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MG MGB Technical - fuel consumption (new owner )

hellow people . could you help me please. i have a 69 gt. the fuel consumtion wasnt that good. i was getting about 19 to the gallon. I have since balanced the carbs.(one was out quite a bit)now i am getting around 26 to the gallon. is this about right? while i was balancing the carbs i checked the colour of the spark with a colour tune. on pressing the throtle the colour was blue but i had a yellow flame every now and then. i turned the needle on the carbs untill the flame stayed blue under acceleration. this was done on both carbs. one thing i notised was that no matter how far either way i turned the needles i could not get the flame to go yellow all the time. In the end i set the needles so the flame was blue under acceleration.
my questions are
1. what is the average mpg for a unmodified gt
2. should i be able to get the carbs set rich so the flame is yellow all the time.
3. which way do i turn the needles to lean and richen the mixture.
4. does it sound like i need a rebuild kit for the carbs. ( they are a unknown quantity as far as servicing is conserned. )

Many thanks for your help Andy
andy

Depends on conditions. I regularly get mid 30s, and that includes speeds of 80 and vigourous acceleration. I have never used a Colortune on the MGB twin-carb set up and don't see how it can be used successfully without having more than one or continually moving it from cylinder to cylinder, even if you know what colour you are looking for. You have to balance the air flow at idle first then balance the mixtures using the lifting pins to check. Screw the jets down to richen the mixture and up to weaken. Lift the piston with the lifting pin about 1/32" of an inch and listen to the revs. If they immediately fall the mixture is too weak. If they rise and stay risen it it too rich. It should momentarily rise and then settle back. You have to listen carefully, it is only a slight change. Each time you alter one carb you have to go back and recheck and probably slightly readjust the other as they are interdependant. Have a look at http://www.mgb-stuff.org.uk and click on 'Spanners' and 'SU Carbs' for the full story. It takes care and patience to get it right but once done properly and not fiddled about with afterwards they will retain their tune for many thousand of miles. If you *do* subsequently alter idle speed or mixture afterwards *always* adjust both carbs by the same amount in the same direction or you will muck up the balance and have to go back to first principles again. A rebuild kit only inludes certain parts, it won't solve worn bushings if that is what your carbs have, you will have to have them rebuilt properly. Only if you can't get the correct mixture close to the starting point i.e. with the jet screwed 12 flats down from being flush with the bridge, or can't get it right at all would I consider a rebuild, and that would only be after looking for other problems like a vacuum leak first.
Paul Hunt

thanks for the advice paul. The lift pins. are these the little things by the side of the dash pots(on the left i think.). If it is these mine are stuck solid. could this be the cause. some one mentioned these to me before but they were not 100%
andy

Andy,
I think if you got a response from most UK MGB owners, you would find that 26 mpg is about the average. Paul has always managed to get more out his car than the majority of us do, quite how I've never figured, although it's probably a combination of having both carbs and distributor in perfect order. My own car has old carbs and dizzy (in reasonable order but not like new) and was tuned by none other than Peter Burgess himself on his dynomometer and it still only gives mid twenties in normal driving, although over 30 mpg is achievable on a long run in top overdrive. The B is a heavy car with 40 year old technology, so I don't reckon that's too bad.
Mike
Mike Howlett

Andy - the lifting pins are on the same side as the float chambers i.e. on the opposite side of the rear carb to the front carb. If you unscrew the damper cap but leave it in place you should see it move as you lift each pin up. One stuck pin I could imagine, but both would be surprising. Stuck lifting pins wouldn't contribute to poor economy but they would prevent setting-up by this method.

Mike - must be the way I drive them ...
Paul Hunt

thanks for the advice people. both lifting pins are stuck solid. with this in mind i think i will rebuild the carbs as they are a unknown quantity. i am also going to put some K+N filters on while i am at it. will i need a different size carb needle for these air filters?
Once again many thanks people. Andy
ANDY

Yes Andy, it is usual to richen the needles slighty when using K&N filters - they pass so much more air than the standard filters. Try a No.6 in your HS4 carbs.
Mike
Mike Howlett

Andy, I have a 74 B with HIF carbs, I don't know if this tuning procedure works with the 69 carbs, but here it is. I've never had any success with tuning the carbs by using the lifting pins. I used to use a color tune after balancing the carbs with a balancer, but as Paul indicated thsi is very hit and miss. The most accurate way that I use now after rebuilding the carbs is the one recommended in the manual and the one detailed on the tuning plate on the bonnet closure panel, and that is "lean best idle". To do this you need to ensure that both carbs needles are set to the same distance below the bridge of the jets and that both carbs are correctly syncronized, and the timing is spot on. Attach an accurate digital tachometer to the coil and read the rpm at the base setting. Adjust each carb by a 1/4 turn of the screw, or in your case by one flat of the adjusting nuts and take another tach reading. Adjust both carbs at the same time until you attain the highest idle speed and them richen both carbs by one flat of the adjusting nuts. You need to rev the engine to 2400 rpm between adjustments to clear it out. VEry small adjustments make great differences.

Andy 74 B GT
Andy Preston

Thanks guys you have been very helpful. I will take all this info and use it to tackle my carbs. Once again many thanks eveyone. ANDY
andy

This thread was discussed between 22/08/2002 and 27/08/2002

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