SOHC.co.uk Forums > CB350/400
Won't return to idle/slight oil seep...
taysidedragon:
--- Quote from: Trigger on July 23, 2023, 12:34:30 PM ---My experience I never had problem driving highly tuned engine cars with fuel pumps from the UK, crossing the Austrian alps, the full length of Yugoslavian mountains and crossing the northern Greek mountains. Diving a Noble M12 or a UVA sandrail that had twin choke Webber's ;)
--- End quote ---
I think you need to get to much higher altitudes before you'll notice any loss of performance. The highest roads in the Himalayas, for example. For most of us we'll never need to worry about it.
McCabe-Thiele (Ted):
On a related topic all my bikes & cars drove much better after a decent downpour of rain - probably explains a brief period when ICE manufacturers toyed with water injection.
Mikep328:
The not-returning-to-idle seems to have been caused by the high-idle stop being out of adjustment! The manual calls for .012"/.3mm clearance and there was none. It was holding the throttle slightly open which got worse as the engine heated up. I adjusted the clearance per the Honda specs and, once the engine was good and warm - which takes more time than I would have thought - I adjusted the idle stop screw to 1200 RPM. From that point on the engine returned to that RPM when the twist grip was returned to the idle position. But the idle is a bit rough and the engine still bogs when opening the throttle quickly.
further re the bog - I haven't found any info in the Honda service manual or a Clymers that mentions changing the position of the needle to adjust the mixture in the range the needle controls. I'm NOT saying there isn't any info, just that I haven't found it. I'd think that raising the needle might reduce/eliminate the bog. Any thoughts/experience re this?
I'm definitely going to pull the carbs/check/adjust everything out but we're going out of town on Tuesday for a week so that will wait until we return.
Deek:
I've felt the loss of power in a modest vehicle at 9500 feet altitude in Colorado. There are suggestions of losing 3% of power per 1000 feet of altitude gain, also that engines can run on lower octane fuel in higher altitudes than elsewhere. In certain parts of Colorado fuel stations have sold petrol with octane levels as low as 85.
I guess the impact of altude is felt more on lowly poweredengines. Presumably manufacturers set engines up to run optimally at some average altitude above sea level. No idea whether it varies by manufacturer or intended market. Tibet might be require some adjustments with the lowest city over 9000 feet and an average altitude over 14000 feet. Probably not many 400 fours there though.
K2-K6:
--- Quote from: Mikep328 on July 23, 2023, 04:21:04 PM ---The not-returning-to-idle seems to have been caused by the high-idle stop being out of adjustment! The manual calls for .012"/.3mm clearance and there was none. It was holding the throttle slightly open which got worse as the engine heated up. I adjusted the clearance per the Honda specs and, once the engine was good and warm - which takes more time than I would have thought - I adjusted the idle stop screw to 1200 RPM. From that point on the engine returned to that RPM when the twist grip was returned to the idle position. But the idle is a bit rough and the engine still bogs when opening the throttle quickly.
further re the bog - I haven't found any info in the Honda service manual or a Clymers that mentions changing the position of the needle to adjust the mixture in the range the needle controls. I'm NOT saying there isn't any info, just that I haven't found it. I'd think that raising the needle might reduce/eliminate the bog. Any thoughts/experience re this?
I'm definitely going to pull the carbs/check/adjust everything out but we're going out of town on Tuesday for a week so that will wait until we return.
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Good progress with sorting out basic throttle rest position to get at least a consistency there.
This really is tied up in idle circuut competency, they lead the mixture in with a gradient to fully "knit" together with the main jet and needle flow in getting a seamless transition. Without at least verifying the carbs and their adjustment it's hard to get decent view of what is happening.
With those engine, wide open throttle with high gear and low speed is not where you want to operate, but given the correct gear ratio, then wacking the throttle open shouldn't give that bog, it should just run up the revs without complaint.
The scenario that gives the "bog" is insufficient fllow/metering of idle circuit to get the fires going, then to bring the response fully onto main jet. If this doesn't happen and it's gone too lean from idle giving insufficient mixture, then further opening the throttle slides just exacerbates this by then dropping vacuum to also not pull fuel through main jet route. One needs the other to lead it. Usually you need to close the throttle a little to help increase the vacuum under the slide, then it'll start to pull up through the flat spot but you'll normally have to baby it to get through there.
Focus on impeccable idle circuits, then the rest Usually falls into place on these.
Also, idle curcuit are absolutely key to getting smooth idle rpm and quiet running. One out of kilter can make it sound like a bag-o-nails.
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