SOHC.co.uk Forums > CB500/550
Sooty plugs.
K2-K6:
At least you're getting a response Johnny, an indication that those circuits are operational.
I'm presuming PD carbs, with screw adjust on engine side of throttle slide ?
Honda manuals for slightly later carbs (I've not seen K3 manual) give very specific routine and details for setting these adjustment.
Furthermore, they indicate that factory settings are individual down to each carburettor, then if the carbs are stripped and reassembled, there's need to pass through that routine as factory, subsequently record the final setting position by nothing in the service notes as baseline from there onwards.
They are not "set all adjustment the same" carburettor, that's not accurate enough.
This is where the fine adjustment of idle "synchronization occurs and not the syncing that everyone focuses on. The workshop synchronization just sets the slide air parity, being too course an adjustment to fine tune idle fuelling. Its a part of the process, but not the whole process. Fine combustion parity comes from then making adjustment to these idle circuit flow when everything is set correctly.
Even fuel injection system mirror this aspect on more sophisticated installations.
Johnny4428:
--- Quote from: Sesman on October 03, 2024, 08:39:02 AM ---Hi, Johnny.
Out of interest which carbs are you using and what size main jets are installed? I presume you are running with the standard airbox, using a standard exhaust?
--- End quote ---
It’s a standard set up with 4 into 4 pipes and factory air box set up with oil breather recovery thingy.
The carbs are PD46A with standard mains.
Johnny4428:
--- Quote from: K2-K6 on October 03, 2024, 09:23:58 AM ---At least you're getting a response Johnny, an indication that those circuits are operational.
I'm presuming PD carbs, with screw adjust on engine side of throttle slide ?
Honda manuals for slightly later carbs (I've not seen K3 manual) give very specific routine and details for setting these adjustment.
Furthermore, they indicate that factory settings are individual down to each carburettor, then if the carbs are stripped and reassembled, there's need to pass through that routine as factory, subsequently record the final setting position by nothing in the service notes as baseline from there onwards.
They are not "set all adjustment the same" carburettor, that's not accurate enough.
This is where the fine adjustment of idle "synchronization occurs and not the syncing that everyone focuses on. The workshop synchronization just sets the slide air parity, being too course an adjustment to fine tune idle fuelling. Its a part of the process, but not the whole process. Fine combustion parity comes from then making adjustment to these idle circuit flow when everything is set correctly.
Even fuel injection system mirror this aspect on more sophisticated installations.
--- End quote ---
Yes Nigel the screw on the engine side. What you have written above makes complete sense and I have to walk away from trying to get these multi carb set ups all set the same. I’m quite happy to have one screw at 3/4 and the other three 1/2 turn out if that’s what it takes! 😜
K2-K6:
Thought I'd copy this text to thread that I've included in another
"While I can see the reasoning, I think this deserves expansion to understand.
The amount of fuel added to the air going under the slide at idle us adjusted by the routine that Honda gives for setting airscrew idle mixture. In other words it can't be wrong if its set correctly.
It can go wrong if set badly, or IF the idle circuit, both jet and screw controlled air passage, change after its been correctly set.
In effect, you can't get too much gap under the slide as it should always be proportionately accompanied by the correct amount of fuel. This is exactly what the idle circuit is for.
I intend discussing in carb thread, when I get to this bit
That Honda routine specifically targets fuel air ratio delivered by the idle circuit against the set position of the slide. If that setting then becomes inappropriate, the mixture, for that cylinder, moves out and away form how you've set it and usually lean with the fuel jet orifice being compromised. So you do get too much air, but not from overall position of slide, but failure of delivery fuel jet to maintain flow at defined rate.
To make clear , and I'll shout this bit :) THERE'S NO REQUIREMENT TO SET THE AIRSCREW EQUALLY ..... THE HONDA METHOD VERY SPECIFICALLY TARGETS PARITY OF COMBUSTION AND ACCEPTS DIFFERENT SETTINGS FOM CARB TO CARB ON THE SAME ENGINE.
If you carry out the setting routine to the manual description, it will "show" you if there's any impairment in that specific circuit by needing a setting that just doesn't fit the expected.
It does this by you making observations of the fuel air ratio as you turn the screw and listen to the combustion.
That must be one of the most under used parts of their manuals .... but so important in setup.
Everyone goes straight to carb synchronization, missing the substance and reasoning contained within this little routine that they give."
To emphasise, it's a very innocuous instruction written into manuals by Honda, but absolutely fundamental in their setting i believe.
