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MG MGB Technical - Another Carb Adjustment Question

I have been through the archives and cannot find the answers to my questions. So I did look before I started this thread.
My car - 1973 MGB. Stock carbs - professionally rebuilt two years ago - or about 2000 miles. The car has been running great. Has never failed to start on the first try, no matter the weather. However, my gas mileage has stunk for a long time, so after a lot of research decided to try to lean out the carbs some.

The car was hot. I turned each of the air/fuel screws 1/4 turn counterclockwise. The car started hesitating and bucking and stalling. So I put the screws back where they were and the car still ran awful. So after reading a lot of threads I decided to unscrew the jets about six turns and then screw them all the back in. Then I backed them out about two turns and started the car. It ran and idled fine. So I kept unscrewing the air/fuel mixture 1/4 turn at a time till the car started running bad at 3 1/2 turns. Therefore I screwed them back in 1/4 turn. Now the car runs fine.

Here are my questions:
1) How do I know if the carbs are really adjusted correctly?
2) What would have caused it to act weird in the beginning?
3) Did I do it correctly by starting at 2 turns out?
4) Should I have done this with the air filters on or off? I had then off - they are the stock filters.


Robert Browning

My suggestion is to take the car to a good tuning shop with a rolling road. If you get the right place you will be amazed as to how much better the car goes. The last time I took the GT to our local tuning shop (only 40 miles away) they found another 20 HP plus at the wheels. It was abit disconcerting to stand beside the car as it was drived at 120 mph on the rolling road but these guys are superb. Unfrtunately Gillingham in Dorset is a biot far for you.
Bob Marshall


Looks like you did good. If your car runs fine now you should be proud of your accomplishments. Only when I run out of answers do I use the BBS. Always good to try something yourself, make it work, and be happy.

Sorry I can't answer your questions above, but someone will that is more knowledgable in this area than I am.

If you think fine tuning is needed than take Bobs advice.


Ray 1977mgb

I have to say Robert that what you have done is no more than 'poking and hoping' and as such has a negligible chance of ending up at the correct setting, with twin carbs. It isn't particularly complicated to set them up, but you do have to things in the right order, in the right way, and not move onto the next step until the current step is correct.

Basically you set the top of the jets *flush with the top of the bridge* with the mixture screws, then screw them in two full turns to lower the jets. This is the starting position and should be enough to get the engine started.

Then you slacken the throttle and choke interconnecting bar clamps and adjust the idle screws independantly to give equal 'suck' on both carbs at idle, as judged by a listening tube, gunsons CarbBalancer etc.

Then you richen or weaken one mixture screw to get the highest idle speed, then the other, then go back to the first and have another tweak as the two carbs are interdependant and changing one carb will have a small effect on the other two cylinders. If the idle speed is now too high i.e. more than 1000 rpm unscrew both idle screws a little *by the same amount* to get it down to about 800 or 900. This is the coarse setting. If you have gone a long way from the starting position, or one carb has gone a long way more than the other, a defect (e.g. vacuum leak) is indicated which must be found and fixed before you can go any further

Next you raise each piston in turn with the lifting pin about 1/32" (note that the lifting pin itself goes up by a lot more than this before it reaches the bottom of the piston) and listen to what happens to the idle. If the revs increase and stay increased the mixture is too rich. If the idle tends to stall immediately it is too weak. At the correct mixture the idle will momentarily rise very slightly then settle back down. This can be quite difficult to judge, but if you turn the screw in until you start to hear it is too rich, and out until it is too weak, then correct will be about mid-way. Do this to the 2nd carb, and again you will have to go back to the first and probably make a small adjustment, then back to the 2nd and ditto, until both are correct with no further changes.

Then tighten the throttle clamps, checking that both carbs give equal suck off idle as well as on.

Then adjust the choke spindle clamps and fast idle screws such that both give the same suck on fast idle, and the correct fast idle speed, and both enrichment valves start to operate at the same time. I think this is indicated by an arrow on the fast idle cam on the HIF.

Finally adjust both idle screws by the same amount to give the correct idle speed,

Air filters if in good condition should make no difference, the volumes of air at adjustment speeds are very small compared to full throttle.

Then don't fiddle with them any more! If you do need to make any changes, *always* adjust both carbs by the same amount in the same direction, or you will have to go back to the beginning again.

None of this can be done until the valve clearances, spark gaps, points gap/dwell, timing etc. are correct, and if there are any worn components or defects causing irregular running or misfiring these must be fixed first.

Note that a vacuum leak can give the effect of having correct mixture at idle e.g. good starting, but a rich mixture when running i.e. poor mileage.
Paul Hunt 2

Thank you very much. It looks like I know what I will be doing next Saturday!

I really apprecaite this site and the help it brings. Thank you to everyone for your continued help.

Robert Browning

This thread was discussed on 21/05/2006

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