SOHC.co.uk Forums > CB750

Fuel level cb750k2

<< < (2/2)

Martin6:
Yep, you described the way an air screw works.

I wrote to Mark directly, to double check what he had written and he confirmed:
"The idle-air screw in those carbs (all of the 750 roundtop carbs) controls the amount of fuel-air mixture that gets pulled up for each intake stroke. It is already mixed as fuel & air inside the tiny tube on the top of the idle jet, where those holes are, when air from the little brass orifice at the back of the carb's bell met the rising fuel in the jet's metering hole. Then, the position of the screw admits more (or less) of this mixture during the intake suction."


I am testing this now. I had 2 lean plugs and 2 sooty. The lean I opened up 1/8th turn. The sooty, in 1/8th. I have found two of my lean showing plugs are tan and one of my sooty plugs is improved. The other is still sooty, so I am trying a further 1/8th in.

All on non-ethanol fuel, but general riding, not a strict plug chop, as I believe my needles are in the right position, stock jets, airbox, 341 exhaust etc. Points set, timing checked, ditto valve clearances. Carbs are cleaned, synch'd and float levels triple checked. Although I do find float levels a little tricky.

So, not conclusive, but the early signs for me is following this advice has given me a small improvement.

I apologise if this has taken us away from@Pauarc's original problem. It could be relevant. Not difficult to just give it a try, after adjusting the floats to spec. 👍

Pauarc:
Adjusted floats all within 1mls now no leaks at carb rubber it was still popping check exhaust copper washer it was leaking now no popping and air screw set to 1 1/2 so I think it was exhaust washer

Martin6:
  :)👍

K2-K6:

--- Quote from: Pauarc on April 17, 2024, 02:47:44 PM ---Adjusted floats all within 1mls now no leaks at carb rubber it was still popping check exhaust copper washer it was leaking now no popping and air screw set to 1 1/2 so I think it was exhaust washer

--- End quote ---

Good to get to the bottom of it, probably beneficial to have that odd one out float chamber now matching the others too.

The idle airscrew adjustment does allow for difference in individual flow within Honda set-up routine, being the real icing on the cake in regards to carb balance and smooth low speed running.

This is exactly what modern fuel injection systems do for smooth idle rpm, modulation of cylinder mixture by monitoring of the cylinder speed individually through ecu. Honda did this way before by observation of rpm and singular adjustment of idle circuit to get them smooth.

Navigation

[0] Message Index

[*] Previous page