KZ900 carb help

[1975-S1C]

Hello chaps, I could really use your sagely advice again, and some pointers on what things to do to identify the problems(s) my bike has.

The bike has been bored to 1015, is running an Delkevic 4-1 and had a non-standard air box with a car filter in it. It does have standard pilot, main and needle (with circlip on 3rd slot). It’s always sooted plugs ever since I got it, as well as popping and banging through the exhaust on the overrun, and farting through the carbs particularly on idle.

I’ve cleaned the carbs thoroughly and switched out the non standard air box for a new repro Z1 item with a new filter in it.
Rob (Zed1015) kindly replaced my choke plunger seals with Viton (did a great job, highly recommend) and this did definitely seem to make it run a bit better, but still not right, so there must be other issues to sort too.

I figured that the pilot circuit was maybe rich and that the air screws needed to be adjusted, so I went out this morning to see if I could get it dialled in.

Started off with the screws at 1 3/8 turns out as indicated in the manual. At this setting the revs don’t immediately drop back to idle when throttle is released. Tweaking the screws further out makes it worse, and it takes a long time to drop back to idle, and idle was actually very high. Any further out than that and it splutters and dies. I’m assuming this indicates that anywhere out from that starting position of 1 3/8 out is too lean.
I therefore turned the air screws back in to just 1 turn out, at this setting it’s lumpy, like when it’s running on too much choke, and idle revs drops right down low, is erratic and it farts through the carbs. It also stumbles on throttle at low revs. I figured that was the point at which it’s too rich.
I figured that the sweet spot must be around 1 1/4 turns out, but that just seemed to bring me back to revs taking a while to fall away as well as still seeming too rich.

Got home and checked the plugs. Sooty !

I’m confused as hell ! There must be some other issue(s) that’s causing the rich running and stopping me finding the right idle circuit setting.

Any suggestions very welcomed, as I’m loosing what little hair I have left !

[warren3200gt]

Hhhmmmm, had very similar issues once on a 650.
Everything standard, airbox, exhaust, main jet, pilot jet, needle, good choke plunger seals, good carb to head rubber carb mounts, no air leaks.
Eventually after several frustrationing days of head scratching and another full carb strip I noticed the emulsion tubes had no needle jet fitted to the top end.
Could have kicked my self for not checking on the initial carb clean.

[Ultim8pc]

You say you have a Z900 but fitted a repro Z1 airbox.
I have come across a few Z900s with that combo and they don't run right.
Can you confirm you have the correct 26mm Z900 carbs - ID Z1200?
Also, please take a look at an article I wrote that goes over all the things you should check when your bike isn't running properly. Often, it's a combination of issues.
https://airevalleyclassics.co.uk/badrunning.html

[1975-S1C]

Yes a repro Z1 air box. I do have the original Z900 one that came with the bike that I could refit if needs be, even if only to rule that out.
I forgot to mention that it has Boyer bransden electronic ignition, and a brand new battery.

[1975-S1C]

It feels like there is probably more than one thing at play here so I’m going to go through absolutely everything one thing at a time to check my work. I will report back if I find anything or have any further questions.

What I’ve checked / done so far:
- new Viton choke plunger seals, courtesy of Rob
- new battery
- all jets / needle / clip position confirmed as standard
- cold compression test gave 142 / 152 / 146 / 149 after about 6 kicks with throttle open (I know it should be done hot, but the weather !) do these seem good / bad / irrelevant as it should be done with a warm engine ?

Yet to be checked / done:
- purchased manometer tool to set ‘actual’ carb fuel heights instead of just static measurement of float height
- new carb to block rubbers ordered
- new plugs ordered
- new springs for auto advance unit to be ordered
- new o rings for air screws to be ordered
- may treat myself to a colourtune to ensure pilot circuit is set correctly for each carb
- re-check timing with the strobe

I may throw the standard z900 air box back on too just to eliminate that, as I currently have the repro Z1 on it.
It’ll still have the Delkevic 4-1 on it for now, but if it starts to behave itself, I may treat it to a new 4-4 !

If anyone can think on anything else that should be on the ‘To Do’ list, please do let me know

Thanks
Paul

[warren3200gt]

Difference between cold/hot comp readings is 10-15 psi so yours look fine.

