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MG Midget and Sprite Technical - Rebuild - fuel economy

I recently rebuilt the 1275 in the 1971 midget. I installed a fast road cam, high compression pistons and had it bored to 0.30 over. I also had the distributor rebuilt and curved for the above and had the S.U carbs professionally rebuilt.

The results are stunning. The car runs better than it did new (I am the original owner) and after 1,000 miles, the engine is idling smoothly and has responded well to fine mixture and ignition tuning. What has most surprised me is the drop in fuel economy. When the car was new, it got 30 MPG+ but now I am getting just over 20 MPG. The difference is worth it for the extra fun but it is a stretch on a 6 gallon tank.

The Mercedes ML350 SUV is no longer the least fuel efficient vehicle in the stable, and it runs fine on the lowest octane grade.
Glenn Mallory

Hi Glenn,

I remember reading /hearing somewhere that the original tune on SU's tended to run as lean as possible on part throttle to get better fuel consumption, running lean typically costs you a bit of throttle response - it might be worth fitting a wide band lamda sensor in the exhaust and just checking the part throttle fueling.

It mighe be possible to lean the carbs out when you don't need the power ?

Mal
Malc Gilliver

USA MPG figures are lower due to the smaller gallon but I’d still expect better than 20mpg on a mildly tuned engine. In fact in some cases it could even give better mpg than a standard engine. Has it been set up properly on a dyno?
John Payne

Glenn,

Your fuel consumption figures do seem high. My similarly tuned Frogeye records about 30-35mpg using a 3.7 ratio diff.(I think UK gallons are larger) I don't generally do over 60mph when touring so that may also help. A check on the fuelling may be needed.
Bob Beaumont

Glenn,

I have a recently rebuilt 1275 with a mild cam and head work, Datsun 5 speed and 3.90 gears in my Frog with a down draft DGV Weber and I get about 28 to 30 mpg. Even before the rebuild with SUs I was getting 22 mpg. As Bob stated, you might have a look at the fuel system.
Martin

It's only a rough guesstimate, but I got around 35mpg (UK) from my 1500 on a 4 hour mixed A road drive from Worcestershire to North Wales this summer. I don't drive particularly slowly, but I do have a 3.55 diff.

In nearly 15 years of ownership it's the first time I have thought to work out the economy, as I have always been of the opinion it's a car to have fun in so economy is irrelevant! I was pleasantly suprised.

20 (US gal) does sound low, that's 24 miles per UKGal. I get more out of a 3 litre Alfa (although it is slightly more modern, 90s).

Malc.
Malcolm

Recently returned from a 1450 mile round trip to Denmark and achieved 39.7 mpg. Always achieves high 30s / low 40s on trips to Europe.

1293cc @ 81 bhp [modified head, lcb exhaust, Piper 270 cam, HIF 44 and 3.7 diff] set up on a rolling road.

HTH.
Doug Plumb

You have all made me feel better, as I was concerned about not getting more than 35mpg. 998cc, Kent 276 cam, big valve Metro head, HIF44 etc. Economy will improve now as roof is up most of the time, and even more when hardtop is on.
Les Rose

Glenn--Just a thought, if you had the carbs reco'd, maybe they've poked the wrong mixture needles in there-
William Revit

The new carb needles may not be ideal for high altitude running. I believe that the car came with AAB needles. I tried leaner needles but the originals gave the best all-round running condition at altitude (5,000' above sea level) and on the coast. I will check to see what is installed and view the profile against the originals on the Berlin chart.

The car runs so well. My main complaint is having to fuel so often. 6 Gallons won't really get me very far down the road in this part of the world.
Glenn Mallory

Another little possible is where you have your vac. advance connected to and are you disconnecting the hose when adjusting the timing(if using a timing light)
The trap is having the hose going to manifold vac. but leaving it connected while doing the timing ,it'll idle fine and toddle about ok but retard off at full steam
just a thought

willy
William Revit

I have put about 2,500 miles on the rebuilt engine with economy improvement. The mileage on the highway has been as high as 35MPG, consistently 31 MPG, but is still lousy in city driving.

I had the carburetors thoroughly rebuilt and today I pulled a needle to check the code. It is marked 3, while the originals were AAC. What surprised me is that the N° 3 needle is about 1/3" shorter than the originals.

Today I adjusted the mixture using a CO analyzer (at an official MOT station) and ended up raising the jets about 1 1/2 flats to achieve the 2.5% CO shown on the under-hood label as the emission target.

The car runs well following a compression ratio increase, more aggressive road cam and recurved distributor. All of this with the object of obtaining an increase in midrange torque (a success by any measure).

The shorter needles have me wondering if this is not at the root of poor fuel economy, and perhaps the great acceleration. I am temped to just accept this as it is due to the great running condition.

Has anyone replaced AAC needles with a N°3?
Glenn Mallory

Glenn;

According to this chart, '3' is a RICH needle for a 1071cc Cooper S.

https://www.7ent.com/pages/articles-tech-tips/chart-carburetor-needle.html


anamnesis

I'm a bit confused
(normal condition,please don't send flowers)
AAB--AAC would be spring loaded needles I think and No.3 a solid needle so there must have been some parts swapping going on
So-back to basics
What size are the carburettors normally they'd be 1 1/4" HS2's
What colour damper springs are in there
And---are they solid needles/carriers or springy ones
William Revit

Compression ratio increase generates stronger pulsing in the inlet tract. More aggressive camshaft could either mitigate or add to that.
Stronger pulses draw disproportionately more fuel and so require thicker needles (or smaller jets in fixed choke carbs) to keep the AFR correct.

A couple of examples:
When my MGB was running 12:1 (originally intended to suit a race cam) with a std camshaft fitted for a while, rolling road sessions found that GZ needles were required to get AFR correct. If you check the needle charts you will see these are much thicker than the std FX needles. Economy was awesome thanks to the greater combustion efficiency of the higher CR.

We had another B in recently which was hopelessly rich and running HS6 carbs on the usually required SY needles. Even going to thicker ST left it still too rich. Investigated and found CR was over 11:1 on a fairly conservative camshaft. Eventually, using on-board AFR diagnostic and road testing, profiled our own needles a bit thicker again than ST and everything came right.

You now have your mixture good at idle, but the shape of the needle will determine whether it is right everywhere else. No 3 needle is 1.6 thou thinner at station 3 (steady 30mph) and 1 thou thinner at station 5 (highway speed) than AAS, this translates to 15% and 8% richer than the AAS needle, yet both are the same size at idle.

If you haven't done it yet, I suggest you investigate AFR under load. One of the most susceptible load conditions to higher CR is the light throttle cruise - steady speed on either urban or highway - and these have great influence on fuel economy.

In my experience I have yet to find a negative effect of the shorter needle length but will happily defer to anyone who has and be very interested in the circumstances.
Paul Walbran

The "3" needle is in a spring loaded mount. Seems identical to the original AAC. The mounting/spring end is identical. Damper springs are colored blue. Carbs are HS2.
Glenn Mallory

I agree with Paul that determining AFR at speed is the task at hand. The "3" needle is seemingly richer under acceleration and load and this is probably enhancing performance at the cost of economy.

Next step, if nothing else due to the fact that it is simple, is trying leaner needles. I think I have AAZs in stock.
Glenn Mallory

AAZ would be worth a try--they look about right to me
William Revit

This thread was discussed between 18/09/2023 and 30/10/2023

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