1850 fueling

[Oli_88]

Afternoon all.
Chasing an irritation with the fueling on Lolo.
Standard 1850 engine internally, stock fuel pump, stupid 2" sidepipe exhaust, stock carbs with 38mm long trumpets, ram air foam filter socks, AAU needles, not very old red non waxstat jets, 30w oil in the dashpots, red springs in the dashpots, floats with viton tipped needles, set to close at 3.2mm between float and lid, drill bit across the flats method. AEM analogue AFR gauge fitted. Recently had new spindles fitted to the carbs after finding a massive air leak.

Ignition is a standard AC Delco dizzy, accuspark electronic ignition, accuspark solid state 12v coil, accuspark stealth black box programmable timing kit, fairly new plugs gapped properly (also tried with slightly wider gaps with no change), leads are fairly new, new dizzy cap and rotor arm on the way.
Timing set at 12° @ 500 rpm
15° @ 1000rpm
16° @ 1500rpm
17° @ 2000rpm
20° @ 2500rpm
23° @ 3000rpm
26° @ 3500rpm
29° @ 4000rpm
33° @ 4500rpm - 6500+
Vacuum set to start adjusting timing at 5Hg, with a max of 12Hg, it won't go upto the 20Hg the original AC Delco one does. With 20° of advance maximum. I have also run the car with the vacuum advance turned off completely, with no change.
Rev limiter set at 6400rpm.

If I set the car to idle with the AFR gauge at around 13, then raise the lifting pin up and listen to the idle speed, it will stumble and then conk out seconds later. If I set it to where the lifting pins are happy, a slight stumble and then return to idle it sits below 12 afr and the plugs end up very black.

Secondly, when the mood takes me and I want to open it up a bit, seems to hit a very harsh rev limit at around 5400rpm, shown on both the rev counter and the ignition box display.
I initially thought this was down to needle choice, watching the AFR gauge plummet to 18 when it happens. I already had ABN needles fitted, so swapped in some AAUs, with exactly the same result. Pulling the choke out with the ABNs it somewhat, but with the AAUs being significantly richer again, I'm unsure where to go.
I think going significantly richer again on the needles would be a mistake, I actually think the AAUs might be too much.
The obvious answer is don't rev it so high, but where's the fun in that?
The last two options I can think of are an unhappy fuel pump at high rpm, but then I doubt pulling the choke out would help, or the rev limiter is saying 6400, but is actually cutting in 1000 rpm early, which I doubt more.

Car is only run on Vpower, from the same garage as my daily.
So far I have changed float heights to almost 5mm, the weight of oil in the dashpots from 10w40, 15w40, 20w50 and now 30w. Gapped the plugs to 0.88mm and then back to 0.65mm. Put the timing back to factory standard figures, copied the 123 number 8 curve as best as I can. I haven't yet confirmed the fuel pump is still putting out the correct pressure, the fuel filter is still clean. Jets are clear, fuel rail is clear. Timing has been checked at idle and upto 2500 rpm to match up with the black box display.
Any suggestions?

[yorkshire_spam]

If it was me I'd start looking for fuel flow problems.
If you can jerry-rig an appropriate low pressure electric fuel pump in place of the standard mechanical one. Also if you can - get a fuel pressure gauge in the feed to the carbs that you can see as you hit the 5400 "limit".
Check for issues with inline filters - for the sake of a short test run - remove them.

The reason I say this is that my 1500 Spitfire lost 4-5 HP at peak and was falling away around 500 rpm lower on a dyno run - the only change between the sessions? Fuel filter.
Removing the filter put it back to how it was on the previous run.

I'd say in your case for one reason or another at the high RPM the pump can't keep up - or is cavitating or simple not reacting fast enough on the return spring to pump effectively. So the level in the float chambers drops and no matter what you do with needles or floats - it'll lose power.

Just my 2p worth as an uneducated amateur.

[Oli_88]

I'm 50/50 about it being fuel flow related. I think if it was the pump not keeping up, then pulling the choke wouldn't resolve it, as there'd not be anymore fuel in the float bowl to supply it.
But then there's the possibility where it sat for such a long time, the fuel pump got gummed up and hasn't fully blown it all out. Though I have had it apart, so who knows.
Also realised that a 6400 rev limiter is a bit excessive, even though the rev counter redlines at 6500, 6000 will do.
Will also try it without the fuel filter.
Going to modify the fuel rail this week to make it more even front to back. The original having to feed the rear carb from a 90° bend, seems insane.
Will also attempt to get it to do it somewhere I can abruptly switch off and coast to a halt, so I can look into the float bowls and see what is or isn't in there.
Half tempted just to order a new pump and throw it on, I've virtually replaced everything else!

[Oli_88]

New theory.
Took it for a spin after fitting new fuel hoses, the second lot after the first batch of 6mm braided hoses disintegrated immediately, annoyed.
Watching the vacuum advance while driving, on the overrun, there is absolutely no vacuum advance at all, I had some while cruising steadily, a lot on a fast idle and then intermittently.
I'm wondering if there could be a vacuum leak that only rears it's head at full load, say from the brake servo hose.
This would explain why pulling the choke "fixes" it, albeit temporarily.
I'm going to cap it off and run it again. I've checked the breather system, which I have always hose clamped anyway and couldn't find anything with carb cleaner.