Backfires through carb

Hey guys, I'm having some trouble getting my '81 KZ750 to start and was hoping to get some suggestions from those of you geniuses out there. Right now it will turn over and almost sound like it's starting but will backfire through the carbs. Is this caused maybe by a valve being out of spec? Anything else I should check?

Thanks!

First thing to do is a tune-up. Make sure the valve clearances are within specs, if the bike has been sitting for a while make sure carbs are clean and jets are clear. Put in fresh gas and make sure air cleaner is clean, spark plugs are the correct ones and gapped.

What's the history? Did it ever start well or is this a new problem. I notice you have 750 project bike in your sugnature so wondering if you have just put it back together ? If thats the case then what you describe could be caused by having the coils wired backwards. You'll have a red wire going to each coil and then either a green or a black wire. Try disconnecting these and put the green to the other coil and the black to the first. I cant remember which goes where except that it does not matter which red goes to which coil but it does matter for the other 2 colours.
Good luck

History is that the original engine in the bike was low on compression, and instead of doing an 810 swap, I just put in another engine. I don't have any history for the old engine unfortunately. I'll try the ignition wires for sure. I couldn't find in my Clymers what order the coils should be wired to the four plugs. I'm pretty sure it's right, but I'd like to be certain. I'll try just the wires for now and if it still doesn't run, which it probably won't, I'll pull the top and check the valves.

One coil fires plugs 1 and 4 and the other fires 2 and 3.
God luck