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MG MGB Technical - Yet another GT with rough running

Can anyone give me their thoughts on this...

My GT started to misfire today and within a few miles had got really bad.

I swaped out all the usual ignition parts as I had just brought some for a service anyway, but this did not help.

On pulling the plugs they are all sooty, I assume pointing to rich running. It was getting dark by this time so I thought I would check the damper oil as it was about all I could see. the rear carb was ok but the front was dry. I topped them up but if anything it made the problem worse.

I pulled the plugs again to check and they were agan sooty.

I pulled each HT lead in turn but they all had the same effect on the running, no one stood out.

The misfire is worse the higher the revs. It starts and idles fine.

Any pointers to my next move? I have to drive the boring car to work in the morning!

Thanks

Graham
G Lewis

Graham. Is it cool enought to require the use of the choke when starting? If so, I suspect the choke has stuck in the downwards position on one, or both, of the carbs, causing an excessively rich mixture. Several of us have had this problem over the years. Pull the suction chambers and the pistons and physically measure how low the jets are currently. Anything lower than about .055" indicates the jet may be sticking.

Les
Les Bengtson

Thanks Les.

I'll check this when I get home. If this is the problem how would I go about fixing it? I am a total novice when it comes to carbs (but a fast leaner).

Yes it does require the choke to start, but it can be pushed in only about 30sec after. (its always cool enough in England lol).

It becomes my main car on Monday so I really must sort it this weekend.

Cheers

Graham
G Lewis

PS: some more info, there is no obvious smoke.
G Lewis

Ok. All ignition parts checked again, dizzy stripped checked cleaned etc.

Carbs checked and seem fine, though the spring which seems to back off the choke was not connected at the bottom, though connecting this has had no effect.

Timing checked and set with strobe.

Still the same problem. It will drive ok for 100 meters or so then get worse and worse.

Last thing I checked was looking at the spark on the plugs. It seemed very yellow and weak. It was the same on all cylinders.

So now I am thinking its the coil? Could this be right? It might explain the soot on all plugs not just the one, is that right?

If anyone could give me their thoughts on this it would be great.

Thanks

Graham
G Lewis

Condenser?

They always seem to be the culprit in the end.

Or is there any chance the fuel pump is oversupplying?
Andrew McGee

Graham,
Put in a new, good quality condenser.
I had similar issues years ago with a Firenza on the road home from Sheffield to London on a Sunday night. It wouldn't run above 50 on the motorway on the drive home, but had run up on the Friday night at 70+. By the time AA got to me I had cleaned the plugs, re-timed the ignition, etc. Chatting with the AA guy he told me about problems with the exact (cheap) condenser I'd just put in! He didn't have a spare, but I managed to get an old used one from a guy in the next services that I stopped in. That fixed the problem.
Cheers
(another) Andrew
Andy Taylor

Thanks for the replies Andrew and.... Andrew.

I have tried 2 condersers with no change at all to the problem.

Every thing on the ignition other than the coil is now new, but with no change. I have also cleaned and tightened all connections I could lay my hands on.

Cheers

Graham

G Lewis

Graham

I hate to confound you but I put TWO new condensers into my midget not long ago with no improvement to a similar problem. I gave in in the end and trailered the car to my local rolling road (and the guy who built the engine 14 years ago).

Onto the diagnostic machine and presto, the spark pattern was all wrong. In went a new condenser and lo and behold a correct spark pattern was showing, the car ran superbly. The we got it back on the rollers and gave it a thoroughly sorting.

I will change it to Electronic Ignition (not an add on unit) that is built into the distributor as and when I can afford to. The race Midget runs one of these systems and I've never had any ignition issue with that.

Sorry, but I think you should try another condenser.

BTW, my everyday BGT got swapped onto a fully electronic distributor the first time it started to play up as like you that's my everyday car.

But if you want someone to blame, I recall I was one of many who said for you to get rid of the modern, depreciating lump of tin and get a classic - stick with it, even though they can be really frustrating it's worth it in the end.

Good luck

Andrew
Andrew McGee

Andrew,

Grrrr it was your fault....lol only joking. This is one of the things I was looking forward to if I am honest, its like a puzzle that needs solving.

I drove my modern car to work this morning. It was so boring....No exhaust note, no skillful gear change, and worst of all not one head turned as I drove by! I am in love with the GT, warts an' all.

I'll pick up another condenser in the morning. Nothing wrong with having a spare even if it makes no change.

Thanks again

Graham
G Lewis

Graham, I take it you are using propper fuel in your car? I use shell Optimax (now V-power) exclusively in my car.
JC

JC, I am using unleaded with castrol aditive. I tend to use Total fuel as its the station on the way to work.

I have only had the car 4 weeks, its been running quite well the whole time until this week.

G Lewis

Perhaps it is not an electrical problem, but rather one of fuel delivery? Check your float levels and for debris or water in the float bowls. If a float valve is getting stuck open, even just a little, it can cause fuel level and fuel mixture problems.

Also, check out your fuel pump delivery. Disconnect the fuel line at the carbs and use a jar or can to catch the fuel as it is pumped out. Use a fairly large jar or can as you need to have a delivery rate of at least a pint per minute. If far more, you might wonder, especially if you are running an SU pump.

