Topic: CB500K2 -- no fuel to cylinder 4

[gmoss]

Folks, need help diagnosing an issue, if you'd be so kind.

After running a few minutes, cylinder 4 (rider's right) has a warm, but not hot, header. Evaporates spit, but I can touch it without too much pain. The other three headers are hot.

Spark plug is new, correctly gapped, and produces a visible spark when grounded against engine block, but removing the spark plug lead while bike is running does NOT produce a drop in RPM.

Carbs fully disassembled and cleaned, floats all set to manual-specified height of 22mm, petcock cleaned, fuel lines all replaced.

I've taken the carbs apart and made sure the cylinder 4 float pin is clean and smooth, if it's getting stuck then that's a few times in a row that it's happening.

What's the next step in getting to the bottom of the issue? Compression test, or is there anything else I should check first?

Thanks,
-George

[denward17]

Waiting for more experienced folks to chime in.....

However, what does #4 plug look like now after running a bit?

Sounds like it could be a little rich on #4 (running cooler) ?

[ekpent]

If you had a new plug and that cylinder was not getting fuel the color would pretty much still be white on the plug.

[jaytee-nz]

You say the carbs were cleaned but there is cleaning and there is cleaning !
Specifically, did you remove the pilot jets and clean all the very small holes out ? Did you also check that all the passages within the carb bodies were clear ?

[Deltarider]

If I have understood you well, symptoms begin not immediately at the start, so we cannot exclude the possibility the floatchamber does not fill up as it should. That could indicate to an impeded fuel flow or a sticking float needle or a float that is somehow hindered in its movement. Recently I have posted a vid that shows what should go on. Have you seen it?
The CB500/550s are very sensitive for a proper fuelflow route, so let us first establish whether everything is standard or not.The correct dimensions for the fuellines are: for the CB500 and earlier CB550s: 18 and 30 cm length for the models with the bowltype petcock and 17 and 28 cm for models that have the newer style petcock. Do not be tempted to have them longer. For the tube that supplies carbs 3+4, use the soft metal clamp in the middle front side as shown in the pic (btw, horizontal is good enough) for proper routing. Ideal inside ⌀ of the fuel lines is 5,5 mm. Outside ⌀ will be around 10mm. Have this and the lines will not kink. Abstain from extra inline filters. Some have been lucky with them, but my experience is they can't be trusted. Often it isn't that filter itself, but the rerouting of the fuel line that - sometimes intermittent - impedes a proper delivery of the gas.
Have you cleaned and - if needed - polished (lightly!) the float's swivel pin, so the float can move freely.
Have you polished (extremely light!) the top of the float needle and its seat. An additive, usually called fuel system cleaner, can just do enough to 'lube' these parts, so they will move freely.
The clear tube test is not my favourite, but in your case it might help to check the fuellevel in the floatbowl is correct. It should be 3-5 mm below the top of the bowl (see that video). In your case that test should be performed with the engine running.
Sure the #4 carb vents well?