Float valve

[Eden]

I use 400 seat valves in all the phbh carbs I use on the motors I build, there is nothing worse than killing a top end due to fuel starvation at high rpm!

[johnnyXS]

I was just browsing through the old threads in this topic area and thought that I ought to update this one with something I have learned since about needle valve restriction/ sizing .

The Old Dellorto Tuning Guide suggests that to determine the correct size of needle valve jet, it should be at least 30% larger than the Main jet that is fitted . I guess its a good a starting point as any.
So for a 125 main jet (1.25mm dia ) the needle valve should be at least 165 (1.65mm )
The needle valve in my 22mm Jetex measures 2mm dia so there should be plenty of spare flow capacity there although I read that some early Jetex carbs had much smaller needle jets which resricted flow rate and had to be drilled out to 2mm

Obviously the final decision should be made by testing at WOT up a long hill to see if there is sufficient fuel flow from the fuel bowl.

At full throttle, the only jet in operation is the Main jet,.. everything else like the pilot jet, atomiser, slide , needle are all irrelevant

If you like living dangerously then you could apply the choke when the engine is at maximum load . If the engine bogs due to too rich a mixture then the likelihood is that the Main jet is too large .
If the engine picks up (Too lean mixture) then either the fuel flow rate is too low, ( due to needle valve too small or restricted fuel tap/filter), or the Main jet is too small or possibly a combination of both.

If fuel /air flow is not the issue for lack of power at full throttle then you need to look at other things like, binding brakes , blocked exhaust, timing out etc.