Topic: #4 cyl doesn't seem to fire

[Ms Tokyo]

76 CB550k.  #4 header is not as hot to the touch as is 1-3.
Here's what I've done to trouble shoot so far:
-pulled the plug and it is black but I wouldn't say too sooty
-turned the engine over using starter and plug against the case and have spark
-confirmed my coils and leads are to 1/4 & 2/3
-confirmed coil/point wiring is correct
-drained fuel from bowl of carb #4 to confirm fuel is reaching carb

bike seems to be running fine but this temp difference caused concern.  if i pull the#4 plug wire while the bike is running, nothing happens and there are no audible differences.  if i pull any of the other plug wires, the bike stumbles and stalls.

i suspect it may be a clogged jet since there is fuel in the bowl and spark but no fire.  Am i on the right track and is there anything else you would recommend?

[bryanj]

Could be plug failing under pressure try a different one

[754]

Switch 1 and 4 plugs .. what happens ?

[Ms Tokyo]

switching plugs makes no difference, #4 still not firing. I pulled the plug cap again and have confirmed spark.  I'm still thinking fuel but i get some out of the bowl when I drain.

I did install a single line petcock in place of the dual that was original because I could not find a suitable replacement for the trashed original.  I have a brass tee in place but am wondering if this is causing the issue.  I have the T in line near the carb bank.  Should I have this higher towards the tank to give the fuel plenty of time and line to split and feed the carbs evenly?  This would make sense to me if #3 was also not firing but fuel is getting to #3 and firing.  Maybe not enough for the pair???

[754]

 Check your valve clearance., on 4.

[Ms Tokyo]

Adjusted all points to spec with no change. Had it running with the points cover off and noticed the 2-3 points arching. Doubtful this is the issue but I am going to reset points and timing tomorrow. I may just go ahead and replace the 2-3 points. When I was setting timing before I bent up the body of the points a little and had to straighten. This may be reason for arching.

[BenelliSEI]

“Unscrew” the end cap, cut 1/4” off the end of the plug, screw the end cap back on. Or better yet, replace that end cap. For a test, trim and switch the end caps on 1 & 4.

[754]

If one valve is staying open for any reason , it likely will not run.

[Ms Tokyo]

Benelli- I had cut off about a 1/2” earlier. The end cap is brand new NGK as are all of the caps.

754 - what’s a good way to check if a valve is open? It was firing at one poi t recently because the plug shows it.

[754]

You only need 3 things.
 Spark
 FUEL
 compression.
Check your valves... same as adjusting them, see if one is off or tight. 
You can basically check compression with your  finger,  it should push it off the hole......or a compression tester.
 Put a good plug in it,  ground against head, turn it over.. check spark
 If it does not have the air box , you can dribble a tiny bit of fuel into 4... carefully, and see if it is a fuel problem.

It's just troubleshooting..

[seanbarney41]

Did you check for a clogged pilot jet?  That would be my first guess.  A tiny drop of water can block a pilot jet.

[Ms Tokyo]

Ok, I went back through the carbs. I put the old float needle and seats back in. I wasn’t happy with the fit of the ones that came in the rebuild kit. Also put the needle clip in the center (3) slot. Did a couple leak tests on the bench and synced before putting back on the bike. She fired up on all cylinders.  I also set the valves to .003/.003 based on a couple posts that I read from hondaman and the pod & 4-2 setup that I have currently. Needle clip was a recommendation I picked up on a YouTube channel called Classic Octane.

Getting a little white smoke at idle, not much. Air mixture screws at 1 1/2 currently. Will start there to see if that affects smoking. If not, what are your thought? Needle clip or valves? Or both? Go back to stock settings?