Odd engine sound

[Bat21]

Okay folks an odd one here.

Started my 80 earlier today, it has not been run for about two weeks. As usual it started first turn of the key.
I let her idle for about 10 seconds and then used the hand throttle to 'idle up' to 1,500rpm.

I got out to close the garage door but then noticed the engine sound changed, sounded like she was running on 5 cylinders.
I went back to unlock the idle up and drop it back down and as I climbed back up into her the snorkel intake was making an odd sound. It sounded like a 'bop bop bop bop' noise my old Leland Terrier truck used to make as the air compressor was filling the air tank.

I dropped the rpm back to tick-over and the 'misfire ' and odd noise from the snorkel continued for about 3 seconds then stopped.
After this the engine returned to normal and drove as it normally does.

it has never done this in the 12 years I have had it.

What could it be?

 

[StarCruiser]

The only thing I can think of is air getting into the fuel line. Not much, just enough after being left. If it does it again, try pumping the primer on top of the filter.

Why do I suspect this? It’s happened after a period of non-use. I had a slight misfire appear not long after I fitted my eberspacher heater. I T’d it into the delivery line to the pump after the filter. Since I’ve turned it off at it’s valve I’ve had no misfire to speak of.

 

[Bat21]

Thanks SC. I hope it is something as simple as that.
I have a Walbro pusher pump fitted inline so I might flick that on for a couple of seconds prior to start up next time it's left for a while.