Setting idle mixture screw on old Hondas
in [Tech]
|
From: mwhyte via MotorcycleKB.com on 18 Jun 2007 15:18 Hi -- I've got a 1972 cb350 (twin) with idle issues that I'm messign around with and trying to correct. In the process of this, I tightened, then slackened off the idle mixture screws on both carbs. I actually don't know where to set them. Is there a standard setting, ie. 2 full turns from lightly seated, or does it depend on other factors? Thanks much, MW -- Message posted via MotorcycleKB.com http://www.motorcyclekb.com/Uwe/Forums.aspx/tech/200706/1
From: Albrecht via MotorcycleKB.com on 18 Jun 2007 15:56 mwhyte wrote: >Is there a standard setting, ie. 2 full turns from lightly seated, or does it >depend on other factors? Yes, there are factors like wear on the idle mixture screws and the relative humidity and temperature and altitude of the place you live, and jet orifice hole area factors that influence where the idle mixture screws should be set. The old slide valve carburetors used a pilot air screw that controlled the air being mixed with the gasoline that came through the idle jet. Turning those screws clockwise richened the idle mixture. Later carburetors came with idle mixture screws that actually adjusted the idle mixture *after* the gasoline from the idle jet had mixed with the air from the pilot jet. Motorcycle mechanics had to learn to turn the idle mixture screws counterclockwise to richen the mixture and turn the idle mixture screws clockwise to lean the mixture. Motorcycle carburetors became more like car carburetors in respect to which way you had to turn the idle mixture screws. When I look at the rubber o-rings on the end of your idle mixture screws, that tells me that they are the latter type of screw, you turn them clockwise to lean up the mixture. Now, back to idle jet area. The diameter of the hole in a typical idle jet will be from 0.3 millimeters to 0.45 millimeters. If you work the old area formula and compare the area of the holes, you can see that a #45 idle jet has an area about twice that of a #30 idle jet. A #45 idle jet would require the idle mixture screw to be almost fully closed to avoid drowning the engine in excess fuel at idle. It looks to me like your idle jets are about #35's so the correct setting to start tuning the carburetors would be about two full turns out from lightly seated. So, what you want to do is start the engine and warm it up and set the idle speed so the engine runs smoothly at the specified RPM in the owner's manual. Then, tighten each idle mixture screw 1/4 of a turn clockwise. If the idle RPM increases, turn the master idle knob down. Then turn the idle mixture screws 1/4 of a turn clockwise and turn the idle speed knob down again. You want to get the smoothest idle possible at the specified RPM. If the engine starts running rough, back the idle mixture screw out 1/8th of a turn, you are done adjusting. If there is no response to turning the idle mixture screws clockwise, start at 2.0 turns out and then add 1/4 of a turn CCW on each screw. Where amateur tuners get into trouble is in thinking that the idle RPM should continue to increase as they turn the idle mixture screws further and further CCW. Once they reach a certain critical point, the idle mixture gets so rich the idle RPM decrease. The exhaust begins to sting their eyes from all the alkyd in the unburned mixture. The only thing the amatuer tuner can think to do is to turn the idle speed knob up. He doesn't know about the three transition ports that are controlled by the throttle butterfly. Turning the knob up prematurely uncovers the three transition ports that are not regulated by the idle mixture knob. The amateur tuner tries to check for throttle response by revving upthe engine, and he discovers that the idle RPM hangs up at 3000 to 4000 RPM for the longest time. So he turns the idle RPM screw down and now the engine wants to stall again. The amateur mechanic gets all confused by this state of affairs. The idle mixture screw and the idle speed screw no longer seem to to what they are supposed to do, once the idle mixture screw is opened up too far. So, setting the idle mixture screws depends upon deciding on a starting point, like 2.0 full turns out, and going back to the starting point if the engine doesn't respond to carburetor tuning as you expected it to. -- Message posted via MotorcycleKB.com http://www.motorcyclekb.com/Uwe/Forums.aspx/tech/200706/1
