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MG Midget and Sprite Technical - Another misfire when starting thread for 2024

Rather than hijack Colin's misfire, I'll misfire my own. 😁

1275 with an hif44, 1300gt head, 25d4 from a 1300gt, 30thou over, hepolite 19320 (cooper pistons I was told at the time yonks ago). 3 branch. Rebuilt circa 20k miles ago.

It's recently become a misfiring pig to start when the engine is cold.

Earlier this year, circa march/april, it began to get a bit 'sluggish' to start. That is, the engine spin was as normal (good battery), but it wasn't firing as instantly as usual, and in fact misfiring.

It's been gradually getting worse.

In hot weather it's a bit better, but not much. It seems to need more choke than it should to get it going, but won't idle well at all on choke.

At the time, I did some basic checks. Points good and not closed. Timing checked and spot on where I always have it. Dashpot oil at right level. Piston not sticking.

I did a compression test, hot and cold, wot, 220psi is even on all 4. It's always been high on this engine, but still I was suspicious of this, so cross checked with the aid of Alan Anstead's tester. Same result. 220 ish on all 4. Not losing rad water, no oil in water, no white mess in rocker cover.

Once running and warmed up, it still goes like stink for what it is. And once hot, happily idles and starts on the button so to speak. Hence I haven't really bothered to look properly.

But NOW, it's REALLY getting hard to start. I hear the pump (points su) tick, and stop as it fills the hif bowl. So not fuel starvation. Doesn't leak petrol. But it just won't start easily without full choke, even in mild/warm weather.

Two things.
1, I have a significant exhaust blow at the collector on the 3 branch. Rusted/split, I need to weld or change it. Can a collector exhaust manifold leak cause a gradually worsening starting problem? Pops like a looney on overrun btw. But vacuum gauge (manifold connection) shows 25 on overrrun, and 18 at idle 1000rpm. So not leaky valves? And anyway great compression in all 4 pots.

2. It seems to be using more petrol than it should, but then again, I'm not really doing decent length journeys to be sure.

I suspect, on no more than gut instinct, something inside the hif, but don't know what, as there isn't much in there to go wrong; is there? Fuel compensator bi metal? Why a problem starting when cold, and not hot when running?

I have no choice now, I have to investigate properly. Considering all I've nentioned above, which I'll check again but expect same results, wtf is it?

So, suggestions please, by way of a quiz game with no prize. 🤣.

anamnesis

I seem to recall that the choke spindle in an HIF has an odd segmented rubber seal that has something to do with fuel control. If they deteriorate you get difficult starting problems. Could explain a progressively worsening fault?
GuyW

Thanks Guy. I'll have a look tomorrow. I guess it's worth me reading up on exactly how the hif choke works then. I've never really explored the hif. Just bunged it on, same spring and piston needle and it went ok. Only thing I've changed is to drill for ported vacuum. But it was fine after that.

I'll also take off the 3 branch and see if it's fixable. I have a feeling it's too rusted to weld. In which case I need to find out who sells the same 3 branch I have, fir ease of replacement with my existing main exhaust pipe.
anamnesis

As per Guy, sounds like the choke if it's going ok hot.
HIF's have got piddly little passages for the choke.They only just get enough fuel on choke even when everything is clean.
You need to pull the choke shaft/barrel out and give it a good cleanup and replace the 'o' ring--and clean out the porting in the housing.--my guess is the end of the shaft/barrel will have white gunk from the fuel caked onto it. could even be stuck in there and pulling the choke isn't moving it.

William Revit

Thanks Willy. Interesting that you mention white stuff. I may be imagining it, but I think it started acting up around the time I read that Esso supreme 99 now contains 5% ethanol.

My Capri's 28/36 weber slow running jet used to bung up with white stuff if I didn't use Esso e free 99.

I didn't think there was anything in SUs to suffer from ethanol (other than any rubber seals). But still I use Esso 99 for the octane, and wasn't too bothered that it's now 5% e too.

Maybe it's a combo of all you mention. It's got to come off to get at the 3 branch, so that's where I'll start looking. Cheers.


anamnesis

Yes, ethanol could have eaten away into that rubber O ring
GuyW

Wouldn't blame the ethanol, we have done a lot of those o-rings and there is no pattern of failure and fuel type. Most are still the original 50 year old o-rings, and fail because they are like me - old and tired.
Paul Walbran

Thanks Paul.

