MGB Occasional misfire
Discussion
My 1972 MGB has developed a random misfire over the last few days. Its had a new set of points a fortnight ago and I've checked they are the correct gaps and haven't rattled loose, the plug gaps were checked at the same time.
The misfire starts at about 3000 rpm under acceleration, its not recurrent, just the occasional miss, it also misses once or twice when cold.
Anyone got any suggestions on what it might be?
The misfire starts at about 3000 rpm under acceleration, its not recurrent, just the occasional miss, it also misses once or twice when cold.
Anyone got any suggestions on what it might be?
well should be either ignition or fuel
previously new CB points have turned out to be poorly made so that could be a conclusion to jump to
(you can never assume that new parts are not faulty or that the parts already on the car are the correct or original style)
it could be something you've done or ommitted to do
you could check that a wire wasn't nicked or squashed when you fitted the new CB points and at the same time if you haven't already done so lube the cam and oil the dissy
did you
clean or replace the rotor arm - new rotors can also be of poor quality - http://www.distributordoctor.com/red-rotor-arms.ht...
clean inside and out of dissy cap and check it
check or replace the HT leads and securely locate them
check spark plugs for cracks
clean and/or adjust the carbs
replace fuel filter
muck in float bowl/needle and seat
intermitent problems are the worse you sometimes need to wiggle things to get the fault to happen
check the owners Handbook it describes and lists the full servicing schedule - http://www.mgocshop.co.uk/catalog/Online_Catalogue...
ETA: reading it again, yes could/might/possibly be new points or fitting but fuelling possibly - unlikely to be it but did you top up or renew the oil in both carb dampers?
previously new CB points have turned out to be poorly made so that could be a conclusion to jump to
(you can never assume that new parts are not faulty or that the parts already on the car are the correct or original style)
it could be something you've done or ommitted to do
you could check that a wire wasn't nicked or squashed when you fitted the new CB points and at the same time if you haven't already done so lube the cam and oil the dissy
did you
clean or replace the rotor arm - new rotors can also be of poor quality - http://www.distributordoctor.com/red-rotor-arms.ht...
clean inside and out of dissy cap and check it
check or replace the HT leads and securely locate them
check spark plugs for cracks
clean and/or adjust the carbs
replace fuel filter
muck in float bowl/needle and seat
intermitent problems are the worse you sometimes need to wiggle things to get the fault to happen
check the owners Handbook it describes and lists the full servicing schedule - http://www.mgocshop.co.uk/catalog/Online_Catalogue...
ETA: reading it again, yes could/might/possibly be new points or fitting but fuelling possibly - unlikely to be it but did you top up or renew the oil in both carb dampers?
Edited by na on Wednesday 26th October 03:10
+1 na
also, i had a similar issue this summer 1. intermittent misfire / ignition fault, 2. apparent misfire @3000+ rpm.
the answer to pt.2, was the SU fuel pump not keeping up with fuel flow to carbs; in fact the symptoms were more like the engine being restricted/loss of power due to not enough fuel supply. have replaced 37 year SU pump with Facet Blue Top + fuel filter between tank and pump + Filter King between pump and carbs...works perfectly.
answer to pt.1,i couldn't find it - so took off, cleaned, nipped-up, & refitted every spade and bullet connector under bonnet and ignition and solenoids. has ran perfectly ever since; so am guessing it was a simple poor connection in the ignition circuit.
also, do check the conductivity/movement within the fuse box. slightly tighten up the fuse connectors as they have a habit of not holding the fuses tight-enough over many years. it's also worth replacing existing 35A fuses with new ones. good luck...
also, i had a similar issue this summer 1. intermittent misfire / ignition fault, 2. apparent misfire @3000+ rpm.
the answer to pt.2, was the SU fuel pump not keeping up with fuel flow to carbs; in fact the symptoms were more like the engine being restricted/loss of power due to not enough fuel supply. have replaced 37 year SU pump with Facet Blue Top + fuel filter between tank and pump + Filter King between pump and carbs...works perfectly.
answer to pt.1,i couldn't find it - so took off, cleaned, nipped-up, & refitted every spade and bullet connector under bonnet and ignition and solenoids. has ran perfectly ever since; so am guessing it was a simple poor connection in the ignition circuit.
also, do check the conductivity/movement within the fuse box. slightly tighten up the fuse connectors as they have a habit of not holding the fuses tight-enough over many years. it's also worth replacing existing 35A fuses with new ones. good luck...
