Z1OwnersClub GB

Looking for some help with my 1975 UK Z900 A4. Bike is totally original but has developed a running issue which I just cannot get to the bottom of. Having recently overhauled the carbs, including refurbished choke plungers from Rob Meggitt, checked the valve clearances and fitted new ignition throughout (plug, points, coils) the bike was running as well as it has for years. I went to use the bike back in June after a layup of about 3 weeks and its running badly. Starts OK but misfires and wont rev. Have now had the carbs out many times and have ultrasonically cleaned them along with blowing out all the jets etc. Once warm the bike ticks over but cuts out on cylinder 2 with the occasional backfire through the exhaust. I can hear the bike trying to run intermittently and removing spark plug 2 causes it to be smooth, all be it on 3 cylinders. I have looked for an air leak at the inlet rubbers between the carbs and cylinder head but cannot see any problem, the rubbers are the originals as far as I am aware but look OK. Now I know I have low compression on all 4 cylinders, in the region of 90-100, but the bike was running OK. I am out of ideas so any help would be appreciated.

First thing I would be checking is corroded ht lead to plug cap on all caps. If you just slightly lift the plug cap on number two so you can hear the spark jumping to the plug does it cut in? If it does then its defo plug or ht lead or ht cap related.
You could try swopping plug caps 2 to 3 and 3 to 2 if long enough and see if the issue moves. Same with plugs to try to narrow it down.

Yep exactly as Warren said. Sounds electrical to me.

just because the inlet rubbers look OK, that means nothing. Test by spraying carb cleaner or easy start around the inlet rubbers while the engine is running. If the engine speeds up, there's a leak. Are all the rubber bungs on the inlet rubbers tight?

However, I agree its more likely to be spark- related

were the plugs from a reputable dealer? I've heard of cheap Chinese copies of NGKs off Ebay that fail quickly

Thanks for the comments to date. I can rule out electrical problems as the plugs, points and coils (Including leads and plug caps) are all new and supplied by Z Power. I have tried the old coils and plugs and the problem persists.

before addressing any electrical or fuel issues you need a healthy engine, if its the low compression thats causing it then they will need sorting. something to thing about sorting in the future. they are a bit on the low side

I suggest you read an article I wrote which may help:
https://airevalleyclassics.co.uk/badrunning.html