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MG Midget and Sprite Technical - Fuel or Electrics?

Hopefully one of you more experienced guys may be able to help?

After a year and a half I am getting to the end of jobs that required finishing after purchasing a 75% restored 75 midget.

A few weeks ago the first decent run after around 20 miles the car starting spluttering and appeared to be running on 3 cylinders, after slowly getting the car back home a few days later there appeared to be no problem on a short run, but then some time later again after 20 miles the problem occurred again but not quite as bad.

Now after changing plugs, leads, distributor cap, points, condenser, rotor arm I went out again hopeful that the problem was sorted, after again 15 to 20 miles the car when running steadily at around 30 mph in 4th the car again started missing, not badly but it will not run smoothly until you put your foot down and then no problem. Again in 3rd when just plodding along the car will not run smoothly until you put your foot down.

Hopefully now after eliminating most of the electrics I assume it is something down to the fuel delivery, I replaced the old tank and fitted a brand new tank a couple of hundred miles ago due to the leaking tank, so I am now assuming it is something to do with the carbs, has anyone else suffered a similar problem?

Tim Lynam

In this current warm spell, it may be fuel vaporisation in the fuel pipe. Most probably on the 1500 where it runs across behind the engine on its way from fuel pump to carbs. 1500's are prone to this, although that is really just a guess at this stage! How about going for an evening drive when its cooler and see if it still does it.
Guy W

Guy, On all three occasions they were warm days, if it is something like you suggested it would be a relief as I have always been concerned that something worse is around the corner.

Maybe when the cooler days arrive I will know the answer.

Thanks for the suggestion, I had not heard of that issue.
Tim Lynam

I'm not all the familiar with the Midgets, but if it has an electric fuel pump in the back, vapor lock in the fuel line going across the engine should not be a problem as it is under pressure, which inhibits vapor lock. The next time the problem occurs, remove the filler cap from the fuel tank and see if that makes the problem go away. If it does, then the vent for your tank is clogged. Cheers - Dave
David DuBois

Do you have a heat shield between the carbs and intake manifold...the answer says alot

Id also check your carb settings make surethe carbs linkage arent arent binding

With the engine cold try the little lifting pin on the carbs ....then try when its hot and missing

The pin is on the innerside of the carb below the dash pot....try googleing su hs4 carb exploded view

It could also just be a vacume leak because of new vaskets need further tightening

Check the valve clearances also... they could be a hair to tight...they heat up, they tend to close

Use a clear fuel.filter... any air bubbles...could be a small hole in the fuel line

When it got hot and the engine started missing did you try to grab the coil ? What happened...brutal burns or just avg temp?

Prop

Prop
Prop and the Blackhole Midget

DID YOU try to pull the spark plug wires one at a time ...did it make any differance... weve seen new plugs bad right out of the box on serveral occassions

What heat range is your plugs if there hot plugs... that can have an effect as well... idiot kids at part stores are not aware of diff temp range plugs and have little intrest in learning

Next time it faults try removing gas cap...if it swoshes or runs better... partially clogged cap
Prop
Prop and the Blackhole Midget

I doubt if it is a blocked vent in the filler cap as that causes a problem within about 5 miles as vacuum develops in the tank. Tim is getting 20 miles or so before it happens.
Guy W

"I doubt if it is a blocked vent in the filler cap as that causes a problem within about 5 miles as vacuum develops in the tank. Tim is getting 20 miles or so before it happens. "

That depends on how full the tank is. If it is full, it will cause a problem in just a few miles, as the fuel is used up, it take longer for the vacuum to develop to the point it will shut the engine down.
Cheers - Dave
David DuBois

I agree guy... but its a fast easy free bee check that has some validity...not something to focas on

Prop
Prop and the Blackhole Midget

Prop, I am all for the quick and easy checks! And David has a point about the impact of how full the tank is.

The other "quick and easy" check I usually chime in on is "does the rev counter needle flicker?" (=LT electrics problem) but on this occasion Tim seems to have checked or replaced all of the likely suspects.

