misfire when cold

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dudload
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misfire when cold

Post by dudload »

morning gents,

my "good" moggie has recently developed an intermittent misfire/stutter when cold that goes away almost fully after the engine warms up!

now, i've got some ideas what could cause this and have already changed the dizzy cap, next on the list to check one by one is:

1) Leads (although they're fairly new)
2) Spark plug gaps
3) Dizzy itself (its electronic, but could swap out for the old points as i've got a new 45D points spare)
4) Compression test?
5) tappet gaps (although checked these not long ago)

What i'm concerned about is if it's a sticking valve or something to do with the head, as it's strange that its only when cold. The engine never stalls or fails, just runs like absolute rubbish at the start for 10min or so!
simmitc
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Re: misfire when cold

Post by simmitc »

I don't want to depress you, but it could be a failing head gasket. Slightly difficult to start, sounds like it's on 2 or 3 pots, and then all kick in and are OK. You might have water seeping into one pot which gradually gets ejected and then that pot runs OK. I would check compression before changing anything else. A sticking valve might also show up with the test - I've known them work OK when checking compression but then stick when you're not looking :-?

Of course, checking for loose connections on the ignition is also good a good idea, but prove logically where the problem lies rather than changing loads of things at once.

Is your dizzy nice and dry? Although you've changed the cap, condensation inside it can cause the same problems, so check that it is dry before your start the car. That way you can eliminate that as a cause.

Oil collecting in a pot after draining down a worn valve guide might cause an issue, but I think unlikely.

Are there any other symptoms? Overheating, oil consumption?

Is it really rough for as long as ten minutes? If either of the above, then I'd expect a much shorter time.
dudload
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Re: misfire when cold

Post by dudload »

thanks! compression test it is, was hoping to go on a long drive tomorrow - just checked spark gaps and one was somehow at 30thou and reset to 25 so that "may" fix it (although i'd be surprised if so as one cylinder not firing would just cause sluggishness i would have thought!)

will let you know results once done. been a while - standard procedure is full pedal, testing on a warm engine with and without oil?
dudload
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Re: misfire when cold

Post by dudload »

whats also quite strange is that it idles ok, but the issues comes when accelerating, especially when trying to pull away quickly. all sorts of stuttering, and yes for as long as 10minutes!
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Re: misfire when cold

Post by kennatt »

you don't need the oil just do the test it will be obvious if its head gasket,there will be very low compressions on one or possible two pots ,the oil test is to check for worn cylinders ,rings etc,put the oil in on second test and if pressure rises it indicates wear,since it seals the rings and piston gap .If a gasket it will have no effect or very little, on the pressure, the oil can't seal such a gap..Have you checked the oil in the carb dash pot,if low or none it causes this fault as it allows the piston and needle to dance about thus fluctuating the mixture at the jet.
dudload
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Re: misfire when cold

Post by dudload »

Thanks Kennat, just popped outside to check a few things. No mayo in oil filler cap , so good there, but unfortunately the coolant level in the radiator was very low which is pointing towards a failed head gasket. Will check compression tonight and let you all know!
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Re: misfire when cold

Post by shoebone »

Slightly different direction and trying to keep it simple ... i'm thinking lean condition and carb dashpot oil ... or lack of ?
dudload
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Re: misfire when cold

Post by dudload »

hello everyone!

so - starting up this afternoon, everything all good and a drive round the block throws up no issues. dashpot oil is there and i've been checking my mixture constantly with a nice chocolate brown on all sparks apart from 4. i wonder if the slightly wide gap on one of the plugs was causing it all along?!

now, for the compression test - all looks good here in my eyes. stats are:

1) 135
2) 130
3) 135
4) 140 (almost 145)

i did those with a hot engine, and as the numbers seemed good, i didn't bother popping oil down there as rings seem good from that.

i'm now thinking moisture in the dizzy causing the misfire - which is strange given i changed the dizzy cap this morning (to try and fix the issue).
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Re: misfire when cold

Post by kevin s »

There's a few dodgy rotor arms around and they do change with temperature as the plastic expands.
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Re: misfire when cold

Post by kennatt »

seeing that you changed the cap in the morning and it runs ok in same afternoon I think you have found the fault and cured the fault .Well done. now watch the water level carefully .
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Re: misfire when cold

Post by oliver90owner »

apart from 4

Don’t keep us in the dark! What colour is No.4 spark plug? Sooty may mean that is the culprit or ashen might mean a manifold leak.
dudload
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Re: misfire when cold

Post by dudload »

Well, going back out this morning and was back to the old ways. Disastrously stuttery for 10mins then smooth as from then on, my girlfriend was saying we should can the trip!

So clearly something still not right and only when cold. Sorry I didn't mention plug 4, it wasn't oily but was definitely black, can put a photo up later today when I get back.

I might nip up the manifold bolts to see if loose and will probably firstly change the rotor next to see if it's the culprit, although I would have thought it wouldn't change when hot or cold.

Lastly would be changing the dizzy as maybe the electrical unit is on the blink - think I prefer points anyway!
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geoberni
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Re: misfire when cold

Post by geoberni »

dudload wrote: Fri Jul 31, 2020 12:11 pm
Lastly would be changing the dizzy as maybe the electrical unit is on the blink - think I prefer points anyway!
I'd certainly have that last on the list. electronics like that should only be working or not, and unlikely to cause a misfire IMHO.
If it was cutting out completely when hot, then I'd say it could be the electronic unit warming up and a bad joint inside it opening up with expansion and causing it to fail. So if it conked out completely, but then fired up 10 minutes later, it could be the electronics.
Two different RAC guys blamed my electronic points for my breakdowns, yet it turned out to be a broken solder connection in the coil.
Basil the 1955 series II

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oliver90owner
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Re: misfire when cold

Post by oliver90owner »

The simple way to check for inlet leaks is to spray around the area with oil and listen for a change in engine note. But perhaps your pride and joy would not tolerate that treatment?🙂
dudload
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Re: misfire when cold

Post by dudload »

problem solved! tried to take it out once again and this time.... dead. getting the multimeter out revealed 0V on the white (now yellow) wire to the coil - it must have been running on one copper thread previously.

spade connectors changed on both wires and now back to running like a dream :) thanks all
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