Topic: running lean...ideas welcome

[KeithB]

1978 CB 750 K with KieHin carbs.The ones with the float bowls SCREWED on! &^%$*PITA
4 into 1 baffled MAC with pod filters.
Carbs are clean and working well as the bike starts, idles and drives just fine and has no flat spots through throttle range.
Plugs are a bit on the white side though.
Just took the carbs apart to change the clip setting on the needles and found there is only 1 groove. It looks like it is a fixed setting. There are set screws on the tops of the sliders which I presume are for balancing, not changing mixture.
Soooo....drilled out (I know, I know!) the jets to 1/16 or about 155.
Still running lean.
The float level measurement was around 14 so I re-set to 12.5. This should raise fuel level in float bowl.
I do have a set of stock intake stacks , a set of 120 jets and possibly a stock air box.
What would be the next move before putting the carbs back on for a road test?

[Duke McDukiedook]

Buy larger main jets and keep the float level at the specified setting.
You may have to buy larger pilot jets also, there is no single magic setting that will put your carbs where they should be.

It will take a lot of experimentation to get your plug readings right, you can't just slap pods on and immediately expect them to work perfectly.

Running your engine lean is not good for the engine in the long haul.

[KeithB]

Running your engine lean is not good for the engine in the long haul.

you can't just slap pods on and immediately expect them to work perfectly.

Of course! That's why I started this thread.

Fortunately, I have not had the huge problems that others seem to have other than the lean mixture.
I just found that there are shim kits available to allow for adjustment of the needle.
Anyone done this before?

[Inigo Montoya]

How much have you changed on your engine? I mean is it a 1080cc bore or something? I run a k&n in the box and I still run everything with the stock jets.
If you are that lean, then something else is going on, I doubt it is the jets.
The later carbs are awesome for tuning without changing jets. The 78 does not have clip positions but it usually does not need them.
How sure are you the carbs are fully clean? Have you checked for air leaks? What plugs are you using and what ignition?
Even running a k&n and a 4-1, not jet changes are needed. Have you turned out your fuel screws? Out makes it richer on the idle and affect a wide range.

Also, you cant go by the dans mc charts and such as those were made quite a few years ago and gas is different these days.
I have found that a VERY light tan, almost white as delivered the best. What most consider to be correct, only really applies to the 69-76 carbs. the 77/78 were made to run and deliver throttle response on a leaner condition.

[Duke McDukiedook]

He is running pods also on it.

You would be surprised at the amount of people that come on and are surprised their bike "ain't running right" since they slapped pods on and touched nothing else on their bike.

[KeithB]

Even running a k&n and a 4-1, not jet changes are needed.

That's what got me asking questions.
The engine is stock, plugs are D8EA's and ignition is stock timing (with strobe)
Possibly I should drop the plugs one temp range.

Have you turned out your fuel screws?

I have not and am not that familiar with the effect it would have on mixture other than idle. Bike idles perfectly as it is.
The plugs are on the white side of tan so I think I'm close but a 155 main jet seems outside the norm and I would expect a more dramatic change.
Carbs are clean...checked them again and all boots tight.
This lean condition is the same across all cylinders so it's not like I have a leak in one or 2 boots.
As for float level, it is posted somewhere here that 12.5 is stock.

[Inigo Montoya]

Stock is NOT 12.5. It is in fact 14.5 +- .5.
I would see about posting pics of the plugs because as I said before, the carbs are made to run lean, leaner than the 69-76 by far.

[Gordon]

I do have a set of stock intake stacks , a set of 120 jets and possibly a stock air box.
What would be the next move before putting the carbs back on for a road test?

The next move should be to put the stock intake components and jets in.  I don't care if someone wants to run pods on their own bike, but do you know for a fact that it's in perfect running order with all the stock parts installed? 