Been clearing my garden. Hope to investigate my Sprite in the next week. If I don't do it soon, I reckon it'll be impossible to start; both my Sprite, and the investigation. Winter approacheth. Lol.
.
anamnesis

I had this problem after a rebuild with bronze valve guides. The car would misfire due to sticking valves when cold and once warm, ran well. I have had my fill of crappy machine work over the years.
Glenn Mallory

Is this a maniflow manifold? Or 'style'? Albeit with the end missing (rusted and blasted) off the collector.







anamnesis

Revived an older lcb thread for my exhaust manifold.

So back to concentrating on the misfire/starting issue.

Having taken the choke apart, the o ring looks 'jaded', but I'm not sure how bad it is. It's still quite soft, but is fuel seeping past it? It doesn't leak externally.

But Guy and Willy were on the money with choke. And Willy strikes again; White stuff found. Not the best picture; but the hole almost completely blocked when viewed from the inside.






anamnesis

I would take tge bottom plate off the carb and clean out the float chamber too. I bet the float is crusted with white crystal deposits.
GuyW

Yep, did that Guy. Almost as clean as a whistle. But I think that's probably because I must have cleaned it in there already, when I replaced the rubber seal in the fuel bowl lid a year or so ago.

Guy you said, -- "I seem to recall that the choke spindle in an HIF has an odd segmented rubber seal that has something to do with fuel control."

Is this the seal you were referring to?





anamnesis

Hif carb cleaned and reassembled with new o ring and outer seal in the starting device, aka choke.

Just before reverting to the Hif and drilling for ported vacuum. I'd bought and installed two viton tips for my hs2's. Since they are now sitting on my shelf and I intend to stay with the hif44; while the fuel bowl lid was off the hif for checking, I whipped out the float and replaced the float valve with a viton tip.

Bunged it on with the new exhaust manifold -- minus the exhaust pipe as I'm waiting for a new clamp to arrive.

It fired up with ease, no more misfire, and idles nicely. Should be chilly tomorrow morning, so a better test. But I reckon it's fixed.

Thanks for pointing me at the choke chaps.

anamnesis

Early start today at 06:30 when it was still chilly. Very small bit of choke, started first time, runs sweet as a nut.

Happy days. 🤣🤣

https://tinyurl.com/yck2xm49

https://youtu.be/slvGKU7HF6M?feature=shared


anamnesis

I'm on the road again; no misfires and no bangs on the overrun. But with the price of the new manifold, I got no money, I got no place to go. 😀.

But there's always a choice. 😁😁

https://youtu.be/dBN86y30Ufc?feature=shared


https://youtu.be/qRKNw477onU?feature=shared


anamnesis

Car is looking smart for you Anam. It's even all the same colour! 🤣

Even got colour coded jacking point bungs!

And yes, to the seal. Probably. I remembered from when I rebuilt my £5 scrap yard HIF that it had something in there other than just a standard O ring. But that was 20 years ago! But a choke fault seemed to match your symptoms. Glad it's fixed. Indian Summer motoring? Take a nostalgia trip to Brighton.
GuyW

Yep, must be getting fussy and tidy in me old age. Lol.

Probably had that choke problem for at least a year and not realised what it was. Don't suppose the carb has ever been serviced before.

Weather turned out very nice. Woke up to chilly, then very dark, started pelting down, thunderstorms with hail, then gradually dried out. At midday a bit of sun, then boom, full on summer again. Nice blast down some country lanes this afternoon.

So mustn't grumble, eh missus jones? even about yer bert's lumbago. Lol.

Lazy Fridee afternoon ----

https://youtu.be/fGkMTV68kDU?feature=shared

When do you expect to have your other Sprite back mobile again Guy?


anamnesis

Indoor Sprite in more or less daily use.
Outdoor Sprite is a long way off being a finished project. Not good for progress, with winter approaching although the last few days have been the best summer days of 2024!
GuyW

I suppose indoor Sprite's a bit to valuable to swap places with outdoor Sprite; in the interests of progress. Maybe under a cover?

No nearby dry/warm lockups you could borrow space in?

anamnesis

My Sprite is going exceedlingly well.

I hadn't driven it for over 2 months as I was away, and it sat in my garage.

Spun the engine, choke in, to get some oil circulating.

Pulled the choke, fired and started 1st time, running very smoothly.

Took it for a nice 80 mile round trip blast to Croydon to see my uncle. New exhaust manifold, new choke o-ring, and white gunge stuff cleaned away, has worked wonders.

If only the sun had been out too, to make it a perfect day. 🙂

https://youtu.be/9wxI4KK9ZYo?feature=shared

anamnesis

Would you please add a photo of your HIF44 set up? I find it helpful to see how others made the carb swap.

