Topic: Cool #4 Cylinder?

[krmtrains]

Hello All,

I'm sure this isn't a new problem, as about everything that could happen seems to be covered in this amazing forum?  In the process of resurrecting our '75 CB550, I discovered an interesting issue yesterday.  I was having trouble getting it to start.  Just turning over, without even a sputter.  I had fuel, and checked all four cylinders for spark, found no spark on 1 and 4.  Checked points, and found no spark on that circuit.  Replaced points and condenser on that side, and that seemed to do the trick--mostly.  Now, cylinders 1, 2, and 3 seem to be running normally (exhaust hot to touch), but number 4 only feels warm.  We did adjust the valves the last time we had it running.  Would a mistake there cause one cylinder to run cooler than the rest? Is there anything else that could cause one cylinder to do that?  I haven't performed a compression test, but this bike only has 6K miles, so I would not think that would be an issue. 

Thanks for your help!

[ekpent]

Chances are it is going to be a bad spark plug or a carb issue for that cylinder,clogged slow jet etc. That is where I would start on those two.

[krmtrains]

The spark plug is brand new.  I did rebuild the carbs, but might have missed something on this one.  I'll have to take them off again and see what I can find.

[Caymen]

I had a bad plug cap. Engine would run fine, but one cylinder remained cool. One of the caps had a crack in it and it was shorting out to the block. I would swap caps with another cylinder and see if the bad cylinder starts to fire. If it does, or if the other cylinder doesn't, then the cap is bad.


Tom

[TwoTired]

If the valves are adjusted too tight, the valve can't close, losing compression or blowing backwards into the carb.  Either or both can prevent the cylinder from firing.

You need to check for spark at the cold cylinder's plug, though.

[krmtrains]

Thanks for the tips.  I made the mistake of assuming there was spark when I had spark at #1.  I'll have to check that. I've had enough motorcycles for the weekend!

[camelman]

Run the engine for a minute, then shut it off and pull the cold plug.  If it has raw gas on it, then you have a spark issue.  If it does not, then you have a carb issue.

Camelman