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Discussion Starter · #1 ·
so I might as well get to it, having introduced myself on the introductions boards less than 24 hours ago.

there is a peculiar thing going on with the bike's idle. first off, the bike runs great, starts on the first or second kick even in the cold (with choke naturally). even though it sounded pretty good to me, out of curiosity, I decided to check the bike's idle speed. I hooked up an inductive tach clamp hooked up to a multimeter and got a reading off of both cylinders. each was ~3500rpm. this seemed kind of ridiculous given the sound, so I did some research, and concluded that maybe the reading was double because of a wasted spark ignition system, so

question 1: does the 79 CB400ti have a wasted spark ignition system? I can't seem to find this answer anywhere, including the shop manual.

so assuming I'm right about that, the bike was still idling over 1700rpm, which is a solid 500 over the factory recommended 1200rpm (+/- 100). So I set to trying to get the idle down. Having changed the oil, replaced the air filter and run some seafoam through, the first thing I did was try to adjust using the throttle stop screw. here is the first strange thing...no matter how far in I turn the screw, I couldn't stall it out. the bike just kept on humming. I can speed up the idle by turning it out (so I know it's actually moving the throttle slide), but I can't slow it past the ~1700rpm mark.

after that, I discovered that the bike was running very rich the hard way, I ran out of gas, probably only got ~90 miles off a full tank. I figured that could be the explanation, maybe the pilot screw was way out and throwing so much gas in that it would idle at any throttle stop opening. so I took to the the pilot screws, being careful to set them lightly, and then turning them out to the factory setting (1.5 turns I believe). Having done that, I STILL couldn't kill the idle with the throttle stop, although it had gone down to about 1500rpm. So I figured I'd try leaning out the idle mixture completely by lightly setting the pilot screws. even then, even with the throttle stop screw bottomed out, it STILL idled, although now pretty close to the factory idle of 1200rpm.

Although i guess I had succeeded in getting the idle down, I didn't like the idea of running the bike with the pilots screws all the way in, it just seems to be asking for some horrible lean mixture problem. so

question 2: what do you all make of this? any idea why I can't seem to get the throttle stop screw to kill the idle, no matter where the pilot screws are? any opinions about running the bike with the pilot screws (ostensibly) set to a very lean mixture?

I'm very sorry about what I see now is a very long-winded post. I'll try to keep it a little more succinct in the future...

Thanks!

ben
 

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While seafoam can do wonders to keep a clean set of carbs clean it usually doesn't work miracles on dirty/plugged ones. It sounds to me that you have some plugged passage ways. Guessing your pilot and slow jets are suspect. Pull the carbs and do a thourough clean (consider getting some dip and immersing them) and rebuild. Be sure to blow through all the passage ways with compressed air.
Oh and welcome. You can find a manual for you bike in the manuals section.
 

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Sensei
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Yes, It uses a "wasted spark" (one coil with twin HT leads)....
I second a thorough carb cleaning, the factory specs are already set very lean.....
Improper carb synchronization will cause some "odd" idling.....check that as well....
:D Steve
 

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Discussion Starter · #4 ·
thanks for the quick replies!

I'm considering doing a carb clean, but I was thinking, wouldn't gummed up jets have the opposite effect...i.e. make the bike MORE likely to stall out at very small throttle stop openings?

as it stands now, the bike refuses to stall even at the leanest idle mixture with the throttle stop closed as far as it will go...I was thinking maybe it was the victim of a poorly executed carb cleaning, maybe the previous owner hit the pilot jet with a too-large wire brush and expanding the jet openings inadvertently...would that make sense?

also, steve, thanks for the confirmation about wasted spark...

ben
 

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Sensei
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Ben, I believe you are confusing the mixture screw (pilot screw) with a throttle stop screw..... On the 400's, the throttle stop (idle screw) is a finger wheel knob between the carbs... It adjusts only the amount of opening of the butterfly plates, and has nothing to do with rich or lean mixtures.....
The mixture screws (pilots) on the other hand, control the fuel/air proportions at idle.... IF adjusting them has no effect, the carb passages are dirty...
 

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Discussion Starter · #6 ·
My post was a little muddled on this point I think...the pilot screws do affect the idle–it goes down as I lean out the mixture and back up if I start turning them out–but even if the screws are fully set (leanest mixture) the bike still idles, even if I then adjust the throttle stop screw to the smallest opening...

I guess what I'm trying to say is that it seems like the bike is getting such a rich mixture even at the leanest pilot screw setting and smallest throttle stop opening that it keeps on idling no matter what I do...

I'm definitely willing to believe that that could be caused by dirty carb passages even if I don't fully understand why...since I understand pretty little about this thing :D
 

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It sure sounds like your throttle plates are not completely closing. I don't know if adjustments to the throttle cables could cause this (I think you had them disconnected already?). The idle adjustment screw has a lot of travel in the "slow" dierction but will only open the plates a very small amount.

[attachment=1:1x0gj9ug]Carbs-b3.jpg[/attachment:1x0gj9ug]

I guess the fast idle linkage between the choke shaft and the throttle shaft could be stuck.

[attachment=0:1x0gj9ug]Carbs-b4.jpg[/attachment:1x0gj9ug]

Otherwise they probably need to come apart for a good cleaning. There have been some kits showing up on e'bay recently for about $45. It's a good deal if you need them, I'd jump on them myself if I hadn't already bought a pair elsewhere.
 

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Discussion Starter · #8 ·
JT-

thanks for the reply, complete with illustrations!

I'm going to check all this out when I'm back in town next week, and I'll post here what I find.
 
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