Thanks na.
Sod's law dictates that it didn't miss a beat for 50 miles today, so I think it might be fuel related.
I'll run through the rest of the checks you mentioned tonight and see how I get on.
I had noticed that the fuel pump was continueing to 'tick' occasionally after start up, so v8250's point on the antique fuel pump not being able to keep up could well be my issue. The engine does feel like its being held back.
Sod's law dictates that it didn't miss a beat for 50 miles today, so I think it might be fuel related.
I'll run through the rest of the checks you mentioned tonight and see how I get on.
I had noticed that the fuel pump was continueing to 'tick' occasionally after start up, so v8250's point on the antique fuel pump not being able to keep up could well be my issue. The engine does feel like its being held back.
How old are your HT leads? Quite likely to be caused by those, as you've removed them to change the points and gap the plugs.
The condenser might be the culprit too. I'd agree that if you can hear the pump, it's likely to be supplying enough fuel. It's not just a misfire that happens when there's insufficient fuel, it's more of a definite cough and splutter.
Good luck! Definitely one of the more infuriating things to try and sort out. Might be worth your while taking it down to a rolling road with a handful of spares to get it done in one go. There are also two schools of thought with these sort of problems - replace the cheapest things first until you've got a good running car or borrow working items from another car and swap them around until you've identified the culprit.
The condenser might be the culprit too. I'd agree that if you can hear the pump, it's likely to be supplying enough fuel. It's not just a misfire that happens when there's insufficient fuel, it's more of a definite cough and splutter.
Good luck! Definitely one of the more infuriating things to try and sort out. Might be worth your while taking it down to a rolling road with a handful of spares to get it done in one go. There are also two schools of thought with these sort of problems - replace the cheapest things first until you've got a good running car or borrow working items from another car and swap them around until you've identified the culprit.
if the pump needs replacing, but your not at that stage yet, personally for anything up to reasonably beefed up the Q&H electronic fuel pumps are fine
very good point about making sure all the car’s wires, connections and earths are clean, secure and protected from the battery onwards and including all fusebox connections and fuses
the original fuses are a very slight different size to modern one and were marked up 17A continuous, 35A blow
you should also include the ignition switch and pump connections and earth – if you have any kind of anti-theft device this should be included
fuel pump ticking could also possibly mean air is getting sucked in through fuel pipes or connections rather than leaking – or perhaps float sticking
with the pump pumping but engine not running tap the top of the float bowls with the handle of a screwdriver as a first stage of sorting floats
the HT leads may play up more in damp whether, same as dissy cap, you could try having someone rev the car whilst you look under the bonnet in the dark for stray sparks with perhaps a spray mister, don’t be over zealous with this
HT leads, to coil especially, can raise from being fully fitted if the rubber cap covers are tight, as you push the cap rubber covers the vacuum pulls the HT connector up slightly
condenser is another thing not to change unless you have to, bit like CB points really
the list of possibilities reduces with full and proper regular servicing as you’ll be replacing things before they cause a problem and as part of the service you’ll be checking items that are not replaced
why did you replace the points?
