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MG Midget and Sprite Technical - SU HIF44 unstable idling

This is a long shot as maybe there are not too many of you using this carb. I have been running the 998 with the HIF44 for over 10 years without major problems, but over the last year the idle speed has behaved oddly. When the engine is warm enough for the choke to go in it's fine for a while, but after about 15 minutes driving the idle goes up. It was hitting 2000 rpm at times, and seemed to be related to the underbonnet temperature. I replaced the throttle spindle and that made almost no difference. I went back to Burlen Services here in Salisbury and they advised replacing the rubber seals for the cold start unit. Indeed the outer seal was knackered, and the idle was back below 1000 after I replaced it - although I could not get it any lower. I thought that was it, but now it has started idling too high again. When hot the engine hunts, oscillating between 1000-1300, before settling to 1300. Sometimes blipping the throttle gets it to drop back, but rarely. If I stop the engine and restart it usually goes back to 900 or so.

I have an 8" ram pipe on the carb so the intake is well away from exhaust heat, and I'm sure there are no manifold air leaks. The engine has a Kent 276 cam, Metro big valve head, K&N cone filter, and A+ dizzy with Pertronix kit, all of which has worked fine for many years. The throttle cable inner has been replaced twice during this episode. The carb has a blue spring, and I'm using the correct dashpot oil. Can anybody fathom this out? I haven't found anything relevant on other A-Series forums.

L B Rose

Could it be something to do with the main jet holder? I think it has a bi metallic strip that alters the level of the jet with temperature changes. I'm sure I've heard of people brazing them up solid. It's been a while since I've had mine in bits so I'm a bit out of touch on the workings. Mine has behaved itself so far!
John Payne

Worth a thought John. Not sure how that would make the revs rise and fall regularly. I think I'll pop the ram pipe off and look at the carb throat while idling. Maybe I'll see the piston oscillating. The trouble is though that the problem corrects itself when I turn off and restart - difficult to rectify an intermittent fault.
L B Rose

Other thoughts are an air leak somewhere or a problem with the vac advance if fitted. Strange how it corrects by turning off and on though. Good idea to have a look down the carb with the ram pipe off. That's a big carb for a 998 by the way!
John Payne

I was surprised by the carb size as well i want to say thats around 1.75 inch dia

A couple of thoughts...

could he fuel pump is over pressurizing the float (does the SU HIFF have floats) is the pump

The fast ideal speed is set to high compared to the ignition timing...some thing i learned several years back, if your to far advanced and then adjust the ideal to proper speed it will hunt as you call it

Another BLAIRING issue that sticks out... pertronix ignition id definatly pull that chip temp and replace with points and condensors for the moment... and look at the insulation as it goes into the chip, also look up what is know as pretronix phazing

The last pretronix chip i learned about phazing, it self advances inside the chip some how

You said you have an A+ DISSY, I dont think thats correct from memory but i could well be wrong

Carb linkage sticking... if your running hot temps and a little greasy on the linkage they can stick slightly and catch without knowing

Also maybe grab the coil after when it does this next time... if its super hot and burns your hand it could be a low tension issue that feels like a carb problem

Agian air leaks... i know you said its not leaking but, we have all been there

And a last note... you mentioned you replaced 2 accel inner cables recently... they could he cheapos but if jot id look at the carb linkage springs maybe over tight that will certianly wear carb parts out... the springs need to be only strong enough to retract the linkage ... it dosent need to be on hard retntion like a gator jaws

Maybe have a look at your dissy guts make sure the springs are working with in speck the dog legs can move freely and the plate inside that holds the chip is still tight and sturdy

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Sounds like either throttle plate not quite centred (very touchy: just a tiny bit out can do this - hold it up the the light and make sure the gap is all gone) or alternatively if the throttle plate is the type with an over-run valve then the valve can leak through its seat. Replace with non-valve type.
Paul Walbran

Thanks Prop, you have given me a lot to think about. Yes the carb is big but I slavishly followed David Vizard's recipe for a road 998. I started with a Piper 270 cam as he advised but that wore out (long story, don't get me started) so I now have the 276 which is slightly cammy but tolerable. The fuel pump is an ordinary Lucas one with Burlen's electronic switching. I doubt it could be pumping too hard.

I did have problems with the ignition advance sticking some while ago, which caused weird idling, but after a strip and clean that was cured. The Pertronix kit failed twice when the negative cable into the chip broke off - not very well made. But that's really a go/no go situation. God, the very idea of going back to contacts! Never heard of phazing - or is it phasing? Yes I fitted an A+ dizzy as all the various original ones I have were worn out. Normally it's held down by a flange but that was turned off in the lathe and a brass collar fitted to suit the original clamp.

