Intermittent misfiring

[MJacobson]

Good morning all,

I have recently had a new clutch and a recon gearbox fitted in my 1098 Minor and they seem to both be working beautifully. I have also recently replaced my ancient oil filler cap with a new one. After a little research I opted for the shorter, non-vented type as I believe this to be correct for my engine's breathing (I have a 1968 reg car. There is an open ended, fixed, swan neck shaped, pipe running down the outside of the rocker chest, and a rubber hose running from the rocker cover to the black air filter, which is mounted on the carb).

I only mention the above because these are the only changes I have made recently to the car, which has suddenly developed an intermittent misfire. It happens only momentarily and seemingly irregularly. It seems to me that the misfiring only happens when the car has been idling for an extended time (a frequent occurrence here in London). When stood in traffic the idling becomes increasingly erratic and rough and then when the car moves the lumpy misfiring happens when accelerating. After a hundred meters or so of running the problem seems to stop and only reoccurs after idling again.

Would I be right in suspecting some problem with the carb float bowl lid and/or the fuel pump?

Any suggestions or pointers on how to fix this issue would be much appreciated.

Also, I'm considering picking up one of the Grumpy heat shields for the warmer months. Does anyone have any experience with these?

Thanks in advance,

Michael

[bmcecosse]

Just maybe the float needle valve is leaking which will only show up at idling - I think this most likely - but have a look to the points gap and possibly change the condenser. But it may be running rich and just fouling up the plugs (especially when idling as above) - what plugs are in there, and what colour are they ?

[MJacobson]

Thanks for the swift response BMCEcosse!

I've got an AccuSpark electronic ignition set-up so I'm hoping the fault isn't located there. I've been toying with the idea of replacing the HT leads just to see what happens.

The plugs are new(ish) NGK BP6ES. It previously was running with NGK resistor plugs which I replaced a couple of months ago for good measure. I inspected them before heading out to work this morning and they were very sooty. I cleaned them up with my wire brush and put them back with fresh crush washers. I weakened the mixture a little but then found there was a slight hesitation on pressing the accelerator pedal. The tail pipe seems a healthy colour as far as I know.

I'll try to weaken the mixture further and run it in and also have a look at cleaning or renewing the float bowl lid.

Any feedback on the effectiveness of the Grumpy heat shield for the carb and fuel pipe?

Michael

[MJacobson]

Just had another thought: is it possible that since I've fitted the correct (non-vented) oil filler cap, the mixture has become richer as a result of less air flowing into the engine? Or would it not affect the mix in the cylinders?

I'm probably showing my ignorance here!

[MJacobson]

It's probably also worth mentioning, now I think about it, that there is frequently a small amount of petrol on the top of the float bowl lid and also around the pipes on the fuel pump (I've tightened these as much as seems reasonable). Presumably this also points to the needle valve leaking?

[IslipMinor]

Michael,

Fuel outside the pump pipes and on top of the float chamber does not sound quite right!

You need to identify the source of these leaks and fix them quick. If the fuel on top of the float chamber is coming from within the carburettor, that will cause uneven idling, but before jumping to conclusions, you do need to confirm that is the source. If it is, the fix is very likely just a new needle valve assembly in the float chamber.

[bmcecosse]

It all points to the float bowl needle valve weeping - when running along this doesn't matter - the engine uses the fuel - but when idling, too much fuel is getting through and it's running rich/flooding. I'm not a fan of the NGK plugs - but they are not the problem here - although good riddance to the resistor plugs. The pipe makes no difference to the mixture. Best to NOT wire brush plugs - although the ceramic is very hard - it can still be scratched by a wire brush. Some say a soft brass 'suede shoe' brush is ok. But frankly it should never need doing anyway.