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MG MGB Technical - Engine dies after 20 minutes

I'm a little rusty on my MGs just now and would appreciate some tips. My '67 Roadster didn't get to its MoT as the engine coughed to a halt after 20 minutes of idling whilst I was checking it over, and wouldn't restart.

I overhauled the LT/HT side of things (points, condenser, dizzy & leads, timing), it started and ran fine, but died in the same way a little later and wouldn't start again. Dwell meter showed the points operating properly, tach registered normally, and after cranking plugs 1-2 were wet, 3-4 dry.

It's running HS4 carbs so I opened the rear float bowl which was full of petrol, blew through the vent pipe and the jet with compressed air but both seemed clear. The piston and needle were moving smoothly.

Before I reassemble, am I missing something?

Many thanks,

Steve



SteveP

Do you have a filter fitted to the fuel system? Cars, before '68 had no fuel filter from the factory. It's easy to pick up small particles up from the bottom of the fuel tank and hang up one of your floats. Also, make certain that you are getting a spark after the engine dies. You may have an ignition coil on its way out. RAY
rjm RAY

This sort of thing comes up often and you always have to ask "Have you changed them fuel tank cap?" The cap must be ventilated. That is the only way air can enter the tank to replace the fuel you have used. Twenty minutes would be about right for the pump to stop being able to draw fuel against the vacuum in the tank. If you remove the cap, does the car work alright?
Mike Howlett

Steve,

After 20' when it dies have you verified if sparks at plug still present? If no it could be a faulty coil overheating..
After a delay when cold again it will start easily
Cheers,

Jean G.
Jean Guy Catford

OK, here's what I know having watched until it died this afternoon:

There is power to the coil throughout

The dwell meter shows an ever reducing dwell in the couple of minutes before the engine dies (from 60* edging down to 10*). The points gap looks unchanged on opening the dizzy.

There's no spark and the engine will not restart for about 10 minutes then starts first time (front and rear plugs are wet after cranking since working on the rear carb)

Same results using a spare coil

I'm officially clueless!
SteveP

the fact that it recovers after a while suggests it's not the points gap (a mechanical setting) but something electrical. A dwell meter is in fact measuring the "average voltage" across the points and a reducing dwell or an increasing voltage suggests the points aren't earthing correctly when closed. When it fails disconnect the -ve side of the coil and measure the resistance from the cable to earth (points closed) and then follow the path to earth measuring at each connection point e.g. input to dissy, internal cable to points, points to base place and the base plate earth cable. Most likely is a faulty set of points, the base plate earth cable or a very strange condenser fault.
MGmike
M McAndrew

should have added.
if the resistance is NOT zero you have found the problem and just need to find the cause!

MGmike
M McAndrew

I have had points that go "gummy" as they heat up. Either the plastic itself changes characteristics, or the "lubricant" on the pivot post turns to glue. Result is that the points close ever more slowly until not at all. Once it cools it all goes back to normal. All matching your meter readings.
The old phenolic points were easy to clean and relube, the unitized ones less so, hence the name morph from "Quikfit" to "Quikacrap".
Same thing can cause high rpm failure, or failure to attain high rpm.
A drop of thin oil on the pivot may help, at least temporarily. I wash them in Brakleen or similar, let dry, and use a graphite oil.

FRM
FR Millmore

Do you have a new black rotor arm in your sissy as the black ones are terrible and arc after so long. Put your old one back in if it's new and retry again. Only ever buy the red rotor arms as they are made of a different material and don't cause problems.
Ste
Ste Brown

Thanks everyone. I'll try to get some more work done on it later.

I've changed rotor, points & condenser so I guess they are less suspect, if not proven innocent. Good to know the red ones are worth using, I'll invest.

The idea of the points earth makes a lot of sense. I'm thinking the averaging effect of the meter would explain why it slumps to 10* before it stalls but when I crank it just afterwards it shows normal dwell again but won't start.

Steve
SteveP

Did you change the condenser before or after you started getting the fault. If before, maybe that is the problem, as the quality control on them is pretty poor these days
Graham M V

I've built up a replacement dizzy and have just had the engine running for an hour. I'm sure MGmike is right and strongly suspect it was the earth connection from the baseplate to the body at fault.

The car has only done a few miles over the last years so I'm sure I'll be back with more gremlins but thank you everyone for your help on this one.

Steve

n.b. You've all heard this before I'm sure, but I put the all new distributor together with new cap, rotor, points & condenser. The engine never fired, in three hours of trying. In despair I filled it with used parts lying around from years ago; the engine was running smoothly before I'd finished turning the key. It does make diagnosis difficult.
SteveP

Similar issue this year on first start. Not sure if applicable but on this side of pond, 10% ethanol standard and fuel stability problematic. Entire complete fix was siphon out all fuel, add 5 gallons (American) fresh gasoline (petrol) and off I went. Just a suggestion worked for me without a lot of work or money
Jon Levine

I am also frustrated by the same symptoms experienced by our other MGB drivers. Mine is a 1980 LE roadster and after many months I am still struggling to find a solution. I have already replaced the plugs, leads, complete distributor (now 123 electronic type t eliminate points), ignition coil, ignition relay, petrol pump, fuel filter and checked the HIF carbs for free flow but still the car runs for two or three miles before spluttering starts again.

However, someone mentioned sticking valves which made me wonder. When the engine cools down it makes an occasional 'click'. I had put this down to contraction somewhere but could it be a stuck valve returning to its rightful position? I am using unleaded fuel with Valve master additive. If stuck valves it is, an unleaded head conversion would probably be justified. Anyone had this happen?
J.F.G. Brown

Steve,

Did you sort your problem?

My MG garage mechanic reckons mine is more likely to be electrical than valves. I replaced original distributor by electronic 123 type because new points on the market do not allow enough clearance between the spring and the fixing bolt so allowing LT short to earth. Temporary repairs with insulation tape to get you home dont last long.

I'm still baffled at present!
Joe
J.F.G. Brown

A 1mm hole in the fuel filler cap and a red rotor arm fixed my car last year. After hours of faffing around I still dont really know what the problem was. I read somewhere else that in fact the black arms are too short and the spark finds it easier to short to ground via the rivet inside the dissi cap rather than where its supposed to go.
Stan Best

The click you hear might be the relay that controls the running on. It is operated by the residual oi;l pressure on shut down.
ss sanders

Anyone with a classic car that runs rough, cuts out or generally misbehaves when hot should try a new rotor arm before doing anything else. In fact try a couple before delving any deeper. Modern rotor arms are rubbish & loose their resistance when they get hot. I have seen this time & time again over the last ten yrs or so.
G Britnell

J.F.G., yes, the replacement distributor has done the trick in my case with good running restored and a couple of hundred miles covered to confirm.

Steve
SteveP

This thread was discussed between 29/06/2012 and 27/07/2012

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