Morris Minor Owners Club

Float levels and piston characteristics will be different between carbs.

Well, I bought the WinSU program, and it recommended the ADS needle, which is almost identical the AAU needle I already bought! Confusion over the mid-throttle weakening device led me to believe it was the wrong needle,

but after trying the AAU needle this morning its a lot better, more power and smooth cruising. Will have to test it on the motorway before knowing for sure, and there's no nasty petrol smell in the car like when using the HS4

Glad to hear it's getting better - you may find some tweaking of the ignition timing will help too - and have you checked the vacuum advance is working correctly on the dizzy? You can also play about with different grades of oil in the carb damper to see what effect that has. I also found I couldn't get my engine to run correctly on the HIF 38 if I fitted an air filter - ANY filter - tried several and every one ruined the running. So I'm still running without - but in good clean Ecosse air of course!

I spoke too soon!

After a few seconds getting up to 60mph it starts surging, It feels like the car is speeding and up slowing down continuously, after ten minutes of driving like that the car just starts getting slower and slower.

The spark plugs are really white, so its much too lean. When I set the fuel mixture, I find a smooth idle, then make it richer until it starts to go lumpy, then back off a little until the idle is smooth.

I've check the vacuum advance (sucking the tube), I advanced the timing until it was pinking, then backed it off a few degrees.

I'm using the AAU needle which WinSU suggests is almost spot on.

What I do notice is when cruising at 60mph there is a whooshing sound from the carb, like it's sucking in air, but not the usual grunt at full throttle. It kind of makes me feel that there is a bad air leak in the throttle spindle when the throttle is partly open.

I'm pulling my hair out over this, even considering returning to the HS4, it ran perfectly, but the float chamber / jet thread is cross threaded, and petrol is leaking out... If anyone has some spare parts I'd appreciate them.

One thing I did notice, in the HIF38 the piston spring is much wider than in the HS4, i.e it fills the width of the piston. What's significant about this?

Just a different design for that spring. Any chance the fuel supply from the pump is being restricted - twisted or tight bend flexi pipe - lack of fuel would give these symptoms. If not that - just try setting it up much richer at idle - and see what happens at 60.

Make it richer didn't seem to help.

The fuel pipe does actually loop over, but its a smooth bend. Could this be the cause?

Is it possible you've somehow connected the HIF piepwork up wrong? The first time I tried to fit one I think I put the fuel into the outlet/ float breather pipe instead! That was fun...

Also, have you checked your fuel pressure/ delivery from the pump? I doubt it's that, but worth checking.

I'd say if the plugs are white then it's definitely a fuelling issue. I'm too tired to go over the whole thread, but have you run without the silly leaning pipework fitted (you've got the daft pipe from spacer to float chamber haven't you?)?

I can't remember what the correct spring would be, but one idea is to add another HIF spring (get one from any HIF car at the breakers). It'll mess up your overall tuning, but in my experience over springing prevents leaning out (though the section of needle in the jet is thicker, the extra velocity of air draws more fuel for a given engine speed) though of course it kills outright performance! Might help diagnose things.

I've probably got a spare float chamber and little brass nut to go with it somewhere if you wanted to go back to the HS4 - Might even be interested in a part chop as I'm after a HIF38 for my Midget Though I reckon you should persevere with the HIF as once it works it's a much better carb.

Spindle wear should be detectable if it's really the issue - Can you move the throttle linkage end up and down at all? A little movement from side to side is normal, but if it clunks around in the bush then it's very worn. I don't think that's the problem though. s the inlet snugly bolted to the head? I've had fun in the past getting inlets and exhausts to bolt up as the manifold flanges are often different thicknesses...

Are you using the correct spring as recommended by WinSU? There's a few different "colours," though the ones I have are all brass-coloured and I had to figure out which was which using a set of scales.

Here's how I have it connected now...

Rocker cover - Breather on the carb body (Solid oil cap on rocker cover)
Leaning pipework removed, capped the end of the vacuum side
Two float chamber breather pipes open to atmosphere

I think this is correct, I'll post some photos in any case... you clever people may spot something wrong.

The HIF dashpot spring is standard Metro (i'm guessing)

WinSU suggests Yellow (Strong) dashpot spring, does this have much effect at cruising speeds? I was thinking this only effected the acceleration.

The throttle spindle does move side-side, and also up-down a smaller amount.

Another point, when it starts surging, pulling the choke out helps, but that's probably only covering up the real problem.

