Backfire

[Rustinmotion]

I’m running a fairly new 4 branch manifold I’d be surprised if it’s coked up but il check if nothing else works

[Othen]

Originally Posted by Rustinmotion

I’m running a fairly new 4 branch manifold I’d be surprised if it’s coked up but il check if nothing else works

In that case the exhaust manifold almost certainly won’t be the issue, it would take some years to coke up enough to cause a problem.

So, start narrowing down the issue:

For how long has the backfire occurred? In particular was it preceded by anything significant?
Does the motor car always backfire, or is it in only certain circumstances (hot or cold engine and so on)?
Does the motor ever run on (diesel)?
Is there indication as to whether the backfire is a high pressure one (from the combustion chamber, a sharp bang) or a lower pressure one from within the exhaust pipe (a dull thud).
Does the motor use much oil?
Have you done a compression test recently?

Alan

[Burdekin]

What's your idle rpm and how have you got mixture set at idle?

Assume no issues with timing? I imagine timing would at idle would need to be way out to cause a backfire.

[Burdekin]

Also what spark plugs are you running. If timing, idle rpm, idle mixture are correct maybe try a cooler spark plug.

[Derek UK]

Originally Posted by Othen

No. My 1963 handbook says the engine needs 97 octane, not 100. I also have a B18d motor with twin SU carburettors. E5 is 97 octane and will be absolutely fine for the OP's motor car.

Alan

Back in the mid 60's I ran my 1961 Amazon 122S B16b on Cleveland Discol. That was 5 star 101 octane. Yes, 97 is fine and our engines run smoother on it than basic petrol. Watch out for pinking, retarding 1 or 2 degrees can help.

[JP 1800]

Most likely a case of old fashioned dieseling, where excess carbon causes autoignition of fuel. What is needed is a decoke, this was a standard maintenance procedure back in the 60s and 70s before fuel injection was common. If you pull out the plugs, set a piston to near top dead centre and shine a torch into the plug hole. You will probably notice the piston tops are black with carbon. you can do this in turn with each cylinder. To remedy this requires removal of the cylinder head for a de-coke. Its relatively an easy but some what involved procedure. Once the carbs and cylinder head are removed you need to remove the carbon from the piston tops and exhaust/inlet valves. A brass wire wheel on a drill is good for the valves and a soft scraper will do the pistons. Be careful as the pistons are alloy and can be damaged easily. You will need to fit a new head gasket when reassembling, these are relatively cheap at £20, however I usually use the full kit and refresh all the top end gaskets. As stated this was common fare back in the day and any competent mechanic or even diy mechanic can do in within a short day.

[Rustinmotion]

I have done more than my fair share of de coke jobs back in the late 70’s it was done almost yearly, usually when the engine ran on due to glowing carbon igniting the mix and keeping the engine running, this is not ( I don’t think ) the same thing, it’s definitely happening in the exhaust. Anyhow spent half an hour in the garage this evening timing is good at 18 degree BTDC at 1500 rpm
Idle was a little fast at 1000 rpm so slowed it down to 700 rpm and I think the mixture was a little rich, lifting the dash pot piston had the revs rising but not dropping so wound the jets in a little
It’s running but I’d still call it a lumpy idle, exhaust note is an even burble and it did not backfire when I turned it off, need to go for a run to be sure
What’s general opinion of using sea foam to clean everything in the top end?

[Derek UK]

Squirting Redex down the carb/carbs was the old method. Makes loads of smoke so be sensible where you do it. A little squirt into each cylinder and left overnight might might also loosen/remove the coke. Still nice and smoky! Modern fuel makes very little coke and what it does make flakes off and goes out of the exhaust. If the head hasn't been off since 1963 then yes there may be some carbon for the Redex to take care of. Seafoam is a system cleaner rather than a decoker, AFAIK.

[Othen]

Originally Posted by Derek UK

Back in the mid 60's I ran my 1961 Amazon 122S B16b on Cleveland Discol. That was 5 star 101 octane. Yes, 97 is fine and our engines run smoother on it than basic petrol. Watch out for pinking, retarding 1 or 2 degrees can help.

So, to be clear for the OP: 97 octane gas is the correct grade for his motor car..

[blueosprey90]

Perhaps you can gleen a direction from these.

Grey spark plugs suggests lean, not rich -

https://fboignition.com/articles/sparkplugreading.html



Back fire or after fire - in two parts

https://youtu.be/XVDXLW3P7q8


https://youtu.be/bgS3PA5r0vY