revs, until it picks up alongside the stock bike at around 3000rpm. So the FCE does what Ivan says it should.

The final test is to ride the bike – and the difference is dramatic. Go into a roundabout on a closed throttle and it’s now easy to find neutral throttle midcorner. There’s no need to be smooth and no wincing as the snatch comes in. There’s no question – the FZ’s snatchy throttle is completely gone, Ivan’s FCE works and there doesn’t seem to be a downside. Tank range is limited by the FZ’s 17 litres, not by any increase in consumption.

So if you own a FZ1 and are bothered by its snatchy throttle, Ivan’s FCE is the cure. And if you’re a fuel-injection engineer working for a bike manufacturer and wrestling with the next bunch of emissions legislation, please remember: the throttle works both ways.“

yamaha_fz1/gen_2_fuel_snatch_problem.txt · Last modified: 2016/07/20 16:13 by ko4bb
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