MOT and mis fires...

[David]

Well I spent yesterday afternoon fitting new plugs (old ones in attached photo, number two cylinder looking too lean maybe?), good quality leads, new disti cap, points and condenser which I'd hoped would fix a misfire I've developed as I start to accelerate(the PowerSpark module toasted itself a few weeks back). I needed to do this to be sure I could get to the MOT testing station.

Well, after all this was on, plug and points gap set, timing checked I noticed that the in distributor points back plate to disti body earth lead had all but come away so I trimmed and soldered this back(photos). I ran the car up until hot, checked everything again, ran up and down the driveway and all was good with the world.

This morning I hop into the car, driver off without issue but the first time I had to accelerate up an incline the misfire reappeared, giving a big bit of right foot it passed through the mis and smoothed out. I managed to get to the MOT station just half a mile away with the mis still happening as the revs built but then clearing.

So assuming there's nothing wrong with the ignition system I suppose I now have to look at;

- Air/vacuum leaks on the carb/inlet (I have new vacuum pipe and fittings to the distributor which I will fit)
- Blocked fuel filter, and then why. Hopefully the tank isn't full of crap
- Sticking carbs (the Strombergs are old and leaking oil through the dashpots slowly but they are topped up)
- Check the distributor for bad signs of wear. (I did notice that the points cam arm was not sitting quite square against the distributor spindle cam lobes!)

Is there anything else you guys can see I've missed and should look at?

thanks,

David.
PS. I suppose I'm suffering these failures now the car is in regular use after being hardly used for years.

[b_caswell]

Re: MOT and mis fires...

I would run the car for some distance and then check the new plugs. If it is only no2 that is running weak I would suspect the inlet manifold to head gasket. My car also looses the dashpot oil quickly causing it to misfire under load but found using engine oil works better for a while longer. Try richening up just a touch.

I would also check the CD diaphragms for damage also the Emission valve diaphragm. Also check the valve clearances 10th inlet and exh.

[Tony Stott]

Re: MOT and mis fires...

Can't you use ep80/90 or thicker in the dashpots? seem to remember my dad used ep in the carbs on his Riley 1.5 - but that was before the Ark set sail!!

[Harry]

Re: MOT and mis fires...

Hi,
I think I'd look again at the HT system, it sounds like you have a short, have you removed the bonnet and reved up the engine in total darkness ? you will surely spot the spark shorting if there is one to be seen, I've been caught out with this one a few times. The cap may be faulty ?

[David]

Re: MOT and mis fires...

I would run the car for some distance and then check the new plugs. If it is only no2 that is running weak I would suspect the inlet manifold to head gasket. My car also looses the dashpot oil quickly causing it to misfire under load but found using engine oil works better for a while longer. Try richening up just a touch.

I would also check the CD diaphragms for damage also the Emission valve diaphragm. Also check the valve clearances 10th inlet and exh.

Thanks Ben. I'll have a look at the inlet manifold and carb gaskets/diaphragms.

I am using engine oil in the dashpots, it takes a few weeks at least to get low. I have the emission control carbs, no mixture control on the bottom of them Unless I try adjusting the needle from within the dashpot.

[David]

Re: MOT and mis fires...

Hi,
I think I'd look again at the HT system, it sounds like you have a short, have you removed the bonnet and reved up the engine in total darkness ? you will surely spot the spark shorting if there is one to be seen, I've been caught out with this one a few times. The cap may be faulty ?

Thanks Harry.

The cap is knew and the behaviour is the same, new and old. I will try the night time test though, that'll annoy the neighbours!

I will also try swapping the original coil back on, it was fine, I only swapped it because I bought a new one as a precautionary measure when I fitted the Powerspark module.

[Gareth]

Re: MOT and mis fires...

Old fuel???

[andyf]

Re: MOT and mis fires...

Thanks Harry.

The cap is knew and the behaviour is the same, new and old. I will try the night time test though, that'll annoy the neighbours!

I will also try swapping the original coil back on, it was fine, I only swapped it because I bought a new one as a precautionary measure when I fitted the Powerspark module.

It was probably the poor earth that fried the PowerSpark module. Was the coil rated correctly? It may be a special one to go with the PowerSpark if you bought it at the same time.

[David]

Re: MOT and mis fires...

Old fuel???

No, it's had two tanks of new stuff through it recently. Good thought tho.

[David]

Re: MOT and mis fires...

It was probably the poor earth that fried the PowerSpark module. Was the coil rated correctly? It may be a special one to go with the PowerSpark if you bought it at the same time.

Good point. I will call Simon tomorrow and ask him. Although it's been running OK for the couple of weeks since we put the old points/condenser back in.

[David]

Re: MOT and mis fires...

Failed the MOT on out of balance rear brakes, I knew I should have fitted both new rear wheel cylinders! Should be all OK on the retest tomorrow!

Fingers crossed the misfire doesn't stop me driving it home!

[milliemarlin]

Re: MOT and mis fires...

Do you think the misfire is related to engine speed or power?
I find it useful to check the fault finding section in the Haynes manual:
Engine hesitates on acceleration
O Worn, faulty or incorrectly-gapped spark plugs (Chapter 1).
O Vacuum leak at the throttle body, inlet manifold or associated hoses (Chapter 4).
Engine misfires throughout the driving speed range
O Fuel filter choked (Chapter 1).
O Fuel pump faulty, or delivery pressure low (Chapter 4).
O Fuel tank vent blocked, or fuel pipes restricted (Chapter 4).
O Vacuum leak at the throttle body, inlet manifold or associated hoses (Chapter 4).
O Worn, faulty or incorrectly-gapped spark plugs (Chapter 1).
O Faulty spark plug HT leads (Chapter 5).
O Distributor cap cracked or tracking internally, where applicable (Chapter 5).
O Faulty ignition coil (Chapter 5).
O Uneven or low cylinder compressions (Chapter 2).
O Fuel injection system fault (Chapter 4).
Good luck with the MOT.
Paul

[David]

Re: MOT and mis fires...

Yeah!

Passed the MOT, just the wheel cylinder needed replacing, and I knew it was marginal.

Also the MOT guy said the car was not misfiring at all on the test drive and it behaved on the way home. That's all well and good but I'm nervous that there's an underlying problem which is going to catch me out some time soon so I'm going to continue with the investigation.

[b_caswell]

Re: MOT and mis fires...

Well done and just keep on motoring but keep your breakdown card in your pocket and all will be well.

[David]

Re: MOT and mis fires...

Well two weeks on and no misfires.

I did replace the fuel filters (it has two pumps for some reason!) which were looking slightly dodgy so I may have just had fuel starvation, who knows. I also relocated them from atop the tank to next to it. Next job to put a floor in the back over the tank.