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MG Midget and Sprite Technical - SU Carbs - question (another)

Gents,

MKIII, 1275 with almost standard setup. I have this car around 17 years and never had to touch original SU carbs, except installation of K&N filters with richer needles and oil level checks (I always used 20W50 fro the carbs). The carbs were always running OK and I learned to live by the rule "do not fix what is not broken."

Few months ago I decided to install simple fuel filter between the pump (electric replacement done by previous owner, sits in the engine bay close to the carbs) and the carbs. Roughly 100 miles after the installation I noticed that under heavy load the car is loosing power. When I released the throttle, problem disappeared and came back with next heavy load again (up the hill or heavy acceleration). My first reaction was to try to remove the fuel filter and voila - problem solved. But, it came back in just a few days, with the same symptoms.

Next "usual suspect" was the fuel pump, resp. fuel delivery to both carbs. Both checked and no problem found - fuel lines are clean, pump is very strong and both carbs were full of fuel. During checking the fuel delivery I removed the air filters and I noticed that piston of 2nd carb (closer to the gearbox)is reacting slowly compared to the first one. When you rev the engine, first piston reacts immediately both ways, up and down. But nr. 2 has got a significant delay in reactions. It even doesn't go the same level up as nr. 1.

What is your advice? How to make sure that both pistons are in synchro?

Thanks, Stepan
Stepan Marek


first: balancing in:
https://www.mgexp.com/phile/1/492119/SU_idiot_guide.PDF


second:
be aware there is a fuel filter inside the fuel tank. You can see it if you remove the fuel sender. It can be cogged by debris from the fuel tank.


Flip
Flip Brühl

Stepan,
before checking to see if the carbs actually need (re)balancing you need to sort the slowness of the rear (nearest the gearbox) carb. As many faults and repairs often just boil down to cleaning and lubricating try cleaning that carb.

Sticking with your tried and tested good method of getting them right and leaving them alone - for a start try just cleaning the piston and needle of the slow reacting carb. Have the car up to full running temperature then remove the air filter and have someone rev the car up and down as you spray the piston and the needle with carb cleaner. to start with you will need to keep the revs up as the carb cleaner entering through the air intake will cause the revs to drop and if that isn't cleared up with revving then you may stall the engine if the revs are too low.

See if that works, if not come back for details of removing the piston and careful cleaning of it internally and needle and limited (area) oiling of the piston.


Nigel Atkins

When you say your pump is OK, have you measured its throughput ? Maybe the filter add some resistance and damaged the pump... Unlikely, but logic if your problem is really linked to your last modification...

I agree with Nigel: clean the chamber/piston as per the SU manual. They need to fall at the same rhythm. You can try to turn the piston chamber by 180 degrees to see if it falls faster (don't forget to check that the needle still falls freely in the jet !)

Otherwise :
Float height ?
Fuel line between float chamber and jet getting perished ?

I did read a lot about carbs lately :) Some regulars will recognise their ideas :p
CH Hamon

This thread was discussed between 25/01/2020 and 27/01/2020

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