Morris Minor Owners Club

Hi, I just upgraded my HS4 to HIF38 (due to a leaking float chamber), but can't get the thing to run well.

Its really smooth at idle, checked the ignition timing and all is ok, but when cruising at part throttle it splutters and caughs, pushing the throttle further, or backing off a little cures it.

I'm thinking this may be an air leak on the throttle spindle, it does slide side to side by about 2mm, is this normal?

What else is odd, there is a hose connected from the side of the carb near the fuel breather/overflow around to the manifold side of the carb near where the vacuum advance is connected. What's that for? I've got the breather pipe hooked up to my rocker cover breather, and the fuel overflow left open. Is this all correct?

Anything else I should check?

Yes - the breather should go to that inlet on the carb. Have you got an air filter on the car ? Try it without! Don't worry about the spindle - it's more likely to affect idle - if anything. Probably needs a different needle in there - which one does it have at present ?

I have an AAL in there right now, it was perfect in my HS4.

I've just learnt the hose i describe is actually a mid-throttle weakening device. Could this be causing my problem? Can I just block off the two connections?

Not heard of this 'mid throttle' device - but yes - no harm in trying that.

The mixture weakening device works by weakening the mixture in conditions of high engine vacuum + cruise. It was intended as an economy device. With the needle that was correct in the HS4, without the device, now in the HIF38 the mixture is possibly being forced to be too lean and generating a lean misfire condition. It is described well in Vizard's book as well as how to build one for the HS4, a real mixture of valves and tubes.
Disabling it should work. The "hose connected from the side of the carb near the fuel breather/overflow around to the manifold side of the carb near where the vacuum advance is connected" can be removed, blocking both connection points. I can't find the diagram of this, so it is only a guess from memory and I don't mind being corrected.

Did you change the needle from 'standard' to the AAL? 'Cos I'm running with a HIF38 from a 1.0 Metro and it has the part throttle weakening pipe at the back and I don't have this problem. Is the pipe intact and not leaking?

I'd agree that a leaking throttle spindle would mainly affect the idle but 2mm sounds a bit much (I don't know the spec. though) - I'd have thought the throttle butterfly would be a closer fit than that.

Are not the needle types different as doesnt the Hif use a spring bias type ?

The carb came with ADS, and to be honest i've not tried it. The idle is very smooth, but I'll try blocking off the weakening device at the vacuum end.

I just it without the weakening device plumbed in, and it seems the problem has gone.

What i've done in the end is run a hose from the rocker cover to vent down to the road, then blocked off the connection near the vacuum pipe, and blocked off the breather pipe (originally connected to the rocker cover). It seems to be running ok now.

I used the dust caps found on new brake cylinders to block the breather pipes. The fit nice and tight.

Good news - but it would be good to have the engine breathe in it's own fumes - why not try again - but with a restriction in the pipe to limit the amount of air (and fume) drawn in.

well the weakening device is a hose from the float chamber to the manifold side of the butterfly.

The breather pipe to the rocker cover is another pipe, but It seems idle better without it connected.

Aye - but the engine is so much less prone to oil leaks if that pipe is connected! The idea of the 'weakening device' is simply to allow manifold vacuum to act on the float chamber and so depress the fuel level when running with high vacuum. Never come across a carb with it fitted - but have read about it and ignored since my main interest is performance !!

Rolling roads (with a knowledgable operator) are the most effecient way of setting up a carb.

most efficient way of emptying your wallet also

Exactly - a good steady hill with not much traffic about will do the job fine.

This carb is actually still giving me trouble.

I find that driving on local roads the car is running slightly rich, but ok. As soon as I do some higher speeds e.g. Motorway, it starts to go lean and missfires. If i pull the choke out a little then it runs ok.

After a fast run, I checked the plugs and they were chalky white coloured. I changed nothing and when getting home after 30mins at lower speed they are black and sooty again.

It really feels like the lean missfire coincides with cruising at a certain position on the throttle. When I pull the choke out a little, it lets me lift off the throttle slightly, and hence runs ok.

The AAL needle i'm using was fine for my HS4, so I assumed it would be the same in the HIF38.

I'm thinking about getting a full rebuild kit, but would like to know what's causing this.

The HIF does flow more than the HS series of carbs, and has a bimetallic weakening device IIRC (the jet height is set by a small metal bit that moves when you turn the adjustment screw and also when it gets warm). So you could well have the wrong needle in there even though they're both 1.5" carbs.

It might be worth buying a copy of WinSU (I know I will if I ever have a tenner spare) and seeing what that recommends.

This page, http://website.lineone.net/~bmmoc/HIF%2 ... %20Mod.htm , suggests an AAA needle on a 1098 with an HIF 38.
If you compare your AAL with the AAA or even the AAM ( http://www.mintylamb.co.uk/suneedle/ ) you will see that is quite a bit leaner in the areas you are experiencing trouble. As to how to get the correct needle,....

Certainly that AAL is too weak. AAA is generally thought to be a good starting point. Just check there is a spring inside the bell, yes ? If no spring the piston will ride far too high giving these symptoms.

That's interesting as the AAL worked fine in the HS4, and the WinSU program suggests the same needle with both carburetters.

Maybe I should test an AAM needle in any case before rebuilding the carb.