Mgb cutting out when hot, out of ideas!

Mgb cutting out when hot, out of ideas!

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Dbest92

Original Poster:

300 posts

132 months

Wednesday 24th April 2013
quotequote all
Hi all!

Picked the car about a fortnight ago, ran lovely for a week/100 Miles no probs, now, however recently after you've been driving say 15 miles, it will suddenly lose power and start to misfire, then slowly dies until it stalls, or will idle on 2ish cylinders for a bit. Leave it 10 mins and it will go on fine for 2-3 miles and then same again!

So far I've replaced; coil, condenser, points, low tension lead. fuel systems and carbs have been stripped and cleaned, fuel pump and red rotor arm were installed recently, has been timed and tuned etc. first time it broke down the rev counter was all over the place, then replaced ht components (conks out same as before) but the rev counter works as it should!

No signs of oil water mix or pressurisation in the radiator, as did think of head gasket?

Any ideas would be greatly received!

Darren

Mr2Mike

20,143 posts

254 months

Wednesday 24th April 2013
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Fuel tank air vent blocked perhaps?

andym1603

1,798 posts

171 months

Wednesday 24th April 2013
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Could it be suffering from carb icing or fuel vapourising from high under bonnet heat?

The Black Flash

13,735 posts

197 months

Thursday 25th April 2013
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Timely post. Mine did exactly this to me last weekend. Fine for 10-15 miles, then starts missing, getting progressively worse until it dies completely. Leave for an hour and it's fine for another 10 miles.
Something similar happened last year so I replaced all of the HT leads, distributer cap, plugs, rotor arm which seemed to fix it, but now it's back.
I limped it home and started looking around, and found that the vacuum unit on the distributer had gone. Dunno if that's the cause of it all, but it's certainly going to have a major effect! New one arrived today, so I'll fix that, retime it and see if it solves it. Worth a look for the sake of pulling off a hose and sucking on it.

nta16

7,898 posts

233 months

Thursday 25th April 2013
quotequote all
Dbest92 said:
it will suddenly lose power and start to misfire, then slowly dies until it stalls, or will idle on 2ish cylinders for a bit. Leave it 10 mins and it will go on fine for 2-3 miles and then same again!
could be loads of things, more info needed

you can either do the step by step diagnostic, such as - does the rev counter twitch when the car misfires?

or the try loads of stuff on the limited info without diagnostics, such as -
vent to fuel tank - as soon as it plays up, pull over safely and immediately remove the petrol filler cap, if the running improves you have your answer, but do replace it again


Dbest92 said:
So far I've replaced; coil, condenser, points, low tension lead.
why did you replace these? particularly the condenser? (you were warned about the poor quality of new items)
and which low tension lead?
did you follow the diagram exactly in the DH for fitting the CB points and condenser the correct way to post?
what about the dissy cap, inside and out?
what about checking the HT leads for condition and fit?

Dbest92 said:
fuel systems and carbs have been stripped and cleaned
and lubricated? new gaskets? any missing gaskets?


Dbest92 said:
fuel pump
have you checked filter and connections (a small leak could suck air in instead of leaking out)
did you replace any of the fuel line?


Dbest92 said:
and red rotor arm
from what source?


Dbest92 said:
has been timed and tuned etc.
were the tappets adjusted?
any chance of mistakes?


Dbest92 said:
first time it broke down the rev counter was all over the place, then replaced ht components (conks out same as before) but the rev counter works as it should!
rev counter judder normally means problems on the LT side, you've replaced a bit of each, see above - does rev counter judder at misfire still?


Dbest92 said:
Any ideas would be greatly received!

Darren
you've changed a lot of items at once, when trying to find the cause this makes it more difficult

btw, there's a good diagnostic flow chart in my Porter Step by Step servicing book I'd assume (might be wrong) the same is in the MBG version


Rob Dicky

206 posts

222 months

Thursday 25th April 2013
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Had a similar problem years ago with a Nissan Micra, it ended up being a collapsed silencer box. The baffles had collapsed and closed up the exit pipe, the silencer was only a few months old. Problems eventually got worse until the engine would only run 10 mins then stall and would have to wait 30/40 mins before it would re start.

nta16

7,898 posts

233 months

Thursday 25th April 2013
quotequote all
a couple of scatter thoughts:

have you a lot of oil leaks or lot of oil consumption?