I only use the slightest film of silicone grease on the cables, which I only replaced as a precaution in case that was the problem. It doesn't appear to be.

The carb springs are the ones that came on the carb.

Air leaks? I'd have thought they would show all the time not just after a run, and not settle with a throttle blip.

Worth checking the coil as you say. It's a Lucas sports one.

Thanks also to Paul. Yes I wondered about the throttle plate and re-centred it the other week. It did have a run-on valve originally but I pulled that out and soldered up the plate.

Oh well back to the garage.
L B Rose

Les, this is a bit of 'outside the box' thinking. What arrangement do you have for crankcase breather with your combination of a 998 set up and an HIF carb?

If as the engine warms up after it's 15 minutes of running time it is begining to breath in oil fumes this could cause the revs to rise. Then when you switch off the crank pressure equalises and the speed resets when you restart, only to gradually increase again as it rebuilds crank pressure.l don't know how likely this is, but it's just a suggestion

Good point Guy. I have the 1098 closed circuit breathing system, with breather cap on the valve cover. The breather valve diaphragm is fine. There is a extra breather connected to the redundant mechanical fuel pump point on the block, with its own oil separator, plumbed into the same valve on the manifold. I have had this set-up for most of the last 10 years and only recently am having this problem.

L B Rose

Les, I don't know - I was just looking sideways for alternative possibilities that might explain the changing nature of the problem!

If both of your breathers are in fact isolated from the intake system by your PCV valve then maybe it couldn't ingest oil fumes then. But then, how does the PCV valve work? Does the diaphragm actually isolate the fumes from the intake? I thought the diaphragm acted as a valve, closing down when suction was too high but opening up at lower levels of suction to draw off the fumes into the intake manifold.

Although it has worked ok for 10 years maybe it is now breathing more oil out to develop this problem. OTOH, I am not even sure that ingesting oil will increase revs! I know oil in the mixture lowers the octane rating but being a hydrocarbon it is flammable and must richen the mixture.

Sorry! More questions than answers! You need input from someone who knows what they are talking about :-) Peter, where are you?

I think this could be pressurisation - running a 998 with a hot cam would usually accelerate bore wear and I'd certainly expect it to be past it's best after 10 years. Usually accompanied with leaks from the rear scroll and even possibly the front pulley. Ingesting oil mist will certainly increase idle speed in line with your description. The usual trick with higher mileage engines was to dispense with the closed circuit system and run the breathers direct to atmosphere - allowing uncontaminated induction, with the sump returning to more or less atmospheric. Maybe worth cleaning the bottom of the engine and putting some cardboard down to monitor any changes. The only odd variable idle experience I've had with the HIF was when the idle screw was actually loose in it's thread. But on that occasion it lost revs rather than gained any.
f pollock

Well thanks Fergus. That's one knowledge-based endorsement for my suggestion!

Les, as a temporary check then, you could disconnect your breathers from the PCV valve and leave them to vent to air. Go for a test drive, repeating your 15 minute + journey and see if it still has the fast idling fault. Don't disconnect at the manifold to PCV hose as this will result in a manifold air leak - and upset the engine revs! Or if you do, then plug the manifold connection.

Disconnecting the breathers may also temporarily increase any rear crank scroll oil leakage but that's a small inconvenience to pay if it identifies your mysterious revving engine problem. You can then set about a plan for a permanent solution. If the test doesn't solve the revving, then you know that my oil breathing suggestion was just a wild herring chase. ;-)

Have a nice Sunday.

So far I've only looked at the dizzy. Cleaned and lubricated the internals, reassembled and reset timing. Perfect idling after a short run up to temperature. I'll do a longer run later this week and see what happens.

Even though over 10 years old the mileage is very low and compressions are excellent. The cars has only been used 1-2 times a week. I do have the rear crank oilseal kit although it doesn't work perfectly and there is a small drip. I've just put a Speedi-sleeve on the crank pulley and with a new seal there is no leak from there. There is virtually no oil mist when taking off the oil filler cap.

After the next run, if the idling still goes up I'll try disconnecting the breathers.

While resetting the timing I decided to optimise it. This entails turning up the idle to 2000 or so and rotating the dizzy for highest revs. Would you believe this turns out to be about 20º of static timing? Very odd. But of course with more advance the engine coughs and stops when starting from cold, but with only 5º it starts on the button. Am i doing this right?
L B Rose

No! The highest idle speed at 2,000 is not necessarily the 'right' timing for the rest of the time. With a 276 you will get a smoother/faster idle with increased static advance up to ~20° BTDC, but apart from that one situation, it will be over-advanced for everything else.

