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MG MGB Technical - HIF 44 Question

Had a problem getting a good even tickover which resulted in me rebuilding both carbs with the overhaul kit and also fitting new throttle shafts.
The problem still remained and was finally traced back to a faulty vacuum unit on the distributor. Changed that and a near perfect tickover can be abtained as low as 4-500 rpm!

However, went to get the car MOT'd today and the mixture on the tester was very lean - CO was about 0.675% so the tester said when picking the car up.

Obviously this is very weak and I just wondered how much roughly does say a half turn of both screws make to the CO?

To be fair the car ran "OK" with the lean mixture and not having access to a gas tester just wondered how much richer would a half or a whole turn of the screw make?
AdrianS

There is not a fixed amount of CO increase per turn - it varies!

Turning 1/2 a turn may increase CO by a small amount and then the next 1/4 turn may see a large increase.

The easiest way to tune without a gas analyser is to use the lifting pins on the carbs - at idle lift the carb piston using the pin and see what the idle does - if the engine dies the mixture is too weak - richen by half a turn on BOTH carbs, repeat until the engine speed rises when you lift the piston, then back off (weaken) a small amount till the idle rises slightly then drops back to normal when you lift the pin.

Check you get the same response on both carbs - if one is richer than the other weaken off only that carb until you get the same response from both carbs.


Another simpler rough way of doing it it to take the dashpots off and set the jets 60 thou down from the bridge - you won't be far off!
Chris at Octarine Services

Thanks Chris.

I did initially set up the carbs as per the SU tuning guide but to be fair, I found using the lifting pins - even moving them very slowly was a bit hit and miss!

When you say 60thou from the bridge - do you mean 60 thou from the venturi alloy or 60thou from the top of the brass insert in the carb body?
AdrianS

Do you have a pair of HIF44s or HIF4s?
Dave O'Neill2

60 thou down from the bridge
Chris at Octarine Services

Moving the lifting pins slowly is going to make using them more difficult, you need to lift the piston with them smartly, but only by the stated amount, i.e. 1/32" in the case of HS4. This isn't critical, until you get used to it and know what about 1/32" feels like you can unscrew the damper cap and look down at the piston and eyeball it.

I've found that instead of measuring it two turns down from flush with the bridge is close enough, it's only a starting point, and you will almost certainly have to adjust one carb a bit more than the other.

If you have twin carbs as you indicate then the interconnecting shaft clamps must be slackened or you will not be able to adjust each carb independently. Then when tightening them again you must ensure that both butterflies start to open together, i.e. air-flow still balanced just off idle, or they will only be balanced at idle, and not when running.
Paul Hunt

USing pins that haven't been used for a good while is often difficult - the pins stick and telling when they first touch the piston is vague at best.

However I find the 1/32 is of no consequence at all - just lift the pistons STEADILY (DON'T SNAP THEM UP!) and the engine will either die, race or stay much the same - perfectly good enough for tuning!
Chris at Octarine Services

Twin HIF 44 carbs

Started again using the 2 turns down from flush method - it was not far off from there I felt.

Ended up with one 2 1/2 turns out and one with about 2 3/4 turns out and had to play with the balance a little after adjusting the mixture.

Car runs very well now!

One final thought, the inlet manifold is not divided in the middle is it? If so how much mixing of the carb outputs occurs in the inlet manifold?

AdrianS

The manifold isn't divided in the center. The mixing, inside the manifold, depends on which carburetor's butterfly valve is open the most. RAY
rjm RAY

As said the manifold is a balance tube connecting both carbs and all four cylinders. Both butterflies should be open the same amount of course, in normal use. Some mixture gets from the front carb to the rear two cylinders and vice-versa, which is why one adjusts the front (say) carb, then the rear, then you go back to the front again which will probably need to be altered slightly, and so on. A couple of iterations is usually all that is required.

If you have trouble training your ear to pick up the exact spot where the revs momentarily rise and settle back again i.e. the correct mixture, richen until you can just tell it is rich, then weaken until you can just tell it is weak, counting the flats between the two. Mid-way will be about right, and the more you go back and fore between the two the better your ear should become. HS carbs are bad enough, but HIFs are almost subliminal and harder to pick up.
Paul Hunt

Or rent or borrow an Innovate Air Fuel LM-1 with tailpipe sniffer to adjust the mix through the rev range
vem myers

Only any good on the 4-cylinder once you have both carbs in balance.
Paul Hunt

This thread was discussed between 25/07/2013 and 31/07/2013

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