Topic: '75 CB550 3rd Cylinder is cold. Help!!

[deathcult]

Hi everyone,  I have a strange problem, and I am a beginner so I hope you with more experience might know what is going on. 

I have a '75 CB550.  It is stock, and it runs great, no shaking, good power, nothing out of the ordinary.  So I have a very very small amount of oil that is coming out of the 3rd exhaust when i ride, it is not smoking and does not smell of gas.  So I start up the bike the other day, and after a minute I checked the 3rd exhaust header for heat, and it was cold, all the others are blistering hot.  So I check for spark.  I am getting a spark but just to be sure I pull the 3rd boot off the plug and start the bike which immediately dies out.  I put the boot back on and it started up great and purred like she always does.  But still the exhaust header is cold.  I don't have a compression tester, but that is my next step i guess, then valve clearance, then the carb.  There is exhaust coming out of the 3rd cylinder, and it feels cold to my hand comparatively.  Can there be combustion at a lower temp?

Has anyone ever heard of this?  What could be causing this?   The bike is indoors during this storm, so it is a perfect time to do some work on her.

Any help would be awesome!!
Thanks

[scunny]

undo the drain screw on #3 carb and see if you have fuel

[deathcult]

ok, and if there is no fuel than there is a carb problem? and if there is fuel than it is something else down the line?

Thanks!!

[TwoTired]

 The cylinders fire in pairs.  If you pull off #3 wire you lose operation of both #3 and #2.

When you took out #3 spark plug, did you notice the combustion deposits on it?  See for comparison:
http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html

Did you actually see spark jump across the electrode gap, and not just at the center electrode to the insulator?
If you swap plugs with another cylinder, does the problem move to the other cylinder?

It is rather common for these carbs to block their fuel jets, or have their float valves get stuck if the bike has sat too long.   Either will starve the cylinder of fuel, preventing proper combustion.

Combustion gasses run from about 1200-1500F.  Most would consider that hot.

I would be certain their isn't a spark/spark plug problem before poking at the carbs.

[1timduke]

   Put a brand new (not cleaned) plug in the 'dead' cylinder, fire it up, if it doesn't warm up the exhaust at all, it's most likely a carb issue, or perhaps the ignition wire spark is weak.   But I'm leaning more toward a bum ring or valve issue if you say there is a slight amount of oil coming out the exhaust.   Oil seeping in will quickly foul a new plug, rendering it essentially dead     
   I traced down the same issue in mine, tore those carbs apart 3 times...what a fun time!   

Hope it helps, and welcome to the forum!

-Tim

[BLUE71TURBO]

If there's no gas- stuck float valve.  If the carb has gas and still no heat on pipe- plugged jets or weak spark or fouled spark plug.

[deathcult]

Awesome, Thanks everyone for all the help. This is just the kind of advise i was looking for.  I will post my results as soon as i get the time to work on her!!