Backfire

[Rustinmotion]

Why does my car backfire once after I switch the engine off? All I can find is it’s un burnt fuel hitting the hot exhaust which makes sense but why is fuel entering the exhaust system, plugs are dirty grey rather than light brown so perhaps a tad rich but not by much
Any thoughts on what’s happening

Steve

[SnineT]

Originally Posted by Rustinmotion

Why does my car backfire once after I switch the engine off? All I can find is it’s un burnt fuel hitting the hot exhaust which makes sense but why is fuel entering the exhaust system, plugs are dirty grey rather than light brown so perhaps a tad rich but not by much
Any thoughts on what’s happening

Steve

Most likely carbon built up or something isn't fully stopping the fuel flow from the pump when you turn off, however this normally translates into run on rather than misfire.

If it's petrol which I'd imagine if must be then I would start with engine firing timing and suspect it highly if you've had a new timing belt, or if you haven't and it's long overdue you may have jumped a cog, does it still idle and run smooth or can you feel or hear a slight missing?


Just noticed it's an old car with conventional electrics so yeah I'd start at the distributor and see if the timing is correct with the old Gunston lamp.

iirc white or grey plugs is lean not rich, I'd start with the timing and get that in properly then check the lifter adjustments in the rocker box.

[Rustinmotion]

iirc white or grey plugs is lean not rich, I'd start with the timing and get that in properly then check the lifter adjustments in the rocker box.[/QUOTE]

Mine are slightly darker than this

[powen1]

Could it be carbs ? what carbs you running ? if they are not closing off properly excess fuel/air will enter the chamber as you turn off the ignition ? or timing

[Rustinmotion]

Originally Posted by powen1

Could it be carbs ? what carbs you running ? if they are not closing off properly excess fuel/air will enter the chamber as you turn off the ignition ? or timing

Twin SU, il check ignition timing and carbs and report back

[powen1]

Check the butterfly are closing shut :-)

[Rustinmotion]

Originally Posted by powen1

Check the butterfly are closing shut :-)

It’s not idling fast

[Derek UK]

Are you using the highest octane fuel you can? Twin carb spec engines were designed for 100 octane and now you have to do the best you can. Yes, check the timing.

[Othen]

Originally Posted by SnineT

Most likely carbon built up or something isn't fully stopping the fuel flow from the pump when you turn off, however this normally translates into run on rather than misfire.

If it's petrol which I'd imagine if must be then I would start with engine firing timing and suspect it highly if you've had a new timing belt, or if you haven't and it's long overdue you may have jumped a cog, does it still idle and run smooth or can you feel or hear a slight missing?


Just noticed it's an old car with conventional electrics so yeah I'd start at the distributor and see if the timing is correct with the old Gunston lamp.

iirc white or grey plugs is lean not rich, I'd start with the timing and get that in properly then check the lifter adjustments in the rocker box.

There are no cam belts on our ancient B18/B20 motors SnineT. It won't be a fuel pump issue either because they are mechanical (when the engine stops the pump stops as well).

It probably is carbon build up though: inside the exhaust manifold where it might glow red-hot with the engine running. When the unburned last charge from the from the one (maybe two due to overlap) open exhaust valve(s) as the engine stops exits, the carbon may ignite it in the manifold.

It would be worth checking the ignition timing first (because that is quick and easy, and eliminates another thing), but if that doesn't sort it a decoke of the exhaust manifold would only take an hour on these very simple cars. If the B18d motor uses a bit of oil (nothing to worry about, all B18s use a bit of oil) then this might be more likely as the oil sometimes solidifies in the manifold. De-coking like this would have been pretty common back in the day, but we have become used to better oils so it happens less.

[Othen]

Originally Posted by Derek UK

Are you using the highest octane fuel you can? Twin carb spec engines were designed for 100 octane and now you have to do the best you can. Yes, check the timing.

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