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MG MGB Technical - Adjusting pins SU HF4

First, yes I never know what to call some of the parts...

Okay, after the 40 minute drive back home last night, I thought the engine would have quite easily have reached operating temperature, etc.

So I decided to check that test on the carbs where one lifts those pins on each carb (SU HF4) to see how it affects idle.

rpm rises & stays up: too rich
rpm rises briefly and drops: about right
rpm falls - engine rough: too lean

So my results:
front carb - lift pin causes a barely noticeable rpm change - I left that carb alone.
rear carb - lift pin engine dies - which to me is worse than "engine rough"

So I turned the card 1 flat richer (downward - clockwise if looking down at carb, counter clockwise if looking up at nut on carb)

No change - still dies when pushing pin up.

I wound up turning rear carb 5-6 flats richer - now it is just like front carb, no noticeable change.

History: Car always smelled too rich, so several hundred miles ago, I was slowly leaning carbs out, 1 flat at a time, Car slowly smelled leaner, not so much of a gas smell. Eventually I wound up turning nut 5-6 flats leaner on both carbs, carefully making sure I turned both carbs equally. Car ran fine, but I forgot about that pin to push up for testing. These carbs were balanced 1-2 years ago using one of those units to put over the carb air intake. All seemed fine, but car always smelled too rich.

All again appears fine, but why would balanced carbs that were equally leaned 5 flats result in this test killing the engine when the rear carb pin was pushed up?

Your thoughts?
R.W Anderson

You should be using the lifting pins to raise the piston 1/32", which is probably best judged by unscrewing the damper caps and peering inside as you lift. The pins will lift a lot more than that *before* they start to lift the piston. You should familiarise yourself with the effects of weak and rich mixture before embarking on a setting-up exercise, otherwise you won't know what you are looking for. But you can't just dive in and start tweaking mixture without balancing the air-flow first, your rear carb may be completely shut with the throttle released! As you have completely messed up any balance that was there to start with you will have to start from scratch.

Slacken the throttle and choke spindle clamps, make sure the choke cams are fully restored, and the fast idle screws clear of the cams.

Screw the jets up until they are flush with the bridge, which is *not* as high as they will go. Then screw down two full turns i.e. 12 flats. This should enable the engine to start and run for the next stage.

You next need to adjust the idle screws to give the same 'draw' on each carb, then tighten the throttle spindle clamps, and make sure you *still* have the same draw on both carbs with the throttle cable just opening the throttle a little.

*Then* you can move onto the mixture. Adjust each jet in turn to get the highest idle speed. If one or other has to be turned *noticeably* more than the other then a problem with that carb is indicated.

Deliberately make each carb rich and then weak as determined from the lifting pin to train your ear for the subtle change in idle characteristics when it is correctly adjusted. If you are having difficulty doing that make a note of the point at which you can just tell it is rich, then how many flats is needed to turn it to where you can just tell it is weak. The correct position will be *about* mid-way between them. Keep doing this to get closer and closer to the point where you *can* tell when it is correct i.e. a momentary rise and then drop back to normal while you are still holding the piston up. It *is* very subtle, almost subliminal oh HIFs, a bit easier with HSs. Do one carb at a time, but after the second carb you will have to go back to the first and fine-tune it a bit more as the two carbs are on a balance tube and hence interdependant. You may have to go back and fore a couple of times.

Then adjust the choke spindle clamps so that both cams start moving at the same time, and adjust the fast idle screws so that both carbs are *still* sucking the same amount when on fast idle. Then adjust both screws *by the same amount and in the same direction* to give about 1000 rpm when the cabin knob is pulled 1/4".

After that, only ever make further adjustments to either mixture or idle speeds by the same amount in the same direction on both carbs, or you will have to go through the whole process again. Done right in the first place, with no defects on the carbs, they should hold there tune for many years and tens of thousands of miles.
PaulH Solihull

I'm going to do a compression test, just so I have that out of the way, and get new air filters. Then I'll head over to a friend's where we can first adjust valves, just to have that out of the way, and then move on to RE-adjusting the carbs back to balance with the tester that goes over the air intake. Somewhere in here I still need to rebuild my distributor.

I will then slap my own hands whenever I think I should "tweak" anything, and live with whatever the resulting gas mileage is.

Sometimes I need a reminder to leave well enough alone.


BobA / Andy
R.W Anderson

When you rebuid the distributor, don't use the Moss Motors bushing. It's too short and gives a comparitively shorter service life. Instead, get the proper length bushing from Jeff Schlemmer at Advanced Distributors. He can also supply the appropriate springs to control the centrifugal advance curve. If necessary, he can also rebuild your vacuum advance mushroom to the proper specs. Click on this:
Stephen Strange

This thread was discussed on 03/10/2010

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