I still favour the fuel vaporisation in the delivery pipes. Running at steady speed and at low revs the fuel is only moving through the hot pipes slowly, and has time to overheat, and at the same time air movement around the engine bay is reduced at the lower speeds.
Guy W

I'd thoroughly clean out the jets ensuring they are not blocked with crud and clean up the needle. Ensure no air in fuel lines - from a loose union perhaps - ensure no air leak at inlet manifold, carb join or from any hoses. Check plugs also....and fuel pump itself to see if it is delivering what it should.
Mark O

Thanks for the ideas on what could be at fault.

Regarding how full the tank was, when the fault happened the first and second time the tank was probably around half or less full, when I went out yesterday the tank was full to the brim, I could not believe how little fuel it takes to fill the tank!

When the tank was full yesterday the problem was not as bad as before but I put that down to all the new bits helping the cars performance.

I have had to change a couple of flexible fuel lines which were badly perished, maybe I have missed one as after changing the perished ones the performance was far better.

But I have never had the carbs apart so that looks like the next job, but it is a bit daunting as the last time I touched any carbs was back around 1978!
Tim Lynam

What we are referancing on the gas tank is once in a blue cheese moon the vent on the gas cap becomes clogged and the gas cant be sucked thur the pump... like a kid drinkijg a cherry milk shake thur a straw that gets clogged and the kid turns purple trying to suck out the cherry blockage thur the straw...the more full the tank, the sooner the issue starts ...so if you drive the car untill its really bad and dies, pull the cap if its under suction it will make a swoshing sound when you release the cap and the car will strart right up agian

I think the fuel lines are a.great place to start... sometimes new hoses (generally speaking) need a few extra twist on tue clamps if there new and have a few short miles on them

Id seriously avoid taking apart or re-adjusting the carbs...a good cleaning yes even pull the dash pots off and clean out aged tarnish from oil setting a long time and install fresh oil in dash pots

But as to the actual adjustment... id hold off till last possiable culpret...setting just.dont change on there own and if they were properly set to begin with ...well now you have exasterbated the problem even further...and still not knowing the true reason for the issue to begin with

BTW...THERE IS A VARY GOOD CHANCE NOTHING IS WRONG....is there a section of sheet metal between the carbs and the intake manifold...if not, GUY nailed it dead on...that little section of sheet metal is not for looks its to shild the carbs from over heating and boiling the fuel into vapor which makes your engine do exactly like your discribing

Prop
Prop and the Blackhole Midget

Prop, sorry I forgot to mention earlier that there is no heat shield protecting the carbs, thanks for your explanation of what its for, a brilliant description, I wondered what they were for!

Are they readily available as I had not noticed them for sale, but then again I had never looked.
Tim Lynam

one here on fleabay

http://www.ebay.co.uk/itm/Triumph-spitfire-mg-stainless-carb-heat-shield-/251305689099?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item3a82fc800b
mark (1977 1500 Midget) Preston Lancs

ps dont forget gaskets either side of it.
mark (1977 1500 Midget) Preston Lancs

Ding ding ding... GUYs the winner, good call guy.

Yeah... evilbay will have them, if not try moss, or minispares on your side of the atlantic bathtube

carb Heat shilds will make a huge differance...these engine bays get brutal during the summer months and with the heat coming off the exhaust manifold, im shocked that even fuel vapor can even survive long enough to reach the combustion chambers

Prop
Prop and the Blackhole Midget

as with other threads it may be an idea to ask for a photo early on to spot obvious things like missing heat shields

with a suitable photo posted this obvious omission would have instantly been picked up
Nigel Atkins

As I thought the issue would be to do with the carbs or electrics I never dreamed of posting a picture of the engine, but in hindsight the missing heat shield would have been spotted sooner, but do most owners now fit them as a necessity and is there anything I should be aware of when fitting one, there appears quite a few springs and cables that will require disconnecting before fitting?