[sean_rfi]

On completing my Z900 A4 I set the pilot air screws to 1and 3/8 of a turn as per the workshop manual. It very nearly ran OK but had the common problem of hesitancy on light throttle or it would momentarily bog down if the throttle was closed and then reopened - - really annoying! A check of the plugs after a reasonable high(ish) speed run revealed that they were a little sooty although not terrible. I then dug out my colortune (older 14mm version - ideal for th Zed). I then very carefully set each cylinder so that I had a just off the yellow, nice blue combustion flame at tick over and 1500rpm. Great, I thought, that's sorted. Tore off down the road and it ran even worse. Another quick check of the plugs showed more soot this time - as expected. I then went back to basics and set the air screws as best I could on rpm/engine note and found that I was much closer to 2 turns than the manual specified 1 and 3/8 setting. A quick test ride confirmed this was a much better adjustment. The bike would smoothly pick up from a closed or light throttle and hardly minded the throttle being quickly closed and opened again. The plugs were a much better colour now. I've subsequently made a couple of very small tweaks based on plug colour, but it has really improved how the bike runs/ rides. Anyway, this is a long winded account to say you may find that air screws need to be adjusted out more that you might think. It is very easy to make the mistake of associating poor running and hesitancy with a weak mixture/lack of fuel.
I hope this might be of some help....

[Ultim8pc]

It feels like there is probably more than one thing at play here so I’m going to go through absolutely everything one thing at a time to check my work. I will report back if I find anything or have any further questions.
What I’ve checked / done so far:
- new Viton choke plunger seals, courtesy of Rob
- new battery
- all jets / needle / clip position confirmed as standard
- cold compression test gave 142 / 152 / 146 / 149 after about 6 kicks with throttle open (I know it should be done hot, but the weather !) do these seem good / bad / irrelevant as it should be done with a warm engine ?

Yet to be checked / done:
- purchased manometer tool to set ‘actual’ carb fuel heights instead of just static measurement of float height
- new carb to block rubbers ordered
- new plugs ordered
- new springs for auto advance unit to be ordered
- new o rings for air screws to be ordered
- may treat myself to a colourtune to ensure pilot circuit is set correctly for each carb
- re-check timing with the strobe
I may throw the standard z900 air box back on too just to eliminate that, as I currently have the repro Z1 on it.
It’ll still have the Delkevic 4-1 on it for now, but if it starts to behave itself, I may treat it to a new 4-4 !
If anyone can think on anything else that should be on the ‘To Do’ list, please do let me know
Thanks
Paul

Compression looks good and you are looking at the right stuff.
As for other things to try, just refer to my article I linked earlier.

[warren3200gt]

The only other thing I would add to the list is poorly adjusted and/or routed dry throttle cables. They can cause all sorts of weird symptoms. Hanging rpm after throttle blip, slow to drop back to idle rpm, but obviously not mixture issues.

[1975-S1C]

The only other thing I would add to the list is poorly adjusted and/or routed dry throttle cables. They can cause all sorts of weird symptoms. Hanging rpm after throttle blip, slow to drop back to idle rpm, but obviously not mixture issues.

Noted Warren. The carbs will have to come off again anyways, so I will make sure to re-lube the cables and ensure they are correctly adjusted.

[gray]

check the advance/ retard springs aren't weak - that causes fluffiness off a closed throttle

[1975-S1C]

check the advance/ retard springs aren't weak - that causes fluffiness off a closed throttle

Thanks Gray. They are on my ‘to be replaced’ list. Appreciate the advice

[1975-S1C]

So, I think we are on the right track.
The Mrs is watching some shite on the tv, so I sneaked off out to the garage for a bit of a nosey.

Used the carb bowl fuel level measuring tool I just bought on the two outer cylinder carbs just to see what they were like. They are both well above where they should be. This despite setting the float height accurately and checking several times. I’ll get the carbs off the bike and make a jig to hold them level in the vice while I get them set up correctly. Feel a bit embarrassed by this school boy error, but just goes to show that float height in the manual doesn’t necessarily mean the correct fuel level. Lesson learned !

As a by product of taking a photo of the fuel levels (my eyes aren’t so good these days at close-up stuff) I noticed when I enlarged the image that the air off-take blank on number 1 carb to head rubber looks perished / cracked. I wonder if this may be causing an air leak and contributing to the issues ? Anyways, they are being replaced so that will be sorted shortly.

I will still check out / replace all of the other stuff on my ‘to do’ list, but feels like progress !
Cheers

[1975-S1C]

Fuel levels of the two outer carbs

[Ultim8pc]

Feel a bit embarrassed by this school boy error, but just goes to show that float height in the manual doesn’t necessarily mean the correct fuel level. Lesson learned !
I noticed when I enlarged the image that the air off-take blank on number 1 carb to head rubber looks perished / cracked. I wonder if this may be causing an air leak and contributing to the issues ? Anyways, they are being replaced so that will be sorted shortly.

Yes, correct float height doesn't always equal correct fuel height.
A perished vac take off blank can leak air and cause issues, so best replaced.

Sounds like you are getting there.