From: mwhyte via MotorcycleKB.com on 18 Jun 2007 16:55 Albrecht wrote: > >So, setting the idle mixture screws depends upon deciding on a starting point, >like 2.0 full turns out, and going back to the starting point if the engine >doesn't respond to carburetor tuning as you expected it to. Albrecht, thanks, as ever, for your comprehensive response. I'm nothing if not amateur, unfortunately. As a result of my fiddling, I've done the following: I found an old chart of settings at this URL: http://oldmanhonda.com/MC/DataTable.html So I set the idle mixture screws at one full turn from lightly seated, on both sides. Then, I fiddled with the idle adjustment screws on the throttle. Both sides are now fully open (ie. maxed out). Result: An idle at about 1100 rpms, which would be perfectly satisfactory IF the bike wasn't now very sluggish and unresponsive. It sputters badly, and is really unresponsive to the throttle. The torque seems cut in half -- which leads me to think I've fouled a plug now somewhere in this process. There still seems to be a steady rhythm of exhaist coming out of both pipes at idle, but I'm not sure that means anything with regards to a fouled plug/cylinder not firing. And I'm no mechanic (obviosly) but it strikes me that the idle throttle screw shouldn't need to be maxed out to maintain an idle speed of 11 rpms. Now what, I'm not sure. I just had my carbs rebuilt, but, I'm starting to think, maybe not very well. I'm going to go back and study your lenghty reply and try to implement some of the things you've said here. In the meantime, any advice with respect to the further details above would be GREATLY appreciated. Many thanks once again, MW -- Message posted via MotorcycleKB.com http://www.motorcyclekb.com/Uwe/Forums.aspx/tech/200706/1
From: Albrecht via MotorcycleKB.com on 18 Jun 2007 17:09 mwhyte wrote: >I found an old chart of settings at this URL: > >http://oldmanhonda.com/MC/DataTable.html > >So I set the idle mixture screws at one full turn from lightly seated, on >both sides. I can see that oldmanhonda calls the idle mixture screw an "air screw". It really makes no sense to me to put an o-ring on an air screw. If oldmanhonda is correct in the assertion that it's an air screw, turning it clockwise will richen up the mixture and the motorcycle will have more power and throttle response as you open the throttle. If oldmanhonda is right, the engine will run more erratically as you turn the idle mixture screw counterclockwise and the mixture gets leaner. If the exhaust gasses smell like alcohol and sting your eyes, that might be because a lean mixture doesn't burn well, and a lot of hydrocarbons are thrown out unburnt. > >Then, I fiddled with the idle adjustment screws on the throttle. Both sides >are now fully open (ie. maxed out). > >Result: An idle at about 1100 rpms, which would be perfectly satisfactory IF >the bike wasn't now very sluggish and unresponsive. It sputters badly, and is >really unresponsive to the throttle. The idle passages and ports may be plugged up with gum and varnish. You can try spritizing them out with Berryman B12 Choke and Carburetor Cleaner in the aerosol can. > >The torque seems cut in half -- which leads me to think I've fouled a plug >now somewhere in this process. There still seems to be a steady rhythm of >exhaist coming out of both pipes at idle, but I'm not sure that means >anything with regards to a fouled plug/cylinder not firing. If a spark plug is fouled, you can see it. It will be wet with gasoline or oil or it will be covered with black soot. Gasoline is clear, oil is oily and black, soot is fluffy and soft. > >And I'm no mechanic (obviosly) but it strikes me that the idle throttle screw >shouldn't need to be maxed out to maintain an idle speed of 11 rpms. You are correct there. >Now what, I'm not sure. I just had my carbs rebuilt, but, I'm starting to >think, maybe not very well. Old timers are still confused by constant vacuum carbs and how they work, compared to the old slide valve carbs. Younger mechanics never knew there was any difference. -- Message posted via http://www.motorcyclekb.com
From: mwhyte via MotorcycleKB.com on 19 Jun 2007 08:16
Albrecht wrote: > >Old timers are still confused by constant vacuum carbs and how they work, >compared to the old slide valve carbs. Younger mechanics never knew there was >any difference. The mechanic who did the work for me is a young guy, probably late 20s. I found a guy near here, an old timer, who's been wokring on these old Hondas since they were new, so I'm going to give him a try ... I checked the plugs and they both look bright and shiny to me. So that seems to suggest something in the carbs -- something plugged or varnished, as you suggest, which would be irritating, seeing as I juts had them rebuilt ... Oh well, the adventure continues. Thanks a ton for all your advice. I'm learning ... -- Message posted via MotorcycleKB.com http://www.motorcyclekb.com/Uwe/Forums.aspx/tech/200706/1 |