if you have the old set you could try putting them back on – when the misfire returns – with the potential poor quality some part you’re better to clean up than replace
I’m sure you’ve seen my servicing lists but if not or you want notes just PM me
good luck, try to reason things out and start on a step by step logical diagnostic (unlike I’ve done here)
very good point about making sure all the car’s wires, connections and earths are clean, secure and protected from the battery onwards and including all fusebox connections and fuses
the original fuses are a very slight different size to modern one and were marked up 17A continuous, 35A blow
you should also include the ignition switch and pump connections and earth – if you have any kind of anti-theft device this should be included
fuel pump ticking could also possibly mean air is getting sucked in through fuel pipes or connections rather than leaking – or perhaps float sticking
with the pump pumping but engine not running tap the top of the float bowls with the handle of a screwdriver as a first stage of sorting floats
the HT leads may play up more in damp whether, same as dissy cap, you could try having someone rev the car whilst you look under the bonnet in the dark for stray sparks with perhaps a spray mister, don’t be over zealous with this
HT leads, to coil especially, can raise from being fully fitted if the rubber cap covers are tight, as you push the cap rubber covers the vacuum pulls the HT connector up slightly
condenser is another thing not to change unless you have to, bit like CB points really
the list of possibilities reduces with full and proper regular servicing as you’ll be replacing things before they cause a problem and as part of the service you’ll be checking items that are not replaced
why did you replace the points?
if you have the old set you could try putting them back on – when the misfire returns – with the potential poor quality some part you’re better to clean up than replace
I’m sure you’ve seen my servicing lists but if not or you want notes just PM me
good luck, try to reason things out and start on a step by step logical diagnostic (unlike I’ve done here)
I meant to put it's sound more like possibly fuel related, though not necessarily pump, something with carbs perhaps maybe
Bs as with a lot of classics like the air filters replaced regularly
even (not this case) things like the petrol cap seal can have an effect
as said there are so many possibilities that you need a diagnostic system
alfa's idea is good to take it to some one with a diagnotic machine but before this it'd be best to do a full and proper service and the checks suggested in this thread
or if you're short of time then other way round, take it to machine first to find fault then do a full and proper service
Bs as with a lot of classics like the air filters replaced regularly
even (not this case) things like the petrol cap seal can have an effect
as said there are so many possibilities that you need a diagnostic system
alfa's idea is good to take it to some one with a diagnotic machine but before this it'd be best to do a full and proper service and the checks suggested in this thread
or if you're short of time then other way round, take it to machine first to find fault then do a full and proper service
na / alfapint
Thanks again, much appreciated.
I replaced the points as the bit that touches the Dizzy rotor shaft was nearly worn away and I couldn't get the right gap. The car ran fine for 2 weeks after. The reason I checked the CB (and the rest of the ignition) becuse I was having some damp start problems, once the CB's were replaced the damp start issue went away. However it has started to come back, so maybe the CB are faulty.
I do have a Pertronic system that has never been fitted, that might be a better solution than CB, I had one on my last B without any issues.
I'll check all the wiring and connections /carbs tonight and I'll get some new leads, as they may have not have like being messed about whilst I changed the CB.
I'll also change the air filters, I still have the original paper type fitted.
Thanks again, much appreciated.
I replaced the points as the bit that touches the Dizzy rotor shaft was nearly worn away and I couldn't get the right gap. The car ran fine for 2 weeks after. The reason I checked the CB (and the rest of the ignition) becuse I was having some damp start problems, once the CB's were replaced the damp start issue went away. However it has started to come back, so maybe the CB are faulty.
I do have a Pertronic system that has never been fitted, that might be a better solution than CB, I had one on my last B without any issues.
I'll check all the wiring and connections /carbs tonight and I'll get some new leads, as they may have not have like being messed about whilst I changed the CB.
I'll also change the air filters, I still have the original paper type fitted.
mac,
get that owners Handbook to see details of servicing and lots, lots more information about your car, (Ref: 0052) – http://www.mgocshop.co.uk/catalog/Online_Catalogue...
