riello f5 burner not firing

Since you changed the motor and the pump you probably missing either spark or fuel.

Maybe disconnect the nozzle line from the pump and see if you're getting oil and check for spark. The electrodes have to be set right.

AS @HydronicMike mentioned you're in this pretty deep. You really need a tech with the correct tools and a combustion analyzer

Your oil supplier must have a tech or know of one if your buying oil from a company with no service.

Tries to start ? Just a flash then locks out ? Try a .75 60 W instead of the B . It could be the mix of the fuel oil .. Years ago we had a standard … I only use W now…

I'm with rick. you won't go broke betting on fuel problem with riello. haven't experienced coupler failure myself but iirc the couple is a nylon D that could give up. but given all the parts replacement and depending how high the level of oil in your tank compared to the burner and whether the supply line goes overhead I also wouldn't be surprised if you have lost prime. maybe you don't mention and you have been bleeding clear on these retrials but are you checking for solid oil flow at the bleeder? I have had more trouble with the air on one riello than on hundreds of other burners in my experience. it did have an up and over oil line although it was well terminated with flare connections and the burner was about even with the bottom of the oil tank but I could never get that thing to prime and I could never identify a source of air. i put in a tee with valves in the oil line right before the boiler and I would run it out of a five gallon can and then I would partially crack the valve to the oil tank so it was still getting plenty of oil out of the can and it would start to draw on the tank line and then slowly over five mins I would close off the 5 gallon can and fully open the line from the tank. and then it would run a few months and air up again. my conclusion was, always two pipe riello. no problems since. maybe a tiger loop would have worked but I was so damn frustrated with this thing, i ran the second pipe and never (fingers crossed 5 years later) had another problem.

but could we catch a breath and go back to the beginning. you started with it sputtered but wouldn't light before you replaced anything iirc. i presume that the boiler had shut down and you mean it sputtered but wouldn't light when you reset it?

i'm a little confused that you later say the fan wouldn't run until you replaced the capacitor because if the fan wouldn't run that's the same motor that runs the pump and it shouldn't have sputtered (you might have heard buzzing of ignition but i wouldn't call that sputtered. or did the symptom change as you went through the various replacements and trials?

I've been working on Riello burners since the Mectron series was common. I don't think I've ever had to replace the nylon coupling (Pump Drive Key). I don't gamble, other than slots once every few years maybe, but I'd bet the ranch that the coupling isn't the culprit. A professional might know if they happened to walk by the burner one day.