Cylinder 2 cold

[shudnobetter]

Hi all. I've a 1990 FJ1200 which persistently runs a bit under power, and rough at idle.

Tried lots of tweaks but just yesterday I used Colortune to set the air/fuel mixture...this revealed all good with 1,3 & 4 BUT no sign of combustion in cylinder 2. The spark can be clearly seen.

Running to normal temperature with the regular plugs back in i find the top of 2 is cool compared to the other cylinders. This seems to confirm that, whilst there's a healthy spark to all 4, there's no combustion in 2.

Anyone had this problem?
Any suggestions?

[balky1]

Yes, but I had a spark plug problem. Something you say you don't. But even if the spark plug fires nicely when out of the cylinder it could be malfunctionate and cease to work when installed. This I also had. Since this is the easiest to eliminate, in your place I would try with a new plug on that cylinder or swap one from the cylinder that you know it works.
Someone else will most probably have an other idea (like carb problem).
One more thing: if your plug is dry when you take it out (after the bike was running) then it is a fuel delivery problem, i.e. clean the carbs, try again.

[ribbert]

If you've got spark, the only other two things you need are compression and fuel. The reasons for the absence of these could be any one of a number of things. Start by finding out which one is the culprit, that will make any advice much more focused.

Perhaps a bit more info on your experience to carry out these checks so we're not telling you what you already know. If you want some tips on how to easily and quickly check them there are plenty of folks here that will help.

Noel

[Pat Conlon]

Clean the idle circuit in your #2 carb

[shudnobetter]

Thanks, guys.

I'm no mechanic but learning as I go.

Here's what I know for sure.

Good spark
Problem confined to this cylinder (tried different plugs and exchanged plugs and leads etc)
No combustion (able to view using Gunson Colortune - spark evident but no flame)
No smell of fuel
Plug relatively cool and totally dry when removed
No air leaks at seals

Am I right to assume it has to be a fuel supply problem and therefore a carb cleaning issue?

[FJmonkey]

You answered your question already, "No smell of fuel". Pat's advice is the best next step.

[shudnobetter]

Very many thanks to all for the help.

In the end it turned out to be amazingly simple (but not before several hours of messing about). The carb float was set wrong height so the needle valve was stuck shut.

Now running nicely on all the 4 cylinders Yamaha gave her.

Strangely frustrating and satisfying at the same time.