lack of power and misfiring after warming up.

[deano69]

Hello, the mog went kaput a few months back, needed head to be skimmed as garage found fault on face and to be honest its not pulled freely since.But more recently its been back to the garage as runs alright but lacks power, doesnt like getting to 50 and on running to town and callig in a couple of places before coming home, she started misfiring on acceleration, feeling like she's dragging then shoots forward with ease again for 2 seconds then back to dragging again.Ended up not being able to get to 40 mph without constant misfires and random, 2 second bursts of freeing up and driving properly, before back to lack of power and misfiring.Garage thought jets needed rebuild but once delivered they found the fuel pump diaghram was rotten to fuel pump repaired/rebuilt and they thought everything was great.I immediately realised that the egine was still, if only slightly, having problems pulling but did get her to 50 but it seemed laboured.Anyway, run into town called at a couple of places and she started misfiring again, not so bad this time but much teh same on way home as before-constant misfiring, lack of power and acceleration which increases as i try to accelerate.This will be the third time shes back to garage for much the same problme.Any ideas?Oh and when head was sorted, entire ignition system was overhauled.

Cheers for time and consideration.

[liammonty]

If the entire ignition system was replaced, I'd say there is a fair chance that that is where the problem lies (you say it began when this work, along with the head skim, was done?). It could be as simple as points closed up, and you should definitely check before spending any money on the carb. The carbs really very rarely go wrong. Other culprits could be the condenser or coil (a topic of hot debate on here, but I have been left stranded by a replacement coil- original ones tend to be quite reliable). Rotor arms can also be a problem, as they too aren't great quality these days. I would go through all these and replace one by one (as they are all dirt cheap) and eliminate them as the problem, having first checked that the points that are in there currently haven't closed up.

[chesney]

Possibly a stuck float in the float chamber? Check the petrol tank filler is vented too.
Could even be some debris in the float chamber, or a partially blocked filter in the tank.
EDIT - sorry, didn't realise the problem started after the head skim

[deano69]

Dont be sorry.Before the head skim it didnt run at all-after head skim and ingnition system overhaul it ran but always seemed to drag if only slightly, wasn't quick of the mark and lacked power, acceleration which only got worse until it started misfiring.Garage thought it was fuel starvation, hence looking at carb and fuel pump, so stuck float ect may be it...?

[liammonty]

Symptoms do tie in with points closing up. Seriously, for the time it will take, I'd eliminate electrical issues, as they are far more common than fuel ones. You could spend a lot of time on the carb, and find that there's nothing up with it. If you do suspect the carb, the easiest way to eliminate it would be to put another one on (second hand) which would be cheaper than buying a new jet etc, an dwould be very quick. Still, you can check the float chamber easily enough without buying anything.

[IslipMinor]

When the head was skimmed was anything else done at the same time to the head?

Will it run 'normally' on light throttle openings? Or is the lack of power across all throttle positions?

[deano69]

As far as i know, nothing else was done to the head.It misfires across all throttle openings but is most obvious when attempting to accelerate at higher speeds, ie from 35-40mph, less obvious at 10-20mph and very obvious after changing up a gear as i open the throttle.Makes for precarious exits from junctions as power sudenly drops and the on coming car gets to you quicker than you thought.

im just asking now cos it'l be going back to garage tomorrow and i want to help point to what may be the problem for them to fix rather than paying for them to do what they think is the problem and then finding its not fixed.

cheers for the input.

[JOWETTJAVELIN]

Was the head tightened down again after the first heat cycle. Water may be seeping into the bores causing erratic running.

[deano69]

Yes was tightened down after x amount of miles later.

[lowride stepside]

asuming that the valve gaps have been set right .tho it does sound like points gap . What colour are the plugs and are they all the same colour ?

[kennatt]

Sounds more like fuel starvation to me,when they rebuilt the pump what did they do,did they clean and reset the points as well as a new diaphragm, there are two one way valves in the pump did they examine these,I would get them to check the fuel flow ie with the carb end of the pipe into a container the pump should give 1pint per minute,and also check the float level is set correctly.

[deano69]

Pump was sent off to a company that repairs them when they found the diaphram was worn out, so ive no idea what they've done, ive assumed they've rebuilt the pump-its supposed to have a two year guarantee.Im gonna suggest they check the points and the float chamber.Took 10 days to get it back last time, not good when its your only transport, so was hoping to not have to go without again.

[davidmiles]

Dean, do you have an in line fuel filter? (fuel pump to carb) I'm seriously considering fitting one to mine.

[aupickup]

in line filters are not needed and can cause problems

[JOWETTJAVELIN]

As aupickup says avoid an inline filter as there are at least 2 filters on the Minor anyway, and I don't know of an SU carb that's been blocked by crud in normal use - the jet hole is too big.

[chesney]

Agreed, the only place debris will likely ever be a problem is the float I would imagine, by which point the chances are that remote you may as well replace the float - cheaper than a filter (I assume, never considered fitting a filter!).

[JOWETTJAVELIN]

Well yes, but sediment collects at the bottom of the float chamber, that is if it gets there in the first place having evaded the pump filter and the thimble-filter at the banjo union!

Greater minds than ours planned all this over half-a-century ago!

[MarkyB]

A lot of fine sediment gets through the coarse filters in the fuel system and either goes through the jet or accumulates in the bottom of the float chamber.
Not a problem until the carb is removed, laid on it's side then refitted.
This puts the years of crud back into circulation and it can all end up in the pipe to the jet.
Not enough to block it completely but enough to give the symptoms you have.

It happened to me after I rebuilt my engine.

[bmcecosse]

To clear any crap - rev up and hold your hand over the carb intake.........
The SU is VERY tolerant of dirt - absolutely NO need for a flow restricting fuel filter.

[deano69]

Finally, after two garages looked at it, 'fixed it', which didnt actually fix it, a third classic restorer and service garage found the fault-the regulator box was putting to much power to the battery, effectively cooking it, 3 cells were dry, hence why it was having great problems starting up and the rotor arm was rusted out.Should be coming back with a shiny new alternator on it tomorrow, hopefully all fixed up ready for the christmas relative visits, not that i'll be buying any more presents for anyone after my bank account reserves have been walloped.At least i know where to take it locally from now on.It seems my resident village garage has charged me for ignition system overhaul, skimming of cylinder head and rebuild of fuel pump that simply have not been done.Isn't that fraud?