20° static timing is too much for starting, but should give a reasonable light throttle performance.

What distributor and advance curve are you using? A 276 needs the equivalent of an Aldon 'Yellow' distributor timing profile, or even a 'Red'. That way you can increase the static to 10/12° BTDC, but still have a maximum of ~30° BTDC at 4,000+ rpm (distributor vacuum disconnected).

The picture below shows the Nodiz ignition map that we have on our 1380, with a 276 cam and HIF44 as well. At idle the timing (including vacuum advance) is 20°+, but with any load or engine speed increase, drops straight back to <20°.

In comparison with the Aldon 'Yellow' distributor that we had before, the idle quality is very much smoother with the Nodiz unit.

Silly question - you have got a good throttle return spring fitted to the HIF44?


Richard Wale

What you may have to do, it pull the carbs and manifold ... clean it recheck it put it all back togather... try it then run it to see if it worked out okay then try the next componet if the carbs dont work out okay

If not, then move on to the next syatem
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Thanks Richard

The distributor is a standard 59D4 marked 41907A (998 Mini). I know the curve isn't right but I could never afford an adjustable type. I did have the car on Brian Slark's rolling road way back and they thought it was advancing OK. It was Slark who set the timing in the way I have described. I would like to get the curve a bit closer to optimal somehow.

The throttle spring is the standard one and is pretty tough. No suggestion of sticking as far as I can see.

Today I sealed the breather intake to the manifold and vented the breathers to atmosphere with a catch tank (plastic bottle!). I can't see much difference to the idling. When hot it sticks at about 1100-1200, then eventually drops to 900 with a rise to 1100 every few seconds. Better than over 1500 I suppose. There is not a drop in the catch tank or the clear tubing that goes to it. I'll run it like this for a bit and see what happens. Perhaps it's what they call 'a feature'!

Prop - the carb and manifold have been apart a few times recently and all that is squeaky clean. New throttle spindle, new seals etc. Can't think of anything else.
L B Rose

Thats why im saying take the arb back apart agian ... this is the last part that can be suspect because it was done just recently about the same time this problem started, something could have gone a miss or un adjusted

Everything that others are suggesting your saying are not inplay on top of that the engine is fairly modifed in directions most of us may not have gone

Thats why i say start with the fuel system 1st cause its the most recent, and give it a complete check out from the fuel pump forward.

If nothing changed

Then i would completely relook over the entire ignition system

If nothing...then the exhaust

Then just keep taking on rhe next system on and on

Eveytime ive had that hunting lopeing on my engine is because the timing was to advanced when i set the carbs high speed adjustment screws or had the floats set to high

To which i think you said that wasnt an issue,

But i think we are all guessing at this point which im not a big fan of, i like to narrow it to a section then narrow it to the problem

So from my perspective with everything being so modified and the carbs was the last known place that was extensivly attended to thats where id start

Just as an example i replaced the spings on my high speed adjusting screws cuz i retapped the holes and put in a larger dia screw cuz of wear... had some issues then got to looking and the dia of the screw spring was wider OBVIOUSLY but it was enough that it was catching on another of the carb parts i filed the carb part off a bit... problem solved

On your set up... if there is not something a miss on the fuel system or the ignition system then most likely will be an issue between the carb settings and the dissy settings

Ond of the Problems is communication we keep learning a little more about how modified your engine is with each time you post... exp i did not know this was a mini morris dissy from an A+ 1098 Engine, and your engine is an inline reg A 889 engine.... so thats something we havent seen before

Btw... please do answer this question regardless... are you using a timing light and is it known to be fairly accurate. We have had that happen here on more then one occasion were we assumed the person was using a timing light and after 100 comments back and forth we found out differantly.... so last establish that 1st

Thanks prop
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Well Les, if disconnecting the breather hasn't helped, then at least you can eliminate the oil breathing possibility.

New tack then - when you reassemble the carb did you check/ adjust the float level? I have the same carb as you and find that it is fairly sensitive to fuel level. I suspect this is due to the relatively small fuel capacity in the integral float chamber design. Though I can't see why that should result in the rev "cycling" that you are getting.

You're not using a turbo inlet manifold are you? These things are so cavernous for n/a engines, petrol collects and tips in giving a cycling idle.
f pollock

I don't know how reliable this information is, but on the 'Mini Forum' the 59D4 41907A distributor is quoted being for an A-Plus unleaded engine, and has the advance specification as:

41907 - 59D4
16º-20ºat 4400rpm
12º-16ºat 3000rpm
6º-10ºat 2000rpm
0º-4ºat 1000rpm

If the maximum advance is 20° crankshaft degrees, then the static advance could be increased to around 10° BTDC. CHECK very carefully for detonation under power right through the rev range.