Tim Lynam

Tim,
unless I'm missing something, these were fitted as factory standard because they are needed as the carbs sit over the the exhaust

have a look at a parts catalogue and see what else you might be missing / not able to reuse / needs replacing like gaskets perhaps insulating blocks, springs etc.

again if you were to put up a clear good quality photo of the area others might be able to advise on what they can see from that photo
Nigel Atkins

Nigel,

I more than likely wrongly assumed they were an add on, but after looking at many engine pics it appears they are the norm and not the exception, which shows what a good spot it was for Prop and Guy without seeing a pic, I will post one over the weekend who knows what else may be missing!
Tim Lynam

I think I've learnt from the BBS that the 1500 heatshileds can fall apart (to pieces?) as they're sort of fibre rather than metal like the A-series but I can't remember the last time I saw under a 1500 bonnet, probably 15+ years ago
Nigel Atkins

This one is listed on ebay

This is for A-series 1275 with Duel SU HS2 carbs

You will need 4 gaskets

Prop

http://item.mobileweb.ebay.com/viewitem?itemId=310298604078
Prop and the Blackhole Midget

After finding that the heat shield is missing but one on way along with gaskets, I thought I would try and find the missing heater valve in the engine, but that also appears to have been left off or has been diverted.

After checking the photos in the manual it looks as thou the valve has been left off and the bottom pipe coming out of the heater unit goes directly to the front of the engine to the thermostat, other than resulting in extra warm feet which is not appreciated in this weather is this normal or could it cause any damage?

Picture below.


Tim Lynam

might need a photo from wing side of altered pipe work near heater (or might not depending on answers)

some would say that the 1500 needs all the cooling help it can get - personally I'd want to be able to turn the heater off as if it's fully functioning you'd either have blistered feet now or a giddy head if you drove with the roof up to protect you from the sun in the present heat

and the car should be able to not overheat even with the temps as they are now, they were exported to hotter places
Nigel Atkins

Tim the front float bowl appears to be on an angle, this may have some influence on flooding issues. Nice clean bay though, well done. I fitted a stainless shield which appears to work well in our hotter conditions, but I'm sure the standard one will work OK as well. The valve goes between the heater and the pedal box mounted to a bracket.
cheers
Rod
R W Bowers

Sorry should have posted this pic earlier, sorry it's a bit dark but shows the general idea. This is where I put it but it's close to the original possy.
Rod


R W Bowers

nice spot on the float bowl there Rod

IIRC the heater tap is as you look at it front the front of the car right: on (brother), left: off - but DH will confirm

Tim,
I did note you got those over length 7mm silicone club HT leads with the right-angle dissy boots instead of straight, that's the same set I offered for postage and I could make mine shiny than new :)

by the look of both engine bays the cars need to get out more :)
Nigel Atkins

You have got a good memory Nigel, after searching the different suppliers of distributor leads on the internet and after getting a sarcastic reply from one of the suppliers regarding costs I decided to try a local motor spares shop who was more than helpful so I gave him a try on all the electrical bits, these were bought at a reasonable cost and a bit of guidance is invaluable to a novice, they were supposedly leads for a Midget but they appear to work well.

Rod, thanks for picking up the wonky bowl which I had never noticed, the holes are there for the heater tap but have been pop riveted up! I have attached a side on view of the (incorrectly?) connected heater and looks like I am going to have to search for the new bits to put it right, you have not got any of these hidden away have you Nigel!



Tim Lynam

Tim,
I got a very poor memory but you asking about leads was quite recent, nothing wrong with the leads you've got they be fine for a few years, personally I prefer the straight connectors to the dissy and also I had mine made to length at no extra cost just for the sake of asking, I got the length slightly wrong on one lead but not by enough to worry me

I'm told the ones in the Accusprark kit can easily pull apart but at the price of the kit something would have to give

I also remember your car as its an early 1500 formerly with 145/70 tyres (and on those never any trouble wire wheels) :)

sorry I don't normally keep any parts but these at least three on here that sell s/h parts

from your second photo the overflow? pipe off the float bowl look like clip could be just passed the end of the connecting pipe and the clutch pipe seems unclipped and things a little knit-one-pearl-one but nothing to worry about

Nigel Atkins

Tim

It looks like the manifold heater pipe has succumbed to the dreaded tin worm, there should be a pipe coming from either side of the inlet manifold that warms the manifold and takes hot water to the heater. Not sure if it's related to your original issue, I wouldn't think so at this time of year.

