Float heights, what is not considered.
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Squid
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Float heights, what is not considered.
The height of the fuel in the Firestorm float-bowl effects carburation during the transition from pilot-jet to needle-jet (emulsion tube). Basic studies on how carburettors work (car or bike) confirm this. Some Firestorm riders complain of a lean situation around that area (about 4000 rpm).
Now, sit your carbs on their side, down on the bench in front of you, with the fuel bowls off, with a straight-edge (eg; metal 300mm ruler) held at 90 degrees to the venturi of the front cyl carb, hold the ruler at the aprox fuel level next to the floats, then on the same angle (horizontal as on the bike), sit the ruler alongside the rear cyl carb, see the float level is different (in relation to the emulsion tube bleed holes) between the two carbs, the front carb in-effect, has a lower float height level, Honda has tried to address this matter by setting the emulsion tube bleed holes 2.5mm higher in the rear cylinder carb, problem is that the effective float level difference is about 6mm between the two carbs, this is because Honda used the same carburettor casting for each carb but they sit at different angles when fitted to the bike!
Now I hear you say "but the float heights are not adjustable in the VTR), the front carb needs the float level raised a couple of mm, this can be realized by shortening the float valve needle, I ground .3mm of the top of the float needle, this raised the fuel level by 1.5mm.
According to my lambda readout, this cleaned-up a 4000 rpm leanness. Changes in fuel bowl level had minimal effect either side of the applicable range (pilot-jet, needle or main), the rear cylinder still liked a touch more fuel on mixture screw, needle & main jet.
John Orchard
Now, sit your carbs on their side, down on the bench in front of you, with the fuel bowls off, with a straight-edge (eg; metal 300mm ruler) held at 90 degrees to the venturi of the front cyl carb, hold the ruler at the aprox fuel level next to the floats, then on the same angle (horizontal as on the bike), sit the ruler alongside the rear cyl carb, see the float level is different (in relation to the emulsion tube bleed holes) between the two carbs, the front carb in-effect, has a lower float height level, Honda has tried to address this matter by setting the emulsion tube bleed holes 2.5mm higher in the rear cylinder carb, problem is that the effective float level difference is about 6mm between the two carbs, this is because Honda used the same carburettor casting for each carb but they sit at different angles when fitted to the bike!
Now I hear you say "but the float heights are not adjustable in the VTR), the front carb needs the float level raised a couple of mm, this can be realized by shortening the float valve needle, I ground .3mm of the top of the float needle, this raised the fuel level by 1.5mm.
According to my lambda readout, this cleaned-up a 4000 rpm leanness. Changes in fuel bowl level had minimal effect either side of the applicable range (pilot-jet, needle or main), the rear cylinder still liked a touch more fuel on mixture screw, needle & main jet.
John Orchard
Last edited by John Orchard; Jul 23, 2024 at 03:05 PM.
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