MGB intermittent running fault??? HELP!
Discussion
So got my project mgb running fine the otherweek after checking the timing, and stripping down and cleaning out the carbs. Tuned it up and took it for its mot.
Was running spot on. Got to Sainsburys, turned it off to get some cash out, got back in and was running super rough, misfiring and would barely pull, so I stopped in a layby.
Managed to limp it home, took 2 hours to do about 5 miles stop starting!
Sounds like its running out of fuel but it had £20 in it.
Checked fuel pump. Pumps 2 pints in about 45 seconds so thats ok.
So swapped the leads, cap and rotor arm for the ones on my mini I know are good, changed the plugs,and tore the carbs down....again.
Found front float to be catching on the side of the bowl, think it was misshapen due to age.
Fitted a new float today, and it was spot on, problem solved!
Tuned it up and was running sweet. But then started to miss very very slightly at high revs, so I went and got some more fuel as sounded like it might be low.
Stuck some fuel in, now it will barely start again and running super lumpy!!!
Could the fuel pump be working intermittently??
I'm all out of ideas, so and help is much appreciated,
thanks Retro.
Was running spot on. Got to Sainsburys, turned it off to get some cash out, got back in and was running super rough, misfiring and would barely pull, so I stopped in a layby.
Managed to limp it home, took 2 hours to do about 5 miles stop starting!
Sounds like its running out of fuel but it had £20 in it.
Checked fuel pump. Pumps 2 pints in about 45 seconds so thats ok.
So swapped the leads, cap and rotor arm for the ones on my mini I know are good, changed the plugs,and tore the carbs down....again.
Found front float to be catching on the side of the bowl, think it was misshapen due to age.
Fitted a new float today, and it was spot on, problem solved!
Tuned it up and was running sweet. But then started to miss very very slightly at high revs, so I went and got some more fuel as sounded like it might be low.
Stuck some fuel in, now it will barely start again and running super lumpy!!!
Could the fuel pump be working intermittently??
I'm all out of ideas, so and help is much appreciated,
thanks Retro.
OP, this sounds like the SU fuel pump. Have had same problem on my B and with previous SU pumps. Now, the purists will shoot me for what I'm about to say...get rid of the SU pump and fit a Facet Blue Top Pump with a Filter King [glass bowl] in the engine bay. Fit an additional standard type fuel filter between the tank and the Facet. Right, public confession over...
...the reason for getting rid of SU fuel pumps is simple. They operate on a cam contact mechanism. These units will always drift out of set-up, and then overtime start to foul up, and with the way MGB's succome to their little rusty habit the point contacts never keep pace with the fuel flow that is required by the carbs/engine. This is like having your engine partially fuel starved at mid-high revs. Electronic SU's are readily available but they are not as good as the Facet units.
The Facet will over fuel the carbs at std flow rate so this needs to be throttled back a bit. Do this by fitting a 6-8mm copper fuel line between the Filter King and the flexible fuel line to the carbs. You can always trim this for correct MGB flow rate by adjusting the Filter King fuel flow rate; a simple job.
Understanding that this is your problem, complete the above and the old girl will go like never before.
...the reason for getting rid of SU fuel pumps is simple. They operate on a cam contact mechanism. These units will always drift out of set-up, and then overtime start to foul up, and with the way MGB's succome to their little rusty habit the point contacts never keep pace with the fuel flow that is required by the carbs/engine. This is like having your engine partially fuel starved at mid-high revs. Electronic SU's are readily available but they are not as good as the Facet units.
The Facet will over fuel the carbs at std flow rate so this needs to be throttled back a bit. Do this by fitting a 6-8mm copper fuel line between the Filter King and the flexible fuel line to the carbs. You can always trim this for correct MGB flow rate by adjusting the Filter King fuel flow rate; a simple job.
