Spark plug 3 and 4 completely black

[donald90]

I have a 1981 CB750 custom currently spark plug #3 and #4 are being completely with a powdery black substance that can easily be wiped off. The first thing I did was adjust the pilot screws on the carbs which adjusts the gas flow. I basically had to turn the pilot screws in all the way and cut off the gas supply to stop the fouling of the plugs. I figured this shouldn't be right so now I just checked my valve clearance and
Cylinder              Intake                    Exhaust
#1              0.004 & 0.004            0.003 & 0.0025
#2              0.003 & 0.004            0.003 & 0.004
#3              0.003 & 0.003            0.004 & 0.004
#4              0.004 & 0.004            0.003 & 0.003

I believe these are within the correct specifications so I'm at a loss as to what is causing the huge difference in gas consumption for the cylinders. This is also under the assumption that Cylinders #1 and #2 are running correctly considering they have lightly brown spark plugs.

Using standard airbox, Carburetors have been cleaned very well.

Any help is greatly appreciated

[LukeM]

Hi Donald, and welcome to the forum.

If you haven't done so already, you're welcome to download the factory service manual from the home page.  It's got some great details on the carbs and how to set them up to factory spec.

You've given us great info on the valve clearances.  You seem to know your way around the bike's motor.
It is probably not ignition related, as cylinders 3 and 4 are on different coils.  Same goes for the ignitiion pickups.  I suggest looking for the following:

Leaking or cracked carb boots.  Check intake and head side.  Sometimes soaking them in a diluted wintergreen solution can seal them up properly if they are rather out of round.
If you had the whole carb assembly blown apart, something may have gotten out of line when you put them back together.
Leaking or cracked vacuum diaphragms.

I'm sure others with more experience will chime in, but this should be a good start.  Good luck, and keep us in the loop.

Luke M

[donald90]

Thank you for your response.

The carb boots are brand new and I blew propane around all of the carb boots and the tachometer showed zero changes in rpm. I am not sure what you mean by head side. also not sure on what the vacuum diaphragms are, i have had the carbs pulled apart completely for cleaning about 8 times in the last 3 years.

[TOOLS1]

Dry black soot means you are too rich. Did you chang the jets?
TOOLS

[donald90]

the jets are stock and pressed in might be  a pain to switch but im not sure. Could the black soot mean to gas rich and normal air or could it be not enough air normal amount of gas.

[TOOLS1]

It could be either scenario. Is there a restriction in the airbox? Are you sure the soot is dry? Is there any smoke coming from the exhaust? If so, what color is it?
TOOLS

[donald90]

Air box is clear, and clean. the soot is dry, the exhaust is blackish.

[TOOLS1]

That is definitely rich. I myself do not have much experience with the CV carbs, but I will bet that there is something missing, or loose in those carbs. We do have a good write up on here about rebuilding the CV carbs. You might want to check it out, and look for missing parts.
TOOLS  

[shinyribs]

No vacuum diaphragms to worry about, per say, since these CV's use pistons rather than diaphragms. But Luk may be referring to the air cut-off's.

One thought, are your choke plates opening on the carbs for that cylinder? I'm trying to imagine what would be separate from 1/2 & 3/4. I know the choke rod is split in to pairs. That's all I got for now.

[donald90]

@TOOLS1   I have been using the honda manual (Downloaded from this site) and I
did use the carb rebuild guide (also downloaded from this site) followed the guide
exactly there were zero missing pieces. I have been using this forum for about 2
years now. I've learned alot from this forum.

@shinyribs  The air cut off valves are brand new.  I have been concerned with how
choke rod works. I feel as if the springs for the choke rod are not the correct tension
but I tested it after connecting the carbs together and flipping the choke plates open
and close a million times before actually putting the carbs on the bike. In other words
the choke plates seemed to be working. How would I be able to tell if the choke plates
were opening or closing without removing the carbs?

Thank you both for your replies.

Also I'm currently stuck on trying to get the tachometer cable back into the housing on the cylinder head cover. the cable will go in part ways but not all the way which then blocks the screw from locking it in place. The cable needs to go in about another quarter of an inch by the looks of it but it will not go any farther in.

[Maz]

Hi Donald,

I had a similar problem after replacing my carbs and I think you are on the right track with the choke plates. Mine operated perfectly on the bench but, once on the bike, 3 & 4 remained closed due to the 'light' tension on the spring that makes both halves of the choke rod turn together. You can check without removing the carbs but it's a bit fiddly. If you loosen the top bolt on the airbox and the hose clips on the rear carb boots, you should be able to slide the airbox back enough to pop the boots off the carb intakes enough to see the choke plates. Unfortunately the carbs will probably have to come off to rectify the problem if those two plates are remaining closed. The spring tension is minimal so any slight mis-alignment between the two pairs of carbs causes binding. I found that if you loosen all of the screws that hold the carbs to the two steel 'rails', they can be moved around enough to line things up properly and allow the spring to do its job.
Also, while the carbs are off, unbolt one pair and move the pairs apart slightly and it's possible (although very fiddly) to unhook the choke spring and pull the hooked end all the way round and then re-hook it onto it's seat. This means that the spring is wound round by 1 extra turn giving much better tension and giving more assistance to choke plates on #3 and #4.

