Topic: Cylinder #2 not firing?

[echo4mike]

Hello Forums,
Newbie here.   Got a 76 CB550F several months ago and have been putting it all back together.  I have not rebuilt the engine (have another, spare, engine... planning on total rebuild of that next winter).  For now, I've just been trying to get this one cleaned up and "running" for now.  Cleaned the carbs myself, and replaced internal bits (needles, float valves, etc, etc).  The engine runs now, but not "great".  I just did a compression test, cold engine, and got 90psi for #1, #3 & #4.  #2 showed 100psi.  All of those seem "low", as 130+ is the preferred target psi for these? (the Clymer manual doesnt actually specify what the values should be, and I'm just going on what I've gathered from the interwebs).
I did not put any oil in the cylinders and retry, to determine if the issue is rings or valves related.

The more interesting issue is that #2 cylinder.  When I pulled the spark plugs, that one looked brand new/clean.  Like it hasnt been firing at all.  And when I run the bike for a few, all exhaust pipes get hot, except that one (is just warm, maybe from nearby heat).  So, clearly, #2 has no *bang*.  I've checked that the spark plug is sparking.  The compression test showed that it does have some compression.  So other reasons for not firing would be...  air/fuel?   If its getting compression, then I'd like to think that it's pulling air in.  So maybe the problem is the #2 carb?  Just not sucking up any fuel at all?  Another potential (I suppose) reason could be if the valves are so out of whack, could it be pulling air in from the exhaust valve during the compression test?  Like it's got compression, but it's sucking the wrong way?

Any advice is greatly appreciated. 
Next winter, I'll start tearing the other engine apart and do a full rebuild there.  Never done that before, but that's what this new hobby is all about - something to do, something to learn, etc.  Seems like a bit of a daunting task, since I've never rebuilt before... but, with the manuals, internet and forums like this, I figure I'll get it done somehow. 

Thanks,
John

Before pic (as it was when I got it) and an after pic:

[OneWheelDrive]

If you've confirmed spark on that cylinder and it has compression, my next guess would be fuel.  When running the bike for some time and pulling the plug, is the plug wet with fuel?  If not, you may have a plugged jet, fuel feed to bowl problem, or your float is too low to feed fuel to the jets.  Easy test is to unscrew the bowl drain plug and confirm fuel in the bowl, while you've got the plug out, do a clear tube test to see where your fuel level is.  If none of the above, pull the carbs and confirm the jets are clear.  Good luck!

[mystic_1]

Do a full tune-up.

Cam chain tension, valve clearances, ignition dwell and timing, in that order.

Then repeat your tests to see if anything changes.

You might also consider swapping the #2 and #3 plugs to see if the problem follows the plug.

mystic_1

[74cb750]

Do a full tune-up.

Cam chain tension, valve clearances, ignition dwell and timing, in that order.

Then repeat your tests to see if anything changes.

You might also consider swapping the #2 and #3 plugs to see if the problem follows the plug.

mystic_1

You might also consider swapping the #2 and #3 plugs to see if the problem follows the plug.
This will let you know if you have a bad wire, which it sounds like to me, because I have had the same problem. It was the wire was worn and grounding out sometimes.

[echo4mike]

Thanks for all the advice guys...  I'll work on it a bit more this weekend and see what I can find out.  Pretty sure #2 plug is sparking without issue tho (pulled the plug out, connected it to the wire and grounded against the engine... sparks as expected). 

Bowl drain plugs are stuck tight, and previous owner(s) nearly stripped the flat head on them... So I'll have to work at getting those off and replacing them.

Been reading up on top end rebuild too... Thinking that, when I get around to it, I'll replace the rings and all gaskets - but keep the pistons, valves, etc (just clean them up really good).  Or is it more advisable to spend the extra $$ and get new pistons and valves, while I'm in there?
Is it also more advisable to just do a complete engine rebuild (top and bottom)?  Check everything, since it'll already be torn apart?   Part of me says, if there isn't a problem with the gearbox, then no need to open it.  But another part says, just do it all. 

thanks again!

[flybox1]

Nice transformation from that bike you bought