Understanding that this is your problem, complete the above and the old girl will go like never before.
if this is the source of the problem and it's a 10800cc road going B only then a standard electronic pump will do the job just as well as the knocking/vibrating Pacets type pump and without the need for a regulator
and personally I wouldn't worry about having an SU label on the electronic replacement
clean fuel filter just before the carbs is always useful
and personally I wouldn't worry about having an SU label on the electronic replacement
clean fuel filter just before the carbs is always useful
na said:
if this is the source of the problem and it's a 10800cc road going B only then a standard electronic pump will do the job just as well as the knocking/vibrating Pacets type pump and without the need for a regulator and personally I wouldn't worry about having an SU label on the electronic replacement
clean fuel filter just before the carbs is always useful
the little std cube-type 12v fuel pumps will not do the job; they pack up at the most inconvenient times. only time one should consider these type of units is if installing 2no units in parallel.clean fuel filter just before the carbs is always useful
OP, i'm not going to blow my trumpet here nor contradict na's notes, but, i know what i'm talking about when it comes to SU pumps. Rebuilt far too many to remember and have changed 24+ SU units to Facet install as above; not one has ever failed. these not only on MG's, but Astons, Jaguars, Jensens, Daimlers et al. they do not fail.
ref' na's note on noise/vibration. if mounted correctly on rubber mounts and washers; the noise is only slight. this also gives you/the driver an immediate technical note should the pump stop i.e. an electrical supply failure. it's horses for courses and your choice...but do give the facets a go; they will stop any fuel pump issues.
ps if the mgb engine really is 10800cc [10.8ltrs] i'm sure we'd all like to see pics of this.
sorry didn't mean to upset you, I didn't make it clear
if it's an ordinary road going 1800 cc (rogue keyboard before) then an ordinary aftermarket electronic pump (e.g. something like Q&H) will do the job - I personally wouldn't pay the extra to have the SU label on the electronic pump
if the car is well into performance then obviously you'll need more
in my experience the cube type can be noisy and vibrate and certainly need the exhaust bobbins type suspension if located in the engine bay and probably elsewhere
if it's an ordinary road going 1800 cc (rogue keyboard before) then an ordinary aftermarket electronic pump (e.g. something like Q&H) will do the job - I personally wouldn't pay the extra to have the SU label on the electronic pump
if the car is well into performance then obviously you'll need more
in my experience the cube type can be noisy and vibrate and certainly need the exhaust bobbins type suspension if located in the engine bay and probably elsewhere
Edited by na on Friday 11th May 19:26
What colour are your plugs? If they are black and wet you might be over-fuelling, either because the choke is stuck on or the float level(s) are wrong. I presume you are running twin SU's, I'd strip them down, set the float level on both, ensure there's no grit stopping the float valve seating and make sure the choke is free to return off. Your description of super lumpy running, makes me think it's flooding.
You've not mentioned a fuel filter, if it's partially blocked this would reduce flow when you need it most.But you would be down power when running not super lumpy but I think your probs are carb related.
Hope this helps
Saxx
You've not mentioned a fuel filter, if it's partially blocked this would reduce flow when you need it most.But you would be down power when running not super lumpy but I think your probs are carb related.
Hope this helps
Saxx
saxxeblt said:
What colour are your plugs? If they are black and wet you might be over-fuelling, either because the choke is stuck on or the float level(s) are wrong. I presume you are running twin SU's, I'd strip them down, set the float level on both, ensure there's no grit stopping the float valve seating and make sure the choke is free to return off. Your description of super lumpy running, makes me think it's flooding.
You've not mentioned a fuel filter, if it's partially blocked this would reduce flow when you need it most.But you would be down power when running not super lumpy but I think your probs are carb related.
Hope this helps Saxx
+1 ...You've not mentioned a fuel filter, if it's partially blocked this would reduce flow when you need it most.But you would be down power when running not super lumpy but I think your probs are carb related.
