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MG Midget and Sprite Technical - Grey smoke & not firing on all cylinders at start

Hi,
I have recently had a brand new set of AUD662 SU carbs fitted to my MK3 midget as the bodies were rather worn on my previous set. I had tried to have the old carbs set on a number of occasions but they were near impossible to balance and set to a useable level. The car runs perfectly except on start up after being idle for a period of time, i.e. over night. When I start the car (with only a little choke), a lot of grey smoke comes out of the exhaust and not all cylinders are firing correctly. After a few minutes this corrects itself and the car runs well.

I have put in a new set of plugs last night incase it was related to a faulty plug/s as the car has been running poorly and very rich for some time (hence the change of carbs). The new plugs have not fixed the issue (well not this morning anyway.

My mechanic assumes this could be that the valve guides are worn in the cylinder head which over night lets oil fill up in the cylinder/s on the piston. Hence in the morning the excess is burnt off....

Any thoughts?

Thanks Matt.
M Lomas

Oil smoke is very distinctly blue, not grey so it doesn't sound to me like a worn guide problem.
White smoke is water (steam) which could arise first thing from condensation and mixed with smoke from an over rich mixture when using the choke it could look grey.

Slightly puzzled why you thought that the old, worn, carbs were causing a rich mixture, Worn carbs almost always result in a weak mixture.

Maybe its time to get it properly set up on a rolling road.
Guy Weller

Matt

How high is the oil level in the carbs? Some manuals indicate the oil should be above the top of the piston/cylinder and others below the top see image item 7. If you top this up above the piston it's possible for the oil to run down the side and into the venturi from where it gets sucked into the engine. Although this is unlikely especially with new carbs it's easy to test for.

Other possibilities, as Guy says, are valve guides, although I'd expect to see smoke when accelerating after a period of overrun.

Bob



R.A Davis

This could be the dreaded oil suck problem. I assume they are the carbs with the oil breather hose connected to them with a 'y'piece. There is a massive thread on this but with no conclusive answer!
Bob Beaumont

It's not uncommon for oil to find its way past worn guides/stem seals overnight, causing it to run on two or three cylinders in the morning.
Dave O'Neill2

gray smoke is a sure indication of a rich mixture...im thinking a badly adjusted choke

another thought is condenstion in the dissy cap (wd40 will cure this)

ocorrosion on the little screws that hold the spark plug wires in the dissy cap..try cleaning those and cutting the wires back an inch or so...

another thought wiuld be a sticking valve aka an exhaust will be the colprete 94% of the time because of excessive carbon build up..due to a over rich mixture over a long period of time

the primis of my thinking is if its not firing at the very start you may have fuel pooling on top of the pistons until it fires and you get the big poof of gray smoke

but I agree..if its oil, then it will burn very blue

prop
Prop and the Blackhole Midget

what is the weather like when you fire the engine up and is the car kept outside

... has there been alot of rain, is there alot of condensation /dew on every thing several hours before or during the start....the reason I lean towards this, is I always have issues on all my cars around this time of year esp if they sit for a week or so unstarted...

its warm one day and its cold the next and just moisture everywhere....its hell on my battry cables and wire connections

so thats why I questioned your dissy cap and spark plug screws in my previous post

prop
Prop and the Blackhole Midget

Thank you all for your quick comments. This is my 1st day on the forum and you have all given me great comments.

My responses are as follows:
Guy - the condensation causing factor sounds plausible because the car was left outside last night and it was very frosty over night and in the AM.

R.A Davis - The carbs were fitted by a professional (of many decades in all things MG) this week so I have not checked the oil levels in the carbs.But I am assuming as they are new carbs this may be less of a possibility...? It is my intention tomorrow to go for a drive and find a hill that I can let it overrun down and accelerate out at the bottom - this will indicate whether there is wear in the valve seals/guides and oil is being sucked in on the induction stroke. I will reply with my findings.

Bob - You assume correct! I need to investigate further as to my previous statement (R.A. Davis)

Dave - I wondered this, however, do the top of the valve guides protrude/ be proud in the head, therefore would the oil be below this level and not seep down past the valve stems during the night? I thought oil is more likely to be sucked in on the induction stroke.

Prop - I will look at the dizzy. I am also going to start the car and turn if off and check the plugs to see if I can see water droplets. I am wondering if this is perhaps due to a perished head gasket allowing water into the cylinders.... Or a combination of that and worn valve guides. In response to your second question, the car was left outside last night and it was very frosty over night and in the AM.

