Firing on three cylinders puzzle.

[Heron]

Hi, earlier this year after giving my ZR1100 some high revs no. 4 cylinder stopped working. Next day it seemed ok.
Yesterday the same happened. My mate riding behind could see some black smoke and the smell of un burnt fuel. Limped home, checked plugs and seemed a good spark. Swapped plugs around , still no joy.
Thoughts: if it’s a blocked jet why would fuel be coming out exhaust.
The shared coil set up kind of rules out them being at fault.
Before I get the carbs off just wondered if you guys had any ideas ?

[Shedman]

From what you describe I would try doing a compression test on the affected cylinder first, if you got fuel and a spark but the cylinder isn't firing then I would at least rule out any compression issue before taking anything apart.

[Heron]

Thanks for that . I will get on Amazon tonight and buy a kit.
Any recommendations for a compression testing kit?

[Shedman]

Thanks for that . I will get on Amazon tonight and buy a kit.
Any recommendations for a compression testing kit?

I just bought a cheap one from ebay, a set of brass plug hole adaptors, rubber hose and a dial, check all your cylinders and compare the outcome. I hope you don't find anything serious and you've just got a dodgy plug

[Heron]

Trip to Mc mart and here are my results.
Usable range : 119 ~ 185 psi.
Cylinder
1 = 165
2 = 170
3 = 172
4 = 165 ( the faulty one)

So lots of head scratching. Good news for rings and valves. This was a cold reading as well.
I think I’ll put fuel tank back on tomorrow and just double check for fuel.
If it present then not sure what’s up.

[hugojose]

How do you know is cylinder No. 4??

[Heron]

It has a cold exhaust whilst the rest are hot .

[Heron]

After yesterdays compression test which appeared to show all was well.
Here is what I got up to today.
First put tank back on. Turned to "prime" and fuel started to pour out of air box on the No4 cylinder side. Turned to "on"
Removed plug, checked for spark with both HT leads off ( spark seemed good.). Turned engine over and held rag over open plug hole. Bit of a dirty stain appeared on rag.
Ran on three for a moment with all plugs in and visible fuel vapour came out of exhaust.
Removed plug 8 connected No7 plug and ( yes I know this is dodgy) turned engine over and ignited fuel sparked out of the plug hole .
Put No8 back in, turned engine over, removed plug and it was soaked in fuel. (?)
Drained carb bowl and some grit was present..
So, Prime causes fuel to leak into airbox. Spark present. Fuel present. An explosion with one plug out. Both plugs in, no ignition just soaked plug. Dirty float bowl.
Looks increasingly that carbs will need to be cleaned but the above still seem puzzling.

All thoughts and advice would be most welcome.

[Heron]

I have an idea what it could be .
Flooding on prime is stuck float. It could be the cylinder is continually flooding and when I caused the fireworks, it was because there was enough air mix for combustion.
So carb strip and clean it is.

[Shedman]

Hi Heron, have a read of this, it was my experience of fuel pissing out of the exhaust, I'm not saying that this is what is causing your problem but it is something to be aware of.
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