Topic: Cold header on 4th cylinder

[NebraskaMechanicDynamite]

Hi,

After revising and selling my first CB500 four, I recently bought a "new" 500 to work on (the condition of my 550 is just too good to keep me busy).
Anyway, I ran into some trouble: after starting up, exhaust headers 1, 2 and 3 are too hot to touch after 30 seconds. Header 4 however, doesn't get any warmer than a rare steak.

Compression on all cylinders is around 134 PSI (cold engine)
All spark plugs are new NGK D7EA's and #4 fires just like the others: significant blue spark.

Could it have something to do with the ignition timing or the carbs?

Hope I'm not double posting. Could not find a similar subject.

[flybox1]

i doubt its the coils, but if you want to absolutely rule them out, switch 1&4 wires. 
if the symptom stays the same (cold #4) move on to carbs and fuel delivery...
remove and clean carbs thoroughly.
check that all mixture screws are set the same, pilot jets are all clean and clear of debris, and float heights are all equal.

[NebraskaMechanicDynamite]

Thanks for the quick reply!
I switched the wires and #4 stays cold, so I guess the carbs are coming off one of these days.

[LesterPiglet]

I would also try swapping plugs, you never know. 

[Stev-o]

Do you have a nice blue spark on #4? [know how to check?]

How old are your spark plug caps?

[NebraskaMechanicDynamite]

No idea how old the caps are. Some of them came loose while swapping, but they can still be fitted tightly to the wires.
I disconnected the caps and put a spark plug in one of them. Held the side of the plug against the engine and pressed the electric starter for a short time. I did this with a couple of wires and different plugs on the fourth one. Every combination gave the same blue sparking result. No harm in replacing wires/caps I guess, but I'm shure it's not the electrical system causing the problem. As said: plugs are brand new.

[Bodi]

Cold, with choke full on and kill switch off, crank it for a few seconds. Remove #4 plug and look in at the piston top. It should be wet with fuel.

[70CB750]

Is there fuel in c#4 carburetor bowl?

[NebraskaMechanicDynamite]

Yes, there was a quite a lot of gasoline in the bowl. About as much as in the other ones I think, but I already drained some of it before I opened the carbs.
On the picture the fourth carburator is on the left. I disassembled them all, but nothing was too gummed up, broken or stuck. Every carb looked the same (I opened some working carburators before) . All mixture screws were turned 1,5 turns outward. Only thing that attracted my attention were the torn gaskets on the main jets. So I replaced them all with new ones of the right size. Any suggestions or remarks before I put them back on? I hate taking of/putting on the carburators 

[calj737]

While you have them off, I'd pull the float valve to inspect it, making sure it's not fouled up. I'd also measure your float heights and verify proper level and operation of the floats themselves. Might also remove the emulsion tubes to confirm they're not clogged up.

You didn't indicate whether #4 plug was wet or not? If it's wet, then fuel is getting to the plug, but not being burned. If not wet, the problem is clogged delivery.

[NebraskaMechanicDynamite]

Plug was dry.
I looked a little better at the carburator and found some white tarnish inside #4, as can be seen on the picture. Any idea what might have caused this? Also the pilot jet was fouled up on the inside. Cleaned it with brake cleaner and ran some air through it until the opening was free again. At the moment I don't have a 6mm socket so I can't get to the needle, but I will clean it tomorrow.
My guess is that the pilot jet is the main problem: when I bought the bike I drove it around for 20 minutes to try it out and the headers were equally hot then. Yesterday was mainly idling.
Floats and float valve look and operate just fine. I can't get the emulsion tube out though. I ran some cleaner through it, but I'm hoping I can push it out from above when the piston and the needle are removed.

[Duanob]

That white powdery tarnish is what happens when ethanol gas is left in the carbs. I had this problem after every winter and every spring I found myself pulling the carbs off for a good spring cleaning. then i finally got wise and drained everything of fuel before putting it up for the winter. Especially focus on the pilot jets as they have a hair size opening. Imagine that white powdery substance can easily clog a pilot jet. You can remove the bowl and push a cleaning wire through the pilots without removing the carbs.

[NebraskaMechanicDynamite]

Indeed turned out to be the pilot jet. #4 gets hot again.
Thanks a lot everyone!
This is a video about carburetor disassembly which helped me a lot:

[flybox1]

Good work 

[LesterPiglet]

White residue can also be caused by water. Corrosion.