MG Midget Forum

Hi all,

I have been losing my mind over this for a few weeks now chasing problem after problem. I have now run out of things to fix yet the car refuses to run right no matter how hard I try. I have fully refreshed all gaskets on intake, adjusted the valves, adjusted the points and rebuilt the carburetors. However, when tuning at the carburetors if i raise the pistons to test mixture the car instantly dies as if it is lean according to the SU tuning guide. And yet it needs max adjustment out on the jets to even run. Although all of this seems lean the exhaust says it is extremely rich with liquid fuel out the exhaust and very smokey under revs.

With fine tuning I can make it run really rich or really lean at the exhaust but at the carbs it always seems lean. And just to run needs all the adjustement out on the jets. I am lost.

Any tips or knowhow appreciated.

you have loosened the four linkage bar nuts prior to adjusting the carbs so you adjust each carb seperately i assume

yes all linkage disconnected, and reset to base settings. at which point I had to wind the jets all the way out just to make it even run. or if I hook back up the choke it needs full choke and still sputters like its lean.

Did you just create your account? Or better question, just buy your car?
How old is the gas?

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79' Midget with a Weber Carb and more beer than gas.

Took it down to the petrol station last week so brand new. I have had the car around a year but I have been trying to get it done recently and it is fighting me at every step.

I'm not combustion expert, nor do i have SU carbs, but you're clearly not getting an efficient burn in the cylinders. Is she clackety? How's the oil? Milk under the cap?
To me, and I'm an idiot, but if the engine is tip top, Sounds like an exhaust leak maybe? She's suckin air from somewhere?

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79' Midget with a Weber Carb and more beer than gas.



Edited 1 time(s). Last edit at 2024-04-25 12:33 PM by BigRedMidget.

Unlikely to be liquid fuel out of the exhaust. Is it still present when the exhaust has warmed up? Have you checked the compression in each cylinder ?

You took it down to the garage so I assume that the timing is good.

I gave up trying to tune by raising the pistons. I think I was lifting the piston too high all the time.

My MGB has a slightly different carb but the principles are the same.
I started by checking that the base of the needle was flush with the body and setting the Jet flat with the body then unscrewing it 12 flats.
It should run at that, then balanced the carbs with a flowmeter. The old fashioned way of checking the colour and condition of the plugs gives a good indication of the quality of burn all be it not an instant check. Adjust by one flat at a time and take it out for a run. Slow method I know but it will get you in the right field.

If it not running at that , ( you mention max adjustment to get it to run, ) might be worth double checking that it was reassembled correctly.

yes thank you all,

I am just going to have to go back through it all again for the 5th time hoping i missed something silly. I am planning on ordering a flow metre next pay cheque. I was just hoping this was something someone had seen before as a specific issue saving me doing this again.

I feel your frustration with the seemingly contradictory results after all the 'tuning' experiments.

A real head scratcher, but no escaping the fact that fuel in exhaust equals way too rich.
Or as suggested... little or no combustion.

I'd start by reverting to the standard number of flats down, and checking floats for proper adjustment to bring fuel level to the correct height below jet deck.

Perhaps floats maladjusted or sinking?



Edited 1 time(s). Last edit at 2024-04-26 05:29 AM by Drewski.

Do you have the correct # needles in the carbs? The wrong needles can mess with you mind.

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Si Vis pacem, para bellum

When was the last time you replaced the condenser? Just ruling that out.

If I were taking a wild guess, then I think you have a vacuum leak and the carb jets have to be set way down to compensate for the lack of vacuum signal across the ventury. At idle the engine will be lean but at higher speed the leak becomes less consequential and the engine will be very rich at higher rpm. This is what happens when the throttle shafts are worn and we set the idle rich to compensate.

You changed the gaskets, but did you resurface or check the manifold for flatness? Same with the carb to manifold flange. They do warp. Have you checked the intake manifold at each end to make sure the metal plug is intact and not leaking?

This flange leaked, you can see why.



A vacuum gauge is a cheap and useful took when doing engine diagnosis. Lots of videos and write-ups on how to use them and what they can tell you.

I have a 1970 Midget, so might be a bit out on this. I would look for vacuum leaks particularly in the area of the suction covers,and around any emission control stuffand the oilfiller cap. Pleasedo not take this the wrong way, but are you sure it si petrol and not derv you got at the petrol station? A friend of mine got it wrong in his 60 year old Jag -it went for a while and then got rougher,but always sounded that it was about to start
Good luck
Stuart

have you considered a vacuum leak, my B had a bad diaphram seal at the Brake master/ vacuum booster

All things being equal...

Obviously, something is out of whack for it to require a full rich condition to keep it running. The only thing I did not see here is synchronization; this goes along the same line as the problem of vacuum leaks others have mentioned.

What I mean by "all things being equal" is that since you have two carbs you must think of your engine in two parts. They can run differently front and back but are connected by a single crankshaft. So, if they are not equal, they are fighting each other. All the suggestions made in previous posts are good. You must start with equal compression on all cylinders. If the compression is off due to worn rings or some other condition the engine will not draw fuel and air equally through the carbs. If the compression is equal, then you must answer the vacuum leak issue. Check the manifold gasket by using a spray like starter fluid around the head, top and up underneath. If no change in the way your engine acts then look at other places a leak can develop; the throttle shafts, vacuum hose, diaphragm in the vacuum advance on the distributor, etc. I had my vacuum advance hose get too close to the exhaust manifold and it deteriorated the rubber and developed a crack and a leak, the engine ran like crap. If you have checked the compression and it is good; for vacuum leaks and that is all good; I would check the synchronization. The carbs must be drawing the same amount of air and fuel mixture.

Start by removing the vacuum chambers from both carbs and verify that the jets are the same height to the top of the bridge. If they are unequal you must start there. I would bring them both up to flush with the top of the bridge then turn them both down two full turns (12 flats each). Mark the flat facing out with a marker like a Sharpie. Reinstall the vacuum chambers and disconnect the linkage so they the carbs act independently. Back the throttle screws off until they lose contact with the idle lever, then turn an idle screw down until it make contact again and turn it 1/2 turn more. Do this to both carbs. Try to start your engine. You may need help here as you need to turn the key and be at the carbs at the same time. You need a way to check synchronization; whatever tool you use, you are balancing the lift of the pistons in the vacuum chambers. Once you have achieved synch you can lock down the linkage so that the throttles move at the exact same time or together. Now you can work on adjusting the mixture. Some people prefer to adjust the mixture before locking down the linkage; that works too. Make sure when you turn the mixture nut that you turn both the same direction and the same number of flats. Watch the marked flat to help you accomplish this. When you have the mixture correct, make sure your idle is correct. I adjust both idle screws the same amount of turns even though they are linked together. Double check everything. Recheck your timing.

To get smooth power from your engine, all things must be equal.

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Jere McSparran
JMAC Engine Shop
Website: http://www.jmacengineshop.com
email: jmacengineshop@gmail.com
Phone: 217-232-7303