Hi guys...new here, be easy on me LOL

I've got a F250 w/460 with a slight random misfire at low load (slow revving, ect) and idles like it's aggressively cammed. I've swapped plugs, wires, distributor, adjust timing, and cleaned the heck out of the carb but nothing changes at all. Compression is great, no vacuum leaks, and at WOT it performs great; it has only 90k total miles...

It's actually on it's 2nd carburator over the years (due to similar issues in the past causing it to fail Cali smog). This one is probably 12 years old. Luckily I'm in MD now so I don't have to worry about smog...I drove it all the way across the country with a huge trailer

Other than that the engine is 100% stock and body and interior are ****... even got a seat from a '94 F350

So are these carbs just crap? From what I'm reading you can't even adjust the mixtures. I REALLY want to get this thing running smoothly like it should..it really seems like the idle and low load mixture is way off..

 

Have you done a leakdown test? Could be a burned valve or two.

 

Compression checked out OK so I didn't do a leak down..it's my understanding that a leak down is only useful in diagnosing lost compression.

 

Check your EGR valve and make sure it is not partially stuck open. It happened to me once, found a small piece of carbon that was holding the valve open.

 

^^ Good call... I'll check that out.

Any other ideas?

 

What were your compression numbers? If everything else checks out good I'd rebuild the carb or if it's a junk smogger carb put a 750 CFM 4150 on there and call it a day.

 

Is it running rich?

 

I don't recall the numbers off hand but they were all +/- 5% and high. It's a 'smogger' carb I assume, bought and smogged CA for 20 years before driving it out here.

It feels/sounds like a lean misfire but I don't have an A/F gauge to confirm. At WOT it is fine..plenty of power and no misfire.

 

Sounds like compression is fine then. Stupid question, but where is your initial timing set and have you adjusted your idle throttle opening and idle mixture screws?

 

That would be my next check too. Here's a personal valuable tip: Screw the secondary idle screws in all the way with an Allen wrench then turn them back out exactly 1 turn. Make the final mixture adjustment with the primarys, trust me, it works.

 

But my 'smogger' carb's idle adjustments are set at factory and non adjustable, no?

 

You need to pull the caps covering them in the base flange of the carb.

 

The plugs that cover the screws may have been removed at some point. If so, it requires a very small allen wrench to adjust.