1976 Z900A4 Carb issue

[sprinta]

Apart from my 72 UK CB750K2 I also have a 76 UK Z900A4 on which I am having a carb issue that some here may be able to offer some advice on?

No 3 pot is running very rich to the extent that the carbon fouling caused the pot to miss and back fire, which eluded me to the problem.

The other plugs were slightly rich but otherwise OK. Did a 'wet' float level check which should be around 3mm below the bottom flange of the carb body face. They were all high more or less at the body flange.

Carbs removed. Jets ultrasonically cleaned and carb bodies passageways cleaned with carb cleaner and compressed air. All passages as far as I am aware clear. New seats and valves fitted, Kester original Mikuni if available around £80 a set.  are Did a bench wet float level setting followed by static balance, all OK.

Re-fit the carbs did vacuum balance and colourtune, all set up OK so go for a ride.

Rides well but on return No 3 is still black and sooty and the No 3 carb wet float level has gone high again!

Remove float and bend the tang up a bit more, still remains high. Tape float up with bowl off with petrol tap on for around 4 hrs, no leak. Even with the tang bent up such that the float only has to move 1mm to close the valve the wet level remains high.

Take the bike for another ride and do a plug chop at 60 mph, No 3 remains black and sooty, so problems remains at higher rev range.

Plug caps on No' 1,2 & 4 around 4K8 ohms No 3 5K4 ohms, slightly high but not enough I would have thought to be causing the problem?

Have swapped plug leads of No' 2 & 3 (they use same coil so fire at the same time) but yet to go for a ride to see if it is a lead/cap problem?

But I am now running out of ideas. The floats float in petrol and water, but in desperation I have ordered set of replacements, at high cost, but doubt they will make much/any difference?

But the main thing that is totally mystifying me is that what ever I do to the No 3 carb/float the petrol level remains high. It does not overflow but the level is in line with the flange face of the body.

As to whether the high float level is the sole cause of the richness or if it is as a result or a commination of carb and electrical is uncertain at this stage?

If anybody can offer some advice or suggestions as to why what ever I do the float level remains high on the No 3 carb it would be very much appreciated as the carbs in operation are very much the same as the ones used on the Honda's, the main difference being that the Z900A4 has enrichment choke plungers against the air reduction slides of the Honda. The plungers have had the seats replaced with Viton as they were a first possibility of allowing extra fuel in causing the richness, but they have made no difference.

[K2-K6]

Is there the possibility of swapping #3 float and valve with one of the other carbs to see if the problem follows those components ?

[sprinta]

Thanks for your input.

As indicated all the seat and valves are new, but I have already tried the seat and valve form one of the other carbs together with another float from one which is OK.

As an update I have done another ride on the bike since swapping the No 2 & 3 leads and caps and the the No 3 still remains black and sooty. Then fitted a B7ES plug instead of the B8ES and it has remained black and sooty even with a hotter plug.

Compression test show that they are all around 100 psi, not great but they are all the same.

Everything is pointing to the No 3 carb but as to what it may be is a mystery at the moment as is the cause of why it also does not overflow but remains with a high float level whatever I do?

[K2-K6]

Certainly valid to check sparks so you're not caught out by something odd there. I'd be doubtful that a shift in resistance could cause what you see here too.

It does look like a peculiarity with the fuel metering on #3 somehow.

Is the overflow on #3 tested as clear so it can actually overflow and not force an effect I've leak into intake via main jet  ?

Any carb vent to individual bodies that could be impeded ?

[Oddjob]

Any chance the choke mechanism isn't working properly? As in it looks like it's lifting but in actual fact it isn't ?

[davidcumbria]

Swap other components ie needles jets pistons carb tops from other carbs to see if problem moves?

[sprinta]

Thanks for the suggestions, will try swapping some of the jets etc around and see if there is any impact

[Bryanj]

If the fuel level is high it has to be getting in somehow. If left does the level keep on going up and fuel leak out or does it stop at the level you say?
Trying some lateral thinking

[sprinta]

It does not rise enough to come out of the overflow pipe, just goes to the bottom face of the carb body and stops there.

Swapped all the jets, seat float etc from No 4 to No 3 and vice versa and now float level seems to remain at the correct level, some progress, but No 3 plug still remains very sooty at both at a 60 mph chop and low speed.

Will try dropping the needle by one groove and see what effect that has?

[Oddjob]

Dropping the needle by one groove will make it even richer, you need to go up a groove

[McCabe-Thiele (Ted)]

Dropping the needle by one groove will make it even richer, you need to go up a groove

Dropping the needle by one groove thus raising the needle into a higher groove would surely weaken the mixture?

The lower the needle in the body the more it restricts the fuel flow  or are these upside down carbs?

[Oddjob]

Dropping the clip results in the needle sitting higher, this allows more fuel through so the mixture goes richer.

[McCabe-Thiele (Ted)]

Surely that's raising the needle!

[Bryanj]

We have a difference of terminology here, raising the CLIP lowers the NEEDLE and vice versa

[McCabe-Thiele (Ted)]

We have a difference of terminology here, raising the CLIP lowers the NEEDLE and vice versa

I agree with you Bryan to weaken the mixture you need to lower the position of the needle in the slider when at rest i.e throttle shut - this is done by fitting the circlip higher up.

To make the mixture richer you need to raise the needle when at rest by raising the needle - this is done by fitting the circlip in a lower groove.

Not trying to nit pick just trying to be thorough - sometimes more is less.