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MG Midget and Sprite Technical - Really struggling here! 1978 MG Midget 1500 won't

Hi All, I'm slightly losing the will to live with my Midget. I've only had it a couple of weeks and I think the person I picked it up from basically got it running with the choke tied on then found a mug to sell it to (hello!).

A couple of days after bringing it home, it started rumbling and kicking out a lot of black smoke, only running at all on the choke then stalling.

So far I have:

- Removed cleaned and replaced the valves in the carbs and float chambers, along with all of the gaskets.
Replaced the intake/exhaust manifold gasket.
- Replaced the spark plugs.
- Done a compression test (all was well!)
- Attempted to figure out if it's the timing or the coil that might be at fault (although I can't really tell with the coil).

What was a plume of black smoke, it now just cranking with nothing happening at all.

If anyone is able to help, I'll owe you a beer! I'm based in Hailsham, East Sussex if anyone is nearby. I'm loathed to spend any more money on it until someone who knows what they're looking at has done so 😊

Thanks so much.

Tim
Tim Tiernan

Black smoke is usually an excessively rich fuel mixture which is what you might expect on an engine that has warmed up and no longer needs excessive mixture enrichment (choke).

It's possible it only runs on choke because without it the mixture is excessively lean and it won't run at all.

If you have a photo of the carbs it will help and the standard HS4s don't have valves in the carbs. What they have are needles with a number on them, a spring, which may be coloured and oil under the black plastic plug on the top that stops the pistons, that have the needle attached to them rising too quickly and too much would create a full lean mixture.

I'm a long way from Sussex and not really an SU 'person' but others will be along shortly.
Daniel

I think 1500 carbs have poppet valves built in to the butterfly that open when you lift off the throttle. If so the best thing to do is solder them to stop them working.
Also, 1500s have waxstat jets which are notoriously unreliable and usually produce an uncontrollable rich mixture.
I would recommend buying a Haynes manual otherwise you are just flying blind.
I'm nowhere near Sussex so no chance of me helping further but I'm sure there are lots of MASC members down there.
Pictures would be helpful if you can figure out how to post them.
Rob
MG Moneypit

It is difficult to determine at a distance - did it run to drive it home ?

Needing choke to run - possible fuel supply issue ? Check pump is delivering plenty - was there lots of muck in the float chambers ?
If so also check supply pipe from bottom of float chambers to carbs is clear- blow through.
Are the plugs wet ?

Check ignition is delivering spark to plugs . Test a plug to earth etc.

You mentioned it started rumbling- was this whilst driving ? If so what was oil pressure like ?

Hope someone local can take a look and eliminate some of the unknowns.
richard b

Well I did say I wasn't an SU person! The brass butterflies with poppet valves in them can be swapped with plain ones or soldered up as suggested.
Daniel

Two situations which can cause that effect are

1. Jets not centred, and the metering needle is jamming. Easy to check (piston should lift freely) fiddly to correct but needs doing.

2. If the jets are adjusted up so high they are level with or protrude from the bridge. In this position, when the engine is trying to idle the piston (being at full drop) sits on the jet, blocking it off.

This jet adjustment situation is usually brought about by worn jets or needles. Giving choke pulls the jet down to clear the piston and gets it to run - but badly because the jet/needle wear is overfuelling.

We recently had three cars with this fault on the same day!

It is easy to check for: remove the dashpot and pistons, and check the height of the jets. If they are flush with or above the bridge (the adjacent part of the carb body) then reset them to 1.5mm down from flush. (about 1 turn of the adjuster nut)
This is leaner than the usual default setting, but how they were set indicates a rich issue and 1.5mm is a position where it should at least run and can be adjusted from there.

With the 1500, if it has the mechanical fuel pump be aware that most replacements are putting out too much pressure (about double) and this can be another thing that prompts adjusting the jet too far.

If the jets are found to be too high, and if after resetting them as above and adjusting still has rich idle mixture at any position below the bridge, steps I suggest are:

1.Check the fuel pressure - borrow a gauge if you don't have one - at the fuel inlet pipe to the carbs. Should be 3.5-4 psi. Anything over 5 is problematic and will bleed fuel past the needle valve, raising fuel level and bleeding out of the jet at idle/low speeds. Solution is either a fuel pressure regulator or a pump with the right pressure.

