KZ900 carb help

[1975-S1C]

Hi AL, I was mindful to make sure that there was a good rate of flow. The carbs fill fairly quickly and then cut off. As Warren said in an earlier post, they aren’t literally on/off though and you do get some creep over a period of say 20 minutes or so (don’t ask me how I know !), but as he said that’s what the fuel tap is for.

You make a good point about the floats being out of alignment. Again something that Warren flagged up. I made certain to keep checking that the floats were aligned by both measuring and sitting then on the flat part of my vice and making sure they laid flat.

Hopefully I’ve got them close enough now, but if I find I need to tweak them further I’ll be much more confident that I can do so fairly quickly and repeatably.

I hope to finish up the fuel system stuff tomorrow and get the carbs back on the bike, as I need to move on to checking out the ignition side of things. No doubt, I will have a bunch more questions !

[Al]

Warren also mentioned that the brass hinge section is aligned with the diameter of the floats passing through the middle and not oblique to it.
This picture posted by 'Sprint' last year in the thread i linked, you can see that the floats are about the same resting level as yours with the float valves under pressure and closed. What you can also see is the 'tab' on the far side is resting on or close to the post.
This suggests that there is very little movement available to the float to move from its present position and that when the 'tab' stops the float moving,.. the 'tang' will still be in contact with the needle plunger pin.
This was sort of where i ended up but the RS carbs have a cast nipple in the float chamber which messes it up further. It was stalemate for me for a bit.
This is as a consequence of the float level being incorrect despite the fuel level being demonstrably correct!!!!
"Someone may have found it easier to bend the floats rather than the tangs previously if they did it all with the carbs on for example". Do you know what; that sounds like stuff i used to do when i was a youf.

P1020699.JPG (178.73 KiB) Viewed 2816 times

AL

[1975-S1C]

Ahhh yes, I can see what you mean Al. If the floats were bent up or down then that would obviously affect the fuel level regardless of the tang. Another peril I’d imagine is that the floats would have a tendency to touch on the carb body and not move.
Hopefully mine are ok, as I kept checking them as I went along. They did go out of whack a few times when I was adjusting the tang, but I corrected them before I moved on. The floats seemed to have a good amount of movement too. The one thing I was pleased about is that the fuel level went up the tube pretty quickly and then cut off at the right level, so I think the flow rate is sound.
I guess we’re gonna find out soon !

[1975-S1C]

Also, just noticed in that picture, look how much the tang is bent upwards to compensate for the fact that the floats are out of whack !

[Gray17]

Excellent post and great photos too, you did a great job of setting up the float heights, of course proof of pudding is in the running.
Lets hope it runs great now too.
Def a great post to follow for us novices, just need mine doing now!

[1975-S1C]

OK, so I think we all knew what was going to happen !
After Al had planted that seed in my brain I couldn’t rest until I went out and took the float bowls off and had a look to make sure that my floats were aligned, centre line of float through centre line on hinge.

And the results are in ……. They’re not ! Not a single one. They all look exactly like the one in the photo Al posted. I’ve attached the best photo I could capture with my naff eyesight, but as you can see it’s actually slightly worse than where I am holding the ruler in the photo. .

I’ve bent the floats on the first two carbs so that they are aligned to the centre of the hinge pin. Thinking that would make the float height more akin to the manual I reached for the vernier, and now it’s gone up from 28 to 30 !
Are we 100% sure that the floats should be aligned (centre of float through centre of hinge pin) ? If so I’ll go bend the others straight and play around until I get the right levels again.

Does anyone have a brand new Z900a4 float that they could photograph and share ?

Thank goodness we are going to the pub tonight !

[Al]

I am sorry i have misled you. This was the pic he posted on page three of this thread and i have told you the reverse.
That being said; if i had 28mm static float height and my tangs were potentially acting to side load the float needles, i would be tempted to try to get the floats back nearer to 26 mm by bending them and trust that brings the tangs after adjustment nearer to acting perpendicular to the float needle and seat centrelines.
Sorry to put you through that.
AL
Check the float hinge plates aren't bent and the floats aren't bent.
They were as the solid circle when new.

