SOHC.co.uk Forums > CB500/550
CB 500 K1 running issue
Beefington1975:
--- Quote from: K2-K6 on September 22, 2022, 06:11:33 PM ---For some reason it seems to be getting just a little excess fuel at the very low end of carb range.
Setting the screws out further will not change the metered fuel from idle jets at their maximum (roughly half way through carb opening) as that's controlled by the 40 sized orifice. What the air modulation does is to change the "ramp" angle leading to that point by bleeding more air through than fuel at the very base of throttle slide opening.
Honda don't state an absolute for this setting of air jet, more a competent start point to be refined during setup. Obviously this case you can see the response from resetting them (indicating circuits are working reasonably) and ending up at 2.5 out is probably at the far end of range reality, but not unworkable. Probably more assessment is valid here to see if you've any unwanted complications and perhaps see if anything else is out of the ordinary over further mileage.
At least if it runs well you've got a chance to now get more comprehensive view.
The inline filters, I'd definitely not associate with low speed fuel demand and feel confident to discount those in this area of carburetor work. Usually it's peak flow that is of concern with these and inadequate supply volume to replenish float chambers fast enough when flat out and wide open throttle. The float chamber of course acts a constant reservoir to the jets and will ordinarily have no problems at low speed.
--- End quote ---
That makes sense. I'll run it as-is for a while and keep monitoring. Will change the air filter too to rule that out as a factor.
Thank you for the generous help and information - it's paying off and I've learned a lot.
Cheers
McCabe-Thiele (Ted):
The last few of posts got me tinkering with my 500 K1 as it's raining. I have been unable to get it to idle at low speed it just cuts out or runs at over 2k rpm - let off the main stop screw just a fraction and it stops.
I've not done anything to the carbs as I was not too bothered about how it runs as long as I can try out all the gears on a short run.
So I checked the four air screws - they were all set at around 1.5 turns. I moved them to 2 turns then fired her up and let off the throttle stop screw a tad again - she now idles quite comfortably at around 1200 rpm. Yes not perfectly even yet but so much better than it was - gives me more confidence that it will not conk out on a run now.
K2-K6:
--- Quote from: Beefington1975 on September 22, 2022, 06:26:11 PM ---
--- Quote from: K2-K6 on September 22, 2022, 06:11:33 PM ---For some reason it seems to be getting just a little excess fuel at the very low end of carb range.
Setting the screws out further will not change the metered fuel from idle jets at their maximum (roughly half way through carb opening) as that's controlled by the 40 sized orifice. What the air modulation does is to change the "ramp" angle leading to that point by bleeding more air through than fuel at the very base of throttle slide opening.
Honda don't state an absolute for this setting of air jet, more a competent start point to be refined during setup. Obviously this case you can see the response from resetting them (indicating circuits are working reasonably) and ending up at 2.5 out is probably at the far end of range reality, but not unworkable. Probably more assessment is valid here to see if you've any unwanted complications and perhaps see if anything else is out of the ordinary over further mileage.
At least if it runs well you've got a chance to now get more comprehensive view.
The inline filters, I'd definitely not associate with low speed fuel demand and feel confident to discount those in this area of carburetor work. Usually it's peak flow that is of concern with these and inadequate supply volume to replenish float chambers fast enough when flat out and wide open throttle. The float chamber of course acts a constant reservoir to the jets and will ordinarily have no problems at low speed.
--- End quote ---
That makes sense. I'll run it as-is for a while and keep monitoring. Will change the air filter too to rule that out as a factor.
Thank you for the generous help and information - it's paying off and I've learned a lot.
Cheers
--- End quote ---
Takes a little time back and forth to work through things, but beneficial in giving you a clearer picture. At least it appears now workable and you get to enjoy riding it too, so all good.
Hopefully it will stay stable but you may need to consider again if air filter proves to flow a little more easily.
Let us know how you get on.
K2-K6:
--- Quote from: McCabe-Thiele (Ted) on September 22, 2022, 06:34:04 PM ---The last few of posts got me tinkering with my 500 K1 as it's raining. I have been unable to get it to idle at low speed it just cuts out or runs at over 2k rpm - let off the main stop screw just a fraction and it stops.
I've not done anything to the carbs as I was not too bothered about how it runs as long as I can try out all the gears on a short run.
So I checked the four air screws - they were all set at around 1.5 turns. I moved them to 2 turns then fired her up and let off the throttle stop screw a tad again - she now idles quite comfortably at around 1200 rpm. Yes not perfectly even yet but so much better than it was - gives me more confidence that it will not conk out on a run now.
--- End quote ---
Yes Ted, it's a valuable tool to trim slow speed running. So often the manual specification is referred to as an absolute which I don't feel is what Honda meant it to be. These carbs are very accurate/repeatable/concise but with other elements affecting the critical metering of fuel in such small quantities, then valid adjustment is entirely practical.
Their method (Honda manual script) is a very benign but far reaching routine that's often missed when considering setup. In effect, it takes account of, successively;- fuel specification, air flow, valve efficiency in sealing, piston rings, dynamic compression, ignition performance, air density, barometric pressure (height above sea level) to fine trim the the slow speed running.
It is exactly what fuel injection system accomplishes with use of Oxygen sensors reporting from exhaust to trim the mixture for prevailing mechanical and atmospheric conditions. Not in such a repeating timeline, but the same process but using you as it's view of how it's running.
Turning airscrew out (not pd carb, they're opposite usually) brings the mixture toward to lean at probably 14:1 ish when it will start to miss partially, then back in to richen slightly (rpm drop by approx 100) to probably end around 12.7/13.0 to 1 at which point it idles well.
Fuel injection does exactly thus, with short duration trim values to cope with immediately occurring emmisions, then aggregated into "long term trim values" that the ECU keeps updated to maintain overall mixture given the equipment it's working with. If the long term trim values reach certain threshold then the ECU will throw a code giving "emmisions" warning, or needing workshop attention.
Our emmisions code for these would be roughly 1/2 to 3 turns out for nominally general operating conditions. Outside those we would consider then to exhamine components to find cause.
Dammed clever those Kiehin designers :)
Beefington1975:
An update.
I had a very satisfying run out today. The bike performed well, no sign of the previous problems. If anything, I'd say the engine feels more responsive and 'free running'.
Covered about 40 miles this time. Checking the plugs at the end shows a little soot on the end of the threads and the bottom electrode, but the centre electrode and ceramic look ok.
Very pleased to have been able to enjoy a ride without trouble!
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