1981 CB750 Won't Start

[excessivedemon]

Sirs,

I badly need your expertise on my issue with my 81 CB750 - It was previously running fine and great and it was pulling really strong. I went to fill my tank (91 octane - it's the same gas I usually fill up with and it's the same gasoline station I always go to). Right after filling up I noticed the bike was idling somewhat roughly (it was dropping revs every now and then so I had to rev it a bit to prevent it from stalling) anyway, I didn't pay attention to that and took it for a short spirited ride (just roughly under 8 kilometers) before deciding to head back home. Just as I was nearing home and I passed a slowly moving car, the engine suddenly died down. I could not start the bike since then - when I try to start it all I hear is the starter doing that zzzz sound but it's not firing (similar to that sound you get when you try to start the bike with the kill switch on, right now there's no difference with the sound it makes with or without the kill switch on).

I followed the advice on the other threads about similar issues: I checked for spark for all the plugs and there is strong spark (I'm already using iridium plugs by the way). I replaced the fuel filter (it had some dirt). I checked to make sure all fuses are intact, and also checked to make sure all my wires are properly grounded. I tried with and without the choke on, I even removed the kill switch from the handle bar just to make sure it is not grounding but it still does not fire. I tried hitting on the accelerator a couple of times as I try starting the bike (and I can smell the fuel going through the carbs) but it still won't start. What else could go wrong? The starter has been rebuilt and the gears are intact (from a few months ago) but I'll try checking again tomorrow - I seem to doubt the problem is there though since I never had any starting issues before - could it have something to do with my last gas fill up? Is 91 octane bad for these bikes? It seems to have always worked before. We do have 100 octane here but it's a hell lot more expensive.

Thanks in advance.

[TOOLS1]

It sounds like your engine is cranking over, so its not the starter. You have spark, so its not the ignition, or the kill switch. That leaves us timing, compression, and fuel. I doubt you slipped timing, or suddenly lost compression in all 4 cylinders, so it must be fuel. Are the plugs wet? Have you tried squirting in a little starting fluid to see if it fires on it? Do you have fuel coming through the fuel line?
Also on the fuel octane, it is best to use the lowest octane that the engine will run on without knocking, or pinging. The higher the octane rating the slower the gas burns. The only reason high octane gas is associated with power is because compression is what makes horse power, and the higher the compression ratio creates more heat. This causes the faster burning gas will cause pre-ignition.  
TOOLS

[excessivedemon]

Hey Tools, thanks so much for the reply. Plugs are not wet, and it seems to try to want to fire with starter fluid but does not complete (it makes that zzz bup bup bup sound then dies after). There is fuel coming through the fuel line (I just replaced the fuel line and fuel filter with a new one and I can see fuel going through) but I noticed something strange - carbs #2 and #3 are pissing some fuel (not that much, just a small bit) I tried tapping the carbs lightly and it stopped pissing fuel - could it be my carbs are blocked or something? Is there a quick way to maybe flush whatever is blocking without disassembling it (like some compressed air through the fuel line maybe)?