What would cause symptoms similar to a lean lean mix other than an air leak in the inlet manifold/carb (i.e. blip throttle or raise the carb piston pin and engine tries to stall)?
I'm not quite pulling my hair out yet, but definitely seeking a bit of advice. I'm near the end of a 3-year long rebuild (engine and body) and started her up for the first time a few weeks ago. Cam bedded in OK at 2500rpm with a bit of guess work to get mixture 'good enough'. Now I'm trying to get the mixture and idle set with a bit more precision before taking her out on the road. Problem is everything points to an inlet manifold air leak but I can't find one.
A bit of background... engine is a 1380cc which seized with slipped crank thrusts shortly after I moved to the USA (brought it with me from Scotland only a few months after buying it). I stripped the engine apart, sent the block crank and head to the local machine shop for 'remedial action' then rebuilt it myself. Carb is HIF44. Cam is an AC Dodd RS+. Compression ratio is marginally over 10:1. Maniflow LCB and 1.75" exhaustIgnition is via Megajolt V3 running a map supplied by AC Dodd for running in (as was the needle recommended by AC). AC has given me a lot of advice so far, but I don't want to pester him too much and wear out my welcome 
OK, the symptoms:
- Cam should be able to idle at 950rpm even though it'll be a bit lumpy, but setting it anywhere near that low and it's in danger of stalling (i.e. way more lumpy than I'd expect and hunting a bit)
- The only reasonable idle rpm I seem to be able to set is somewhere in the 1200rpm range
- Blip the throttle, or raise the carb piston pin and the engine comes very close to stalling
- Even if I set the mixture screw to the extreme end of rich I can't overcome the problem (and I don't want to leave it there for running in... and run the risk of bore wash!)
- I have a AFR gauge with wideband sensor. I feel most comfortable if it's showing approx 13:1 at idle (maybe a tad rich?) but it's bouncing around a bit more than I'd expect (a good +/-1). Is that much bouncing normal? FYI... The O2 sensor is fitted in the Y collector of the LCB.
- Blip the throttle and the AFR swings from extreme rich to extreme lean, then takes a good 5 seconds before recovering
- As far as I can tell the carb is good. I stripped and rebuild it with an SU kit - even replaced the butterfly throttle disc a few days ago as the old one wasn't sealing as well as it should. Float is set correctly.
- I've squirted WD40, carb cleaner, freeze spay, etc around the carb and manifold and that method shows no sign of an air leak. Only time the revs die is if I give a quick blast of freeze spray directly onto the air filter
- Breathers... when connected the symptoms are even worse, so for the time being I've blocked off that port on the carb (FYI I'm using a vented oil filler cap)
- Fuel is good... supplied via a Facet solid state pump and Filter King regulator (and I've set & checked it is delivering at 3.5 psi)
- Megajolt ignition map... the idle area was originally set to 14deg, but increasing that to 18deg has helped a little
- Megajolt VR sensor is in the correct position (i.e. No1 at TDC and the missing tooth is 90deg from the VR sensor... no offset compensation needed in the map)
- I'm confident I set the cam timing correctly... but is there an easy way to accurately check it without stripping the engine down?
- Engine temp seems OK (gauge showing on the cool side if anything), and oil pressure is 75psi at idle dropping to 50psi when revved)
So before I try a different HIF44 (I have a spare I can get shipped from the UK) what else should I be checking? Yes I should get this on a rolling road, but finding one here in the USA with an operator who knows SUs and Minis is pretty much impossible 
Cheers,
Colin.