M Lomas

Matt,
Good luck with your investigations. I was suggesting a combination of condensation plus a slightly rich carb setting, but your road test will soon show this up.

Sounds to me like it needs an Italian Service! ;-)
Guy Weller

ahhh... id slow down a bit, and think about this a bit...youve made seveal statments that are a bit concerning

1st this is not a big issue worth losing sleep over... your only getting a poof of gray smoke and a cly or 2 isnt firing right away. then every thing works fine...as meat loaf would sing 2 out of 3 aint bad trust me, you own this car for very long the issues that will wake you at 3am will come in due time....id say, give it a wash and vac and enjoy the drive

most likely with fresh plugs and some warmer weather, will cure everything we are suggesting thats condensation related

if you had a blown head gasket you would have far more serious issues...so thats a definate non issue

as to worn valve guilds...even if it is, I wouldnt go that hassle till you had to, then at that time do a full head build with peter burguss . id be shocked if there were more good valves then good in all the 1275 heads

if the smoke is blue and it is the valve seals then that is an easy fix.. just 4 valve seals on the intake

the fact that the car spent one night outside last night is not an issue... if it lives outside year round is what trend we are looking at

something to remember also... if the engine is really cold, its going to give off alot of what looks like smoke till it warms up and some times because the fuel Is more dense in cold weather...it can have a gray tint of smoke/steam until it warms up

I just wouldnt make this an issue just yet ...esp if its curing its self with in a few minutes...it when it keeps blkwing gray smoke and missing on a cly for the entire drive weather 30 min or 12 hours

so yes... change the plugs and wait another 2 months and see what happens

by that time... im sure you will be looking at a tranny rebuild or a snapped rear axle, or rotted seal in the MC cly or frayed accelerator cable or a....ect ect hahaha

prop
Prop and the Blackhole Midget

Matt, hi,
did your mechanic just fit new carbs (as instructed), as a full set up would be check adjust - tappets, CB points, plugs, timing and mixture in that order, best done after or as part of a full service

their great debate on the type of oil and it's level in the carbs but if your going back to the mechanic you probably be best to stick with what he's put in which is usually clean engine oil or SU (20 SAE) Damper oil

John Twist (he of excellent videos) says 90 weight oil (not as thick as you might think) so that what I'm going to try next and I think I remember being told 30 years back that if when you pull the damper (as you lift the cap and rod up) and can feel a resistance that fine

for the time being if you can see oil in the carbs don't bother topping up as like the rest of the car if it's properly serviced and in good condition it will find it's own good running order level and brand new carbs should be in the best of order

your hill test should help
Nigel Atkins

I am going to surprise myself with this comment, but Prop is talking a LOT of sense!
Actually, it seems that Prop is making quite a habit of talking sense these days, I have noticed it in one or two of his other posts lately. Quite confusing!
Come on Prop, tell us what you really think is wrong with it!
Guy Weller

hahaha...

well even a broken clock is right twice a day...it happens

but nigel is correct...id think the mechanic would do a complete tune in on all systems...otherwise with new carbs, all the other settings are going to be off a bit

so i think id check with the mechanic to see what was done when he fitted the new carbs..because almost every thing will be a smidge out

this is one area nigal is dead on about a 36,000 mile tune in...definatly buy the hand book if you dont have one

prop
Prop and the Blackhole Midget

Matt,
another good one from Prop, especially as he mentioned getting a copy of the Drivers' Handbook if you haven't got one as even long term owners can learn from it - Ref: 0057 - http://www.mgocshop.co.uk/catalog/Online_Catalogue_Handbooks_5.html

in it is the schedule for servicing and a run through or check to the 36k-mile service if you haven't already done so
Nigel Atkins

I heard the Pope Horhay said a special mass, driving the demons from props body! It was an Exercism causing p soup to be sprayed everywhere! Now that he is fixed, Its time to lay hands on his cylinder head and ask the almighty for a miracle!

Oooohhh! Release the Hounds! Im ready for the shark tank!

http://www.youtube.com/watch?v=lpyg94OzHK0

I couldnt resist, My will is weak!

Oh lord maybe I need an exercism too!

Ha ha ha
Steven Devine

This thread was discussed between 21/03/2013 and 24/03/2013

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