Topic: yet another running rich question

[John]

Sorry, running rich seems to be quite the popular question right now but this question is kinda long to just tack onto someone's thread.

My 71 cb750  runs and idles pretty well ( i start every post with this sentence which begs the question - why the hell am i messing with it - oh well) It seems however to be running rich. I have been doing a lot of testing (timing, synching carb etc.) as well as some short rides around chicago and the plugs are always sooty black. 3 and 4 give me hope; They have some brown/tan spots but 1 and 2 are covered with carbon. Compression is right around 175 on all 4 . Valves and timing are spot on. Carbs have been "berryman-ed" so many times it's a small miracle that there is any metal left. Float bowls are not overflowing. Last weekend i emptied each float bowl and measured the contents. 1 and 2 were a tiny, tiny, bit higher. I lowered those floats but it didn't seem to affect anything. Oh, also the jets are 40 /120. Jets are all new (keyster). Needle and needle jets are new  (also keyster i think). Air screws are all 11/2 turns out. What else? I have stock air box, 4  into 4 hm300s with baffles and a wiseco 836 kit. Is it possible that it is as simple as turning the air screw out to 2 or 21/2 turns. It just bothers me when things get too far from spec. It makes me feel like there is a problem that i am overlooking.  If anyone has any ideas of possible things to check please post them. I'm going to attack this thing this weekend (depending on how things go i may mean that sentence literally : )

thanks again to everyone for all the help,
john

[750deepsouth]

On my K2, with similar symtoms, I turned the idle mixture screws out to 2 turns - huge difference.

[TwoTired]

There is a school of thought that says increasing the bore size and thus the volume of air of the engine mechanicals, increases the volume of air sucked through the carbs.  The increase in volume of air, lowers the pressure, relative to atmospheric, from one side of each jet or metering device in the carb, to the other side of it.  The jets, therefore draw more fuel than they did with the lower cc sized engine.  Unless you increase the bore size of the carbs, too, you may need to REDUCE the jet sizes.

Or...

There are also air metering jets in the carb that aren't commonly changed.  One of these air jets is adjustable as your idle screws.  Another, is fed from a small tube you will notice is in the intake side of the carb bore.  This tube feeds air to the emulsion tubes located between the slide needle jet and the maiin needle jet. Somewhere along this inlet tube is an air jet.  It is not one that is replaceable but is drilled during manufacture.   Exact specifications for the K2 are hard to locate.   The Honda shop manual lists a #120 air jet (pg.225) for the F model, which by the way has a 105 main jet.   And a #150 (pg 257) for the K7K model, which has a #120 main jet.  But, these are also very different carbs between these models.

Then there's the emulsion tubes that use the air from the air jets.  The emulsion tube itself has holes drilled into it, and the sizes and placement along its length govern air premixed into the fuel before in enters the main venturi.  I haven't found any documentation about these hole size specifications for the CB750.  But, there were differences between the CB500 and CB550 emulsion tube hole sizes (.7 and .9 respectively) I assume to be attributable to the bore size increase. (same carb bodies)  But, I admit I don't know this for sure.

At any rate, if your plugs are going sooty, you can certainly change the airscrew settings easy enough.  If you turn them out too far, you'll probably notice a loss of throttle response from low rpm.  You'll just have to turn them back in a little bit.  If your plugs still get sooty after that, then slow jet, needle, and mains will probably need addressing.  Or, you could experiment with air jet and emulsion tube hole sizes to get more air into the premix.

Sounds like fun, eh?

[John]

Lloyd,
Thanks for the answer (i am constantly amazed by the knowledge of people on this board and their generosity of time answering my questions). It's an interesting idea that the 836 with it's additional vacuum might draw proportionally more fuel. I think that may be right (my idea before installing was that air and fuel would increase equally). I am still thinking about this one. I would love to compare jetting notes with people who have a similar setup (perhaps another post). My plan is to track down an accurate tach (what are people using - ah yet another post) and work on fine tuning the idle screws this weekend.
thanks again,
john

[eldar]

it may not bet the best test but for short term idle and rev, remove the air filter and see what happens. Also with the filter in, do some high speed running after cleaning the plugs, then cut the engine at high rpm. then take them out(mechanics gloves are great for this to prevent burns!) and see if they are still sooty. If not very bad, then you should look at your idle circuit. I think this procedure is called a "plug chop".

[Bob Wessner]

as well as some short rides around chicago and the plugs are always sooty black

I agree with Eldar. The riding you describe does not sound like you would get an accurate reading on the plugs, at least as far as the high speed jets are concerned so it may well be your current issues are surrounding the low speed jets/air screws, etc.