Raising that needles.

[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]

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.

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.

[K2-K6]

The overheating is not really an issue.

To seperate it from running an engine under load (dyno work) in static air is important.  That's not reasonable.

Idling they'll effectively do all day without technical problems as the fins are specifically sized to cope with this by radiation.  Certainly I've sat with very little air movement going into and out of bike race meetings in summer temps for extended periods without ever having a problem.

The heat coming off the engine definitely increases,  but that's what fins are supposed to do.

The PD carbs are very finely metered (one reason why those setup are generally good for fuel consumption) and this proceedure may tell us something in understanding the issue. It may end up showing different set point for fuel in use as likely it's now running e-fuel, unless specifically avoid by OP.

It'll be interesting to see what the manual set up routine gives in the way of screw settings.  Certainly testing like this is reversible and effectively no cost to judge it.

[deltarider]

I agree that the danger of overheating, like in the international forum, is often exaggerated. Some live their moment of fame by not only echoing others you must have a fan, but in some cases added you need two. I never felt the need to use one like when syncing. Procedure was done within 20 minutes though.
The procedure you have described is good ofcourse... in theory. More than once I have scanned the internet hoping to find a video, like on Youtube, where it is demonstrated with success.
So far: not one. So anyone is invited.

[Johnny4428]

Appreciate and understand all your interesting comments. You know what, if my 550 never runs any worse I would be quite happy. But just thought that I could improve on that midrange hesitance. I guess fuel was different and with everything new in the 70’s things were different, especially with settings being quite critical.

[K2-K6]

Thoughts to hopefully add a little to this and maybe influence that hesitation.

It's something that all petrol engines and their different metering systems have to cope with. Running at constant demand takes a prescribed amount of fuel in mixture balance to be effective, this is insufficient to get the motor to accelerate quickly.
Most system on opening the throttle do the opposite in metering terms, in that as the throttle restriction is opened it causes the metering to move leaner and can prevent the motor accelerating as commanded.
All the different systems cover this in a different way. Plain old type slide carbs run too rich most of the time to avoid this, CV type hold the floating valve (through damping) to keep the vacuum constant as their name indicates, can even be set to give raised vacuum at this point of acceleration to help increase fuelling. Fuel injection, monitoring of throttle position applies additional fuel if the rate of throttle opening is measured as faster than mapping equivalent to avoid this lean stumble. PD on 750 F2 uses accelerator pump to dump additional fuel into carb if you increase throttle in fast application.

These other PD type rely more on idle jetting to give some increase to help. As it's brought up from closed throttle under load the idle circuit more quickly hits saturation in delivering more fuel at that point through it's balance against venturi depressions. That circuit is very accurate in it's original design specification to give emmisions clean idle in that period on manufacturing, and so there's very little "fat" to work with. What may seem like very small changes can really help in setting the idle circuit to try and remove hesitancy in driving.

Some things you could try in assessment to see if you can improve.

Use R plugs and R plug caps together, set the gap to minimum book specification and ride it to see if there's any change. This appears to improve firing at marginal mixtures (slightly lean under throttle roll on) that can exist to give that flat spot hesitancy.

Using something like a piece of fleece material taped over the airbox inlet just to see if you can bias the mixture in encouragement to slightly richer bias. This to assess at low cost and work levels to see if it improves it and what colour the plugs then run.

As you note, it's more annoying then anything else,  but won't take a full step on moving the needles.  This shows it's really close to not existing, so dealing with small margins.

Hopefully something to play with and see if you get something out of it.

[ST1100]

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In the meanwhile you are plaguing the engine with prolonged idling and possibly overheating.

I plant two office fans on HIGH in front while tinkering with carbs & settings... (also driving exhaust fumes out the gate...  )

My (new) electronic synX has a pretty sensitive/precise tachometer...

https://www.louis-moto.co.uk/artikel/synx-synchroniser-classic/10034539

To some degree down-pipe temperatures are also an indicator for rich/lean mixtures...

[Johnny4428]

Yes indeed, cylinder 2 is different. Went for a run today with a set of Denso resistor plugs that I purchased by mistake. And removed plugs after the run, the plugs accross cylinders 1, 3, and 4 are pretty consistent but there seems to be that sooty cylinder 2! The run itself was no worse or better with the R plugs. I’m going to swap over leads 3 and 2 and try a run just for interest that might at least eliminate a dodgy lead. Also ignition timing advanced a little to be nearer the mark.