2. If fuel pressure is OK, check the needle valves for wear and correct size (should be 70 thou hole, not 96. 2mm drill bit is a good gauge - passes through 96 but not 70)

3. If both the above are OK, renew the jets. Make sure they are properly centred when you fit them, as per the manual.

4. If the symptom persists then renew the metering needles.

You might like to do the needle valves, jets and needle at the same time and be done with it.

Paul Walbran

Sorry not close but happy to help as remote support ;O)

In situations like these I like to go back to basics and recheck everything! Starting with the ignition system.

1500's have a ballast ignition system with a resistive wire in the loom. These have a history of failing resulting in various/obscure starting/running issues. So, first action is to confirm you have a good spark at all plugs. I find that bypassing the ballast (for a short period of time) eliminates a number of potential issues e.g. ballast wire (although thinking about it, this is bypassed whilst cranking), ignition switch etc. To bypass it all, run a 12v feed wire from the battery +ve directly to the coil +ve. Do you have points or and electronic ignition system?

Then check the static timing; I can't remember the exact number but anywhere around 10deg should be sufficient to get it running.

Once you're happy with the electrical side, baseline the carbs.

If waxjets are still fitted, replace them with standard. As you've already cleaned the float chamber needle valve, check the float height is correct to ensure the fuel level in the jet is correct. Then check the carb piston moves up and down freely. The 1500 should have biased needles which gets around the need to centre the jets but these can cause wear in the jet! Check the jets for wear and replace both the jet and needle if any is found.

IIRC setting the jet 1.5 turns down from bridge should get you in the ball park for start up mixture. But as Paul says, one turn should be enough to get it fired. Compromise on 1 & 1/4 lol

As others have said, check the pump is delivering fuel. Pressure isn't so important at this point as long as it's not flooding the bowls to the point that fuel comes out of the overflows and/or the jets directly in to the intake. This can be checked with the dashpot off while cranking. Also checking for the presence of butterfly poppet valves and their condition i.e. stuck open or not.

Also, make sure the choke fully releases and correctly applies when pulled e.g. the jet is moved down and the butterfly is opened via the choke cam.

If all that doesn't get it started... a ticket from Edinburgh to Gatwick, an overnight B&B and I'll be there in no time at all lol.

best of...
MGmike

M McAndrew

All good advice, but if you are not familiar with setting up and adjusting the carbs as others describe, the Burlen web pages have clear and useful downloadable instructions sheets for the different su carbs.
GuyW

Burlen site:
https://sucarb.co.uk/technical/technical-su-carburetters

And to add my 2p worth. If the car was running when collected then it seems unlikely that the problem is electrical. Avoid the scatter gun approach of changing too many variables at one go. Check the obvious, like points gap and for a visible spark at the plugs but otherwise leave the electrics well alone for now.
GuyW

Hi Tim,

Saw your posts on the Facebook group and I get the feeling the carbs are not set up correctly from the videos and pics you shared there.

Good thing is that with a little bit of research you can learn how they work and set them up yourself if no one in your local area can help.

I had to strip mine down completely and rebuild as part of my restoration and I learned along the way. Got mine running well enough and I’m no expert.

However as others here have said could be ignition / spark or a combination of both.

But don’t give up you’ll love it when you get to the bottom of the issue/s.

The guys here are very knowledgeable and helpful, so keep asking the questions.

As I mentioned on Facebook I like the University Motors channel on YouTube with John Twist. As a beginner I found the videos really helpful.

https://youtube.com/@universitymotorsonline?si=P2iwwFr9hgykyHwh

Good luck

James


James Paul

I once bought a 1500 that would only run if you pulled the choke out.

It turned out that one of the carb linkage clamps was loose and only one butterfly was opening.
Dave O'Neill 2

Just to add to Paul and Mike's comments:
Waxstats can be problematic and cause very odd fuelling problems indeed, but you don't need to replace the carbs.
The waxstats themselves can be simply removed and discarded, but you'll need spacers to replace them.
It turns out that a couple of 1p coins is just right.
The idea of the waxstats is to compensate for variations in inlet charge temperature by fiddling about with the jet height, but sources vary on how effective they are. My own view is that they are not particularly helpful.

Just my 2p.
(YSWIDT!)
Greybeard

Good point Dave, that can be a sneaky and unusual fault that isn't immediately obvious. And nice and simple to fix.
Paul Walbran

This thread was discussed between 07/08/2025 and 09/08/2025

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