Misfire/Backfire/Low Power Issues

[SKU3410]

Morning. Recently went for a drive. First 5 miles till I stopped ok. Car normally settles quite quickly but it has a slight misfire on start up. But on restart which took a lot more effort than usual, had a large misfire and then developed into a back fire. At the same time the engine became really down on power. Decided to head straight home. Barely made it up hills. When I parked the car my son noticed spitting blue flames before I switched off. The left rear silencer was glowing red thru the left side of the fan tail. Did some research and could be one or more of several things but just wondered if anyone had a similar issue that would help me know where to start. Or any suggestions would be handy.
Thanks
Kev

[MarekH]

Dear Kev,

It can't be "one or more of several things" - you are pumping unburnt fuel into and back out of the cylinders without igniting it first. The hot exhaust then ignites it.

I'd take a look at your ignition and why it isn't lighting the mixture in the cylinders where it is supposed to. Then I'd check whether too much fuel is somehow going in relative to airflow.

kind regards
Marek

[abowie]

What car do you have?
As Marek says the exhaust is getting hot because unburned fuel is detonating in it.
Likely cause is a timing/ignition fault and I'd pursue that first. Too much fuel is the other possibility but I'd have my money that it's spark that's your problem.
Cheers, AB

FWIW have a run through my "No spark" algorithm for the 6 cylinder car.

No Spark

[SKU3410]

Thanks guys for your help. I have a 1972 series 3 red Fhc. Will start with ignition. I think there are carb issues too tho. Still learning. Thanks again.
Kev

[D COUPE]

Sorry for not reading your post earlier........I have the same issues and it is usually damp in the dizzy cap.

After a little lay up the damp condensates in the cap and all hell breaks lose when you try to go anywhere.

My cure is to remove the offender and dry the dizzy out with a hair dryer. I always carry a new cap where ever I go......

Worth a try

Dave C

[SKU3410]

Ta Dave. Planning some time on it weds. May start with that then. I'm as bald as you can be so will borrow the wife's dryer. I must remember to post results
Kev

[D COUPE]

I just remember thinking after a few mile, things just cant go so badly so quickly. The common denominator was the dizzy cap.....

DC

[PeterCrespin]

Ta Dave... I'm as bald as you can be so will borrow the wife's dryer. I must remember to post results
Kev

Sorry, but I think you'll still be bald :-(

As for the misfire, an IR thermometer will hrlp show which cylinders are misfiring, which can be a strong diagnostic for carbs or electrics on a V12.

[SKU3410]

Well chaps it would seem it was the Distributer cap contacts. Needed cleaning. Cant check on a run yet but fired up and ran smoothely. Thanks for all your help. Lets hope.......
Kev

[Adamski]

I think you've had a lucky miss and hope it was just as simple as that. Having battled with this issue for months/years and now nearly solved the issue I suspect your ignition is breaking down due to heat. Heat is an issue in itself especially if you have older Opus ignition and even replacements. The retard unit on the distributer could be part of the problem giving retard when it should be inactive due to the thermostatic vacuum valve once warm. These are unreliable and are not shutting vacuum off when they should and the retarded timing at low revs and tick over caused overheat.(No longer available) Also various vacuum pipes may be perished and possible you are still drawing hot air from the above the exhaust manifolds due to another Heath Robinson set up of sensors and flaps. There is good sense in scrapping the complete emissions package that UK S3's had as its rubbish.
Modern petrol also has played it's part in excessive heat and you may do well at some time to fit high flow radiator and multi blade fans and a complete go through of the complete cooling system.
I will in the near future do a write up on how I have battled with the heat issue on my V12 in my many years of ownership .

[SKU3410]

Finally got out on a long run after fitting brand new distributor cap. Went really well. The old cap didn't have a gasket which may not have helped and the old one contacts were corroded. Thanks again for your help.