1975 RD250B backfire

[sjeffs01]

Hi, ive recently bought a 1975 RD250B, I've had the carbs stripped, cleaned, jets checked & float height set. It now runs much better as before it would only run on choke, now it runs pretty good, but still bogs down when accelerating from a start under load or on a hill, and as you accelerate it's ok upto 5000rpm then holds back and starts to backfire,if I reduce the accelerator it clears and accelerates, but then holds back and back fires again when I open it up more.
The bike as far as I know is running standard jets, but has two K & N style cone filters fitted by the previous owner instead of the standard air box.
Should I look at getting the carbs jetted up to match the air filters or look for a standard air box which I'm struggling to find. I'm not interested in making the RD faster, I just want it to run smoother so standard would be the best option I think??
Any advice will be much appreciated, and any ideas where to source an original or aftermarket air box.
Thanks

[gavink]

If you have K&N filters then you will definitely need to jet up the mains, maybe pilots and tubes, I’d be fairly confident if you take the plugs out they will show it’s been running lean. The filters should also be oiled and not dry.
Perhaps the previous owner ran these because they couldn’t find an air box., in my experience years ago If everything is standard then running filters was difficult to get correct carburation.
I would also check the exhaust port, exhaust and baffles for carbon build up as this would severely affect running (I’ve just got an RD350(A) and it’s badly coked).

[beardy]

Just to rule it out I’d test your high and low speed windings. Hopefully you have a manual with the readings.

[reedpete]

K&n with solid back face are the death of Rd carburation unless on the end of a long tube.
Swap for foam pods and jet up or better just put it back to stock.
£100 for an airbox, £35 for y boot, £25 for a filter