Z900A4 Black plugs

[sprint]

My UK 1976 Z900A4 is completely std, with std carbs, jets, air box etc and original exhaust. Fitted with NGK B8E S plugs which are constantly black and sooty, see attached. It is also fitted with electronic ignition.

Carbs have been stripped and cleaned several time together with the float levels but all the plugs remain black and sooty. It ticks over nicely at around 1000 rpm but stutters and stalls when coming off tick over or when closing the throttle and opening again when coasting etc. Above 2K rpm or so it picks up cleanly and rides well.

Any suggestions as to the likely cause and possible remedy?

[zed1015]

Very possibly choke plunger seals.
I offer a repair service and fit new ethanol resistant seals if you need them done..

[sprint]

Thanks for the reply.

Is it likely that all 4 would have the same problem at the same time?

Is there a way to identify as to if this is the actual problem?

What is involved with doing the seals and is this something I can do at home or do the carbs have to be set to you and what is the cost?

[zed1015]

Yes! At 40 years old all the seals will be worn regardless but some can leak worse than others randomly across the four cylinders..
Repairing them will either entierly cure or at least reduce your problem depending on whether you carbs have another issue but having them done always makes an improvement and the new Viton seal prevents against damage from the fuels ethanol content which the originals can't do..
The symptoms among many others depending on the severity of the leakage are sooty plugs, low mpg, poor idle, rough running, plug fouling when sat in traffic or persistent low rpms etc.
The seals are captive in the plunger head and were not a servicable part so need specific tooling to replace which i have developed.
They are £12.50 per plunger + return post ( £53.50 per set of four returned) and only the bare plungers need to be sent not the whole carbs.

[sprint]

The bike idles smoothly at around 1K rpm and runs OK above 1K5/2K but the other symptoms are present.

Will see if there are any other suggestions and have a bit more of a play around myself, but if there is no improvement then having the seals done would be worth doing.

[sprint]

Can the plunger assay's be removed without having to remove the carbs?

[gray]

weak auto advance springs? this would affect all 4 carbs equally

[zed1015]

Can the plunger assay's be removed without having to remove the carbs?

They can be but with the exception of number 4 it's a near impossible pain and fiddly job with a specially modified, bent and slimmed 14mm spanner that takes much longer than pulling the carbs off.

[sprint]

weak auto advance springs? this would affect all 4 carbs equally

Thanks. Whilst it has electronic ignition I do believe that it still uses the sprung loaded advance/retard mechanism, but not sure what impact this actually has? Does it still rely on the mechanical ad/rt or does it have it's own built in electronic ad/rt?

[sprint]

Can the plunger assay's be removed without having to remove the carbs?

They can be but with the exception of number 4 it's a near impossible pain and fiddly job with a specially modified, bent and slimmed 14mm spanner that takes much longer than pulling the carbs off.

Thought as much!

[gray]

some electronic ignition systems eg Dyna and Newtronic certainly, maybe others too, they still use the original mechanical advance/ retard unit . Springs aren't dear at Z Power considering the great difference they can make if they are stretched by a mm or so

[Hack]

I have this same issue on my Z900A4.

Has always been like it for the 18 years Iv'e owned it: plugs are blackened but never enough to foul up and cause an issue, does bother me though.

I thought I'd read that the recommendation was to switch to a hotter plug - NGK B7ES

Just in the process of a rebuild after rebore so not feeling like trying anything new until it's settled down, although I think even KHI suggested B7ES for running in and "less enthuiastic" use, which is me all over.

[warren3200gt]

I have this same issue on my Z900A4.

Has always been like it for the 18 years Iv'e owned it: plugs are blackened but never enough to foul up and cause an issue, does bother me though.

I thought I'd read that the recommendation was to switch to a hotter plug - NGK B7ES

Just in the process of a rebuild after rebore so not feeling like trying anything new until it's settled down, although I think even KHI suggested B7ES for running in and "less enthuiastic" use, which is me all over.

Don't know about 900's but khi on 650's changed the plug recommendation from 8's to 7's as 8's carbon fouled in everyday use.

[zed1015]

I have this same issue on my Z900A4.

Has always been like it for the 18 years Iv'e owned it: plugs are blackened but never enough to foul up and cause an issue, does bother me though.

I thought I'd read that the recommendation was to switch to a hotter plug - NGK B7ES

Just in the process of a rebuild after rebore so not feeling like trying anything new until it's settled down, although I think even KHI suggested B7ES for running in and "less enthuiastic" use, which is me all over.

Unless a plug chop was performed at high rpm the plug condition only indicates the a/f ratio of the pilot circuit at idle.
Sooty plugs are possibly due to pilot mix and incorrect mixture screw settings but most likely choke plungers seals are causing the rich running although there are other less likely possible causes.
A hotter plug shouldn't be needed.

[Al]

Hack, the amount of excess / un-necessary fuel you have used over the last 18 years could have bought you a colourtune. Currently around the £40 mark. If you get rich readings at idle speed with the colourtune and cant dial it out by the time you reach 2,000RPM using the air / fuel screws, you likely have worn choke plunger seals assuming everything else is ok; Correct size pilot jets and not worn out, not overly restrictive air filter, standard settings for needle and needle jets etc.
Can i ask what reason for the rebore,... was it because you had very many small vertical grooves in the faces of the bores?

AL