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kambites

Original Poster:

39,673 posts

104 months

[news] 
Wednesday 31st January 2007 quote quote all
I have a 1974 MGB roadster (late chrome-bumpered jobby) which has a mechanical problem I have, thus far, had no luck diagnosing. It's been happening for ages now and several garages have looked at it and proudly announced that they've fixed it only for it to happen again next time I drive anywhere.

The symptoms are thus:


The engine starts fine and runs fine for somewhere between 15 minutes and half an hour (depending on what it's doing). Then I start to get a slight hesitation if a floor it from idle (in neutral)... the engine splutters and the revs drop slightly before it picks up and revs freely. This gets gradually worse until even touching the throttle lightly causes the engine to stall. Eventually it wont even idle. Once it's died it's a while before it'll start at all and hours before it'll run properly again.

Nothing is obviously overheating, the water temperature is fine and the underbonnet temperature seems normal. The fuel supply is fine and the carbs aren't getting hot so I don't believe it's vapour-locking. I'm pretty convinced that the fault is electrical but don't know how to (1) prove this and (2) work otu what component is at fault.

Any help would be much appreciated.

wadgebeast

3,856 posts

94 months

[news] 
Wednesday 31st January 2007 quote quote all
Sounds a bit like the car's overheated in the past and warped the head. Once the block gets back up to temperature again, the gasket fails causing the lack of compression, stalling and failure to run again until it's cooled down.

Gasket set is pretty cheap and it's not a hard job. It'll cost you about £40 to have the head skimmed.

Worth having a look along the side of the block immediately below the head to see if you can see discolouration from the escaping steam.

Your water levels of course remain ok, because the seal is there for that first hour of driving, and then you can't run it long enough to get the temp back up and lose more water.

Doubt if it's electrical; if it was the leads or something, it would run erratically from the start.

wildoliver

7,374 posts

99 months

[news] 
Wednesday 31st January 2007 quote quote all
Right think I know what it is.

Firstly check the headgasket, get the engine nice and hot, and look around the head-block seal and see if any water is escaping, or any sign of water escaping previously (brown residue), then switch off and let cool, check for sludge in rocker box and breather pipe (5min job) and see if water level and oil level have gone up/down, also check for scum on surface of header tank.

I think all that will be fine as I don't think it is headgasket, the heads are hard to warp in a B and generally don't cause that problem when they do.

Then for the cost change the coil and condenser(if fitted). Try and see if it has solved.

If not then I suspect it is your fuel pump. It is a notorious problem on B's, and starts to manifest itself like this, runs fine for a while, then starts to pack up, next time the engine dies switch ign on and see if you can hear pump ticking away like mad. If it is then it died and is trying to refill the float chamber as the change in PD from being switched on and off has jerked it back in to life, if there is no sound from it at all open boot and tap it with a screwdriver handle, firmly but not hard, see if it restarts, if it does then this is the problem.

Come back after trying that.

kambites

Original Poster:

39,673 posts

104 months

[news] 
Wednesday 31st January 2007 quote quote all
Headgasket: possible, I'll check this evening. I replaced the head about 5 years ago with a reconditioned (peter burgess) one. I'll check the torque on the head bolts and look for leaks, etc. and repost.

Coil/condenser: The last garage I took it to replaced these and it didn't solve the problem although they said it did allow them to restart it immediately after the replacement. That's what made me think it was perhaps something else in the electrical system damaging or overheating the coil.

Fuel pump: I don't think this is the problem - if I look at the fuel filter (it's a glass cased in-line one) there is fuel in there, although there's some gas (air?) too. The other thing that makes me doubt this is exactly how it fails. It initially stutters at low revs and full throttle which sounds like over-fueling (or a weak spark) rather than under-fueling to me.

wildoliver

7,374 posts

99 months

[news] 
Wednesday 31st January 2007 quote quote all
If it's the fuel pump then it will start to stutter at low rev's and allow you to rev it up and then die.

The fuel filter depending where it is mounted could have air in it all the time.