[eurban]

Mains dominate the mixture from 3/4 to WOT (wide open throttle) throttle.  Have you tried running the bike at close to WOT.  It will probably sputter and die as a 155 main is way too large for your application.  Consider stepping away from power tools and focusing on some details instead.  . . .First, you are absolutely positively wasting your time with carb tuning if your carbs basic functions and settings are not worked out first. Air leaks, improper float levels, impaired idle circuits (you did pull out your pressed in pilots and cleaned them right?) proper synchronization, functioning accelerator pump, etc etc must all be worked out before messing with main and pilot jet choices as well a main jet needle position (you can shim the main jet needle with tiny washers if you need to richen the mix in mid throttle ranges).   Engine mechanicals, and the ignition system must be up to snuff too. . . .New plugs will not look magically tan after a few minutes of running so consider how much you want to rely on that info to make changes to your carb tuning. Also used plugs will not magically clean themselves and reveal current conditions once you make changes. Comparing a plug to the various charts you will find online or in manuals requires that the plug has been use for good while and it only gives you an overall picture of your carb tuning, and won't really tell you specifically if your main jet, or main jet needle position, or idle jet /mixture are chosen correctly.  Plug reading (with new plugs, as when you do plug chops) is possible but as mentioned, it is difficult with modern fuels and you really have to know what your are looking for.  Google for "reading spark plugs" and you might be surprised at how involved it is.  For instance a proper main jet plug chop involves a new plug installed in a warmed up motor, bike quickly taken up in to the higher gear ranges and run at WOT for a good 20 seconds or so followed by pulling in the clutch, killing the motor and coasting to a stop. Plugs are pulled and examined deep down in the well where near where the porcelain center meets the metal body of the plug. The carbon ring that appears and its thickness from the metal body tells you how your mix is.  Some cut the plug apart to see this or use flashlight and a magnifying glass. Honestly I have difficulty, reading plugs and I have tried alot. . .My experience is that the #35 pilots jets on the 750 with PDs are up to the task of handling most exhaust and intake modifications.  They and the passages that feed them must be absolutely clean.  With the Pods (what kind?, as TT is so fond of saying, "Pods" is not a sufficient specification) and exhaust you will likely need to shim the main jet needles mildly to get the correct mid throttle mix and will likely need to go a few steps larger on the main for the 3/4+ throttle performance. This means in the low to mid 120 range not anything remotely close to what you have created with your drill!  The main has an affect in mid range throttle performance so you should get it sized correctly (tested at close to WOT) before deciding to shim your needles.  You can use your choke to help you diagnose whether you are lean or rich at various throttle positions as applying slightly will richen the mix and then the result in performance can be evaluated. . . .You could also put back on the stock airbox, return the main jets to stock and likely have a nice running bike that makes power through a wide range of RPMs and throttle positions.

Edit:  Here's a link to an old but excellent article on spark plugs.  Reading for mixture is discussed towards the end . . . .http://www.strappe.com/plugs.html

[KeithB]

There is much to consider in that last post. I appreciate your extensive response eurban.
I know the drilling was a crazy thing to do but I just wanted to do something way off the mark to see what changed.
It was an experiment only. Honest, I do understand how the carbs work. Bikes are not the only carb experience that I have had.
This post from "kandrtech" ...The factory manual indicates that the float setting should be set at 12.5mm.
You are saying this is incorrect and should be 14.5? It seems they were at 14.5 to start with. No problem to restore.
My question is, would a 12.5 setting make the mixture a bit richer or is float level not relevant to mixture?
The bike was on the road the whole of last season and I have run the bike easy and hard up to redline and there is no flat spot or hesitation or any kind of power problem. No pinging either.
Actually, the  bike runs great. I just thought the whitish tips of the plugs indicated a lean mixture and might indicate she's running a tad hot.
I guess that just considering the colour of the tip is not satisfactory.
It seems the '78 is much different to my '72 in terms of fuel mixture.
I am aware that the 77-78 models were setup to be leaner and it looks like the only way to richen  the mixture would be to shim the needles.

