To recap, the first drive with the new carb was incredibly rich and obviously needed some work, but I wasn't really surprised, and wasn't too worried as everything is adjustable.
Based on the fact that low rpm and mid rpm cruise was incredibly rich, with WOT also definitely on the rich side I leaned out everything.
Stock settings
Primary Jet: 72
Secondary Jet: 82
IFR: Unknown - suspect its .032 or .033
IAB: 72
MAB: 33
Pump cams: Pink/Pink
Getting fuel when needed clearly wasn't a problem so I left the pump cams alone. Revised setup was as follows:
Revised Settings
Primary Jet: 68
Secondary Jet: 82
IFR: .030
IAB/MAB unchanged
After several days of being busy, I finally got the chance for a drive. The result - idle screws were now functioning in the 3/4 to 1 turn range, an improvement from 1/2 turn as before. My low speed cruise below 2000rpm was now mid 13's, with very very light acceleration it would shift to the low 14:1 range momentarily and then settle back down to mid 13's with a constant throttle position. This is great!
Harder acceleration, (but still above PV vacuum level) got the mains on which lowered things a little towards the 13:1 area. Harder acceleration (PV active) and it nosedived towards 11:1. WOT was still incredibly rich at between 11:1 and 11.5:1, sometimes dipping as low as 10.8:1 but I had left the secondary jet unchanged so this wasn't too surprising. My aim was to get cruise/light accel sorted first.
This all suggests that the power valve channel restrictor is too large. When the PV is active below 6.5" vacuum, a shed load of fuel is getting dumped in so these were the next target. I stripped the front bowl off and took a look a the PV. I was a little worried as the Quick Fuel SS series does not come with adjustable PVCRs so I would have to find some suitable wire to stick in to reduce the area. Imagine my surprise and happiness when I removed the PV and saw these:
Lurking underneath the PV valve orifice (the big hole in the middle) are two adjustable jets, the same 6-32 size as the other adjustable restrictions in the rest of the metering block! I was slightly suspicious of this carb from the start, as SS series should have red anodised metering blocks and this one is clear. I think what has happened is someone has specified a carb for a particular application then for some reason decided not to buy it, but the shop had already made it up, so they sold it on (to me!). I tried to find out what PVCR size a 750-AN and found anywhere between .055 and .062 depending on the source on the internet. I found some handy bits of wire of various diamters and determined that these PVCR's were approx .055", or somewhere around 1.5mm.
I already had some unused restrictors that were the original IFR's (.032") that I replaced, so I drilled these out to .040" (1.0mm) and used them as new PVCR's. I figured if I started small it would be obvious what the effect was - I'm hoping the effect will be leaner on part throttle accleration, then I can always drill them progressively larger if I went too far lean.
I reluctantly left the secondary jets at the stock 82 size so I could properly evaluate the effect of the PVCR change. Only make one change at a time! The next test drive will be interesting.
As an extra note, at the end of my test drive yesterday for the last 1 mile or so, on very light throttle cruise the AFR was off the scale lean and it was misfiring all over the place. On application of more throttle it would clear up and run normally so I had to do the last mile nailing it then coasting. I'm slightly confused as to what the cause could be. I pulled all the plugs and they are all a little dark from the rich mixtures I've been running but otherwise ok. All plug leads check out fine, timing is locked down and unchanged, vac can is working normally. Can't think of anything else at the moment other than some crap managed to get in the carb and blocked a channel. I've stitched it all back together ready for the next test drive so I'll see if the problem has gone away.
Later edit - this was resolved with fresh fuel, it got to the bottom of the tank of fairly old fuel.
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