the cam in the dissy and points pivot post should be carefully greased, plus sparse oiling of dissy
the new CB points may be OK, damp suggests dissy cap, HT leads or electric connections – but it might not be theses things
electronic ignition heads if installed correctly – so that the delicate leads are not pinch, nicked or scuffed – are IMO usually a lot better than points but they can’t help with the often very worn rest of the dissy
don’t change the HT leads for the sake of it, check them but if they are more than say 3 years old they are so inexpensive you could change them anyway – change the leads one at a time so as not to loose the firing order and also mark up the new leads and dissy cap (1 is nearest radiator)
you can get paper filter cheap too I think, Mann filters from motor factors they’ll also have Mann filters fuel filters at low prices too so no reason not to change both air and fuel filters at least once a year – you will notice the difference than clean air filters make
don’t forget cleaning or replacing the rotor arm (with a good quality one) clean by rubbing the end with a clean pencil rubber and to clean and check or replace dissy cap
remember to record HT lead position to keep correct firing order, clean outside of dissy cap with a cloth and the inside with a different clean lint free cloth, inside check the sprung carbon, the cap for faint cracks and the four stubs for burning
get that owners Handbook to see details of servicing and lots, lots more information about your car, (Ref: 0052) – http://www.mgocshop.co.uk/catalog/Online_Catalogue...
the cam in the dissy and points pivot post should be carefully greased, plus sparse oiling of dissy
the new CB points may be OK, damp suggests dissy cap, HT leads or electric connections – but it might not be theses things
electronic ignition heads if installed correctly – so that the delicate leads are not pinch, nicked or scuffed – are IMO usually a lot better than points but they can’t help with the often very worn rest of the dissy
don’t change the HT leads for the sake of it, check them but if they are more than say 3 years old they are so inexpensive you could change them anyway – change the leads one at a time so as not to loose the firing order and also mark up the new leads and dissy cap (1 is nearest radiator)
you can get paper filter cheap too I think, Mann filters from motor factors they’ll also have Mann filters fuel filters at low prices too so no reason not to change both air and fuel filters at least once a year – you will notice the difference than clean air filters make
don’t forget cleaning or replacing the rotor arm (with a good quality one) clean by rubbing the end with a clean pencil rubber and to clean and check or replace dissy cap
remember to record HT lead position to keep correct firing order, clean outside of dissy cap with a cloth and the inside with a different clean lint free cloth, inside check the sprung carbon, the cap for faint cracks and the four stubs for burning
I have had the same issue on a couple of my MGB's in the past, both times it turned out to be the carbs. They were the later type with the fuel chambers under the carb. I changed then for the earlier type with the float chamber on the side (was lucky enough to have a few spare sets laying around) and the problem went away. I never got round to refurbishing the faulty sets so never did determine why they caused the problem.
Think I have found the fault, a 12v wire from the dizzy had been between the oil filter and engine, I had moved it to the other side of the filter when I was inspecting the dizzy. Now its been moved back to its original path the missfire has gone.
It's probably a loose connection or poor insulation on that cable thats been the issue.
It's probably a loose connection or poor insulation on that cable thats been the issue.
mac,
it might be that or just a coincidence
if it is that then you want to make sure the wire and connections are clean, secure and protected otherwise if could lead to a breakdown at the least convenient time
if it’s the spade connector that is loose on the connecting peg it may just need tightening by carefully closing it up more, or it may need cleaning up, just taking a tight fitting spade on and off a few times could be enough to clean it – you’ll soon know when you first take it off and look at it if it needs more cleaning
if it’s a wire where it goes in the spade connector cut the wire back a little and replace the connector with a new one
if it’s just a nick in the wire’s insulation somewhere and you can’t replace the whole wire best to sleeve it with some shrink wrap insulation, this may mean also renewing a spade connector
it might be that or just a coincidence
if it is that then you want to make sure the wire and connections are clean, secure and protected otherwise if could lead to a breakdown at the least convenient time
if it’s the spade connector that is loose on the connecting peg it may just need tightening by carefully closing it up more, or it may need cleaning up, just taking a tight fitting spade on and off a few times could be enough to clean it – you’ll soon know when you first take it off and look at it if it needs more cleaning
if it’s a wire where it goes in the spade connector cut the wire back a little and replace the connector with a new one
if it’s just a nick in the wire’s insulation somewhere and you can’t replace the whole wire best to sleeve it with some shrink wrap insulation, this may mean also renewing a spade connector
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