Trouble shooting engine start up

I can't figure out why my #4 cylinder won't fire. Carbs were dissassembled and thoroughly cleaned. Bike has a new battery with 12.71v at terminals. Compression cold seems to be workable at 130psi. A thimble of oil in the cyl has proven that the valves are better sealed than the rings because the copression jumps up to the 170's after a tad of oil in there. First, I went directly after weak spark. I did just check voltage at the coil's primary and saw almost a full volt drop to 11.76v. So, I straight shot a hot wire right to the coil primary. Same deal, the #4 header can be held indefinately while it's running. #4 is not firing. It gets a little warm, but just proximity. The other three pistons fire right up. One cyl shy, it has a bit of a struggle idleing, but will hold an idle. Compression with hot cylinders dropped alot. #3 was like 75 or 80 hot, the others were a bit higher. I swapped out the #1&4 coil just to eliminate that as a possibility. Spark plugs have been swapped around, still no #4 firing. I cleaned and reset the breaker points before all this. What next? Pull the carbs and make sure there isn't some ity bity grain of dirt blocking up a pin hole? Is there some simple way to see if fuel is getting into the #4cyl through the carb like it should? It's got compression, it's got decent spark- it must be the fuel delivery or timing.

If you have pod filters, you could try removing the #4 pod, and spaying starter fluid into the #4 carb while the bike is idling. If the motor revs, then you'll know it's getting everything but fuel.

stock air box, but good thinking. One advantage to pod air filters. I just went out and bought new spark plugs, maybe that will be it. It seems like I need to dissassemble the carbs and look for blockage. Here is the bike:

When you switched coils did you have to keep the same plug wire on #4 due to it's length? Have you actually seen the #4 spark plug sparking? I ask because even if the coil is good, and #1 has spark, it could have a bad plug wire or cap on #4 that keeps it from sparking.

yes, I have checked actual spark visually several times, the new spark plugs I just installed make no difference. Still headed towards carb look see.

have you tried putting a timing light on the "bad" cyl to see if it is firing under a load, also does the pipe get warm if you bring up the rpm?

Sounds like the carburetor pilot jet is clogged or no gas in the carb. Of course you can remove the drain screw towards the bottom of #4 carb and see if gas comes out. I had a big fuzzball somehow go down and plug one of my carbs fuel inlet once.

Same thing I told him Ron, but good luck with that! As Ron said, just take off that float bowl for #4 and check to see what's up in there. By the way it was really windy out there, almost lost the bike in 1 of those on ramps.

The bowl is filling with gas. I will try to clear the jets by removing the bowl while the carb set is on the bike.

BSKZ650, I'm not sure how to check if the #4 cyl is firing under load with a timming light, which is one tool I have yet to buy. How does one do that? It occurred to me to try to rev the throttle and see if different jets sart supplying gas at higher RPM's. Same deal, #4 header stays relatively cool.

check the choke well in that #4 carb

I took the carbs off so it would be easier to take apart the #4 carb, but I couldn't get the choke plunger out without taking the #4 body off the assembly. That is the only part I could not pull apart from the bowl side. I cleaned and ran copressed air through everything else, reassembled and started it again. Same thing. I'm getting pretty discouraged. I'm not sure what to do next. It seems like #4 is getting gas since the rubber carb duct had some gas on it like the others when I removed the carbs. I don't think it's timing because #1 cyl (same firing time)runs well. The engine sounds good when it is running, you wouldn't know #4 was missing if you didn't feel the header. I'll check coil secondary resistence with caps off, but wouldn't that also directly affect #1 in a similar fashion? I'll check compression again, too.

Have valve clearances been checked recently?