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MG MGB Technical - Will I get home tonight?

Right,

The car (70 BGT) was running fine yesterday. It did the 20 miles each way to work no problem. In the evening I did my weekly oil check and topped up the oil (it tends to use a bit). So, all was well in MGB world.

Got up the morning and it started fine. By the time I was on the motorway (1 mile) the choke was in and all was not well. The power felt down and the engine sounded rough. As I came off the motorway things got worse, with me having to rev constantly at lights to stop the engine stalling, and the engine sounding "lumpy". The last bit of my journey was on dual carriageway and again the power was poor and it sounded like there was a pulse in the engine. I have not had series problems with a car before, but I'm guessing it was almost like all cylinders weren't firing properly.

So I finally got to work, and immediately my thoughts turned to getting home later! The problem is I'm playing squash 20 miles away, and then I have a 30 mile trip home. So quite a distance to limp! I have had a look under the bonnet and don't see anything obvious out of place. I have had fuel pump problems a few weeks ago which have not reoccured, but the pump ticks happily when I turm the ignition key, so I don't think it is that (having said that there didn't appear to be much fuel visible in the fuel filter, should there be?). What would happen if one carb was not providing fuel? I have check the level in one float chamber and it was fine (I think, about 3/4 full when float removed). I will check the other side at lunchtime. I also checked the points gap, which was OK, but as I was already late I had to get to the office.

I have a reasonable tool kit and a haynes manual with me at work. So, what can I check at lunchtime? Suggestions please!

Cheers
Iain
I D Cameron

It is usual for the fuel fliter not to contain much fuel.


Check the distributor cap for dampness or tracking inside.

Check the points for gap.
Chris at Octarine Services

Whoops - didn't read that you had checked the points - that was my first thought on your symptoms - could also be a failed rotor arm or a failed condensor - have you got spares or a Halfords nearby?

If you want to talk - 0780 1576731.
Chris at Octarine Services

Chris,

Would a condensor fail quite suddenly? It is only just over 2000 mile old, so I would have hoped it would not yet have failed. The dizzy cap was dry inside.

Another suggestion has been to check spark plugs haven't got carbon build up. I'll do that at lunch.

If I'm still stuck then I may give you a call later. Thanks for the offer.

Cheers
Iain
I D Cameron

I had a similar problem at this time last year. Car ran rough, needed choke, wanted to stall etc. Turned out to be the heater valve dripping onto the king lead and dizzy cap. I guess suddenly being opened and shut after a long summer in the off position had caused the seals to fail.
steve
S Coulson

How long does the fuel pump tick when you turn the key? What I'm wondering is whether your recent fuel pump problems have allowed debris to reach one of the float chambers. If so, then maybe the needle valve isn't shutting off and so one carb is running way too rich - leading to the lumpy pulse you noticed. Trying switching on the ignition, but not starting the engine, and see if fuel drips out of the chamber overflow.
Miles Banister

Condensors can fail without warning and can even be duff from new!

Also check the coil bakelite for damp and dirt - tracking to one of the LT posts is not unusual especially in damp weather.
Chris at Octarine Services

Checked the spark plugs at lunchtime. They were covered in black soot and oil residue (despite being only 2000 mile old). Two of them broke when being removed (at the top - they had been put in to the correct torque and were removed with the correct tool) so I think they may have been a little dodgy any way.

So, where does the oil come from? Well, the compression is good and when I diconected the breather pipes (this is a UK 70 BGT with HS4 SU carbs by the way) I was able to collect a fair amount of oil (1/4 pint in a couple of hundred miles). So it would appear to be coming through the breathing system. Why? I don't know and am open to suggestions!

Fitted the new plugs and away I went, back to normal power and performance. So I will make it home tonight, although there is an underlying problem to be identified. Thanks to those who have offered help.

Cheers
Iain
I D Cameron

It is normal for oil fumes to be sucked through the breather system - that is what it is for.

Excessive oil being carried through is usually caused by excessive piston blowby, absence or choking of the wire mesh in the front tappet chest cover.

With the engine running at idle, remove the oil filler cap - the engine should falter - put you hand over the filler neck and the engine should steady and you should feel a gentle suction on your hand.

If the engine doesn't falter and you feel the engine blowing against your hand then there is excessive piston blowby.

If the suction is strong then there is a problem with the carbs breather system - are the plastic restrictors missing between the rubber hoses and the carbs?

Dirty air cleaners can also lead to excessive suction on the breathers.

Chris at Octarine Services

Chris,

I have been giving this some thought and had come to the conclusion that there are only two ways for the excess oil to get to the carbs. It must be either 'sucked' or 'blown'.

Being 'sucked' suggests to me that not enough air is getting into the carbs. I have K&N filters in traditional housings, so I'll need to have a look and see whether they are still clean.

To be 'blown' must be some type of raised pressure in the engine. However, a recent compression tes showed 175-180 psi for all cylinders. Does this preclude piston blowby?

I have recently replaced the wire mesh tappet cover, so I know this is in good nick. I'll have a go at your little test on the oil cap (which was replaced 2000 miles ago). Is this best done on a hot or cold engine or does it not matter?

"are the plastic restrictors missing between the rubber hoses and the carbs?" This is the statement that concerns me most! I have a metal pipe that comes out of the tappet cover. This connects to a rubber tube that conects to a plastic Y-piece. This then connects to a short rubber tube to the front carb and a longer rubber tube to the rear carb. I don't have any restrictors as far as I can see. Should I have?

Cheers
Iain
I D Cameron

Chris probably is better to answer this question about good Comp Tests precluding blow by. I will say, however, from experience with another car I once owned, a collapsed (broken) oil control ring expander on one cylinder gave pretty much the same symptoms as you mention. When I tested that car, comp tests were good, but one cyl. plug was the tell tale being more sooted than the others. I finally took the engine apart and there was the collapsed ring expander. I got lucky, no damage to the cylinders or pistons. However with 130K miles on the engine (a Nissan), I rebuilt it anyway. FWIW
Bob Muenchausen

Broken plugs - some time ago I broke three in one go, after never having broken any in over 20 years. Mine were Champions with the ribs on the ceramic having flat tops and sides with sharp corners, whereas older Champion plugs had ribs with radii and curves. Always used NGK or Bosch after that.

Generally all tests are best done on a warm engine, the choke and incomplete combustion can hide/cause effects.

I don't have any restrictors on my 73 either and it is the first I have heard of them. One chap recently spent weeks diagnosing excessive oil coming through the breather, eventually only replacing the front tappet cover cleared it. More recently someone else had it despite replacing that cover (or was that you?). Overfilling could cause it.

Was the other float chamber OK?
Paul Hunt

Paul, haven't yet checked the other float as the plugs fixed the running problem.

The plugs that broke were indeed Champion. Have replaced with NGK.

I have recently replaced the tappet cover with one which was definetly in decent shape before fittng.

Other suggestions for the cause of the oil getting into the breathing system welcome!
I D Cameron

Just double checked - it was the HIF4 carbs that had the plastic restrictor fitted onto the carbs before the rubber pipe.

HS4s just have the rubber pipe.

You can still suffer from piston blowby even with seemingly good compression pressures - it depends on the CR of the engine, an 1868cc engine should have pressures around 200psi so 175 - 180 would be a bit low. A 1798cc engine should have pressures around 160 - 180psi.
Chris at Octarine Services

Chris,

It is a standard 1798 (to the best of my knowledge), although it does have a gas flowed (stage 2?) head.

Just been and tried your oil cap bit. Taking it off made no notice able difference, except from more noise escaping from the hole. When I covered with hand I did feel a gentle pull. So what does that mean!

Cheers
Iain
I D Cameron

Sounds all pretty normal except perhaps the mixture is a little rich.
Chris at Octarine Services

Iain. One thing no one has mentioned to check is that the jets are returning properly to the full up position after the choke is used. I, and others, have had them stick in the downwards position. If that has happened, to could explain some of your symptoms. Les
Les Bengtson

So did you get home, Iain? !!

Mike
Mike Standring

Iain,
Les beat me to it. You said in your initial post that "the choke was in and all was not well". It could be that the jets are sticking down after the choke knob is pushed in. This would cause a rich condition and gradually foul your plugs.
Bill
Bill Boorse

This thread was discussed between 11/12/2003 and 12/12/2003

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