I shall re-install the 120's and try a stock air box to see if it has any effect.
I will also stay away from the power tools

[BobbyR]

I have the same bike with a stock airbox and 120 mains. I am running a bit rich which is OK. Unlike the earlier carbs the 78 screws are true mixture screws not air screws. Unless you pulled the pressed in tubes, the carbs are not clean.

[scottly]

Just for grins, try posting a pic of your plugs..

[Spanner 1]

+1 on the float level@ 14.5mm for the '78 carbs ( PD 42B )...... also have'nt had confirmation about the slow jets being removed and cleaned, I believe these bikes will run off the idle jet/ accelerator pump and main jet purdy good with the slow jet blocked IMO., but leaning-out the mix considerably without the slow jets 'input'....

[dave500]

your saying the motor runs well,,no flat spots,,good power throughout,,try a colder plug.

[KeithB]

I must admit that hearing that others have these bikes running rich makes me think I might have missed cleaning something properly.I will get into the carbs and clean again, and again, and again...
A colder plug is something I will try as well as adjusting the air screws.
The bike runs very well and it's not poor performance that got me looking at the fuel mixture.
I just don't want to run too lean for too long as I have seen what that can do to valves .
Pic of plug to be posted later.
Thanks for the suggestions

[BobbyR]

I must admit that hearing that others have these bikes running rich makes me think I might have missed cleaning something properly.I will get into the carbs and clean again, and again, and again...
A colder plug is something I will try as well as adjusting the air screws.
The bike runs very well and it's not poor performance that got me looking at the fuel mixture.
I just don't want to run too lean for too long as I have seen what that can do to valves .
Pic of plug to be posted later.
Thanks for the suggestions

Keith, those are not air screws, they are real mixture screws. The earlier CBs had air screws. Our CBs have state of the art  30 year old technology. 

[KeithB]

Our CBs have state of the art  30 year old technology.  Grin

LOL!

I'll give them a try.

[Inigo Montoya]

You would be surprised at the range of adjustment from these fuel screws.

[shizzomynizzo]

your saying the motor runs well,,no flat spots,,good power throughout,,try a colder plug.

I remember reading somewhere that a HOTTER plug will richen, and COLDER plug will, in fact make it leaner. Can anyone confirm?

I am experiencing a lean condition on my kz550 and am trying to work it out too.

Thanks

[Gordon]

I remember reading somewhere that a HOTTER plug will richen, and COLDER plug will, in fact make it leaner. Can anyone confirm?

The heat range of the spark plug has no effect on the air/fuel mixture.  It can have an effect on how much of the deposits left on the plug are burned off, but changing plugs to fix a lean/rich problem will only help with one fairly minor symptom of the larger problem.  

[shizzomynizzo]

Gordon,
On the KZ, I replaced the plugs with NGK, had ND before. There is not a problem with power, starting, or anything like that. The plugs are a little whiter than I would like though. I don't know that I would even call it lean. Would you worry about that? I just don't want to burn a hole in the piston.

BTW sorry to threadjack!

[Gordon]

Gordon,
On the KZ, I replaced the plugs with NGK, had ND before. There is not a problem with power, starting, or anything like that. The plugs are a little whiter than I would like though. I don't know that I would even call it lean. Would you worry about that? I just don't want to burn a hole in the piston.

BTW sorry to threadjack!

I don't like to see any white deposits on mine.  I tend to err a little into the dark brown most of the time.

[KeithB]

I went to a hotter plug and my '72 and it worked very well.
They are a bit rich to start with and the hotter plug  cleared up a bit of a fouling problem.
Still a bit brown but engine starts and idles right off the bat.
I'll make some adjustments on the '78 this week and let you know what happens.

[Inigo Montoya]

Just remember, the 77/78 are made to run leaner than the earlier bikes. So in many cases, what is perfect for the early bikes is to rich for the later ones.

[KeithB]

The screws that you are referring to are on the underside of the carb at the exit side, right? (only ones I could find)
So, which way for more fuel? Screws in or screws out?
The Honda manual explains that these are pilot screws for setting idle but you suggest they will affect low throttle positions as well?