[1975-S1C]

No worries Al. You are trying to help, and I really appreciate all the help I’ve been getting on this.
I will circle back with the two carbs I’ve already messed with. I will shoot for the elusive 26mm in conjunction with the right fuel level.
This’ll be a job for tomorrow now as I’m off out now, then to the pub later.
Cheers for now.

[Al]

Thank you. Since i feel responsible for driving you to the demon drink, make yours a double and send me the tab.
AL

[1975-S1C]

Couldn’t resist another fiddle before I go to the boozer.
Set one of the carbs I’d already bent the floats on to a height of 26mm with the tang in a dead neutral position. I did this by bending the floats back, and then a tiny bit more.
Tried it,
Almost spot on !
Minor tweak to just make the tang barely ‘feel able’ above the level of the float hinge. Perfect. Plus the flow rate it quick to get it there.

I’m on a roll I thought, I’ll do another. That one doesn’t want to play so nice, so that’ll be for the morning, but I think I’ve finally got my head around it. Thanks for being patient!

[Gray17]

Oh dear, bet your well gutted after all that time wasted spent setting them up so perfectly too. Still sure a few beers helped!
Forgive my ignorance but exactly what is the problem with the way they were before, is it that the floats stick, or dont operate correctly?
Just wondering why if they were set up to correct height it actually matters?
Thanks

[1975-S1C]

It is a little frustrating, but it’s a learning exercise for me. I think the main advantage of having the lower float height seems to be the flow rate of fill (replenishment rate) , which seems to be quite a bit quicker than how I had it before where it would creep up on the level much more slowly.
I’ve been busy all day so not had chance to work on it today, but hoping to get it all sorted before the weekend is over.
Fingers crossed !

[1975-S1C]

Well it’s been a while since I lasted posted on this.
Had to park it whilst getting my fathers house ready for sale to fund his care home

Anyways, where was I ? I managed to finish up getting the float heights about right. Carbs all fitted back to the bike and Z1 air box with brand new filter in it. There are new head to carb rubbers, plus all of the things I did in the earlier pages of this thread. New air screws are set at 1.5 turns out.

Having ordered new springs for the advancer mechanism, I set about changing them, only to find that there actually aren’t any. I guess that must have been removed when the Boyer Bransden ignition system was fitted.

I started the bike and warmed up the engine. Using a strobe timing light the timing at low rpms seems to be at the TDC mark as opposed to the F mark. I advanced it a bit, the bike seemed to rev from idle nicely instead of bogging as it did before, but there was some popping from the exhaust.

Decided to take it round the block to see how it went. It pulls away very cleanly from idle, which is so much better than before, but after a few minutes of running it’s taking some while to return back to idle and also backfiring on the overrun when the throttle is closed.

It’s frustrating, as it feels like I can’t be a million miles away from it being right, but I think I’m reaching (or maybe have already reached) the limits of my knowledge/ability, and as it stands the bike is barely rideable.

As always, any help / suggestions gratefully received

[warren3200gt]

Slow to return to idle will be an air leak on the carb to head rubbers. If it did it all the time, even from cold, it could be poorly adjusted throttle cables but as its on when Warm something is expanding or softening with heat and allowing an air leak.
Popping on over run is an air leak in the exhaust system. On overrun excess fuel is ejected directly into the exhaust. Without an air leak it evaporates in the exhaust fumes. With an air leak the next exhaust cycle will ignite it. The excess fuel needs an air source to ignite.

[1975-S1C]

Thanks Warren. After pondering your first comment I thought ‘how could that possibly be, I just fitted new carb to head rubbers, but then I remembered that in order to get the Z1 air box in I had to remove the air cleaner stay, which I was planning to re-fit afterwards, but in my haste to go take the bike for a spin I didn’t. I wonder now if when the bike warmed up the combination of the rubber being a little softer and the unsupported weight of the carbs and air box would have caused an air leak somewhere ? Anyways, I will definitely fix that straight away so that I can rule that out.

I’ve also consulted Google and it seems a weak mixture can also cause elongated time to revert to idle. I haven’t played with these since replacing them with new, and they are all set at 1.5 turns out. Perhaps a quarter turn in may help ?

I’m fairly confident that the throttle cables aren’t to blame, as they snap back nicely.