My money is on the fuel pump.
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wadgebeast

3,856 posts

94 months

[news] 
Wednesday 31st January 2007 quote quote all
A fair point with the fuel pump. I've only had to replace it once in the last 10 years, but it was a much more erratic failure than yours. Hiccuped a lot for no reason, finally packed in on the same day. But yes, the old ones (especially mechanical pumps) aren't renowned for reliability. It's not a hard job (just dirty) and not particularly expensive either.

As I posted earlier, it's definitely worth checking the head - I had exactly the same symptoms a few years ago and that's exactly what it turned out to be.

kambites

Original Poster:

39,673 posts

104 months

[news] 
Wednesday 31st January 2007 quote quote all
The fuel pump is an electrical one mounted under the rear wheel arch and it's not very old. The fuel filter is mounted in-line, about a foot from the carbs.

wadgebeast

3,856 posts

94 months

[news] 
Thursday 1st February 2007 quote quote all
My fuel filter is in the same place as yours and always has a small bubble in it. Doesn't cause any snags at all.

If it was an electrical fault, it would be there from the start and not just when the engine is warm. It could be a mixture problem, but I tend to get someone knowledgeable to sort that out for me as it's less frustrating in the long run.

Did you have any luck seeing if it was a head gasket failure?

bikemonster

1,188 posts

124 months

[news] 
Thursday 1st February 2007 quote quote all
The car runs fine when it's cool but dies when it gets hot. Has the mixture/float levels been looked at?

If they have then you may have a coil or condensor that is giving you a hard time.

Start with swapping out the condensor and see if that sorts out the problem.

After that, try borrowing a known good coil.

It would also be helpful if you told us what the garages who have looked at the car have acshly done.

GreenV8S

23,471 posts

167 months

[news] 
Thursday 1st February 2007 quote quote all
An ignition fault causing a weak spark could result in a misfire that gets worse as the load increases, some of these faults do tend to get worse with increasing temperature. However, the misfire would continue as long as you kept the load on rather than just being transient. The transient nature of the symptoms to me suggests fuelling, and the fact it happens when you open the throttle suggests under fuelling.

The Burger

2 posts

52 months

[news] 
Thursday 25th March 2010 quote quote all
Hi,
I wonder if you ever got to the cause of the problem ?
I bought a 1977 MGB about 6 years ago, and I have had the same problem.
The car has been in my garage, as I have had no luck sorting it out, A
Local garage said they had and they had not.
Yeterday I started playing with the car again, and swopped the Coil twice, But with another coil, after 15 - 20 mins the problem returns. At 1st I thought perhaps someone had bypassed the Ballast resistor wire, but I checked the Voltage, and it is correct, for starting and running.
My Fuel Pump due to lack of use needed a Kick, but when the Fault appears, banging the Pump Does Not eradicate the problem. I have taken off the Fuel Filler cap incase it was a Vacuum issue. Someone a while ago told me to Bin the HIF 4 Carbs and replace them with HS4's , however There are a good many post 74 care working fine.
The one strange thing, is If the car stops and will not start, replacing the coil causes
the car to start and run normally. Can 3 Coils be Faulty ?
I am baffled, the next check I will do is to see if the Ignition Relay is failing to Hold-In
maybe due to its coil failing after a period of time, therefore depowering the ignition LT circuit.
re the Comments on a Head problem, During the Dealer visit, the car had a Lead Free Head fitted.
I hope you finally sorted out your car, and can give me a Clue what maybe wrong with mine.


The Burger

2 posts

52 months

[news] 
Sunday 28th March 2010 quote quote all
Hi,
Someone suggested swopping the Rotor Arm, I didn't think this would work as the Distributor was replaced by the MG Specialist I sent the car to.
Anyway, It seems to have sorted out my problem.
The original Rotor Arm looks Perfect, I can see no signs of Tracking.
BUT there is a lot of info on the I/Net re Poor quaility Rotor Arms.
There is a USA company making higher quality ones, and the UK Distibutor!!
is " The Distributor Doctor "
I shall be ordering one of those tomorrow.
I hope this can help someone else who has been plagued with this problem, as I have.
I wasted many hours, and money assuming, as it was new it was ok.
Regards
Keith
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