SU pumps are pretty low pressure and delivery relatively lackadaisical. If an old SU pump, there may be some issues and perhaps you will get lucky and get a response from David Dubois of this BBS. He is our resident SU pump expert.
Bob Muenchausen

Graham. Yes, it is possible that the coil is at fault. The easiest test it to install a new coil. However, since you will be reading this tomorrow morning, your time, please allow me to suggest to you my website, www.custompistols.com/ which contains a couple of excellent articles on the fuel system by David DuBois, which may be of use to you if, as Bob M suggests, you have a fuel system problem. Dave's e-mail address is hot linked to the article(s) and he should be able to answer any fuel system related questions you may have. I have some tech articles on trouble shooting the ignition system and spark plug wires which might also be of benefit to you. A weak spark can be either a bad low tension ignition problem, a bad coil, bad plugs, bad wires, a bad rotor (very common, especially with Lucas rotors which are grounding out to the distributor shaft) or a bad distributor cap.

The articles available should, I hope, allow you to narrow the focus of your investigation.

I understand, since my MGB is my daily driver, what it means to not be able to start the car to get to work. I will use mine to get to work tomorrow. Please contact Dave through his hot link if you find any fuel system problems. You, should it be necessary, can contact me either through my website or by posting here. I will check again tomorrow night.

Les
Les Bengtson

Just my 2 cents, of course. I think it's running rich.
Tom

Thanks for the replies. Even as I sleep there are people on the otherside of the world helping me, don't ya' just love the web.

Les I had actually been reading your site last night. Its got good stuff on it.

I just looked up the part No's for the coils on the midget and the GT. They are the same, so I'll swap them and test that after breakfast. If not I'll do some more reading and crack on with the fuel system.

Cheers

Graham
G Lewis

Right, I swapped the coil with the one from the midget(currently in restoration) if anything the problem got worse. As I drove back into my drive my neighbour from down the street(also a midget owner in even longer term restoration)asked abot the problem and suggested I try another coil anyway as he had one.

It was a renult one but the ratings were the same. Put it on and guess what the problem has cleared up! The car is a little sluggish, so I am going to clean the plugs and check the timing again later.

It seems a weak yellow spark and sooty plugs might mean your coil.

Thanks for all the help everyone.

Graham
G Lewis

Graham

I'll have alook at my coils this afternoon (race Midget, road Midget and BGT) I think that they are all high performance ones - or maybe that's just some marketing ploy to get me to spend more money.

I know one of them has a black body instead of the usual ally finish (oh no I sound like my wife now - what kind of car was it - a red one!)....

Whether they give out a bigger kick to the distributor I'm not sure, but they do seem to work better than the other ones did.

Andrew.
Andrew McGee

Andrew,

I need to buy a new one now, so I might go for a sports coil(a gold one). Thinking about it I need to buy two as the Midget one is also dead.

MOT in a week, at least this is sorted as the unburned fuel would not have been too good on the emmisions I'm sure.

Graham

G Lewis

As far as I recall the emissions test for our cars is a visual one and will only fail if there are clouds of smoke coming out of the exhaust. Anything else should be waived.

So if the tester fails you on emissions I would question his judgement and ask him to check his test procedure.

But make sures they cleared you on everything else first.

Andrew.
Andrew McGee

OH NO! I am cursed...See my new thread 'No compression on 2 and 4'

Graham
G Lewis

Sorry Graham, that's way beyond my comfort envelope.

Let's hope the pistons and valves have touched.

Keep us informed

Andrew
Andrew McGee

Well, the head should be off tomorrow andrew, I'll let you know.

Just re-read - you said you hope they have touched! It's all your fault again! haha.

I'm on teh beer now and removing spiders for the wife.

Graham
G Lewis

Oops sorry, wouldn't have wished that on anyone!

Especially after more than likely destroying my race Midgets engine last weekend at Zandvoort.

Although I won't be investigating that one either, it will have a long journey back to Folkestone for examination.

Crossed fingers on both accounts, yours an dmine.

Let us know.

A.
Andrew McGee

Good luck with that Andrew. DId you have a site with your race midget on? I think I remember seeing it? I am still in two mind if I should build mine to race. Not sure I have the time or money just yet though.

Just been looking, engines are not cheap! I have rebuilt a montego engine in the past, it ran lovely afterwards so I know I have it in me. I am no expert (yet) though.

Graham

G Lewis

Have a look on the Midget BBS and the two threads that relate to Zandvoort have a load of stuff in there about the racing.

We were out at Zandvoort racing with FISC EuroTour in my one and only outing this year, I'd previously raced with them for the last five years.

http://www.fisc-europe.org

In a nutshell, I qualified third, on the fourth lap of the first race I challenged for second and the brakes failed (big time) and I ended in the tyre wall. Brakes repaired (and front of car) for race two where I started 20th, at the beginning of lap two (I was told I was 12th) the engine went off, game over!

Being honest though, I tried running the race Midget onthe road when I first started out, it isn't really practical, hence why I built a road Midget (using some tweaked bits out of the constantly evolving race car). It more than likely is a one way trip if you do convert it to race spec.

regards

Andrew.
Andrew McGee

Graham. The week spark sounds like ignition wires, bad rotor, or bad cap. Check all of them, but especially the rotor. Lucas had a bad batch a few years ago and they are still in the supply system.

Glad it is running better, if not perfectly. Time and hard work will make things better.

Les
Les Bengtson

This thread was discussed between 14/09/2006 and 17/09/2006

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