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MG MGA - carburettor adjustments

I've recently put new cork seals in my carbs to stop a very slight petrol weep below the jets and after reassembly and adjustment of the engine slow running I keep getting a 'rhythmic beating' sound that was not present before. I've done all the usual setup things for the carbs so am not quite sure how to resolve this. I've heard this rhythmic sound from other engines in the past and it may be that the carbs are still out of tune. If this is the case is it the airflow or the jets that need attention?
J H Cole

John

Am I right in presuming you took the carbs off for the job? When you say 'I've done all the usual setup things for the carbs' does that include balancing the airflow?

Steve
Steve Gyles

yes both carbs totally disassembled then rebuilt with new washers & seals. slow run airflow balanced with hose pipe etc. Jets adjusted with gunsons colour tune (that I find very vague in terms of settings) and also by piston lifting procedure -none of this necessarily means that I've got it right. I normally end up adjusting the jets by the colour of the sparking plugs.
J H Cole

John, the 'rhythmic beating' sounds like a rich mixture (eight stroking). I've tried 'Colourtune' but found that 'the book' method was a lot better. What happens when you lift the pistons? I use the lifting pins, very gently, and look for a just-perceptable momentary increase in revs which then settles back to where it was.
Lindsay Sampford

Lindsay, need the car running at the moment but will have another go this weekend and report back.
J H Cole

John,

Do you have a dial caliper? First make sure the fuel level below the bridge (the ramp in the bottom of the carb body where the needle goes into the jet) is about 1/6 of an inch down. You can hunt for the float level by removing the piston chamber and the piston and then moving the jet up and down to find the fuel level. Once you find the level, measure with a dial caliper (the end should extend as the jaws open, use the end to find the depth) and look for around .12 to .20 down from the bridge.

Once you have the fuel level right, set the jet at .070 down and as you adjust your mixtures move both exactly the same, clear the carburetors once an adjustment by blipping the throttle. Check by raising the piston 1/32 of an inch by turning a screw driver blade under the piston. You are looking for a 50 rpm rise, and then falling off. If you do this you will probably end up at .063 down when the jet is up against the adjusting nut.

Double check your airflow, and you are done.

warmly,
Dave
Dave Braun

Lindsey,just finished adjusting my carbs again -jets and airflow as per manual. This time I think their better synchronized and the engine is happier, no more 'beats'. I don't know what I did wrong in the first place but as you say I think the mixture was very rich in the first carburettor-the plugs were really black.
Dave, I have to plead guilty to not fully understanding your description. Surely the petrol level is a constant set by the floats, is a dial caliper the same as a dial gauge? I like the idea of setting the jets about right at the start and then fine tuning only with the jet adjusting nut as opposed to me wondering all over the place with the jet at the start. Not sure if I'm ready for your technique yet. Thanks.
J H Cole

John,

The petrol level may not be constant with the setting of the floats. The level in the float bowl will be constant, but only as far as your float weight and the depth of the needle valve in the lid. The washers and other casting factors at the float bowl attaching end will cause a variance of the float height as well. As long as it isn't sloshing over the bridge, it will work, but setting is more difficult with the depth too low, which I find the 7/16 bar method results in.

A dial caliper looks like the attached picture. The end that sticks out the bottom moves with the jaws and can be used for depth checking.

warmly,
Dave

Dave Braun

John

A tip that has worked for me when just replacing the jet seals is to use the external depth gauge, as shown in Dave's picture, to measure the depth of the jets below the bridge before dismantling. When I reinstall I set them up to the same depths. It has saved me having to revert to the original set-up procedure of 6 flats etc, then tweaking. If I change needles and other bits, or more comprehensively dismantle the carbs I revert to basics. It's worked well for me.

Steve
Steve Gyles

I wonder if one of the jets was sticking slightly after using the choke. If you suspect that, you can reach underneath each carb to the bottom of the jet link and push upward. If you feel any movement it is a problem, and takes only a minute amount to upset the mixture(s)

Ralph
Ralph

This thread was discussed between 12/05/2011 and 15/05/2011

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