XJ750 #3 Cylinder Not Firing??

Ok this is weird.
I fired up one of the bikes today and it was running kind of flubby.
I pulled each plug wire and no difference on #3.
Pulled the plug and it was alittle wet but not soaked or oil fouled like I'm used to seeing. Tested the compression and it was at 148. Well within reason. So I tested the spark. Getting spark and that seems OK. I figured well maybe the carb might be acting up. Pulled the vacuum line on the carb mount and fired it up and added propane to see if the cylinder would start to fire, No change. So now I put in a fresh set of plugs and the same thing. Now I checked for vacuum leaks and no change with propane or carb cleaner around the carb manifolds. This is really strange because I set the valves and syned last fall and it was running perfectly. It sat all winter with the carbs drained and foging oil in the cylinders.
I must be over looking something here.

MN

 

You could try this - check to see if number 3 is sparking - remove plug - ground base to cylinder head and turn over - watch for spark make sure that spark is not close to top of cylinder threaded hole in case of ignition - also remove all caps then each rubber seal and make sure all brass inserts up inside the insulators are tight with a slot screwdriver - bigfiz targeted this for me a few weeks ago when my number 2 was not firing until engine warmed up - it needed to be tightened - let me know - good luck

 

I would also get the bike revving in the 2500 - 3000 range and check to see if that exhaust pipe gets hot. If it does, it means you've got either an air leak in the carb to engine gasket, or ........ you're not getting any fuel from your idle jet.

 

Pull the carbs and run a wire into the idle jet
Put it back on and go!

 

If the plug is wet, then fuel i not likely suspect. What does the number 2 plug look like? Sounds suspicious of weak coils. I would check the resistance on the coils and go from there.

 

spark plugs are cheap, get four

 

poor connection in the spark plug cap? good luck Mn

 

Ok
Put in new plugs , no change
Pulled carbs, whistle clean inside.
Checked float levels , pulled jets and pilot jets. All clear
Ohmed coils all in spec.
Removed vacuum line from petcock and manifold. Found slight trace of fuel. Put a vacuum gun on line and tested integrity of vacuum side of petcock and found leaking fuel through and into carb manifold.
Installed carbs and set to prime. Put spare plug on vacuum port and bike runs like champ.
Going to place an order for rebuild kit for fuel valve.

 

So you're saying #3 wasn't firing because it was being flooded with gas from the petcock? - Hadn't thought of that before. Good investigation!

 

The strange thing was the plug was wet but not oil soaked. I did not run the bike for a long time so it didn't flood too bad. It was just rich enough to make it miss and the exhaust smelled like the choke lever was on. The exhaust pipe was cold and I just though it was strange because it ran so good before I put it away. I guess it's the original parts in there that didn't stand the test of time. 30 years of service , I can't complain too much.