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Low rev misfire (My theory)
LBMEFM - 27/2/12 at 06:50 AM

I posted the the question below yesterdy and thanks for the answers and the general feeling that it's a fueling problem. I have thought about it and now have a theory. Towards the end of last year I extended the air intakes on the original Fireblade air box filter with tubing up into the bonnet scoop to allow it to draw in cool air. I think by doing this I may have caused the problem. As we know the piston goes down on induction, creates a vacuumn which in turn draws air across the jet and pulls in fuel. On my new design the tubes in the scoop have air forced into them which in turn pushes air into the cylinder which may reduce the vacumn effect, therefore, less fuel is "sucked" in, causing fuel starvation and spluttering. What do you think?


Low rev misfire

MK Indy - Fireblade 900cc on carb's

Slowly making my way through the list of over winter mods/faults etc. One I can't check, as the car cannot be road tested, was a mis-fire on pick up. On track days I would put my foot down and it would momentarily hold back with a misfiring splutter around 2000rpm then clear itself as it went through 5000rpm or so. Any thoughts of what it's likely to be so that I can change any faulty parts. The plugs were new at the start of last year, maybe I should try new leads or coils. What do you think? Barry

[Edited on 27/2/12 by LBMEFM]

snapper - 27/2/12 at 08:12 AM

Bikes have used ram air for years, unlikelly.
To test run without airbox, no spluttering it's the airbox
air sucked or air pushed its still air mass moving past a hole and it's the pressure difference at the interface that sucks air out
I THINK

ReMan - 27/2/12 at 08:38 AM

If this was the cause, then a turbo'd engine would not work

I'm interested because I was considering doing this myself, but I'd have thought that its more likely just caused a weak spot, in the rev range due to the increased airflow.
Try blocking one of the 2 tubes off
Or another possibbility is that the bonett duct does not get any aiflow at low speeds due to the aero on the car?
Or it may be all coincidence and one or more of he carbs is dirty!
Coils and leads are more likely to fail at high revs
Plugs would be worth checking and swapping to eliminate, if you've got an old set kicking round



[Edited on 27/2/12 by ReMan]

MakeEverything - 27/2/12 at 09:37 AM

My experience of bikes is that the airbox is calibrated to work with the engine perfectly. By changing the airbox, you may have to change the map for fuel as you say. Have you considered that by changing the airbox, youre reducing the vacuum pressure within which is sensed by the ECU for fuel?

I would do two things;

Get a power commander or aftermarket ECU.
Have the injectors cleaned.

Dusty - 27/2/12 at 11:18 AM

quote:
TextAs we know the piston goes down on induction, creates a vacuumn which in turn draws air across the jet and pulls in fuel.

I think you have that the wrong way round. If I remember my physics from school in the early sixties air isn't sucked by a vacuum into the cylinders. It moves over a pressure gradient from high to low, pushed by the pressure differential between the two. The physics of normally aspirated is just the same as turbo or supercharger. The descending piston creates a low pressure area, atmospheric pressure (or whatever the turbo or supercharger produce) is the high pressure area and the inlet tract just a connecting tube between the two. This pressure pushes the air in and there is a dropping pressure gradient as it passes through the inlet tract and into the cylinder. The velocity and mass of the air passing through the carb venturi along with the pressure gradient from inlet opening to cylinder, determine the pressure of the air immediately over the main jet and the main jet contains fuel, behind which is atmospheric pressure 'pushing'. Again the differential pressures between air and fuel determine how much fuel will be pushed out of the main jet, not sucked
Sorry, rambled a bit and it may be garbage. It was a long time ago. What I was saying is the air and the fuel are both pushed to where they are going not pulled. If you do something to increase the push behind either one without a similar change to the other then the mixture ratio is going to change. Alterations to inlet tract, longer, shorter, wider, narrower, bends, steps, temperature, venturi, etc will all result in change of the air pressure over the main jet.

snapper - 27/2/12 at 12:56 PM

Fueling and tubos, either fuel injected with MAP sensor to adjust fueling when on boost or the thread on here about the pilot tube
Or here. Pilot tube
Nothing's simple when it comes to carbs and pressure as carbs don't understand pressure just air flow, that's why they get confused.

ReMan - 27/2/12 at 01:01 PM

Errr PITOT tube is not a smelling mistake