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MG Midget and Sprite Technical - engine dies on part throttle

It's a long story (so bare with me) but it started with a bad missfire that happened at random making the car undriveable for a few seconds then it would clear and work ok --- until the next time.

Anyway, I replaced most things ignition and carburation wise and it wasn't until I replaced the condensor that the fault cleared. By that time all the new bits meant the engine ran ok but wasn't back to it's old self.

I've had this car over 3 years now and it has up until this incident been 100% reliable. I even went to Le Mans Classic 2014 in it and apart from a dead alternator (in the middle of France) it behaved impecably.

Lately, the engine has been as rough as a bag of nails so set about sorting it out. While idling I took off the ignition leads one by one and 3 and 4 made no difference to the roughness, the engine ran as before. There was a big spark to plugs 3 and 4 so I don't think it is ignition related. Taking the leads off 1 and 2 stopped the engine. So, Headgasket? Checked compression and all were consistently 180 psi, so maybe not HG failure.

I looked at the carbs and noticed the rear carb was opening later than the front on the linkages. So I sorted that out, no difference, it still ran rough. I took the dashpots off and noticed the rear jet was quite a bit higher than the front jet. So, I made the rear one richer to make the heights the same. Now the engine runs a lot better, both pistons rise and fall in unison and the piston lifting pin on both carbs do the same thing.....but there is now a problem that has me stumped.

With the engine idling, if I rest my foot on the accelerator very lightly and try to raise the engine speed a little above idle it immediately dies. If I do it harder it hesitates for a fraction of a second then the engine speeds up. On a test drive the engine runs as well as it did before but I have had some embarrasing incidents when pulling out of junctions where the engine just dies for no reason. If I do the same but with enthusiasm it sort of hiccups some times but doesn't die.

BTW, idle is a bit erratic so I'm just about to clean all the linkages and set it up again but while I'm doing that some advice would be welcome.

Rob
Rob aka MG Moneypit

Dashpot oil low or too thin?

Dashpot piston detached?

Inlet manifold air leak?

Perforated float?

Ignition timing?

Duff lead/leads?

From what you say, it looks like the fault is on one or both of the rear cylinders, therefore indicating a rear carb problem.
Bernie Higginson

fuel level in the floats?

mog

Hi Bernie. Dash pot oil, topped up with engine oil but didn't change anything. They didn't really need topping up but I did, just in case.

Dashpot pistons ok, floats ok. Not sure about ignition timing but it doesn't pink on load so I assume it's ok.

Leads, I think are ok. I get a big fat spark of all leads when held close to it's spark plug.

Which leaves Air leak. Funily enough I took the K&N off and it's baseplate to clean the linkages. While fidling about I thought I would check manifold nuts and found one or two I could nip up half a turn. The center one (the hardest to get to I fould loose enough that I could rotate the washer underneath, so I tightened it up. Also when I put the baseplate for the K&N back I noticed one of the gaskets was on backwards so covering up the air balance hole to the rear SU. I replaced both with new and made sure they were the correct way round.

So now it runs better than ever BUT I can still stall it using the accelerator. It's a lot better than it was, but it still does it.

The idle is now steadier and it no longer pops on the over-run which it has been doing for a while. That must have been the loose manifold nuts.

I think I should now replace the inlet gasket just in case the loose nuts have damaged it.

Mog, yes that did occur to me but they look ok, no holes and new valves so I think it is air leaks. Well I hope it's air leaks because that is simple to rectify with new gaskets.

Rob
Rob aka MG Moneypit

So when you push the lifting pin on the back carb and let it go... what does it do... 1 of 3 things will happen

Does it

1. Speeds up then runs normal

2. Dies almost automatically

3. Speeds up and then dies

Id say check your choke linkage that its not sticking

also pull the butterfly linkage shaft and look for wear

Prop
Prop and the Blackhole Midget

Is the points gap right?

The ignition will function 'reasonably' even if they are malajusted but under certain circumstances there's a loss of usability when the gap changes

Just a thought
Bill sdgpM

Prop, number 1 is what happens.

Bill, the points gap is a little wider than normal. I checked with a 15 thou feeler and it was wider. I might have adjusted the gap but it started to rain and it's a right PITA anyway so I left it. I will try re-gapping tomorrow, when the weather is better.

Rob
Rob aka MG Moneypit

Try checking the vacuum advance tube too.
Nick Nakorn

Rob, does the rev counter needle flicker and dance about as the engine stumbles, or just drop with the drop in revs?
Guy W

Hi Rob

A few years ago on one of our Halloween runs my car started to misfire badly at low revs and would not idle.

