A 2V, 2100 is too small for a 390 if performance is of concern. 390's were higher compression than the 360. but yes it should run.

What do you mean by a check valve in your vac advance ? lose it.

. the easiest way to check for most leaks is to slowly cover the air horn with a rag. if it speeds up find the leak, if it tries to die look for another problem.

If you decide your carb is toast I'd recommend getting an intake and a 4V like you should have. keep your foot out of it and your MPG shouldn't change at all.

 

your truck is Right Hand drive, Euro built Ford?
Is there EGR exhaust gas recirculation valve on you engine? EGR stuck with carbon, leaking diaphram,
and or theh Vacuum source hose to EGR is not on venturi vacuum.
should have Vacuum venturi source to distributor, should be vacuum when throttle is opened, no vac at idle
Some old engines use a temperature switch to only supply vacuum when at operating temp, & they may not be working cause the are Age worn, & may leak vac.
A Vacuum Can resevoir are known to leak, vacuum can may be a plastic ball also, these hold a vacuum source for vacuum operated accesoried - ie ac/heat blend vacuum motors on air blend doors. Heater vacuum blend doors are prone to leak if Age has taken its toll,
some old trucks had a vacuum control Ac/Heat switch in Dash Cluster that would cause intermittent problems.
Some vacuum advance units have an adjustable Allen wrench set screw to adjust diaphram tension, you may or may not have, setting was to allow advancing sooner rather than later, adjustments for high altitude in the old days of performance tweaking!
Every thing you have done may be the best you can get for the old truck.
Wear & tear sometimes cant be tweaked to perfection, The only way is to remove the radiator cap and drive a new truck on to it, a costly option!
GOOD LUCK, MAY YOU DO WELL!
13mm
I think you fixed it as good as it gets, but some Vacuum things that might still affect its performance.

 

Thank you for the advice everybody.
After more testing and searching, I've finally found that I have a small vacuum leak from the transmission modulator on my C6. It wouldn't hold vacuum and the lines contained transmission fluid. I'm going to replace that soon and go from there.
I'm also currently trying to confirm whether my brake booster is good.

I'll report back once I have some progress!

 

 

I'll certainly be interested to find out. It's not a huge vacuum leak - I can build it up to 20" on my hand pump but it will drop to ~10" in about 1 second.

I'm still concerned though that this won't fix whatever is causing the truck to idle low/rough once it's good and hot.

 

If all else fails, re-Check CARB as a last resort - Old fuel - or bad load of corner store cheap gasoline will cause poor eratic performance, and can set A CEL-CheckEngineLight on vehicles that has engine controls
Run out/burn a tank full OR 2 of QUALITY BRAND NAME High Octane fuel, sometimes a good fuel additive like LUCAS helps to clean some of the small passages in carb. change fuel filter often.

I have had to take carbs apart to re-clean all orfices and passages with compressed air & B12 chemtool spray to get the gummy sludge out of carb to get them to run corectly.
I have used fine copper wire to probe orfices and blow clean. Just taking the top of carb off to B12spray it and air blast it may be all that is needed.
Check all vacuum line to the end source for leakage, Vacuum Leaks are A PITA!
good luck may you do well

 

 

Thank you once again for all the help guys - apologies that I've not replied sooner. I've not got around to ordering all the parts it needs yet so not much to update on.

Interestingly, for some reason it has been running a bit cooler this past week. With it running cooler, it idles 100% perfectly all the time!

 

I've got some more updates - and some more questions!

My vacuum modulator arrived today so I've got that replaced. I've tested the lines and they hold vacuum nicely. I figured I needed to re-tune the carb so I took the opportunity to check a few things. I found that the I'd set the float level a touch too low, so I increased that to where it should be.
Whilst playing with the mixture screws, I noticed that there's a vacuum port on the carb that was completely uncapped! I have no idea how I hadn't spotted it.

Anyway, it was a ported vacuum so I've capped it off and adjusted my idle mixture. I'm now down to four turns out to get maximum vacuum. This still seems too many turns, but it idles very nicely and pulls 18" of vacuum at idle in park.

After taking it for a drive it is much improved with the hesitation, but it is definitely still there. One thing is that I've got my idle speed set very low (I don't have a tach) - is it possible that my idle could be set too low, even though the truck doesn't stall?