Hope this helps Saxx
so as suggested changed some bits but still got the problem.
it ran ok they other morning, so booked her in for an mot, then got about a mile and started spluttering again. It has no poke and if you touch the throttle under load just dies. In neutral/stationary it will rev gradually to about 3.5K then wont go any higher feels like a rev limiter.
over the weekend I replaced the following but still wont rev:
NEW SU TYPE FUEL PUMP (purely to save time and effort but thanks for the facet suggestions/maybe a future mod)
NEW CONDENSOR
NEW CAP LEADS ROTOR ARM
NEW COIL
PLAYED WITH ADV/RETARD TIMING WITH NO AFFECT
NEW VACCUM ADV UNIT & CHECKED MANIFOLD WAS SUCKING OK
STRIPPED DOWN DIZZY TO CHECK WEIGHTS WHERE OPERATIONAL
RAN WITH NO EXHAUST TO SEE IF IT WAS BLOCKED, STILL THE SAME THOUGH
This morning to eliminate the carbs/fuel issue I swapped the su's for a WEBER ofF another mg we have here (but obv no vac adv feed) and its exactly the same, foot to floor it cuts out, gentle throttle it will rev to 3500 then 'revs hit a wall'.
I have checked for broken valve springS, and valve opperation looks fine.
It does look to run very rich, blue smoke when reved to 3.5k.
Im all out of ideas, and the only thing I can think of now is a chipped valve??
Can anyone confirm / oppose my thought before I pull the head off,
swiftyly running out of patience.time& money
Thanks Retro
it ran ok they other morning, so booked her in for an mot, then got about a mile and started spluttering again. It has no poke and if you touch the throttle under load just dies. In neutral/stationary it will rev gradually to about 3.5K then wont go any higher feels like a rev limiter.
over the weekend I replaced the following but still wont rev:
NEW SU TYPE FUEL PUMP (purely to save time and effort but thanks for the facet suggestions/maybe a future mod)
NEW CONDENSOR
NEW CAP LEADS ROTOR ARM
NEW COIL
PLAYED WITH ADV/RETARD TIMING WITH NO AFFECT
NEW VACCUM ADV UNIT & CHECKED MANIFOLD WAS SUCKING OK
STRIPPED DOWN DIZZY TO CHECK WEIGHTS WHERE OPERATIONAL
RAN WITH NO EXHAUST TO SEE IF IT WAS BLOCKED, STILL THE SAME THOUGH
This morning to eliminate the carbs/fuel issue I swapped the su's for a WEBER ofF another mg we have here (but obv no vac adv feed) and its exactly the same, foot to floor it cuts out, gentle throttle it will rev to 3500 then 'revs hit a wall'.
I have checked for broken valve springS, and valve opperation looks fine.
It does look to run very rich, blue smoke when reved to 3.5k.
Im all out of ideas, and the only thing I can think of now is a chipped valve??
Can anyone confirm / oppose my thought before I pull the head off,
swiftyly running out of patience.time& money
Thanks Retro
eddy, you have my sympathies here, these type of faults can be hugely frustrating. when changing multiple parts it's often the case of going round in bloody circles. my recommendations for what it's worth...start with a clear head and clean sheet of paper...slowly and methodically re-check the following in this order:
1. static and advance timing - have you strobed the timing?
2. although you've replaced the fuel pump, do you know what fuel flow rate you're getting at the input to the carbs?
3. re-check su fuel pump settings, or even better get any 12v constant flow fuel pump and install. this will overcome pump points limitations.
4. bypass complete fuel lines with a temporary pipe run to carbs - make sure you've got a good filter before pump and before carbs.
5. have you checked the fuel filler cap? if not, change for another as these can throw up this type of fault.
6. from your description another thing that could be happening is that under load you've got an electrical breakdown. this can be due to load, and/or, heat. re-check/swap coil ht + sparkplug leads.
7. coil - again under load, coils can fire up perfectly, then breakdown [fail charge + discharge] under load or due to heat.
8. 4no fuses + fuse box. i had a simlar fault as yours last year [but without the blue smoke!] remove and re-pinch all electrical wire terminals for tightness + the actual fuse holders themselves + replace the 4no fuses; here, the fuse wire can be in intermittent contact with the fuse caps; this can throw up all sorts of spurious faults.