any coolant leaks, like from heater valve?

the second one is less scatty

as you see without step by step diagnostics you could look here, there and everywhere - there could be many solutions offered but odds are that very few will get to the cause without further information and investigation

don't give up come back with replies to above particularly if the rev counter still judders with misfire then all can progress from there

nta16

7,898 posts

233 months

Thursday 25th April 2013
quotequote all
another thought as it hasn't been mentioned and as it's lighter nights -

as well as looking at rev counter for judder at misfire

and pulling over somewhere safe to so then taking the petrol filler cap


if that don't work and the engine has stopped by itself because of the problem then take the plugs out one at a time and note the colour of each and it's cylinder number and whether the plug is wet or dry

(number one cylinder is the one closest to the cooling radiator)

if it was very dark you could lift the bonnet with the engine running and look for stray sparks

Dbest92

Original Poster:

300 posts

132 months

Thursday 25th April 2013
quotequote all
Ill look into the fuel cap, over the weekend am going to take the car onto some quiet roads to do some investigating when it happens, interesting regarding the vacuum unit, ill take a look

I've kept the old condensor but it was changed just so I could rule it out, points were ok so seems both condensers are ok, I took it back to the mg specialist who did the points so pretty certain they were done properly, fuse box was also cleaned. I've borrowed another cap and leads so will try those at some point if the fault persists. Regarding the low tension lead, it's the one that attaches to the distributor body to the points post, was in a very poor way!

Carbs again were done by the mg specialist so again would be pretty sure they were done properly. Fuel pump was new when I got the car 2 weeks ago, and the garage tested then fuel flow and was very healthy, but I've not completely ruled out the fuel pump yet.

Again the rotor was from the garage, I did query it and it is one of the decent ones,

The very first time it happened the rev counter went nuts, however recently the rev counter has been steady and shown the true engine speed throught the misfiring etc.

No oil and water mix or consumption at all, both are level and clean, I am trying to go through it methodically to rule things out.

Interestingly when I started the car today the wipers, fan and radio didn't work, these haven't been used since the problems started so can't confirm then this problem started, but wondering if they're related. I found the inline fuse, replaced it with a 35A and nothing worked and the car wouldn't start, or even turn over. A new 25A and car started but no wipers etc. cleaned the original and wipers and all are back, haven't road tested it yet to see if the fault remains.

Will also add that the engine can take a while to 'catch' from cold starts

I do intend to try a few things out when the fault occurs again so rule things out

Thanks smile

Darren

Edited by Dbest92 on Thursday 25th April 23:45

nta16

7,898 posts

233 months

Friday 26th April 2013
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Dbest92 said:
interesting regarding the vacuum unit, ill take a look
see John Twist video on testing dissy vacuum


Dbest92 said:
I've kept the old condensor but it was changed just so I could rule it out, points were ok so seems both condensers are ok,
maybe, maybe not, difficult things to test - condensers do go wrong but not often, same as CB points they are not part of the annual service, I think one sign of condenser fault is CB points pitting/melting - and misfires biggrin


Dbest92 said:
I took it back to the mg specialist who did the points so pretty certain they were done properly,
horrible nasty things, need checking again after fitting to check they not moved, they also sometimes need lubing, certainly the dissy cam needs lubing - have you checked the points gap?


Dbest92 said:
I've borrowed another cap and leads so will try those at some point if the fault persists
as long as they are definitely known to be in full working order - I've seen a number of times spares given or lent by others that are no good, especially car batteries and the saps spend hours trying to use a poor battery and getting nowhere


Dbest92 said:
Regarding the low tension lead, it's the one that attaches to the distributor body to the points post, was in a very poor way!
I thought that's what you meant - a sign of other things do you think(?)


