Topic: Carbon fouled plug. What am I missing?? I GIVE!!!!

[grumpy]

#1 plug is way sooty (dry carbon, not oil).

Checked the:

float condition - fine, no cracks, not bent
float height - same as the other 3, clear tube method
float needle - works, clean, shuts off when it's supposed to
slow jet - size 40 and cleaned, same as the other 3
bowl stand pipe - OK
bowl overflow tube - clear
fuel rail vent tubes - OK
needle jet - cleaned
needle height - one from the bottom, same as the other 3
needle type - same as the other 3
air screw - Keihin with the holes, 1 turn out, same as the other 3
air screw passage - clean
main - same as the other 3
air box - nothing funky
carb boots - fine

tried a different plug - same thing
plug wire - makes a seemingly fine spark when shorted to the block

Checked all of the above on the other 3 carbs - nothing odd

SO WTF ?


Full moon last night?

[DavePhipps]

Try 1.5 turns out on the mixture screws.
What air cleaner and exhaust are you running?
What size main jet?
Also I'm assuming this is an early 500 or 550 from the carb specs.

[grumpy]

<<====   750k3

standard airbox, K&N filter, cleaned and oiled
120 mains

[Rgconner]

Carb sync?

High relative vacuum would draw more fuel I think.

Move that plug wire if possible, maybe it is arc'ing and not getting a good spark?

Only other thing I can think of is... No, about out of ideas. Check the emulsifier tube? Move brass to another carb, see if the problem follows? Maybe a worn needle?

Have a few beers and piss on it?

[lucky]

#1 plug is way sooty (dry carbon, not oil).

Checked the:

float condition - fine, no cracks, not bent
float height - same as the other 3, clear tube method
float needle - works, clean, shuts off when it's supposed to
slow jet - size 40 and cleaned, same as the other 3
bowl stand pipe - OK
bowl overflow tube - clear
fuel rail vent tubes - OK
needle jet - cleaned
needle height - one from the bottom, same as the other 3
needle type - same as the other 3
air screw - Keihin with the holes, 1 turn out, same as the other 3
air screw passage - clean
main - same as the other 3
air box - nothing funky
carb boots - fine

tried a different plug - same thing
plug wire - makes a seemingly fine spark when shorted to the block

Checked all of the above on the other 3 carbs - nothing odd

SO WTF ?


Full moon last night?

What size motorcycle and what year???

A little more info and I could help you.

What were the floats set at
I do not care about the clear tube method.

Can you tell us the needle clip position from the top from now on? Thanks.
Sounds like the needle clip is on the 4th notch from the top.

Mixture screw 1 turn out.

Stock air box  with K&N filter (oiled).
What kind of exhaust system?

Do you have the 7A style carbs??? OR a different number?

OK...#120 mm  mains.
Idle jet size???

When you say the carbs are the Keihins "with the holes" do you mean with the holes in the mounting plate on the back of the carbs?

I really need to know the serial number stamped on the carbs.

[grumpy]

<------  750K3

When you say the carbs are the Keihins "with the holes" do you mean with the holes in the mounting plate on the back of the carbs?

I was referring to the type of air screw: Keihin OEM with the metering holes (as opposed to aftermarket)

Similar to : http://www.hondachopper.com/garage/carb_info/idle_screw_mod/before.jpg

HM341 pipes

Idle jets are 40s

657A

[grumpy]

I should add:

This is a fresh carb rebuild (about 4 days ago). It wasn't doing this before I cleaned and, uh "fixed" them.

I synched them after the rebuild - but I'll do it again.

I'll try swapping the plug wire with the other side & see if the problem moves.
That's about the only other thing I can think of.

Have a few beers and piss on it?

No doubt.

Carb spray cleaner and a lighter sounds appealing, too.

[scottly]

I should add:

This is a fresh carb rebuild (about 4 days ago). It wasn't doing this before I cleaned and, uh "fixed" them.

Recheck the fuel level with the clear tube. (I don't give a fat rat's ass about the mechanical float level setting )
Is the choke valve opening all the way? Carbon fouled plugs are caused by too much fuel/not enough air, period. Bad spark won't foul plugs, because no spark=no fire=no soot

[scottly]

[
What size motorcycle and what year???

Lucky, READ reply #2....

[grumpy]

Carbon fouled plugs are caused by too much fuel/not enough air, period. Bad spark won't foul plugs, because no spark=no fire=no soot

I was thinking maybe the spark was weak. Enough to fire but not a complete burn.

