Carb backfire

So I recently purchased a 1982 xj650r Seca and have been having a hell of a time getting the carbs tuned. So far all the carbs have been torn down completely, cleaned meticulously, all needles and jets replaced with new ones, float levels set to 3mm+-1, throttle shaft seals replaced with number 11 o-rings, fuel tube seals replaced. Idle mixture screws, Springs and seals replaced and set to two and a half turns out. Valve clearances are checked and all are within spec except for the number 4 exhaust which is .002 out of spec. The previous owner had replaced the entire ignition system so that's all new. Carbs have been bench synched. Still running stock exhaust and airbox. When you first start it, it runs fine for about 30 seconds and then starts to backfire through the exhaust and the carb. I've set the idle mixture screws out to four turns and it's still backfires. Less than 2 and it wont run. And becomes hard to start once it warms up, but if you can get it over 5,000 RPM it runs great lots of throttle response, no back fire, spit or sputter. I've used some starter fluid to try and check for vacuum leaks but haven't been able to find any. Am I missing something? Any advice would be appreciated.

 

Bench synch only gets you in the ballpark. You also need a running sync.

 

Running sync By using a vacume gauge?

 

I'm a proponent of working with the 4-stick mercury manometer (they actually no longer contain mercury)
Some think $100 is too frivolous an expense but if you plan to ride multi-cylinder carbureted bikes in your future I say buy the tool.
An hour of shop labour will charge you this to do it for you anyway.
Now, others have built their own manometers out of glass jars and are happy with that.
The running sync makes a big difference.

 

Gotcha, I tried using a known good vacume gauge but with the backfire issue getting a reading was next to impossible.

 

You said that the ignition system was replaced by the PO.
What was it replaced with?

 

Not sure about brand or part numbers, but both coils were replaced the igniter module was replaced as well as the reluctor

 

I realize timings not really adjustable, but I did check to make sure it was in the appropriate range according to the Haynes manual and it was spot-on

 

If the coils are wired to the opposite connectors that can cause a backfire.
What i'm saying is if 1&4 are connected to 2&3 connector from the loom and vice versa, which will in effect make the timing of the spark out by 180°.
But, a long shot by the sounds of it.

 

The OP has seen >5,000 RPM. Not possible.

 

Well I've got a 4 gauge set up to sync the carbs on the way, so we'll see if a running sync clears things up.

 

Got a running sync completed and it is night and day difference. No more backfires at all. However, it seems lean while riding it around, low on power and just doesn't feel like it wants to go. Plugs are clean, no heavy fuel smell. Idle mixture screws are 3 and a quarter turns out. I'm at sea level, is it common to have to bump the jet size up at all? I have a set of 42 idle jets I was thinking of trying. Any thoughts?

 

Something is wrong if it is lean at 3 1/2 turns.

 

Agreed. Either the diaphrams have pinholes, or there is a vacuum leak somewhere (assuming that the carbs are zestfully clean).