This was due to a faulty distributor cap. Although this was a relatively new part at the time it was poor quality - the terminals were out of position and the rotor arm was not connecting on all four.

Got a replacement (bakelite type - Lucas made in India) from Distributor Doctor which are top quality. Fitted this and all was fine.

Good luck.

Les
Les Robinson

Nick, I'll check that. Maybe the diaphragm has a leak.

Guy, no flickering of the rev counter. It just follows the engine speed as it drops to zero.

Les, I've tried it with two caps but as they were both used stock they could both be faulty, although unlikely.

Some new things to check though.

Rob
Rob aka MG Moneypit

If there is no needle flicker then the fault is unlikely to be anything on the low tension side of the ignition circuit.
Guy W

I've been wrestling with a screw on oil filter fitted to the Frog. Thought I would do an oil and filter change as the engine must be coming up to 1200 miles or so since the rebuild. Two hours to get the filter off? I almost had to destroy the bl***y thing to get it off, leaked oil all over the place. Last time I put it on hand tight but I was wearing Marigold rubber gloves for extra grip, too much it now seems. Next time I'll just use bare hands.

Got fed up with that so went back to the midget. The points were set at about 20 thou so I re-adjusted to 15 and it made no difference.

One thing I have noticed is if I set the idle higher than it should be it doesn't do it. Setting it to about 900 rpm and the problem is abscent. Set it to 750 rpm and it's easy to make the engine stop by resting my foot on the accelerator pedal, to take up the free movement, and open the throttle a fraction.

Only visually checked the vacuum piping. It's all there and no sign of leaks or broken connections.

With the idle set to 900, if I rev the engine for 10 to 15 seconds then release the accelerator it drops to about 1100 rpm then spends the next 10 seconds or so slowly dropping down to 900 rpm again.

Not sure what is going on? I'll leave it for now with the idle at 900, at least it is driveable like that and runs really well.

When I've got time I will replace all the gaskets on the inlet/exhaust/carb side of things and get new needles and jets and see if there is an improvement. After that it's a carb rebuild I suppose, but that will be after I replace the hood.

BTW, another thing I have noticed is with the fuel filter. It's a transparent one so I can see the fuel flowing, but every now and again there is a small air bubble in the fuel. Very small, less than a mm diameter but I see 2 or 3 every 20 seconds or so. It must be on the suction side of the fuel pump. Hope it's not the fuel pick-up pipe inside the tank!!!!

Rob



Rob aka MG Moneypit

might be a sticking needle? check the jets centered, and the piston drops with a good click as it hits the bridge.
mog

One thing I have noticed is if I set the idle higher than it should be it doesn't do it. Setting it to about 900 rpm and the problem is abscent. Set it to 750 rpm and it's easy to make the engine stop by resting my foot on the accelerator pedal,

You should be no less then 800 rpm and ideally at 1000 rpm

Anything less then 800rpm and your oil pressure pump is on a tea break

It sounds like you have a vacume leak from what all you mention

Have you pulled the butter fly shafts and looked for wear

Just a thought... with the idel set so low, and your using the cars tach you could be way off on your idling rpm by 200 rpm putting you around 500 rpm, at that pace it wouldnt take much of a carb imbalance that a small goose of the accelrator and the weaker lean carb(rear) could be over powered by the front more richer carb...even a dash pot piston spring being older or wealer then the other would come into play at a very low rpm idle

So id say low idle and worn butterfly shaft is were id look next

Prop
Prop and the Blackhole Midget

####BTW, another thing I have noticed is with the fuel filter. It's a transparent one so I can see the fuel flowing, but every now and again there is a small air bubble in the fuel. Very small, less than a mm diameter but I see 2 or 3 every 20 seconds or so. It must be on the suction side of the fuel pump. Hope it's not the fuel pick-up pipe inside the tank!!!! ####

Hmmmm, thats very odd, there should be ALOT more air then that inside the fuel filter... at least 1/3 to 1/2 of the space inside the fuel filter should be air...im just purely guessing to much fuel pressure you should be at 1.5 to 2 psi

Agian guessing... maybe with to much fuel pressure is the reason why the engine takes longer to come off when the gas peddle is released

Prop
Prop and the Blackhole Midget

I think that Prop is right about an air leak at the butterfly shaft.

Try this test:
With the engine running spray a volatile fluid like carb cleaner directly onto the exposed end of the butterfly shaft. If there is an air leak there it will suck the fluid in which will richen the mixture and the revs will increase.

