Topic: #3 cylinder perplexes owner to no end.

[1timduke]

     I've got something up w/ my '75 550.    The #3 cylinder is acting a fool.   I've got stock jets, airbox, air screw settings, 4 in 4 exhaust.   Only have about 800 miles on a top end rebuild.   The problem is this; at idle, I get little or no combustion on #3 cylinder, i.e. cold header, rev it up and I get some heat on the header.   So today I broke the carbs down, cleaned 'em all up and put them back on, and fired it up...no heat on #3 cylinder header    So I thought, hmm. maybe I've got a bad ignition cable...hooked up my timing light to the cable, fired it up and the light flashes like normal.   Pulled the #3 plug and it looked like it had fuel on it, not oil though.   I checked the valve clearances about 200 miles ago and they were good (bike was hot today, I'll check 'em this weekend)
     I rode about 30 minutes today to see if it would heat at all, I could feel some surging at points about 3000-5000 RPM as it felt like I was getting a little more power.   Occasionally at high RPMs I got a nice backfire from the #3 pipe.   After the ride I felt each silencer and it was as I suspected, 1, 2 and 4 were hot, #3 was not as hot, but did have some heat to it.
     So I'm figuring I've got only a couple of possibilities...I missed something in the carbs and I've got a clogged jet, I've got a bad ignition cable, resulting in weak spark (is it possible to test this yourself w/ only a kick start???), maybe something else???   I'm eliminating timing b/c it only affects one cylinder and I have spark on the cable, I'm gonna check the valve clearances tomorrow so I'll probably eliminate that too.   
    Am I on the right track here, or have I missed something really obvious, or not so obvious?   Anything would help     I just moved from Atlanta to San Diego and haven't really met anybody yet so I don't have anyone else to look at it and bounce off ideas.                                               -Tim

[scunny]

swap ignition leads and plugs 2&3 to see if this could be your problem. valve clearance should also be checked.
then it's onto the carbs, but since the plug is getting wet I think your problem lies elsewhere

[TwoTired]

All the items on the tuneup list should be checked.  Should I ask about a carb sync?

Since you had top end work recently, I think a compression check is on order to see if it has enough compression at idle to fire the cylinder.  If the rocker adjusters aren't paid proper attention during cam cover installation, valves can be bent, leading to poor sealing valves.

Changing the spark plug won't hurt.

The symptoms you describe are often cause by a clogged slow jet, though, as it doesn't get enough fuel to fire at that throttle position.  While a bit of a pain, the jet can be removed and cleaned with carbs still on the bike.  It is a little easier if you remove #4 bowl first.

Cheers,

[1timduke]

Ah yes, swapping the leads!    That'll tell me right away if it's the lead.   I've read that you can reuse the lead if it's bad in the coil by cutting it real close and drilling it out, then epoxy the lead back in (or am I dreaming...)   Thanks a lot guys, updates to follow probably after the holidays.   And I'll be replacing that plug too!    I gotta find someone here that has a sync gauge as well, or go find one online.   I don't want to spend too much, but which is better, the gauge type or the mercury kind???         -Tim

[WHALEMAN]

Try unscrewing the plastic plug connector. Check it for conectivity and resistance level. I have seen where the internal resister has corroded and will not make connection at idle with the engine running but would still spark when tested. Also idle engine in a dark garage and look for leakage. Dan

[zoo mob]

Check your float level on that carb, I had a very similar problem last season that I hunted and hunted for, only to find not enough fuel was getting to the carb.