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MG Midget and Sprite Technical - 'Simple' carb tuning query

Hi all,
Setting up my SU carbs on my Mk III midget.
Balanced using the carb - balancer gauge
Front carb ( nearest rad) set up fine by colourtune & lifting pin sounds.
Rear carb set up similarly gives a fuel rich flame & tries to die after lifting pins.

I set them so it was jet to highest point on both carbs, then back 12 flats then lifting pin & listened for transient rise then back to normal run speed.

I have read & re read HBOl & also my Autodata book.

Have I missed anything ??
Thanks all for any thoughts
Cheers colin
colin frowen

I would strip and clean the rear carb, jets, check for air leaks etc etc. Basically I would not assume the carb is as should be. Personally I have found colourtune to be extremely inaccurate. Far beeter to read the SU manual - which I think is on the web somewhere.....

how high are you lifting the pistons with the pin?
Trevor Jessie

Oggers is right. Don't assume anything. In particular, check the needles are both the same number and check the float level in both.

Also... you may not get a perfect balance due to ring qnd valve wear wear qnd exhaust carbon build up and even valve regression into the head even worn valve rockers or a replacment dashpot piston to ported racing heads can effect carb balance.

But i agree start with rhe carbs 1st.. tear down clean probably plan on replacing the butterfly shaft and all new gaskets

And see what that does...but i think id shoot for getting them close and toss Ferrari perfection to the street corner with the hoes

To me its hard to beat and my favorite is the rubber hose method ...just a 1/2inch inside diameter rubber hose 2-3 feet long and put it just inside the venturi listen and then place the hose in the 2nd hole thw exact same way as the 1st carb then match the rear carb to thw front.

Toa lessor extent the cig ash tray carb balancer ... there just to fiddle for my taste but they do have there following

But if you want to be special aboit it get 2 vacume gauges and make up 2 sheet metal plates with foam on the backside put a vacume port inthe middle of the 2 plates for rhe vacume gauges and sandwich the 2plqtes togather and drill several holes in the plates thwn bolt the plates to the carbs (foam agianst carb) and then just compare the 2 vacume gauges
Prop and the

Check float height. It should be between 1/8" and 3/16" measured from the bowl lid ridge to the center of the float (slide a drill bit between the two). If the float on the rear carb is high, you will get a rich running condition, lean if it is sitting too low.

Getting float heights right is step one and preceeds adjusting jet height and balancing.
Glenn Mallory

Closely inspect the top half of the needles. The rear one will probably have a larger flat patch worn in at the top caused by slightly less centered needle/jet combo.
Alex G Matla

This thread was discussed between 05/02/2017 and 09/02/2017

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