OK, after I ride it back from work tomorrow I'll recheck the plug and see what it looks like.
If sooty then I'll put in a clean plug, let it idle, and check the plug.
Then another clean plug and I'll ride it around the block a few times in 2nd at about 2,500.

Then recheck floats and synch as needed.

Then I'll mow the lawn and take out the garbage (Monday is trash night)

[dave500]

check the cap resistance.

[Duke McDukiedook]

Like Dave said check the cap.

[grumpy]

how do you do that?

[dave500]

youll need a multimeter,set it on ohms,and you want 5kohms reading through the cap only youll have to screw it off the lead.

[grumpy]

Is the choke valve opening all the way?

yeah

youll need a multimeter,set it on ohms,and you want 5kohms reading through the cap only youll have to screw it off the lead.

OK - that's easy.
Will do.

[phil71]

If it didn't do it before the carb rebuild, and the carb rebuild is the only thing you did, then I bet a jet vibrated out and is sitting in the bowl.

[Duke McDukiedook]

If it didn't do it before the carb rebuild, and the carb rebuild is the only thing you did, then I bet a jet vibrated out and is sitting in the bowl.

Makes sense, I thought that too but thought that cap would be a quick check.

[grumpy]

plug cap reads 4.90K
damn, I was hoping that was it.

all jets, needles, other brass bits have been rechecked.

didn't get a chance to check synch. I'll do it tomorrow.

this is getting annoying.

[lucky]

I have been reading all of this and keep wondering just how full the float bowls are?

The floats have a measurement they are supposed to be set to.
It even says in the Clymer's workshop manual on Page 60- #7 Check and adjust the float level as described under Float Level, adjustment,Chapter six.

See Figure 13.

THEN AFTER, that #8 Describes marking the float levels and using  a clear tube method.  AFTER the float levels have been set.

And there is another forum member that is having the same exact problem (rich black plugs), and the float needles were changed from metal needles to rubber tip float needles. A float needle problem.
But the bike is running extremely rich no matter what remedy or electrical or mechanical parts are investigated.

I would like to see the op put in new float needles and seats and adjust them like the workshop manual says to.

[scottly]

THEN AFTER, that #8 Describes marking the float levels and using  a clear tube method.  AFTER the float levels have been set.

That is to find a base-line level mark. The base-line has already been established as 3-4 mm below the top of the bowl, Lucky.
Grump, once again, check the level with the clear tube. If it looks good, drop the bowl and try to figure out how all the extra fuel is getting into that cylinder.

[Spanner 1]

This is my new thing for all carb complaints; ' is the needle jet seated all the way up in the carb body ' ?
What ?... the needle jet is the metering jet that sits all the way up above the ( screws out with the main jet ) emulsion tube. The slide needle sits in it and effectively closes the main jet @ idle , unless it's not seated and 'leaks' fuel @ idle with slide all the way down.... check that IMO.

[grumpy]

fuel is, or was last  checked, about 4mm from the top of the bowl on all 4.
that's like 24mm on the Honda slide rule IIRC. maybe it was 23mm. I forget.

26 in the slide rule had the bowls fill to about 8mm from the top and made it lean out at about 4,000-6,000 revs and 1/3 to 2/3 throttle. It hunted, especially uphill. Raising the fuel level cured the hunting.
NOTE: This was before the rebuild. The fuel heights are the same now as before the rebuild - and the plug was fine then.

OK, re check synch, float needle operation (again), needle jet (again) and fuel heights (again).

Next will be M80s in the carb throats.

[lucky]

8mm would be .314 thousandths.

Quote "26 in the slide rule" Not sure what that means. Is that a metric ruler you are refering to?
26mm is 1 .023 inches. Very close to 1 inch.

The float height is not the level of the gas in the float bowl.

If the gas was 8mm from the top of the float bowl that is just a little over 1/4 inch from the top of the float bowl!  Seems overly full to me.

But I do not know what the height of the gas is in the float bowl when the float is adjusted properly. I cannot see it.
But whenever I have removed float bowls they never are more than 3/4 full.

[w1sa]

Plugs do foul up from poor running ignitions., regardless of how good the a/f ratio and feed is.

Are the emulsifier tubes (sideways holes) definitely clear?

Check the valve clearances (for that cylinder at least)

Trim or preferably replace the plug leadand new plug

Swap coil config to eliminate coil as any sort of problem.

[PeWe]

Plugs?
My CB750 does not like anything else than OEM plugs. NGK D8EA. I have used DR8ES-L too.

I don't know the exact difference except R, resistor to filter disturbances.