If there is a significant leak there, then a gentle throttle opening will suck in air and drastically weaken the mixture, causing it to stumble. A temporary fix is to put a blob of silicon over the end of the butterfly shaft. You could try that to see if the stalling problem is, at least, temporarily cured.
Guy W

Prop, there is an air bubble in the filter, not very big and certainly not 1/3 of the space in the filter. it's a small inline filter not secured to anything. As the engine vibrates the bubble shakes and dissappears into the float chamber. It gets replenished by the small bubbles. It's the bubbles that worry me as it inplies there is a leak on the suction side of the pump. I will have to investigate and hope it's not inside the tank.

Guy, I will try the carb cleaner trick. I actually bought some today to clean the linkages.

Until all this trouble started the engine was quite happy to idle at 500 rpm (as indicated by the tacho) and didn't exhibit the current problem. Maybe it's just reached a point where it can't cope anymore. Just age catching up with it, a bit like me really!!!

Prop, the fuel pressure sounds plausible. I don't have an Su fitted, the car came with a Facet up to 4psi pump, but then again it ran until recently with absolutely no problems.

Rob
Rob aka MG Moneypit

Rob,

You and Nick discussed checking the vacuum advance for a leak, but I didn't see that you have done the check. Do you have the vac adv connected to the manifold or the carb. I think that if it is connected to the carb, the closed butterfly cuts the vacuum off until the butterfly just starts to open. If I have that right, then at a low idle when you tip the throttle you could be introducing a vac leak at low rpm, thus killing the engine.

As to Guy's suggestion of silicon, you can also use heavy grease.

Charley
C R Huff

If its pulling air bubbles through then check the condition of the rubber fuel hose upstream of the pump. If it is perished or not sealing fully it may not leak fuel out under gravity, but can draw air in under suction, especially if you have a higher suction pump than normal.

I doubt that this is causing your faltering engine, as fuel starvation would have greatest effect when there is a high demand (high revs). But its worth sorting out and eliminating anyway.
Guy W

Oh for the love of god, come on Rob !!!

We are not mind readers, we dont have the ability to see.the car... You HAVE to tell us when the car has been modified

We JUST SPENT 18 postings talking about YOUR carb issues and the entire time your giving us the "impression" that you have SU carbs ... YOU DONT HAVE SU CARBS !!!

You waited 18 postings to share that little jewel ....why? Is it some sick twisted sexual fetish you.

what carbs do you have or is that something that we dont need to know ither

And btw ever piece of info we spent 18 threads helping you on... is trash can, none of it applies to your situation, because we all were lead to belive your carbs are SU. So the lack of space in your fuel filter and any referance to fuel pressure is of absolutely no value to you and does not apply in the least of any fractional percentage

Seriously... I dont understand this... and it happens all the time, and its not people that just bought the car and knows nothing about them, its people that actually know the carbs are not orginal

Seriously rob, you knew we were talking about SU carbs the entire time.. what was it you were hoping to attian by listening to us go on and on about SU diagnoses, knowing full well that you dont have SU carbs...id love to know the answer to that question, and you owe everyone here that replied to this thread the answer to this question

Gez, can some one please donate a few soap boxes to my home please.. mine are badly worn out.

Prop

####Prop, the fuel pressure sounds plausible. I don't have an Su fitted, the car came with a Facet up to 4psi pump, but then again it ran until recently with absolutely no problems ####
Prop and the Blackhole Midget

Calm down, its the pump thats not an SU
mog

hmm yup that was my take as well - Prop cancel that order for soapboxes! :-)
a reid

HaHa. Hey Prop, you remember Dana Rosanna Dana Anna(or something like that, Gilda Radner I believe) from Saturday Night Live? If you do, it is now time for you to come back and respond, "Well, never mind".

Charley
C R Huff

Prop, the pump is not an SU. The cars has twin 1 1/4 SU's.

Anyway, I checked the vacuum advance and it's ok. A bit of sucking and a well placed tongue and the vacuum held until I got bored. When released I can hear the dizzy returning to normal.

I tried the carb cleaner trick and when sprayed on the front carb accelerator linkage it made no difference. On the rear carb however, the tickover decreased noticeably, presumably while it digested the extra fuel it sucked in past the bearings.

My new needles arrived this morning so I fitted them. Not sure when the jets will be here but I fitted the new needles anyway.

I'm getting totally inconsistent jet settings so I suspect I have an air leak somewhere. I'm away in Cornwall for a week from Saturday so I'm running out of time with this problem. I may have to shelve it for a while and replace all the gaskets involved when I return.

Rob



Rob aka MG Moneypit

Haha

Sorry about that... new soap box order cancelled

Prop
Prop and the Blackhole Midget

Two nations separated by a common language?