So - making it richer (with choke) does help - if there was no petrol there it wouldn't make a difference. You can add as an experiment the internal spring from your HS4 and see if that helps - or indeed try it with just the HS4 spring. Also try blocking up the breather pipe that was connected to vacuum. Wouldn't worry about the throttle wear - may upset idle but won't bother running.

winger300 wrote:
The HIF dashpot spring is standard Metro (i'm guessing)

WinSU suggests Yellow (Strong) dashpot spring, does this have much effect at cruising speeds? I was thinking this only effected the acceleration.

IIRC the Metro 998 (HIF38) uses a red spring - Which is a lot softer than the yellow.

And the spring gives resistance to the piston - Meaning it not only has some effect (the damper has the largest effect) on the acceleration, but also determines the piston height and so air/ fuel ratio.

So yes, I think the spring is your problem. You might be able to find a yellow spring in a Maestro/ Montego/ go faster Metro HIF44 at the breakers if you're in a rush and can't wait for a new one to be mail ordered

I see, another symptom i'm getting is that when the engine is cold, it idles smoothly, but if I rev it hard it freely revs, and then stalls, You can catch it by pressing the throttle a little before it stalls. This is probably dashpot spring related also I guess?

I checked the spring in the dashpot, it has a yellowish colour, but I think you are correct, it is a Red type spring.

My old HS4 has a Red type also, So I'm not convinced that is the problem, and a stronger spring would surely push the piston lower (leaner), which is not helping my problem.

So, to get the the bottom of this I thought I should learn a bit more of the science behind the SU carb, and came across an explanation of how the piston is controlled by the pressure differential between the manifold and atmosphere.... and then I get to it.... "Suction chamber air inlet (air at atmospheric pressure)" i.e a pathway from the suction chamber to the Airfilter housing.

So, my K&N air filter has obviously been used on a HIF44 before, and there are various holes in the mounting plate drilled, so in my infinite ignorance when changing from the HS4 to HIF38 I thought to myself, why not cover up those unwanted holes with masking tape? And you guessed it, I covered up the air inlets!

I've not tested it yet, but I'm thinking that my taping over the holes stops the piston rising as much as it should, and hence pulling out the choke compensates by lowering the jet.

I'll post the results after a test drive. I feel quite stupid now though!

Alas, it hasn't helped a bit...

I really can't understand what is going on, The car has lots of power at full throttle, pulls up hills really well, but when cruising on a flat motorway at 60mph it misfires. It's actually better if you push it up to 70mph, and pulling the choke fully out makes it better also.

To my mind it feels associated with a particular throttle opening, at one point I can begin to hear the intake hiss at about 30% throttle, and its at this point that it missfires. When I pull the choke out, the car speeds up and I can lift the throttle slightly and all is ok, if I push on past 60mph then I have the throttle past 50% and it runs ok.

I'll correct myself, The stronger spring does in fact make the mixture richer, since the piston is lower, the air velocity is higher, therefore drawing more fuel out.

I'll try to get hold of a yellow spring tomorrow.

The other odd thing I noticed, the piston is very hard to lift by hand, but it also falls very slowly (When the engine isn't running), It's not getting stuck in any places, it just smoothly falls to the bottom, but there's no "Metallic clunk" sound as I've read I should expect. Something wrong with my piston damper?

I had almost exactly the same with my 1340 Mini (544 cam and big valve head) on single HIF 44 carb - but it seemed to be ignition timing related and was cured by cahnging the dizzy and fiddling with the advance curve. Try with the vacuum advance disconnected - just for a trial to see if that makes any difference. better or worse!

My next course of action is to change the piston spring. Is it worth picking up a green spring also to test?

What dizzy did you get in the end?

I just find it odd that the HS4 never showed any of these problems.

Just play about with adding the spring from the HS4 - or just using the HS4 spring on it's own. I amfraid it was just a dizzy from my pile - with the advance stop cut away and various springs tried until it worked! I could get the engine to run fine if i used lots of static advance - but then it wouldn't start on cold mornings - high compression/kicking against the starter. So had to dial in extra mechanical advance to allow small static setting - but the symptoms were as you say - pulled like a train on full throttle but fluffed and spluttered and backfired on steady speed light/medium throttle.

The HS4 spring wouldn't fit as the outside diameter of the dashpot inner damper section is wider than on the HS4, so the spring does slide over.

I can get the HIF spring from a local MG shop though, and I'm fairly sure it's mixture related as the plugs are still white.

Messing with the dizzie is one more variable I can do without right now

Ah right - I thought I had read somewhere that early spring could be used. It was dizzy that sorted my Mini! If the springs don't cost much - worth a go I suppose. Can you get them on sale or return ??

They cost about £2 at Moss (Stockport), that's if they have them in stock.