Dbest92 said:
Carbs again were done by the mg specialist so again would be pretty sure they were done properly.
good but something could have gone since then

the carbs are best set at the end of a full service and after tappets, CB points, plugs and timing - a good clean and check of linkages and all gaskets present

Dbest92 said:
Fuel pump was new when I got the car 2 weeks ago, and the garage tested then fuel flow and was very healthy, but I've not completely ruled out the fuel pump yet.
well it's easy to test as you now know


Dbest92 said:
Again the rotor was from the garage, I did query it and it is one of the decent ones
good because they can be a pig when they're not


Dbest92 said:
The very first time it happened the rev counter went nuts, however recently the rev counter has been steady and shown the true engine speed throught the misfiring etc.
that suggests it's probably either HT side or fuel


Dbest92 said:
No oil and water mix or consumption at all, both are level and clean, I am trying to go through it methodically to rule things out.
sorry Darren but this wouldn't be part of a methodical diagnostics, it would be doing what I and others have done which is jumping from one thing to another

if you can get a copy of (at a sensible price, sometimes eBay) the Porter Manual, if it’s like the one for the Spridgets then it’s very good and comprehensive but might have a few typos in specifications – MGB Step by Step Service Guide (Porter Manuals) –
http://www.amazon.co.uk/MGB-Step-step-Service-Guid...
it's excellent for the servicing and (if same as Spridget one) diagnostic flow charts

even Haynes has fault finding at the end of each chapter that will take you through in an order


Dbest92 said:
Interestingly when I started the car today the wipers, fan and radio didn't work, these haven't been used since the problems started so can't confirm then this problem started, but wondering if they're related. I found the inline fuse, replaced it with a 35A and nothing worked and the car wouldn't start, or even turn over. A new 25A and car started but no wipers etc. cleaned the original and wipers and all are back, haven't road tested it yet to see if the fault remains.
I might be wrong so check your diagrams but I'd have thought each of those had it's own in-line fuse, you have a note that explains the difference between modern and traditional glass fuses

crud on fuses, fuse clamps, the fusebox terminals and connections are a big restriction on the full electricity getting through so like all the earth connections and wires, leads, switches, connectors - all need to be clean, secure and protected

perhaps something common to all three of those items(?) is that they are live from the ignition on(?) so possibly a fault with ignition switch or wiring or connections

some people fit the fusebox(s) upside down on Spridgets so the link underneath goes to the wrong circuits and this causes all sorts of funnies but I don't know what fusesboxes you have on your car


Dbest92 said:
Will also add that the engine can take a while to 'catch' from cold starts
could be fuel problem, could be electric (see notes on battery and connections and leads/earths, could be something(s) not set up properly

don't forget to check your plugs as detailed in previous post


Dbest92 said:
I do intend to try a few things out when the fault occurs again so rule things out

Thanks smile

Darren
great stuff, remember -
RTfM,
check the bleeding obvious,
keep it simple,
don't forget the basics,
don't assume,
check previous work
check again what you've already checked

make a note of results and try to follow one path through to the end before trying another

good luck

Edited by nta16 on Friday 26th April 07:13

nta16

7,898 posts

233 months

Friday 26th April 2013
quotequote all
nta16 said:
a couple of scatter thoughts:

have you a lot of oil leaks or lot of oil consumption?

any coolant leaks, like from heater valve?
I forgot about these, they're not for HGF

heater valve leak was from others having leaks from the heater valves that run, dripped or splashed on to the dissy cap if I understood it correctly

jimbob82

690 posts

133 months

Friday 26th April 2013
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my guess would be points gap incorrect...smile

chormy

635 posts

195 months

Saturday 27th April 2013
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condenser !

Lagerlout

1,810 posts

235 months

Saturday 27th April 2013
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Get a automotive multimeter and check the dwell of the points, from memory it should be c. 60 deg on a B.

I think it sounds like a breakdown in the HT circuit.

This can occur for a number of reasons.

What is the year of the car?

What Coil are you running?

v8250

2,724 posts

210 months

Sunday 28th April 2013
quotequote all
OP, check your fuse box. MGB boxes can throw up some spurious faults. I had the same fault symptons as yours some while back - it's caused by the following:

1. Check [close up] the fuse connectors for a good tight fit.
2. Renew the 4no fuses + the inline fuse. Glass type fuses can read 'good' when checked for continuity but in fact have intermittent contact between wire and end caps.
3. Remove fuse box and check rear for any movement of contacts - these can become lose over the years.
4. Check the insulation pad under the fuse box - this becomes brittle over time which in turn becomes conductive and can pass a temporary short through to the inner wing and nigh-on impossible to fault find. Replace with 2no new peices of rubber pad - an old rubber car mat is good for this.