It brings the combustion process to breakdown from being too lean, then instructs to move it back from that point toward richer in creating a competent base point .
It may look like a single until four cylinder engine, but really its a "platoon" of near 125cc singles on a common crankshaft, thats from a carburettor point of view.
Their routine effectively contains a "calibration" of each carburettor idle system, taking into account any production tolerance that may be included in manufacture of the parts.
Its very highly likely they'll be different from one to another when settings are finely resolved.
deltarider:
--- Quote from: K2-K6 on October 07, 2024, 12:12:59 PM ---Thought I'd copy this text to thread that I've included in another
"While I can see the reasoning, I think this deserves expansion to understand.
The amount of fuel added to the air going under the slide at idle us adjusted by the routine that Honda gives for setting airscrew idle mixture. In other words it can't be wrong if its set correctly.
It can go wrong if set badly, or IF the idle circuit, both jet and screw controlled air passage, change after its been correctly set.
In effect, you can't get too much gap under the slide as it should always be proportionately accompanied by the correct amount of fuel. This is exactly what the idle circuit is for.
I intend discussing in carb thread, when I get to this bit
That Honda routine specifically targets fuel air ratio delivered by the idle circuit against the set position of the slide. If that setting then becomes inappropriate, the mixture, for that cylinder, moves out and away form how you've set it and usually lean with the fuel jet orifice being compromised. So you do get too much air, but not from overall position of slide, but failure of delivery fuel jet to maintain flow at defined rate.
To make clear , and I'll shout this bit :) THERE'S NO REQUIREMENT TO SET THE AIRSCREW EQUALLY ..... THE HONDA METHOD VERY SPECIFICALLY TARGETS PARITY OF COMBUSTION AND ACCEPTS DIFFERENT SETTINGS FOM CARB TO CARB ON THE SAME ENGINE.
If you carry out the setting routine to the manual description, it will "show" you if there's any impairment in that specific circuit by needing a setting that just doesn't fit the expected.
It does this by you making observations of the fuel air ratio as you turn the screw and listen to the combustion.
That must be one of the most under used parts of their manuals .... but so important in setup.
Everyone goes straight to carb synchronization, missing the substance and reasoning contained within this little routine that they give."
To emphasise, it's a very innocuous instruction written into manuals by Honda, but absolutely fundamental in their setting i believe.
It brings the combustion process to breakdown from being too lean, then instructs to move it back from that point toward richer in creating a competent base point .
It may look like a single until four cylinder engine, but really its a "platoon" of near 125cc singles on a common crankshaft, thats from a carburettor point of view.
Their routine effectively contains a "calibration" of each carburettor idle system, taking into account any production tolerance that may be included in manufacture of the parts.
Its very highly likely they'll be different from one to another when settings are finely resolved.
--- End quote ---
As usual I have read your post with much interest. A few remarks if you will allow me. I don't know what Honda manual you refer to, but the 'Honda method' is definitely not in the CB500/550 Shop Manual. In that manual the airscrew setting data is not even consistent. On one page it is 1 turn +/- 1/8, on another 1 +/- 3/8. The content of your post is in theory correct ofcourse, but in practice it's almost impossible to perform the method with good results and so far I have I not seen a video on Youtube or whatever channel that demonstrates the method succesfully. I find that telling and I invite anyone to make one. Over the years I've 'played' with my airscrew openings quite a bit and let me confess that I find it extremely difficult to arrive at an optimum. I should never have touched them on that campground in Greece in 1980. Have a look at the pic below. It shows the scale of an exhaust gas analyzer, typical for that era. As you can see, the yellow field is quite extended and I doubt such a meter can be of much help. To my knowledge Honda did not specify what % CO to aim for (idle) for our models. Monitoring manometers whilst synchronizing, won't help you either. Increments of even half a turn will not result in a changed underpressure of that cylinder. The only thing I can think of, is to put, after syncing your carbs, a rubber hose in the exhausts and listen carefuly and see if you, by comparing and turning the airscrews, can arrive at identical sounds of all four. A leaner carb will produce a lighter sound than a richer which has a much fatter 'thump' sound. If someone has managed a perfectly mixture at idle CB500 4-4 and - more important - a linear acceleration*, I welcome a video with a good recording of the sound that can serve us all as a benchmark. ;D
* As far as the CB500 model for the European continent, I read the slow jets as auxiliary jets that happen to double as idle jets
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