Topic: Leaning out mixture question

[jn895]

I took my CB750K5 out for a ride yesterday and today after having worked on it for quite a long time (about a year on and off).  Yesterday it ran well; today, however, the cylinders stopped firing one by one until finally it wouldn't start.

I pulled the plugs to have a look and they were all black and sooty at the ends.  I ground them against the engine and hit the starter but only one of them (the one from the last cylinder to die) produced any spark, and only then after a few attempts.

So, should I assume the mixture is too rich and try to lean it out? 

Some relevant other info:  I have been playing with the idle screw a lot, because the idle seems to be all over the place.  I'll set it for 1700 and it will idle there for a while, but the next time I start it up it will be way lower, so I turn the screw in, but then the time after that it idles way too fast, etc.

When I went to the store to buy the spark plugs that are currently there (after having fouled out the old ones after leaving the choke on too long) I asked for NGK D8EAs, but the store didn't have any.  The guy gave me some he said were equivalent, but I'm not sure they are.  Will definitely get NGKs now.

The cylinders died in the order of 4, 3, 2, 1.  I thought maybe it was a coil problem, but everything was working fine yesterday, and besides from what I've read on here the coils control 1 and 3, and 2 and 4.  The bike does have one other electrical problem (right side turn signals don't work) but it seems these are probably not related.

Battery is fully charged.  Tank is full of gas.  Carbs and petcock have been cleaned recently. 

Any thoughts?  If I'm right that I just need a leaner mixture, can this be done without rejetting the carbs?

[Johnny5]

Dirty air filter?

[jn895]

I just put in a new Honda filter this morning along with some new plugs, but they sooted up again on the way home.  No so sooty that the bike isn't starting, but pretty black and I'm sure if I keep riding it they'll foul out completely.

Should I try to lean out the mixture with the air screw?  I haven't ridden it over 40mph so perhaps the problem is the idle mixture?

[Mdub]

Actually, the coils spark 1-4 and 2-3.
The air screw only effects idle and low speed.
Pull a float bowl and a jet holder (6mm socket) and see what size main jets you have.

[Chester345]

Just to be sure, you are only moving the idle screw when the bike is good and hot right?  You could yoyo forever if 1 time you do it hot and the next cold.

Where are your mixture screws now?

Definitely sounds like a gremlin in there though...

[jn895]

Perhaps I'm not letting the bike warm up enough before I set the idle--I'll try that and see if it improves things.

I'm going to go out and pull the carbs now.  Here's a side view; can you guys tell me whether A or B is the screw for adjusting the idle mixture? 

And here's a picture of the whole mess, as well.

Thanks guys, I'll post an update after I have a look at the jets. 

[HondaMan]

Those are 657 carbs. Set the idle air screws to 1.0 turns to start, on all. (Doesn't matter if they are "A" or "B", or even if they are "081" or "064", they are all the same units). Consider going in to 7/8 of a turn, if you are stuck with ethanol-enhanced fuels (like we are here in Colorado). These will act like a richer fuel when the engine is warmed up. This will cause some cold-bloodedness until the engine has run for a minute or so, though.

The idle SPEED is set by the large screw on top of the throttle rack: don't use the idle mixture screws to adjust the idle speed. 

[Mdub]

I  think he was which one is the air bleed screw, that would be 'B'
'A' is the screw for the synchronizer port.
Mike

[jn895]

Thank you Hondaman and Mike.  Hondaman, please excuse my ignorance, but what does it mean to set the screw "1 turn to start?"  Does this mean to turn it all the way to a particular direction, and then out or in one turn? 

[tlbranth]

what does it mean to set the screw "1 turn to start?" 

turn clockwise until seated (it stops turning) - don't over tighten. Then back it out 1 turn

[jn895]

Cool, thanks for clarifying that for me. 

I took the carbs off today and had a look at the main jets.  They are 105s.  This confuses me since stock is 120, but a smaller jet would make the engine run leaner, wouldn't it?  It is still more baffling when I remember that, when I bought the bike, the airbox was off... if the guy was running it without the airbox, wouldn't a 105 jet be super lean? 

[tlbranth]

I'd be inclined to thoroughly clean the carbs & set the float levels. Also make sure your needles have the clip in the center groove. See how it acts then.

[jn895]

Update:  rrrgh.  Float height checked, carbs cleaned very thoroughly, new air filter installed, air screws set at 1 turn out, and main jets are, if anything, small rather than too big.

Plugs still black after a couple miles.  I'm now thinking maybe it is not an overrich mixture problem but oil fouling the plugs.  Maybe the piston rings need replacing?  Hopefully not, but I can't see how the problem could still be in the carbs.  It has sat for a while... maybe I should just try to get a bunch of miles on it and see if the oil problem goes away?  I'm getting whitish blue smoke out of the exhaust, not black.  Thoughts?

[TwoTired]

Jet's will remain marked #105 even if someone drilled them out to a larger diameter.

You should find out what clip position your slide needles are in.

You should also verify the air jet passageways are clear to both the idle/pilot jet and the mains emulsion tubes.

[Spanner 1]

I'd bet money that those 105 jets are drilled out, as TT says. With pods and those 4into2 drag pipes, 105's would be too lean.........