9. in fact, carefully go to every ignition electrical point and tighten up contacts including earths.
10. although this doesn't account for blue smoke, in what condition is the earth lead to gearbox? this must be good a tight.
if you want to p.m. me feel free and we'll talk on the phone.
1. static and advance timing - have you strobed the timing?
2. although you've replaced the fuel pump, do you know what fuel flow rate you're getting at the input to the carbs?
3. re-check su fuel pump settings, or even better get any 12v constant flow fuel pump and install. this will overcome pump points limitations.
4. bypass complete fuel lines with a temporary pipe run to carbs - make sure you've got a good filter before pump and before carbs.
5. have you checked the fuel filler cap? if not, change for another as these can throw up this type of fault.
6. from your description another thing that could be happening is that under load you've got an electrical breakdown. this can be due to load, and/or, heat. re-check/swap coil ht + sparkplug leads.
7. coil - again under load, coils can fire up perfectly, then breakdown [fail charge + discharge] under load or due to heat.
8. 4no fuses + fuse box. i had a simlar fault as yours last year [but without the blue smoke!] remove and re-pinch all electrical wire terminals for tightness + the actual fuse holders themselves + replace the 4no fuses; here, the fuse wire can be in intermittent contact with the fuse caps; this can throw up all sorts of spurious faults.
9. in fact, carefully go to every ignition electrical point and tighten up contacts including earths.
10. although this doesn't account for blue smoke, in what condition is the earth lead to gearbox? this must be good a tight.
if you want to p.m. me feel free and we'll talk on the phone.
Ok so took the head off this afternoon, and think it was the head gasket gone and water was getting into cylinders 3 and 4 but mainly 3, hence the wet looking plugs.
At least I think I know what the problem is now, however cylinder 3 looks pretty horrendous (hence the smoke) so may be time to try and source another engine/block.
I guess this is what you get when you buy an old car that has been stood for years, and begs the question why was it stood up in the first place!
At least I think I know what the problem is now, however cylinder 3 looks pretty horrendous (hence the smoke) so may be time to try and source another engine/block.
I guess this is what you get when you buy an old car that has been stood for years, and begs the question why was it stood up in the first place!
you could also try posting on the MG BBS MGB Technical forum http://www2.mgcars.org.uk/cgi-bin/gen5?runprog=mgb... for ideas to see if it's worth continuing with your present engine/block
na said:
you could also try posting on the MG BBS MGB Technical forum http://www2.mgcars.org.uk/cgi-bin/gen5?runprog=mgb... for ideas to see if it's worth continuing with your present engine/block
+1 if block and head decks level, you can clean up in-situ including piston crowns; i.e. de-coke, then fit new gasket and re-assemble. could you post pics for us to have a look at? thing to check before cleaning decks is to inspect deck+gasket for where water leak is from; it's normally quite clear. if liner's/cylinder #3 really shot then you're knackered and will have to opt for rebuild or replacement engine. do post pics for all to see 
that's a nasty one to find, I had something like that once but not on a car, can't remember what it was though
I had an intermitent blade fuse once looked fine tested fine cost me £120 tow off the motorway
replaced a brake bulb once for a neighbour the new bulb worked but then it went out checked bulb holder and wires they were fine checked new bulb it was fine but that side brake light was intermitent, turned out when the bulb filiment got hot one end of it disconnected then when it cooled a bit it went back until it heated up again
so sometimes visual checks work better than meter tests or practial tests other timesit's the other permutayions of the three
I had an intermitent blade fuse once looked fine tested fine cost me £120 tow off the motorway
replaced a brake bulb once for a neighbour the new bulb worked but then it went out checked bulb holder and wires they were fine checked new bulb it was fine but that side brake light was intermitent, turned out when the bulb filiment got hot one end of it disconnected then when it cooled a bit it went back until it heated up again
so sometimes visual checks work better than meter tests or practial tests other timesit's the other permutayions of the three
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