Engire Backfires under load

Gents
I am asking for some help from the brains collect. I have an old 81 KZ750E1, that i have rebuilt and customized to a hardtail bobber. The engine itself is as good as new, full rebuild, rebuilt carbies. Now, i have custom drag pipes with 6 inch baffles and she sports K&N pods on Mikuni carbs, The carbs have been rejetted to suit the pods and for the drags pipes, Now she did run a little lean and i found that i had split one of the carbie mounts, all 4 have now been replaced. She starts OK, idles great, revs out beautifully, but, when i take it for a run, it goes ok in first gear, 2nd gear is ok, but as soon as i hit 3rd 4th and 5th, under full throttle, she backfires and pops badly thru the drag pipes. It used to run reasonably ok before i changed the carby mounts, even rode it to work a few times, 40kays to and from from, but i am at a loss at to why just now she backfires so badly. It wants to run, i know it does, i can feel it.
Question, as she is still using the original ignition coils, with new HT leads and plugs, is it possible that the coils maybe failing? thus causing the backfires or am i looking at this wrong and it may well be the carbies....any advice much appreciated.
Hobes.

Pics of the plugs.
Steve

OK, tonight.

When I rejet, I wait for 100 or so miles to see if it will lean out. That happens when the engine is running better and needs more fuel.
Steve

Ok, pic of 4 realy black plugs. This is a give away that it's running rich, i know that, so, if i run a hotter plug, will that help?

ok, after doing some research, i have found that a hotter plug will indeed help out. BUT, i will also have to check the condition of the coils. The leads are new as are the plugs, but as this engine is not using normal breathing and exhausting, i will have to adapt. so, if anyone has any further advice, which will be much appreciated, it will try a new set of plugs and get back to you.

Further to this, it says run B8ES plugs as standard, so ill be running B7ES and get back to ya.

Running hotter plugs will not fix the problem, it will only mask it for a bit longer. Definitely check the ignition system. The coils must get full battery voltage and the coils and plug wires shoul fall within the specified resistance. Evenso, a weak coil can seem ok when the bike is first started, but may act up as the coil heats up.

Have you double checked the carbs for proper fuel level? You say the carbs were properly jetted, but as black as those plugs look, I would be very suspicious.

Thanks texas.
i will check the ohms readings on the coils and leads, Not being a carby expert, i am only going off what i have read about in other forums, ie, using pods and drag pipes, the biggest issue is getting the jets. Coming out of the US, they take upto 4 weeks to arrive. I may have to get a few other options for the main jets, as this i feel is what maybe causing the most issues.

While you are waiting for the jets, you can check the fuel level. If it is too high, it can cause a rich condition. Search this forum for clear tube method.

mate, i have already done the fuel level, using said clear tube, i am right on specs, this was one of the major things i checked before putting it all back together. I have also done a mechanical check on carby syncs using feeler gauges. spot on. But i will get them checked later when it is running a tad better.

ok, i still have the same back firing issues, but i have had a thought, is it possible that i have jetted this thing way too far( yes i know it is running rich) that the carbies are being pulled dry or atleast drained to far for the petcock to keep up, that it is causing backfire due to low fuel? especially when under full throttle in third gear, it really wants to go, but it seems to starve, maybe, i am just thinking out loud and want some feed back....

Thanks in advance.

OK, i had a brain snap last night.

I think the route of problems is, i had being trying to solve a too lean issue when all the time it would have been caused by a split carby mount. This alone would have been causing it to run too lean. This would have caused the rev hang. I had been looking for the answers to solve the rev hang and tried to rejet the carbs without finding the splint mount. So i will be going back to std jets and see if there are any further issues, I know i may have to go up in jet size for the pods, but not to the extremes that i have at the moment. Again, this is just thinking out loud, but after reading some postes here about carbies and other related issues to back firing, this maybe the solution.
Again Thanks for listening and your input.