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Conquest misfiring when warm - any ideas?

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Hafffy
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Joined: Fri Jul 14, 2017 4:30 pm
Location: Twickenham

Conquest misfiring when warm - any ideas?

Post by Hafffy »

I had my Conquest engine rebuilt 2 years ago and recently it's started misfiring badly when the
engine is good and warm. It's fine for 15 - 20 minutes and then it starts back firing and missing
to such an extent, it's so rough that it inevitable stalls at low revs.
Does anyone have any ideas what this could be? Leaky head gasket?
Any help greatly appreciated. Haffy

Norfolk Lad
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Re: Conquest misfiring when warm - any ideas?

Post by Norfolk Lad »

It sounds like the condenser breaking down a common problem with what is sold as a condenser, a good quality one will last such as Distributor Doctor or similar suppliers.

Stan Thomas
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Re: Conquest misfiring when warm - any ideas?

Post by Stan Thomas »

One things for sure - it is nothing major or expensive!!

Certainly go for the "condenser" (an Anglo Saxon term for a capacitor) - but it is more than likely to be the coil - for as coils ripen into old age, they have a propensity for the internal insulation to break down - the usual sign being when they get warm, as in your case.

Check of course you have the correct points gap - and with the points open, see if you can "rock" the cam to alter the gap - which is indicative of worn distributor bearings.

Let us know how you get on.

Stan.

Stan Thomas
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Re: Conquest misfiring when warm - any ideas?

Post by Stan Thomas »

Just had another thought ...................... Check if you have suppressor HT leads fitted (the centre core resembles "string" in lieu of copper wire) as these will go high resistance after time.

Try just changing the one from the coil to the distributor given you say you have a chronic misfire rather than one individual cylinder. Also, remove any in-line suppressor if one has been inserted in the same HT lead.

Hafffy
Posts: 4
Joined: Fri Jul 14, 2017 4:30 pm
Location: Twickenham

Re: Conquest misfiring when warm - any ideas?

Post by Hafffy »

Thank you for your replies. A while back, I fitted electric ignition and a new distributor cap and leads so
they are in good condition. However, I'll check your suggestion (Stan) about the HT leads. I've also now replaced the coil and it is
running better but it is still a bit lumpy especially when going from low revs to higher revs. It's ok when it gets there. I thought maybe the problem could be a malfunctioning SU carb - though I've cleaned them. Or even water vapour in the petrol? Possibly even a
bad exhaust (it needs replacing and there maybe no back pressure ). Could any of these cause the lumpiness/hesitation as I rev?
Thanks again for your help. Haffy

Peter Grant
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Location: Sydney, Australia

Re: Conquest misfiring when warm - any ideas?

Post by Peter Grant »

When I first drove my DE after many, many years on blocks, my mechanic advised me to put about a half litre of methylated spirits into the tank to burn off any water in the fuel.
It meant that I left a plume of white vapour down the road but only the once. Didn't seem to harm the running of the car.
Cheers
Peter Grant

Phillmore
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Re: Conquest misfiring when warm - any ideas?

Post by Phillmore »

It might be worth checking your valve clearances when hot just to rule out any problems there. Any leaking valves will have more of an effect at tick over than when being revved.
Andy

1954 Conquest Mk1, 1956 Conquest Mk2, 1957 Conquest Century Mk2, 1955 Austin A90 Westminster

Hafffy
Posts: 4
Joined: Fri Jul 14, 2017 4:30 pm
Location: Twickenham

Re: Conquest misfiring when warm - any ideas?

Post by Hafffy »

Thanks again for the advice. I've bought some Wynn's dry fuel - a solution which goes into the tank to
make any water combustible. (If that doesn't work, I'll try the meths. For the car, not me!)
I'm also going to look into replacing/cleaning the tank. It's original and probably full of gunk that
could be obstructing the flow. Anyone ever cleaned a tank?

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theoldman
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Re: Conquest misfiring when warm - any ideas?

Post by theoldman »

While you are at it, treat it to a nice new set of NGK BP6ES plugs - about a tenner a set off Flea Bay................
Normal for Norfolk

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