I hope this helps and you don't get too many sore knuckles getting the carbs off and on again!

Maz

[shinyribs]

donald90 wrote

 I feel as if the springs for the choke rod are not the correct tension but ....

Those springs feel super light. Much lighter than I expected,too. They aren't nearly as strong as the actual throttle return springs.

donald90 wrote

 How would I be able to tell if the choke plates were opening or closing without removing the carbs?

You can feel in between the carb bodies with one finger and feel the rod rotate that connects the choke plates when you pull the choke cable. You should also feel them close when you push the choke cable back in, obviously. If they are sticking in either direction you should be able to roll the rod with your finger and see if it moves.

donald90 wrote

Also I'm currently stuck on trying to get the tachometer cable back into the housing on the cylinder head cover. the cable will go in part ways but not all the way which then blocks the screw from locking it in place. The cable needs to go in about another quarter of an inch by the looks of it but it will not go any farther in.

The inner cable probably isn't lining up. I find it easiest to hook the tach cable to the valve cover first, with the inner cable pulled out slightly. Then twirl the inner cable as you slide it back in. When it engages you will know it because you wont be able to twirl it anymore. Then hook it up to the tach. The engine side of the inner piece is shaped like a flat blade screw driver ( looks like _ ) so it has less chances of lining up on accident compared to the gauge side, which is a four-sided square drive.

 

The inner cable slides in and out freely of the outer sheath. Now would be a good time to give a cleaning/lubing while you have it apart. It might not really need it, but it can give you a much smoother needle at the gauge if it's well cleaned and lubed.

Hope this helps.

[donald90]

The choke plates are not binding. The connecting rod in between carbs 3 and 4 for the choke plates spins freely for both open and close.

[shinyribs]

Bummer. That would have been an easy fix.  

I don't have any other suggestions right now, but I'm still trying to think....

Are you certain you reinstalled the two rubber plugs for the mid jet in 3 & 4? That could cause a rich condition, I think?

[LukeM]

On the CV carbs, inside the cap at the top, is a very large very thin rubber diaphragm and a long spring. As these carbs are going on 30+ years, there's bound to be some wear and tear on that material. Pinhole leaks or tears will cause problems with vacuum in the carb.

Just one more thing to look for.

After thinking about it for a while, dry soot means classic rich condition and good spark.  Wet black soot would mean rich with weak spark.  That's why I didn't suspect ignition or spark wires or such.

Luke M

[Maz]

LukeM,

if I'm not mistaken, these carbs don't have rubber diaphragms, they're metal ones attached directly to the slides which rise and fall inside the carb top.

Maz

[shinyribs]

Yep, these particular CV's dont have rubber diaphragm actuated slides like this.



They are the piston type.



There is a sealing ring that under the cap that, if not sealing, would do exactly as Luke is describing.

[donald90]

How would I test if they were not sealing properly, I feel as though its unlikely that 3 and 4 would be bad at the same time.

On Fri, Mar 13, 2015 at 11:45 PM, shinyribs [via Honda CB750'S] <[hidden email]> wrote:

Yep, these particular CV's dont have rubber diaphragm actuated slides like this.



They are the piston type.



There is a sealing ring that under the cap that, if not sealing, would do exactly as Luke is describing.

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[Maz]

Hi Donald,

something is almost certainly amiss inside the carbs. Taking them off and removing the float bowls is the only way to check. Take a look at the rubber jet plugs inside - They go extremely hard with age and for the cost of a few pounds (dollars), they are well worth replacing if they feel very hard. They should be a tight fit, otherwise they are not doing their job and will let fuel through.
You say the cut off valve diaphragms are new, but when they were fitted, did the spring and tiny 'O' ring go back in?
You really need to check that all of the jets in the carbs are tight and that the main jet tube is pushed home firmly - When you look through the carb with the choke plates open, you should be able to see the tip of the brass tube (the bit that the needle slides in and out of) sitting slightly proud of the bore.

With regard to the vacuum chamber sealing rings, so long as they are smooth and not broken/damaged, they should be fine. They should have a lip on one side.

One last thought, are 3 and 4 flooding? Check the overflow tubes to make sure they're clear, including the brass overflow tube inside the float bowl. If any of these are blocked, the only place for overflowing fuel to go is straight into the engine. If they are flooding, the floats and valves need to be examined.

Maz

[LukeM]

Based on this information, I withdraw any and all comments on this thread.
We now return you to your regularly scheduled repair discussion.  

Luke M