If in doubt replace with a new fuse box - they're cheap as chips and take seconds to fit.

nta16

7,898 posts

233 months

Sunday 28th April 2013
quotequote all
v8250 said:
Glass type fuses can read 'good' when checked for continuity but in fact have intermittent contact between wire and end caps.
I've found this with plug-in fuses too - they look good, might even test as good but then are intermittent in use

Dbest92

Original Poster:

300 posts

132 months

Sunday 28th April 2013
quotequote all
Cheers for the advice

After some further investigation it seems to point towards the points closing up, checked them today and were way to close (10thou ish) set them up a bit wider that the books say 18fhou instead of 16 to allow for thrm closing slightly, went for a good 20 mile run driving it fairly hard to test under load and was fine, will keep an eye but thinking new points? (Only of the gap keeps closing up?), they have been set a few times but didn't expect them to close up so soon, but lesson learned!

This could explain the poor cold starting I rekon (fine when hot though)

Ill also get some more fuses for spares and they current ones look old

Furebox was cleaned and connectors replaced a few days ago by a garage.

Also today the coil ran alot cooler, the other day, just after a few mins of idling, the coil was red hot, too hot to touch, but today was warm but easy handleable, could the tight points gap cause the coil to overheat and fail momentarily?

Interestingly the rev counter did drop a few times but started working again after a few seconds? No hesitance or missing during this though.

Don't want to say for sure it's sorted but it ran fine for a decent journey today and it seems like this could have been a likely cause!

Thanks

Darren


nta16

7,898 posts

233 months

Sunday 28th April 2013
quotequote all
you might have other small issues relating to this or you might not (and issue with ignition switch or connections there or elsewhere for the three items not working before - remember check, clean, secure and protected wires and connections, simple work that can save a load of hassle)

Darren, to remind you again, don't change CB points unless you have to, clean them and reset them (to book spec) - then, depending on your use of the car, after a while week or two or some hundreds of miles check the points again and after this check leave them alone until you check them at the next service

this check of CB points after they've been fitted is necessary I've found

also at fitting as per Driver's Handbook you need to grease the cam and depending on type of CB points they might need lubricating or not - don't get any oil or grease on the CB points

only buy NOS CB points and/or from a good source like the Distributor Doctor http://www.distributordoctor.com/

if the CB points keep closing up then it may be they're a rubbish modern made set so replace with NOS

same applies to condenser, don't change it unless you have to then get NOS and/or from a good source like the Distributor Doctor http://www.distributordoctor.com/

keep your previous condenser if it was old (not modern) in case of other problems later

I've never been a fan of CB points but if you get a good set and they're installed correctly and checked and serviced as required they can be OK

keep up the servicing work and regular driving as just because the engine starts and remains running doesn't mean it's running at its best potential and then there's the much more important systems and components on the car to consider but keep at it and you'll get closer to good running sooner

Edited by nta16 on Sunday 28th April 21:05

DocArbathnot

26,954 posts

182 months

Monday 29th April 2013
quotequote all
I may be way off the pace here.

But if your distributor was knackered and the shaft cam thingy was wobbling about this would make proper adjustment nigh on impossible. Would it be possible to borrow a known good dissy to try? (MG club member that sort of thing)

nta16

7,898 posts

233 months

Monday 29th April 2013
quotequote all
you've let the cat out of the bag there, I wasn't going on to that unless the points wouldn't stay gaped

I hinted at this earlier but didn't want to add to Darren's list unnecessarily

Darren,
dissys can be well worn, even when they were brand new they were worn after only a few years but that doesn't mean yours is more worn that those on the average British classic

as said IF badly worn then the timing, vacuum advance and points can be effected making it difficult to set the car up and keep it set - but I'd have thought your MG specialist would have mentioned it if it was bad

another reason (and another hint) I put the Dissy Dr link is that he rebuilds dissys but save your money if it's not required as you might need it for more urgent repairs

once you've had the car running reliably with a good mileage and all year round use then you can look at the expense of such things, either a rebuilt dissy with CB points or a rebuilt dissy with electronic igniter or better still a fully electronic dissy top and bottom (like the 123)

you also have to take into consideration the matter of balance, if you have a new fully electronic dissy but well worn carbs

bear in mind the engine runs and it's one of the least important things on the car, to start with get it running reasonably but concentrate on the brakes, tyres, steering, suspension, lights, windows and mirrors and once these are all sorted return to engine, carbs, etc.

just frequently driving the car and keeping up with the servicing will improve the whole car and your driving and expectations of it