A bit of diagnostic help please.

Ok, I give up, I'm stumped, I know I'm missing something or something's and as much as it pains me, I need a little help.

I'm just about to list a 2001 Ford courier (Mazda b2600) for sale, but the one thing holding me back is an annoying stumble/misfire/backfire when it's reved from idle. The fault isn't constant in the sense that I can't get it to do it every time, but it's consistent enough that I can basically guarantee it'll do it when I've got a perspective buyer looking at it, or worse still 5 minutes after they've gotten it home, leaving me looking like a lying scumbag. It's got just shy of 200000km on the clock, it's running a 2.6l g6 engine same as the b2600i's

So far I've changed the plugs and leads, it's got a new fuel pump and filter, the tank had been drained and fresh fuel put in. It runs fine anywhere above idle, it idles fairly smoothly for a large capacity 4cyl, but still feels just a tiny bit off where o feel it should be

I've checked timing with the timing locked and it's at 5°btdc +/-1° at 750rpm as per what it's supposed to be. Using my timing light, the spark seems to remain constant even when it does fault, so I'm thinking fuel or sensor problem.

Being non obd2 and not having a check engine light make it a little more of a headache than it may have other wise been. I'm assuming there is a way to get codes from it, but haven't really found much information on this particular animal.

My potential culprits are things like o2 sensor, air flow meter, coolant temp sensor or possibly fouled injectors. 

Because it's something I'm selling I'm trying not to just throw parts at it and would really like to get to the bottom of what the hell is going on.

Clogged injector would be my first bet. 

+1 for clogged injector. Put some Techron by Chevron, the black bottle, in the tank and drive on the highway til you burn half a tank or so. I always throw a bottle in on any long, 200 mile trip. 

Unless it’s an electrical problem with the injector. 

In reply to EastCoastMojo, Sergio : It makes the most sense for it to be a clogged injector, especially given the other fuel system dramas. 

It's had a batch of injector cleaner through it, which seemed to help, as has premium fuel. But it looks like I might need to pull them and get them cleaned and flow/spray checked. Just didn't want to wear the cost of off board cleaning only to find out I'd missed something else.