Also, what do you consider as the benefit of the ported vacuum source?

Thank you, Karl


Karl Thompson

Hi Karl, I'll take a fresh picture tomorrow.

I drilled my carb to achieve ported vacuum, because the 25d4 distributor is designed to advance with increased vacuum. If you run manifold vacuum, then it advances way too much at idle, and won't run smoothly at idle and low rpm.

But if you can get an mg metro distributor, it runs properly on manifold vacuum, because the vacuum can works in the opposite way.

Guy here has one I think.

anamnesis

The problem with an induction manifold connection is that it gives maximum vacuum at medium to high engine revs when the carb butterfly is closed. i.e. On over-run, engine at high revs but foot off the throttle when slowing down. The butterfly snaps shut and manifold depression sky rockets!

Porting at the upstream side of the carb butterfly works the opposite way, with maximum vacuum when accelerating at wide open throttle which is just when you want maximum advance.
GuyW

Something Guy said triggered my memory. And I remembered we discussed this at length not so long ago, in 2022.

Karl, go into the technical archive for 2022.

Scroll down the list, and open the thread: 'Poor idling with 276 cam' started by Les Rose.

There is a thorough discussion on the hif and ported vs manifold vacuum, towards the 2nd half of the thread.

Try this link, it may work. If not, then scroll the 2022 archive for it.

https://mg-cars.org.uk/cgi-bin/or17?runprog=mgbbs&mode=archiveth&archiveyear=97_2022.dat&access=&subject=97&subjectar=97&source=T&thread=20210323151719245065

For what it's worth, here is my hif installation.

Not as neat as yours, and I haven't used the water way connections on the inlet manifold.

My hif originally ran manifold vacuum/advance. It wouldn't idle well at all like that. Then I drilled it to tap the air filter side of the butterfly for the distributor advance, as Guy describes. And now it runs as it should.



anamnesis

Thank you for the photo and information. I will certainly read up about it. AC Dodd, via Youtube, was my guide when i did me set up. It seems my carb already uses the ported vacuum and it has always run well, once dialled in.

Which manifold is that and what are the various connections to it? I wanted to use the water connections as I did, very occasionally, experience what I believe was carb icing with the HS2s.

Karl Thompson

No brand on my manifold. I can't remember now what it is, or where I got it. It has a servo take off (unused) and the other pipe off it is connected to my vacuum gauge.
anamnesis

Interesting link back into the BBS archive from 2020, Anam. I had forgotten that we had that lengthy discussion. A good reminder that we have, or used to have, proper and informative technical Spridget discussions here on the BBS.
GuyW

Very interesting reading, thank you.

This video, plus the Vizard tuning book, was my guide.

https://m.youtube.com/watch?v=prX1NXUfljM

Apart from a truly alarming engine vibration on first start up - WD40 as the damper oil for reasons I can't remember! - and having to cut the top off the dash pot to be able to close the bonnet, I have been delighted with the carb swap. And that is against properly sorted HS2s.

The car starts first go, runs like a modern and idles smoothly at 800rpm. I also like it being much less cluttered under the bonnet. I have an extended length stainless heat shield and a Mangoletsi manifold.

The needle is a BDL, as the engine is very similar in performance to an MG Metro (non-turbo).
Karl Thompson

Spanned 2021 and 2022 Guy. But I was surprised it was so long ago. I thought it was late last year, or even early this year. Yep a good example of some very good tech threads.

Just read through my posts and jogged my own memory on having the metro manifold. That's worrying, if I'm beginning to forget stuff like this. Lol.

Wd40? Interesting. Who suggested that? I tried 3in1 this year, and it's way too thin in summer. So I reverted to what I always used, 20/50. I've read 3in1 mixed 50-50 with 20/50 is pretty well perfect. Cheaper than the 'correct' oil from su burlen lol.

Yep most people I think are happy to have converted to an hif, and mine certainly performs well now I've got the dis' advance on ported. But I really only did it on a whim, and was never unhappy with twins; although my twins being original 1966, are somewhat worn out now. 😁



anamnesis

Yes Anam, 2021, not 2020. I have fat fingers when typing on a phone virtual keypad!
But it's a while since we had such a good technical discussion.
GuyW

My memory is failing and I got the wrong mix of numbers and letters - 3 in 1, not WD40. Probably from this archive which has always been my go-to resource. It was a bit light and it's now half engine oil.
Karl Thompson

This thread was discussed between 06/09/2024 and 13/12/2024

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.