82" GPz750 backfires then no start

I just rebuilt carbs off a 1982 kz1000 for my 1982 GPz750. I put in 115 main jets and 37.5 pilot jets, I got a couple new floats for it and just cleaned the needle jets. I had a local shop adjust the floats as I was having a problem doing it myself. The problem I'm having is the same as before I rebuilt the carbs. I try to start it, it turns over, then I get a very loud backfire. It doesn't start. I got it with the carbs but they weren't installed. I'm not a mechanic. The guy I bought it from said that "it should run" with the carburetors he included with it, I just have to "install them". I figured, "I can do that". Well Summer is almost over, I've rebuilt the carbs, paid to adjust the floats, reinstalled them and BOOM, ALL I GET IS A BIG BACKFIRE. I'd appreciate any thoughts on my next step. Thanks, Marty

What did he do to the motorcycle? Since he had the carbs off, it suggests he was possibly doing other things to the motorcycle.. Did he rebuild/work on the head? Check to see if the spark plug wires are all going to the correct plugs..always check the easy things first...a quick guess about the backfire suggests timing issues.

I would check the timing.

Well, that's a start, I got 2 responses on timing. I changed all the plugs, maybe I put the wire on the wrong cylinder. I don't think they fit any other way, do to the length but I will check. What is the easiest way to tell which wire goes where?

One coil will fire cylinders 1 and 4 (usually the longest HT leads) and the other will fire cylinders 2 and 3. The wiring diagram is not clear on which coil is which.
One coil will have a green wire and a red wire connected and the other coil will have a black wire and a red wire. You can try swapping the green with the black.
Good luck

Quick and easy dianostic: get some carb cleaner or easy start spray, give it a blast into the airbox or over the filters. It wire fire up on that even if the carbs are bone dry. You will know if it is fuel or igntion and then can move on.

Did you put the tap to 'prime' to fill the carbs? backfires are usually weak mix or timing.

I'll echo the timing guesses. Don't know much about the GPZs but on my KZ750 the barrel on the timing advancer will go on 180 degrees off of where it should be. I got mine backwards once and I think it behaved like what you're describing.

Tried spraying carburetor cleaner and got nothing. Could the battery be too weak. I don't know if the voltmeter is accurate or not but it reads 10 volts at most and it doesn't sound like it's turning over very strong. I charged it on the low setting over night and still reads low. Like I said, I've been assuming the meter on the bike is wrong because the battery was suppose to be new but maybe that's all it is.