Bob
R.A Davis

Like bob mentioned there soud be pipes comming out the inlet manifold.
On the heaterside there should be a T-pipe (on its side) where one hose is connected on the top and the other from underneath.

Heatshield should be installed!!
I had the alloy one but there is also a gasket-like one available.


Arie de Best

Not sure that all of the 1500's used the carb heating pipes. I think the very early 1500's didn't use that system. Lots of people end up by passing it anyway, for a variety of reasons.

Arie, do keep up! You are getting slow in your old age. Prop identified the missing heat shield ages ago!
Guy W

Thanks Guy, so mine may not be the only tapless pipe diverted Midget, it would certainly help avoid searching for pipes taps etc now that I have found that by simply pulling out the heater control inside the car, the hot air that blasted the feet which is not too welcome in this weather appears to be stopped.

Tim Lynam

Tim,
Sorry I may have misled you! I think they all had a heater tap so that you can turn off the hot water going to the heater core. Its just that most 1500 plumbed the water through a pipe set into the alloy of the carb inlet manifold. It was a system to speed up the warm up period for starts on a cold winter morning (remember?) And it also stabilises against temperature variations. But some of the early 1500's (yours too, perhaps?) didn't use that system. But they still used the heater tap!

The properly set up system would enable you to turn off the heat, but then open up the air flaps to direct fresh air through the ducting and into the footwells. Or not so fresh air if you are in slow moving traffic and too close to the exhaust tail pipe of the car in front!
Guy W

Guy

Not that it's that important but I don't think Tim's is an early version, the early one's used a capillary tube temp gauge, in Tim's picture it looks like an electric sender.

I used a stainless 2 part heatsheild as per link below, looks good and seems to work.

http://www.ebay.co.uk/itm/Triumph-Spitfire-MG-Midget-SU-Carb-Heatshield-For-1500cc-cars-made-in-STAINLESS-/251308366926?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item3a83255c4e#ht_375wt_1115

Bob
R.A Davis

Interesting point Bob. And well spotted for the electric temp sender.
Though I see that Tim's car is a Feb '75 with the later electric sender, and Aries' car was an Oct '75, apparently with the earlier capillary tube system. Perhaps Tim's has been replaced ? So many things get changed in nearly 40 years its a job to know what is original.
Guy W

Oh well as long as the diverted pipes do not cause any running issues other than poor warm air flow, I suppose I will have to put up with it until I come across the parts.

A new heat shield has arrived today, unfortunately I did not come across the fancy one Bob has suggested, now just waiting for the gaskets and I can look forward to fitting, hopefully it will not be too difficult.



Tim Lynam

The lower carb mounting nuts are difficult to get at. And even more difficult when you refit with the heat shield, as you will certainly find out! ;-) Its one reason the heat shields get left off.
Guy W

Tim, Arie's is a different model to yours with a bypass hose coming off the thermostat housing otherwise I think the same. You should have two diverter flaps in the footwells either side to divert hot air to the demister slots, this may alleviate the hot foot syndrome you've got. IMHO you may need some heat to the manifold in winter to speed up the warm up. I replaced all my pipe work with copper because of the rot, if you do this be aware that you are introducing another metal into the system e.g. use the best anti corrosion product, I use an organic non Glycol product which is brilliant. We don't have freezing issues down here so you may have to use Glycol based. HTH Cheers Rod
R W Bowers

Thanks Guy, those two bolts are the ones I was worried about, but never thought of the hindrance once the shield is fitted, oh well I would rather hear of the problems rather than, only a 5 min job quoted and two hours later to be cursing because I cannot get all back together.

Cheers Rod for your guidance, I should not have too many worries about poor starting on cold mornings, the Midget will be wrapped up in the garage during the cold months! But if anyone has a spare tap and bracket I will gladly purchase if one for sale anywhere.

Tim Lynam

Just a follow up to a problem from several weeks ago, after taking the advice kindly offered on here the problem of the spluttering and poor running after around 20 miles appears to have been fixed after fitting a simple heat shield, a right pain to get the bottom two bolts tightened up, but well worth the pain in the end.

Thanks to all for the advice.
Tim Lynam

This thread was discussed between 14/07/2013 and 26/08/2013

MG Midget and Sprite Technical index

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