quote:

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Originally posted by IanG1
Trying to get my head around an annoying problem I have with the fuelling of the 919 Fireblade engine in my Westfield.
Spec is standard blade airbox and paper filter, some form of jet kit is fitted as the needles are adjustable, currently clip in third groove from top and seating and half thickness washers are in place. Main Jets are 132 and air screw is 2 turns out. Exhaust wise normal megablade headers to a modified straight through R1 end can.
My problem is a massive flat spot/hesitation from 4 to 5K car physically jerks and holds back then once past this point it will fly up to the red line with no problem. It only seems to happen when you plonk your foot down wanting the power, it will cruise at these revs with no problem. If the loud pedal is used less vigorously then you can nurse it through this rev range without the same pronounced effect.
I have read after numerous searches that the middle air correction jet needs to be blanked off next to the intake trumpets as part of a jet kit, mine are not blanked so wondered what the effect of this was. I dropped the needles one notch yesterday to see if things improved or worsened but it then rained so not managed a test drive yet.
Any other advise or pointers appreciated as to which way to go.

Cheers

Ian

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quote:

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Originally posted by Andy B
Thought I would try and put the fireblade fuel supply issue into context.
The blade carbs require a very low fuel pressure to operate effectively in fact the pump should be viewed purely as a means of keeping the float bowls full at all times.
Now if we think of the bowls as the reservoir from which fuel is pushed up the main jet by atmospheric pressure as a depression is created by airspeed in the main throat, we can immediately see the huge problems that can be caused by over pressuring the float bowls. Bearing in mind that a single circlip position on the needle makes a huge difference to the fuelling of the mid-range just by lifting the taper slightly, imagine the effect of pushing fuel under pressure up the main jet, which is what happens all the time when using the facet pump. A 122 main jet can be made to function just like a 132 because of the pressure behind it.
In an ideal world you would use a regulator to control the pressure but unfortunately these regulators are far from reliable, a simple bump in the road can send fuel pressure soaring or drop it to almost zero. This is mainly due to the fact that inside it is purely a diaphragm and spring.
Experience has shown that no matter how you set the regulator up it will cause issues. The problem is that bike engines seem really fast even when they are off song and I have lost count of the number of people who we have transformed the power delivery for just by junking the facet and regulator and jetting correctly.
To demonstrate this at its extreme, a racer at Donnington left his ignition on at lunch time, the facet pump pressure forced the floats down sending fuel up the emulsion tubes and down the cylinders. On his return the engine hydraulically locked and having taken out the plugs he cleared the lock and went on to race....... for 1 lap. After the engine went bang we drained nearly 7 litres of fuel and oil from the sump The same car used to use nearly double the fuel load during a race of its nearest competitor using a self regulating bike pump.
My advice would be to always use a bike pump and do not use the facet and regulator - it does not work
Hope that helps
If anyone needs more info just call me on 01449736633
Andy

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quote:

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Originally posted by Andy B
Thought I would try and put the fireblade fuel supply issue into context.
The blade carbs require a very low fuel pressure to operate effectively in fact the pump should be viewed purely as a means of keeping the float bowls full at all times.
Now if we think of the bowls as the reservoir from which fuel is pushed up the main jet by atmospheric pressure as a depression is created by airspeed in the main throat, we can immediately see the huge problems that can be caused by over pressuring the float bowls. Bearing in mind that a single circlip position on the needle makes a huge difference to the fuelling of the mid-range just by lifting the taper slightly, imagine the effect of pushing fuel under pressure up the main jet, which is what happens all the time when using the facet pump. A 122 main jet can be made to function just like a 132 because of the pressure behind it.
In an ideal world you would use a regulator to control the pressure but unfortunately these regulators are far from reliable, a simple bump in the road can send fuel pressure soaring or drop it to almost zero. This is mainly due to the fact that inside it is purely a diaphragm and spring.
Experience has shown that no matter how you set the regulator up it will cause issues. The problem is that bike engines seem really fast even when they are off song and I have lost count of the number of people who we have transformed the power delivery for just by junking the facet and regulator and jetting correctly.
To demonstrate this at its extreme, a racer at Donnington left his ignition on at lunch time, the facet pump pressure forced the floats down sending fuel up the emulsion tubes and down the cylinders. On his return the engine hydraulically locked and having taken out the plugs he cleared the lock and went on to race....... for 1 lap. After the engine went bang we drained nearly 7 litres of fuel and oil from the sump The same car used to use nearly double the fuel load during a race of its nearest competitor using a self regulating bike pump.
My advice would be to always use a bike pump and do not use the facet and regulator - it does not work
Hope that helps
If anyone needs more info just call me on 01449736633
Andy

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