Richard Wale

Thanks again guys.

No it's not a turbo manifold. I bought the carb and manifold as a unit from the scrapyard. What I find odd is that I can't get the idle below 900. Originally I could turn it down until the engine faltered and stopped. I would normally put this down to an air leak but that wouldn't account for the erratic behaviour - it would leak all the time, to the same extent.

Yes I have a timing light tucked away somewhere. I didn't try that as the problem seemed to be carb-related. Taking Prop's advice, I'll strip the carb again and ensure there is nothing wrong there.

L B Rose


A couple of things to check on the carburettor:

Disconnect the throttle cable and make sure that the 'fast idle' screw is well clear of the 'choke' cam.

Start the engine and let it idle. Unscrew the idle speed screw until it is clear of the body and press gently down on the on the screw to try and close the butterfly - can you get the idle speed to drop?

Secondly, is there a vacuum connection to the carburettor? If there is disconnect it and block the connection to the carburettor - does the idle speed drop?

If there is no vacuum connection to the carburettor, is the connection on the carb blocked off properly?

Richard Wale

Richard, I've already checked the fast idle screw in the way you suggest.

Ah, I had not thought of the distributor vacuum can. I'll look at that next time I get the engine warm.

Les (why can't I edit my profile to show my first name?)
L B Rose

You should be able to do it by clicking the 'Customise' link above, make your change on the 'Customise' page and remember to click on the 'Save options' on the bottom of that page.

Cheers, Nigel.
(not) Les B Rose

Wait wait WAIT.... lets back up a bit

You just wrote, about 3 post back when i asked you if you have a timming light anzd you said this...

""""" Yes I have a timing light tucked away somewhere. I didn't try that as the problem seemed to be carb-related. Taking Prop's advice, I'll strip the carb again and ensure there is nothing wrong there.""""

You mentioned way back earier inthis thread that you have done ignition work on the dissy and cracked the engine up to a high advance to the pojnt that it was great as long as it was ideling and not under load.

You cant do any of this type of work without a timming light... you need to dig it out where ever its at, it needs to be tested on a known good running engineif if its still good then we need to hook it up to the engine and start over from your very 1st post...everything that has been posted is now an official waste of evrryones suggestions and time

I cant belive how often this happens here ... we spend a lot of time working the problem, only to find there is not the right tools being used

Im not upset myself, maybe others are, for me ive just learned to accept it, from a group perspective from now on we need to ask if they got a stobe light, is it known to be qccurate with in the 1st 3 or 4 postings

But all is not lost... we can work with this just let us know when you get your timming gun out and tested for accuracy... if its not accurate, then you will need to source a known good one then come back to the BBS And we can start over and know what we got so we can work the problam and not be guessing

At least this time we are only 25 comments in and not 200 or more like sometimes with everyome scratching there heads wondering what the heck...haha

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Sorted. Following Richard's advice I blocked off the distributor vacuum feed and the idling is now correct. So the vacuum diaphragm is leaking. Might also explain why the fuel consumption seemed to be high, but I do mainly town journeys so I rather expected that. Sorry for the red herrings but knowledge is always power!

Now, where do I get a new vacuum can?
L B Rose

<<Other thoughts are an air leak somewhere or a problem with the vac advance if fitted>>

Kudos to John Payne. More or less the first reply you got Les!

Were to get a new vac can for the dissy.... probably a new dissy, but id try moss 1st for the make and model the dissy came from they may sell individual dissy parts

Congrats on finding the culprete

NP... Water under the bridge

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I found a new vac can on eBay, for princely sum of GBP7.95! Was quoted over GBP40 to rebuild the old one! Thanks again chaps.

L B Rose

Id do the rebuild at rhat price, even throw in a few extra dollars to have it recurved to fit the engine

8 vs 40, if you got a worn out vac can then what else is getting tired and needs a good cleaning and mayne replacement

You already got time into this project might as well do it right the 1st time and have a great running engine

It needs to be curved anyway its all expermential ... so now is the time to go all in, esp with the expermential carb set up your going with ... probably would equal another 10 years of problem free driving

But if you just replace the can then how tight is the points plate, are the dog springs getting worn and streatchy is there play in the shaft ect ect ect

So for the price of a wonder woman movie ticket plus small pop corn and soda pop you can have all those potential issues resolved

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This thread was discussed between 08/06/2017 and 18/06/2017

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