Classic cars forum & vehicle restoration.

Finch661 · Posted: Wed Oct 07, 2009 9:56 am Post subject: improving MPG

i have been using my mg recently, as my mini is currently getting ready for a paint job, and i have noticed that i am getting about 20mpg. the mg is standard 1500 twin HS4s, bar i have exposed filters (standard paper) and have replaced the points with a electronic box. i have tried leaning the mixture, but this then mucks up the idle speed, and usually stalls the car.

is there anythign i can do to increase the mpg. would a carb service kit help?

cheers Rolling Eyes

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1985 Mini Mayfare (1275)
1976 Mg Midget 1500

Nic Jarman · Posted: Wed Oct 07, 2009 11:18 am Post subject:

Is your choke returning properly? Are the butterflies working together and otherwise is the engine running OK. I do a 15 mile trott to work and I get around 30 - 35 mpg but I have leaned the carbs and I got 40 mpg on a long run.
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1936 Morris 8 Series 1
1973 MGB roadster
1977 MG Midget 1500
Dax Rush

Finch661 · Posted: Wed Oct 07, 2009 11:50 am Post subject:

as far as i know, it all seems to be ok. i had the car professionally tuned about 5 months ago. the mechanic said that the carbs look ok, though he couldnt inspect the float bowls as the screws have siezed.

i have noticed that the exhaust is really petrolly (if thats a real word) i could lean the mixture and increase the idle speed maybe...
then again maybe its time for some new carbs?
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1985 Mini Mayfare (1275)
1976 Mg Midget 1500

ukdave2002 · Joined: 23 Nov 2007 Posts: 4104 Location: South Cheshire

If the needles are not central in the carb they can wear the jets oval, which will make the carbs impossible to set up, a stuck needle valve in the float chamber or a float with a hole in would also cause it to run very rich. Both quite cheap to fix on those carbs.

Are the plugs very sooty?

Dave

Finch661 · Posted: Wed Oct 07, 2009 1:19 pm Post subject:

i will have a look at the plugs tonight. i would love to be able to be able to achieve at least 35mpg Smile

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1985 Mini Mayfare (1275)
1976 Mg Midget 1500

Finch661 · Posted: Wed Oct 07, 2009 4:22 pm Post subject:

i have just been offered a pair of reserviced twin hs4s for £50 Smile

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1985 Mini Mayfare (1275)
1976 Mg Midget 1500

Peter_L · Posted: Wed Oct 07, 2009 5:08 pm Post subject:

Finch661 · Posted: Wed Oct 07, 2009 7:35 pm Post subject:

well the mechanic tried to unscrew the floatbowl screws, but the head of one of them snapped off, so he decided not to continue. as far as i know, the carbs are the original, and the previous owner never really did any maintainance
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1985 Mini Mayfare (1275)
1976 Mg Midget 1500

Peter_L · Posted: Wed Oct 07, 2009 8:05 pm Post subject:

If those screws shear then it one heck of a job because the casting is so soft compared to the screws.

Some jigging to hold the carb solid and a drill press, but even then it's not a done deal.

Good luck with the refurb units.

Finch661 · Posted: Wed Oct 07, 2009 9:39 pm Post subject:

yeah thats what i thought, best sorting out the carbs. the car is still a work in progress, slowly i am getting there. electrics are almost there, just need to do some body work, and get ythe economy up Smile

i will let you know how the new carbs go
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1985 Mini Mayfare (1275)
1976 Mg Midget 1500

Rusty · Posted: Thu Oct 08, 2009 11:36 am Post subject:

Idle mixture is easy to check by just lifting the pistons and listening, but a lot of boy racers put "go faster" needles in the carbys (recomended by their mates who "think" they know) to pour in more fuel. If someone in the past has put the wrong needles and or dashpot springs in it then you need to take it to a carby bloke who realy understands SUs and has at least a gas analyser and preferably a chassis dyno so you can see whats happening under operating conditions. If the needle profile is wrong it could be running very rich at mid range to flat out and yet idle mixture could be fine. There will be a default needle for your aplication and the number of the needle is stamped into it so the mechanic can check it fairley easily.
Back in the 70s we had a spate of cars coming in with bad fuel consumption and the reason turned out to be that some boy racer type convinced all his mates it was a good idea to pull your SUs apart and file a flat on one side of the mixture needle to pour in more fuel ! It certainly achieved that but they only gave about 15 MPG.

Finch661 · Posted: Thu Oct 08, 2009 3:00 pm Post subject:

like a lot of the car, i dont know what has been done to it in the past, what bits are new, modified, knackered. i can only assume that the carbs are the originals and they havent been serviced at least in the last 5-10 years. the car was sitting in a garage for about 3 years, and it took a fair bit of work to get her running and mot'd. not bad for my first project car
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1985 Mini Mayfare (1275)
1976 Mg Midget 1500

Penman · Joined: 23 Nov 2007 Posts: 4756 Location: Swindon, Wilts.

Hi
If one of those screws has sheared you might be getting fuel leaking out of the joint when underway, particularly on corners/bends, which has evaporated off and left no visible trace when you take a look after stopping.
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Bristols should always come in pairs.

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Finch661 · Posted: Fri Oct 09, 2009 10:41 am Post subject:

thats a fair point. my plan is to replace the carbs (bought some reconditioned ones), replace the fuel pipes from the pump to the carbs, and replace the pipe from the end of the master fuel line to the pump. i have already replaced the tank and master fuel line (with no sign of leaks) that should remove the possiblity of a leaky fuel line or carbs!
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1985 Mini Mayfare (1275)
1976 Mg Midget 1500

scott_budds · Joined: 20 Nov 2008 Posts: 175 Location: Norwich

I had a similar problem with my 1972 Midget. One question for you, have you changed your plug leads? I changed mine for a set of performace silicone ones. I had no end of trouble, I had to have the carbs set so pure petrol was comming from the exhaust! Couldnt work out what was going on until my friend who was a mechanic came round one night. He said start the engine and was looking in the engine bay. He spotted there was a small air bubble in the new silicone King lead. Spark was earthing across the body so I was loosing 90% of my spark plug power. I quickly changed it with the old solid copper one and the difference was amazing!! So sometimes new is not better Just thought it might be a help.

Buddsy
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Im looking for an Elan plus 2 for my next resto project...if you see one think of me please!!