Rob
Rob aka MG Moneypit

Sorry about picking up a comment from several replies back, but for getting off a tight Midget oil filter, I recommend this:

http://www.halfords.com/motoring-travel/tools-diy/tools-tool-kits/halfords-advanced-professional-adjustable-oil-filter-remover

I spent considerably more than the two hours that you did, removing the oil filter that came with my recon engine, two years ago.

Chain wrench, two different strap wrenches and a couple of improvised bodges, hundreds of tries and much swearing would not budge it. But that tool shifted it on the second try.

It's expensive for a single-purpose tool, but would have saved a huge amount of grief if I'd had it at the start.
Pete AC

Pete, the usual trick is to take your largest screwdriver and bash it right through the filter from one side to the other, then heave. Never fails.
David Smith

David

You beat me to posting a great tip... been there done that, luckly only once... and yes its a mess

Prop
Prop and the Blackhole Midget

My little trick for removing the spin-on oil filter is to wrap a worm-drive hose clamp around it positioned as such that the drive nut is forward and in line with the triangular opening through the wheel well. I then take a 3/8" socket on the end of a long extension, put it through the opening in the wheel well, engage the adjusting nut on the clamp, and tap the extension with a mallet until the filter begins to move. Once loose I remove the clamp and spin the filter off by hand. Much cleaner than impaling the filter with a screw driver (been there, done that). Hasn't failed me yet.

This works well on a left hand drive but not sure how it will work with the steering shaft in the way on a right hand drive.


Paul Noeth

Paul, that's an abnormally large worm-drive camp (what we call a Jubilee clip) which not a lot of people here would have handy, and secondly, what do you do if the clamp is slipping round the filter as it would in Pete's case as he said strap wrenches were slipping?
Yes we know the screwdriver is messy and to be avoided if possible but it does work 'when all else fails'.
David Smith

"that's an abnormally large worm-drive camp"
That's two clips joined together in the photo. I have never failed to remove a filter with a chain wrench. The links dig in and give good purchase on the can.
Mike Howlett

Paul... thats a great idea,

we call em hose clamps here... and they come in all sizes

Prop
Prop and the Blackhole Midget

David, if the clamp begins to slip, just use a ratchet handle and tighten it a little more, the socket and extension is already in place. Never had a problem with it slipping.

Mike you are right. I didn't have one long enough in my junk box so I just joined two short clamps together. I tried to move the filter with a strap wrench but could not get it to budge. I do not have a chain wrench so I tried the hose clamps. I think tapping on the extension works well to break the seal, and as a bonus you don't get red in the face or bust any knuckles. I've only used it a couple of times because the engine rebuild is somewhat new. I'll have to see how it works over the long haul, but for now the clamp holds a revered spot in my kit of specialized Midget tools.
Paul Noeth

Prop, we call them the same here but there are several different types of hose clamps. Just trying to make myself clear.
Paul Noeth

Cool trick, Paul.

I have found one down side to the screwdriver trick (besides the mess). If the filter is so tight that a good filter wrench won't budge it, you may find that the screwdriver just continues to cut through the body of the filter instead of unscrewing it.

Then you have a oily pile of jagged metal with no strength anywhere except for the base plate of the filter.

Once that happens, you have to resort to a cold chisel on the base plate. So, before you go to work with the screwdriver, you might want to be sure you have enough room to use the cold chisel on the base of the filter.

Charley
C R Huff

Rob,

I just now realized that this oil filter stuff is all thread drift on the back of your part throttle problem.

Oh well, you said you were off to Cornwall for a week anyway, and we have to entertain ourselves with something till you get back.

Charley
C R Huff

Thanks for all the advice from the knowledge collective.

I found the problem which is shown in the attached photo. The missing bits were not stuck to the inlet manifold so I think the engine ate it.

While I was curing this problem I replaced all other gaskets and put new jets in the carbs.

The car is now back to how it was when I bought it. I can get it to smoothly tick-over at less than 600 revs with no sign of part throttle dying.

Of course I'm not going to keep the slow idle but have run out of time today to get it fully warmed up so I can check the mixture settings etc.

Rob


Rob aka MG Moneypit

Good news. It usually turns out to be something simple that that we ignore because it is too obvious to be the problem. Gotta be something much more expensive than a gasket ;-)
Martin Washington

Good to hear a happy conclusion to this. Well done!

You will of course remember that Bernie suggested in the very first response that it might be an inlet manifold air leak. (along with several other possibilities!)
Guy W

This thread was discussed between 28/07/2015 and 16/08/2015

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