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MG MGA - setting up rebuilt SU H4 CARBS

I AM IN THE PROCESS OF A COMPLETE REBUILD OF MY 1962 MK11 1622--FROM THE GROUND UP. engine IS COMPLETLTY REBUILT AND AM NOW REBUILDING THE CARBS--AFTER INSTALATION TO THE ENGINE--HOW CAN I SET THE THE CARBS UP TO RUN CORRECTLEY--ANY IDEAS.
mike bane

Which carbs do you have? There should be a tag on the float bowls which specify a specific model.

The balancing of carbs is very straightforward. Do you have a Bentley manual?
Jeff Schlemmer

Hi Mike. Setting up SU H4 carbs isnt really that difficult, but it does require the knowhow. If you do a google search on the Internet, you should find lots of information on how to set up and tune SU H4 carbs. Before tuning the carbs, you need to make sure that engine compression is good, and that your ignition system is set up spot on to proper specs. Trying to tune your carbs before doing these things is essentially useless. Before trying to tune your carbs, you need to verify that the float levels are set correctly, and that your jets are PERFECTLY centered. Failure to verify these things will cause you no end of grief! When these things have been checked and verified, then you can warm the engine to operating temperature and tune the carbs. Please note that the carbs are joined together at the throttle shafts and choke linkages. These connections must be temporarily disconnected while the carbs are being tuned. Essentially, the carbs are adjusted individually, and this cant be done properly when the thtrottle and choke linkages are joined. Carburettor mixture is adjusted richer, by lowering the jet via the jet adjusting nut, and conversely leaned by raising the jet. Carbs are synchronised by adjusting the throttle screws on each carb. Each carb must suck in identical amounts of air so the carbs are "balanced" To do this balancing, you can use a unisyn or similar tool, or use a short piece of heater hose. THe heater hose is used as a stethescope to listen to the air intake "hiss" at the throat of each carburettor. When the carbs are properly adjusted they will both be sucking in equal amounts of air, and the hiss heard through the heater hose tube will sound the same. Listening to the hiss of air intake requires a good ear. A unisyn balancing tool gives a visual indication when both carbs are drawing air equally, so that would likely be a better way for you to do a first carb balance. There are SU carburettor manuals and video tapes that give step by step SU tuning instructions, and might also be worth looking into, along with the internet information. Hope this helps. Glenn
Glenn

Mike

As Glenn says, plenty on the internet. This is one that follows along with what he says:

http://www.mgcars.org.uk/cgi-bin/gen5?runprog=mgoc&p=emg/basic4.html

Steve
Steve Gyles

You need to start with a static setup.

Take off the dashpot covers, and then screw the jet adjusting nut up until the jets are level with the bottom of the carb (ie the various bronze rings are all at the the same height. Then screw the adjusting nut down 6 or 7 flats (I think, could be 9) (same on both carbs). Replace the dashpots and loosen the clamp bolts for the link at one of the carbs (no need to do both).

Then start the car and follow the instructions in the car club web page link from Steve to settle the final tuning. Make sure that if you use the hose trick to listen and calibrate the air flow that you hold the carb end of the tube at the same place AND depth into the entry of the carb, otherwise the note will be different (this is where the word "tuning" originates).
dominic clancy

Dominic

I follow your static procedure on the first carb, then measure the depth of the jet with an external vernier caliper. I then set the second carb jet to the same depth rather than counting the flats. This way I find the (fuel) balance between the carbs is always spot on and any richening/weakening is always the same number of flats on each carb - a trick Bob West showed me.

As a short cut on subsequent carb strip downs I measure the jet depths before dismantling and use this as a datum on reassembly. I never had to alter a flat when I replaced all the seals 6 weeks ago. Spot on.

Steve
Steve Gyles

CAN ANYONE TELL ME IF THE NEEDLE AND SEAT OF THE SUH4 CARB- IS THE SAME AS THE HS CARB --I HAVE A SET --THEY LOOK THE SAME --BUT ARE THEY??? IF NOT WILL HAVE TO GET NEW.
mike bane

This thread was discussed between 23/04/2006 and 25/04/2006

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