MK 2 auto choke & missing issues

Hi, my MK2 3.8 auto runs rough at first start on the auto choke.
When the otter switch cuts out it runs fine apart from a nagging miss.
The points are new & only done about 100kms.
Any suggestions guys? Crook Plug?

In the first instance, I would check the condition of the spark plugs, and ascertain if any are not firing by disconnecting leads one by one

next, as you say it is “rough” on ASC startup, one may wish to try altering the richness of the ASC, which is done by turning the small hexagonal nut, clockwise to make it more lean.

Keep track of the position where it started, in case thats not the problem, you dont want to introduce anew problem

My ASC almost always runs a little rough, it’s usually way too rich until it gets really cold and is too lean. A few days in the year it seems to match the weather perfectly.

Check the carbs with the lift pin when the engine is hot.


Thanks Tony, I’ll check that out, sorry for the late replies.

David, I’ll check that too thanks

awg - I’ve checked the plugs with the engine running & all seam OK. (ie: engine speed & idle changes when individual leads pulled off one by one)

Adjusted the ASC hex nut clockwise 1/4 turn, so I’ll try the auto choke again in the morning when cold & see if it’s made any difference.

davidsxj6 - David, sorry, I’m not sure what you mean by the carbs lift pin?
I thought you meant the carb piston dampeners which were both a little low on oil, so I topped them up. Is that what you were referring to?


Nope, the lift pin lifts the carb piston fractionally. It’s under the body of the carb (has a weak return spring). The idea is you lift the pin about 1/8 inch (and hence the piston) at idle and the revs should rise momentarily and then fall back a little to a steady idle. If they fall, the carb’s too lean and if they increase and stay there it’s too rich. The Factory Manual and other sources describe lift pin use more fully.
Also check your distributor cap for cracks, damage etc.

Paul is right, do check the distributor cap carefully for hairline cracks. I’d wager that the misfire is either the rotor button or a sticking valve. Buying a good quality red rotor button from Martin at The Distributor Doctor in the UK is a good start. May as well get a decent condensor at the same time as most of the generally available ones are rubbish!


OK Paul, I found the relevant page in the manual, so I’ll go through & check that operation. for both carbs

Thanks Tim, I’ve not heard of the RED Rotor before or that web site but looks interesting. Freight kills us here in OZ though, but I will check the dizzy cap as well.

Pierre, just for your information, it’s OK to reply to all posters in the one response, keeps the thread a bit tidier and shorter.

Does it really? And he did that before, I‘m perfectly happy with how he replies?

Someone said confessor, meant condensor, good to have a spare. The lift pin needs to be pushed until it touches the piston and then another mm or two.Adjusting the ASC clockwise makes it leaner because the needle can fall further down into the jet, maybe that helped. One thing after another though, check the mixture first and then throw parts at it, and look at cold and hot operation as if they were completely seperate issues, for now. They might be unrelated.

Pierre is free to take the information and either use or ignore.

It can be confusing if you‘re being replied to as part of a response to several people and it never reaches you because you get no notification, so grouping several answers that are not just „thank you“ is tidy, short and useless. Maybe it’s me but a shorter thread has no inherent beauty. Anyways, way off topic.

Yes, that was me, sadly. Damned spellcheck is most unhelpful but quite funny at times! I need to read, then re-read and re-read again!:roll_eyes:

Righto understand, I’m happy to do group response guys.
After adjusting the hex nut on the ASC a 1/4 of a turn clockwise it seems to run a little smoother on start-up now.
Once hot, I lifted each carby lift pin 1/32" as per the manual & although the front pin was tight initially they are now both free in their movement with no change to engine speed.
If I push them up higher till they touch the piston the engine speed changes dramatically.
David, I’ve just noticed your post where you suggested this is the height to check them at!
So why the variance from the manual instructions of 1/32"?

You are far better off taking off the alloy air intake plenum and lifting the pistons with a fine screwdriver through the opening until you hear a change of engine speed as others have described - drop = lean, rise and constant = rich, rise and fall = just right. You can use a piece of hose to your ear and listen to the openings of each carburettor to ensure that that they have the same ‘hiss’ sound. Balance is crucial. The idle speed is the big screw and the mixture the small one as you have no doubt seen in the Service Manual.

Thanks Timothy, I’ll have another go after Xmas.
Merry Xmas all

Yep. There’s a go on YT called University Motors. Specialises in MGs. He does it with a 3/16" screwdriver as you say. Gets more control this way. I think he might be right.

Hey Pierre, amend your profile so we know where you are. I am in Sydney and Tim is in Melbourne. :slight_smile: