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MG MGB Technical - Intermittent running problem...

Hi there - I wonder if anyone can help me with what I will admit is a very vague question, but which has been driving me mad for years! I'll give all the detail I can...
I have a standard '71 roadster, the only deviation from normal is a set of K&N filters, on the car when I bought it fifteen years ago or so.
The running problem has always been with the car, sometimes happening twice a 'drive', sometimes not happening for months (but always a nagging worry). It has been getting worse - more frequent - with age though.
Driving along, the car suddenly loses nearly all power. The engine does not stop, but sounds like a tractor (probably one or two cylinders not firing). If you pull over the engine will idle very roughly if you work the accelerator, but eventually stalls. However, if you turn off the ignition and immediately re-start the car will (generally) run fine - to the extent that I've even felt the car start to 'fail', and just switched it off for a second and coasted, then continued OK without stopping.
Chasing this down, I have been through the electrics - coil replaced, new leads, dizzy is maintained, car is serviced regularly (at different garages). I have replaced the fuel pump. I have even replaced the filler cap on the petrol tank to see if it was a vacuum/breather thing - it still fails...
Short of replacing the fuel tank, lines, pump (again), filter and carbs I'm at a loss of what to do!
The only clues I can give is that it tends to happen more under load like going uphill, or if the car is cruising along a fast road for more than a few minutes - you can feel the car running out of puff. But switch it off and on again and it's fine for a while! It doesn't run brilliantly in hot weather either, but that may be an unrelated red herring.
The only ideas that I haven't tried yet are a fuel pressure regulator (but recovery is too fast I think for SU's to drain if they are flooding? Plus no smell of petrol in the engine bay or excess fumes from the exhaust, and fault would be more frequent. It's a standard SU pump), or my latest guess is if a fuel line (or similar) is failing somewhere - moving fluid inside the line reduces pressure so the line collapses, then recovers when the flow is stopped? In which case what line should I check / replace? I replaced the plugs last week - old ones were fairly black, but not overly so (short infrequent runs). Replacing the SU's is last resort due to price, so I'd like to know if they were at fault before going down that route.
Would love to get this sorted - I nearly wiped out recently overtaking a lorry when the engine died, and it really spoils the experience as I have no confidence in taking the car any distance. But it's an intermittent problem - it never does it when it's with a mechanic!
Any suggestions would be more than welcome - shout for any more info on the car or symptoms.
Thanks in advance.
R Hope

What happens to the rev counter when the engine dies?
Dave O'Neill2

Hi there - the rev counter still shows the 'true' revs of the engine, there's no wild fluctuations, just the fluctuations of a poorly running engine. The revs drop as the engine won't run properly, until the point that it stalls - the rev counter shows this.
Thanks
R Hope

It could possibly be a failing rotor arm.

The other thing to check when it dies is the fuel level in the float chambers...assuming that your carbs are HS4s.
Dave O'Neill2

How about the ignition switch - does it get hot? Could be a loose connection. Although I think the rotor arm suggestion is a good one - just don't see why switching off then on again would sort it.
Andrew Dear

I would suspect the dizzy, check the advance with a strobe. Or pull it out and check the springs and weights, replacements come in at about £50 - £70 at powerspark with electronic.
http://www.simonbbc.com/distributors/4-cylinder.

If you have an uneven idle and you have to set it high, the carbs are probably worn on the throttle spindles. If you can feel the movement with your fingers, they will need rebushing, remanufactured cost about £150- £200.
c cummins

Looking at the symptoms suggests you haven't got an issue in the ignition LT side as the tacho isn't erratic. Changing the rotor arm is a cheap item and sensible but it wouldn't explain the symptoms. Have you tried pulling out the choke when it happens? If there's a low fuel issue it may be enough to richen the mixture and prove what the problem is. I tried to think of things that would effectively be reset by switching off then on and the only thing I could think of was the fuel pump. I know you've replaced it so we'll assume it's good but the wiring is still the same. My theory is there may be a bad joint in the wiring feeding the pump or the earth wire. This could introduce a small amount of additional resistance that would drop the voltage available to the pump and not allow it to complete a stroke. Turning the ignition off and on might just give it a sufficient kick to get it started. Can you hear the pump ticking when you do a restart?

Bob
R.A Davis

It does sound like a fuel starvation problem, but not a carb related thing, so don't go spending on replacements. Have you a fuel filter fitted? A rusty, cruddy tank can allow the outlet to become restricted and hence starve the carbs. When the engine cuts the debris falls away till the next dam forms. Let the fuel level drop, jack up the N/S, remove the sender unit and drain. See what rubbish comes out!! If it is "messy" the pump may be compromised as well.
Allan Reeling

Hi there -

Thank you all for your suggestions - some good ideas there, some new things to consider too. I'll answer what I can -
1. I'm not convinced about the rotor arm, as it's an issue that's being going on for a while and the arm has been swapped a few times - but I guess could be poor quality replacements. I'll swap it again for a new Distributor Doctor item and see what happens.
2. I'm not sure how I can check the float chambers - the speed at which the car 'recovers' would imply no fault there, but I don't think I could pull over, get out of the car and round the front in time! Suggestions of how to 'fake' this check maybe? (Yes - they are HS4's, and a pair according to the tags!)
3. Ignition switch - I had an issue with indicators failing at the last MOT (last year), and ended up with a professional auto electrician first sorting an issue with a bad earth, then swapping the flasher unit. I guess there could be an intermittent fault here. I'll re-visit the electrics.
4. A fault with the dizzy - I'll run through these checks, thanks. The car idles OK - the fault only ever happens when it's running, and generally under a bit of pressure. So it could be the dizzy advance, but it would have to be an intermittent fault with the dizzy - it doesn't happen every time the car is run. Is such a fault possible / likely?
5. I have tried pulling out the choke, but all it does is flood the car faster than trying to keep the car running by flooring the accelerator and dipping the clutch.
I have thought of the fuel pump before. My first attempt at a fix for the issue was to swap the points on the old fuel pump, then I changed the whole pump - same issue, so probably not the pump itself at fault. The wiring to the pump is a possible - I'll check that. I couldn't honestly say if the pump ticks or not when it stalls - I'm normally swearing too loudly! It works fine when cold - you hear it ticking away happily.
6. Allan - I think that sounds like the most likely cause of all.
I wonder if anyone knows what happens to the fuel at the carb end if the flow is heavily restricted but not stopped at the tank / pump? If a low flow means only one of the pair of carbs receives fuel (or one carb empties as the engine draws fuel faster than it is being replenished), that would explain the 'tractor' noise and running, a trickle of fuel just keeping the engine going on one carb / two cylinders. I presume then that switching off the ignition switches off the pump, so the draw on the tank stops and the blockage falls away. Switch back on - pump works until the fuel blockage happens again.

I've got lots to work on here - I will report back, and thank you so much for your time, knowledge and suggestions. Any more comments or suggestion still welcome though!
Many thanks,
Richard.
R Hope

I think this could be a simple case of a sticking valve. When a valve sticks the engine will sound extremely noisy and also will run very rough, it will normally occur when the engine is running at higher temps and will generally free off when the engine stops and cools slightly

The only issue I have with this theory is that I would expect the tappet to come loose after a few occurances of this rattling, but I suppose if it only occurs every now and then and you do not allow it to happen for too long I suppose it is possible to not come loose.

Has the engine been modified to unleaded fuel? When in the first instances engine builders were modifying cylinder heads for unleaded operation they used the standard valve stem clearances and these proved too tight with the result that many times valves began to stick and eventually many builders recognised the need to give the valve guides more clearance to solve the issue.
Robert (Bob) Midget Turbo

Hi Bob - Thanks for your input, it's not something I'd thought of.
The engine is still running on leaded, I have always used a Castrol additive, and have only ever used that one.
(I had a '79 BGT too for a ten years, that was leaded / castrol additive too, with no running issues at all - I think I might have kept the wrong car...!).

However, I have done no work on the head since the car was bought, other than having the tappets set every now and then, so to an extent it's an unknown quantity in that the previous owner might have abused it. It's lasted well though! The fault has also been happening for years, so if it's a mechanical fault the component causing the issue must be tough.

Thanks
R Hope

Richard,

In you original post you mentioned the possibility of the fuel hose collapsing. I had exactly that happen on my 46 Chevy, and it acted exactly as you describe your MG. So, check the line on the suction side of the pump to see if there is any weak rubber hose. In my case, the hose was not old; it just was not stiff enough.

Also, I had a similar problem with my BGT and it turned out to be a loose rear number plate. The pump grounded to the same bolts that held the plate.

You said you replaced the coil, but do you know for sure that you got the proper coil to match the ballasting (or lack of ballasting) for your car?

You may well find out that Allan is right, but may as well check the easy stuff first.

Good Luck.
Charley
C R Huff

Sounds like one carb not supplying fuel to me.

You can tell if one carb is emptying from what the pump does. If you switch off the ignition while it is happening, then switch it on again and the pump chatters away, then one or both carbs is emptying, for whatever reason. If the pump makes no sound at all, but the car starts and runs normally until the next time it happens, then the carb isn't emptying. However you could have debris in a float chamber which is blocking the jet pipe. The only way to reliably determine that is to have a look, and a 71 should have HSs which is a simple matter or removing the lids and having a look. It could also be something inside the jet pipe, so not visible.

If you can keep it running long enough to disconnect each plug lead in turn and see if either the front pair or the rear pair always make it stall, and the other pair make no difference, then that indicates the carb that could be at fault. However it could also be caused by one or more plugs no longer firing, so again if you can keep it running long enough to clip a timing light (12v powered type) onto each plug lead in turn that will tell you if it is a cap or rotor problem. Could still be plugs, but by this time I assume you have had more than one set installed.

A 71 should have a 12v coil i.e. no ignition ballast, which should measure about 3 ohms across the primary, no lower than 2.4 ohms. If it's the coil, or any other ignition LT problem, then the tach will be jumping about or dropping to zero when it happens.
PaulH Solihull

Hi there from Auz.
I reckon it's your fuel pump. My B did this and turning on/off the ignition gives the pump a flash and a bit more fuel flows thru.
My experience says new does not necessarily mean it works.
Can't agree with the rotor they normally work and just give lousy performance if worn and pitted.


Trackless
J Millsom

All the information above is good and could cause this problem. The junk in the fuel tank and a perished rubber fuel line can behave like that. To add to the list is a loose muffler baffle. Does not happen often but it does happen. The baffle blocks the outlet as you drive then drops away when you turn off. Good luck with it. Denis
Denis4

Hi there

I have only just stumbled over this thread, so sorry I’m late to the discussion. I would start a new thread but it’s so strikingly similar to my own experience that I feel it’s worth adding in here.

The problem that R Hope describes is identical to mine and I can only say that I totally empathise with this owner's problem as I have been experiencing exactly the same symptoms - and some near misses too - as the loss of power is dramatic and sudden.

It's a late BGT (1980) owned for 12 years, and as an occasional fair weather driver I have been trying to get to the bottom of it on and off for around 4 years. It's running a Weber DCOE 45 (no vacuum advance) and it seems to be electrical as the fuel side looks OK.

Key remedial actions:

- I have had 3 sets of points, condensers and rotor arms through it (2 x MGOC, 1 x Intermotor)

- New HT set & dizzy cap (MGOC)

- A new fuel pump to replace a failed unit (QH electronic - Made in Germany! – MGOC)

- A ballast resistor and a new coil to bypass any effects from the length of resistance wire that's in the loom on later cars (MGOC)

Recently my attention has focused on the distributor which has some play if I wiggle the cam.

Several contributors to this thread identify the dizzy as a likely suspect for R Hope’s problem. However I am not totally convinced that this is the cause.

If R Hope’s fault (and mine) can be put down to a worn distributor why does it start and run OK and then really play up when it gets warmed up or under load?

Can anyone help explain?

Thanks in advance

Hal

Hal Ninethousand

Just to give an update, if anyone is following and interested!

I've been having a look at the fuel tank, pipes and pump. Whilst there is a bit of corrosion on the tank and inside it doesn't seem all that bad given the age of the car. Maybe it is the pipes, which all look a bit tatty to be honest. And I guess it could just be one small bit of gunge that's causing the issue.

So - whilst I don't normally agree with swapping old for new needlessly, I see mgocspares are now doing a galvanised fuel tank, so (also considering current ethanol scare stories) I'm going to swap everything from the tank up to (but not including) the carbs, with new - including a new solid state fuel pump I see the mgoc do as well.

So - I'll get that done and report back. If it fixes the issue, great. If not, then I'll (after swearing a fair bit) look at the carbs next. I'm still veering away from it being an (engine end) electrical issue just due to the symptoms and the way it clears itself. But I could well be wrong...

Many thanks again for your suggestions and comments.

Richard
R Hope

Fit an inline fuel filter, one of those little plastic see through ones just before it goes into the carbs as well as the other things.
I think you are on the right track as it sounds like intermittant fuel starvation to me.
best of luck.
Trevor Harvey

It's an expensive and time-consuming way of diagnosing a fault though.
PaulH Solihull

Before foing all that Iwould try running it with an LED across the igmition switch. if it lights up when the power goes theres your culprit.
Stan Best

Hi PaulH -

Well, yes and no! The pipework isn't that expensive, and the only way to test that is to change it over (plus it's 41 years old, probably about time) - and I need to know the pipework is OK before testing the other parts. The fuel pump may be OK, maybe not - other than changing the points (already done) the only way to test it is to fit a replacement. Swapping the tank may be overkill, but then again it's a known rot spot so I'll put that down to future proofing!
Thanks,
Richard
R Hope

"LED across the igmition switch"

The ignition warning will do that anyway. If ignition power goes while the momentum of the car is still spinning the engine, it will light up.

But in any case Richard has already said that the tach continues to read the true speed of the engine, so it isn't an ignition LT problem other than perhaps a condenser, so it must be something like HT or fuel.
PaulH Solihull

This thread was discussed between 29/05/2012 and 25/06/2012

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