SOHC.co.uk Forums > CB500/550

Raising that needles.

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Johnny4428:
Since we are all waiting for some nice spring weather, what a great time to get that bike out of hibernation and get it road ready. I have taken the plunge and raised the carb needles up one notch to try and increase the amount of fuel to engine. The reason for this was poor starting (when cold) then you would have to ride a good couple of miles before thinking of shutting off the choke. Also on higher revs probably 4000 ish to accelerate beyond there was a hesitance and easily corrected by closing the choke thus proving a lack of fuel. I have previously tried different settings on mixture screw also these carbs have been taken off and stripped at least three times. These are PD46a carbs. I had drained the carbs about 5-6 months ago before winter storage but unfortunately I had a bleed from fuel tap which I have sorted now. So there was fuel lying in the carbs but I do have Frost “ethmoix” added to fuel to prevent varnishing. The bike fired up immediately! It’s never done that before in the last thirty odd years especially after winter lay up, and had to open up choke almost right away to keep her running. But boy she is running rich. Will probably have to close down the mixture screws a bit to try and lean out. Then I will see how she goes out on the road when the weather improves. But I do feel as if I am flogging a dead horse here but it is interesting to try something different. What you think Bryanj, chuck these carbs in the skip? 🤣🤣🤣

Johnny4428:
Just an update on the above. Needles now returned to third slot. Not much use if bike won’t idle below 3000rpm. Just too big a jump to go from third to fourth position. Now adjusting mixture screw in 1/4 turn increments to find best position. Haven’t replaced pilots, don’t know if that would help much in that mid range?

K2-K6:
I feel that some of the difficulties come from these carburettor being very well resolved and ultimately accurate in fuel metering which doesn't give much leeway for setting them.

Intrinsically they are the right solution in delivering fuel at the correct rate,  but with acknowledged finicky sensitivity to any error.

Honda merhod of determining the low speed mixture is to work on individual cylinders by moving the adjustment screw while listening to the rpm and monitoring that to bring the highest rpm by turning the screw (this makes that cylinder leaner, and so run faster) past which point if you keep turning it should start to miss fire slightly (too lean) this lets you see the set point (just before the miss fire) to give you a reference point for your engine and the fuel it's running on.

Then, from that point above, you turn the screw to make it richer and drop 100 rpm from the speed you saw (they state a workshop tachometer, but you could use a digital voltmeter with frequency readout ) to now set the mixture slightly richer than that "optimum" rpm you saw in section above.

This is done for each cylinder to give the bike's reference setting. 

I believe it does affect throttle response as the low speed circuit flows roughly 1/3 of total fuel demand, making it effective up to 2/3 fuelling in running terms. That's paired with 1/2 the main jet flow (which it matches) to give influence up to 2/3 flow demand. 

Johnny4428:
Thanks Nigel for making that very clear. I have a timing light that incorporates a tachometer, so could maybe  use that for a bit of individual cylinder fine tuning.

deltarider:

--- Quote from: Johnny4428 on March 16, 2022, 08:03:18 AM ---Thanks Nigel for making that very clear. I have a timing light that incorporates a tachometer, so could maybe  use that for a bit of individual cylinder fine tuning.

--- End quote ---
I wish you success! Although in theory possible, I never managed, that is: not with a CB500/550. Not even with an analogue tach displaying detailed 20 rpm increments. Effects, often delayed (!), are just too erratic and a digital tach is hopeless for this. In the meanwhile you are plaguing the engine with prolonged idling and possibly overheating.

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