I need a bit of advise here fellow forum members.

My 1098 is mildly tuned. Has been bored 20 thou, 9.5 comp ratio, wedged crank, balanced pistons, rods, crank, flywheel and clutch. Has extractors, twin 1 1/4 su's, mild Clive 102 cam, unleaded 202 head. Accelerates and gets along nicely. Has a 29d distributor, no vacumm advance connected as there is no port on the carbies for it. Does not ping. Static timing set at 6 degrees advanced. Su needles are AN, idles well, smooth through the rev range. But, it goes through the juice a fair bit more than my liking. I put it down to the no vac advance. Thinking of going electronic or maybe a 23d distributor from a Cooper S. Would this work any better. Looking for advise on what others think.

S distributor doesn't have vacuum advance...

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did I tell you that I won a trophy?

Get an A/F meter on it. I bet mixture is out.
Or, it needs more advance. Check what max advance is, most tuned 1098s or 1275s needs around 30 degrees, at about 3500 then no more at higher revs. Idle advance setting is way less important for performance.
Vac advance does not make that much difference to economy. And if wrong spec or its mechanism worn, it can cause engine damage.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Your state of tune is very similar to my K some years ago - but I had a LCB extractor & S exhaust. My car performed better than standard but no noticeable increase in fuel consumption with AN needles in twin HS2s.
Are you sure you are getting accurate fuel consumption figures and not relying on the gauge only?
Vacuum advance is intended to operate when the throttle (via accelerator pedal) is opened only slightly as when driving sedately.
SU Midel books suggest D3 needles for twin HS2s on various modified 1098cc A series motors - but I doubt they will be much different to AN needles in fuel use.

Thanks for the replies Doc and Bill. Filled up today and went for a 20 km drive. Fuel gauge went from full to half within 10 km, then hovered around that mark. Perhaps the douverlacki resister thingy behind the speedo is playing up. Will stick a hook into the tank and move the fuel float arm upwards and see what happens. Will clean up the terminals as well. I'll check out those needles also Bill but it does run smooth at 50 mph so I'm a bit baffled that's why I'm thinking distributor advance. Will see a mate tomorrow and throw it on his machine to see what's going on. Got hold of a 23d Cooper S distributor today, maybe refirbish it and see what happens.

Cooper S dizzy advance curve is the last thing you need with modern unleaded fuel. As it keeps advancing all the way to 7000 rpm. Needs serious advance curve mods to optimise it.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Today I went back and checked the basics, no use doing heaps of stuff which could be costly without eliminating the easy stuff first, just as well I did too.
Shone a light into the tank, appears only about a gallon (4.5 ltrs) had been used for the 33 miles driven. Lifted the float with a hook and the fuel gauge needle went to full. Cleaned the terminals on the voltage stabilizer and the fuel gauge. Turned ignition on, needle went to half full, then it started to move around. Fitted another second hand voltage stabilizer and the needle went from 1/2 previously to nearly 3/4. Not perfect, should be just above 3/4 if accurate, but this made me a bit happier. At least if it shows empty it still has a bit in the tank. Works out I'm getting 33mpg or 10 ltrs per 100 km. Still a bit thirsty for a car cruisin around at a steady 55 mph.
So I'll take the good advise from the Doc and forget to Cooper S distributor route for now, will get it on a mate's machine to see what's going on with the fuel and advance, and look at as Bill B suggests, different needles.

Thanks guys.

PM sent with dizzy specs.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Well done. Some good simple sleuthing and a solution in sight. "Knowledge is easily carried."
Fuel consumption checks are best done over longer distances. Fill up the tank till you can see the fuel level; record odometer; refill tank to the same level and record litres from pump and record odometer reading at the refuel point.

Hey Bill, yeah, a 20 k drive is not much when doing fuel economy checks. So taking some time off tomorrow during my retirement (sic) and going for a decent 100 k drive to get a more accurate figure.

The Doc has filled me in with dizzy specs also, will stay with the 29d, it's fine, but will get an electronic module for it as doing the timing is a bugger of a job with the fresh air heater in the way.

Went for a decent 120 k drive at the weekend, filled er up at the end, got 30.5 mpg. Changed out the AN needles for leaner EB and retuned the carbies. There is not much difference in the 2 but boy there is a huge difference in the driving. Feels so much smoother and for the same speed, don't need much throttle opening. Will conduct another test at the weekend to show I'm on the right track. Thanks to Bill B and the Doc.

I wouldn't bother with another test.... A few years ago I was in "record" mode and I kept a couple of years (say 15 000 miles) worth of consumption figures in a spreadsheet. You'll be lucky if you get consecutive figures within 10 to 15% of each other. Over, say, 10 data points you can start to see trends. Even if you think you're replicating the route, wind, traffic, ambient temperature and even atmospheric pressure will vary your engine's performance....and consumption.

I think some of the other comments are less informed than they might be. On a properly set up engine a vacuum advance - under good conditions - can provide up to 30% improvement in fuel consumption. I live in the country and face almost no traffic lights or other such hassles and my consumption dropped from 30 ish to 40 ish mpg after we fitted the vac advance..... which came from a 70/80s Fraud Escort.

Someone mentioned that they would only work when driving "sedately". True, but consider that an S type engine is only providing about 25% of its capability at 50-60 mph...that's a pretty light throttle. My last trip down the Hume to Melbourne worked out at a bit under 40mpg while cruising at 4K revs (which is a bit on the naughty side of State laws .

These days getting the A/F ratio correct is a lot easier than it used to be. I have two sets of needles that give almost identical HP/performance... but one uses a fair bit less fuel...

Cheers, Ian