Check spark plugs are right heat range – examine for signs of overheating.BMW corteco oil seals.
Check carb needles are correct spec & not badly worn at the thick
end and that the mixture is not wildly out at idle. If the needle is
worn at the thick end, richen it up 2-3 flats on the adjuster and see if
that makes any difference to the detonation. If it does, buy new
needles.
Check compression ratio approximately using a compression tester. Roughly, every 20psi is 1 ratio. (Not 15psi as you’d expect since the process is adiabatic not isothermal – the air heats as it is compressed – and so the simple form of Boyles Law does not apply.) Now you know what you are working with.BMW corteco oil seals.
Check the ignition advance rate (see notes below) and adjust if necessary.BMW corteco oil seals.
I’d check where the advance is before doing anything. Re-profiling the advance curve very easy, and a lot easier and cheaper than removing the head, let alone replacing it. I’d always regard checking the advance curve as the first step. All that’s involved is measuring with a timing light what the timing is doing now (vacuum disconnected) and then tightening up the advance springs in the distributor a bit until it does what you want. In higher performance engines a spring change may be necessary, but not for what you are doing. Rough rule of thumb: 9:1 compression look for 25 deg total advance at 2000RPM and 30 deg after 2500 RPM, 10:1 compression no more than 20 deg total advance at 2000RPM reaching 30 deg after 3500 RPM.BMW corteco oil seals.
This is covered in more detail in Distributor advance rate adjustments – compensating for high mileage wear and for modified engines: Setting up ignition timing for modified engines.
If compression higher than standard, try one grade lower in spark plugs.
That’s the end of the simple checks, after that it’s a case of remove the head, check the squish area/chamber size/head type/cooling holes/hot spots etc.BMW corteco oil seals.
The later MGB heads had the same chamber capacity as the early MGB/MGA1622, but had a shallower chamber with less squish area; most had the same valve sizes, but some had bigger inlets so one of these would be the one to get if you could. Refer my web page B-series Cylinder head markings for notes on the various MGB heads.
Lowering the compression ratio is an option for addressing some of these problems. BMW corteco oil seals. However, I’d be reluctant to give away compression ratio as a cure to detonation – compression increases both power and economy. The B series engine functions perfectly happily on 9:1 to 9.5:1. 10:1 is very viable with a little care and setting the advance curve correctly and even 11:1 is reasonably comfortable on 96 octane if everything else is optimised.BMW corteco oil seals.
Check compression ratio approximately using a compression tester. Roughly, every 20psi is 1 ratio. (Not 15psi as you’d expect since the process is adiabatic not isothermal – the air heats as it is compressed – and so the simple form of Boyles Law does not apply.) Now you know what you are working with.BMW corteco oil seals.
Check the ignition advance rate (see notes below) and adjust if necessary.BMW corteco oil seals.
I’d check where the advance is before doing anything. Re-profiling the advance curve very easy, and a lot easier and cheaper than removing the head, let alone replacing it. I’d always regard checking the advance curve as the first step. All that’s involved is measuring with a timing light what the timing is doing now (vacuum disconnected) and then tightening up the advance springs in the distributor a bit until it does what you want. In higher performance engines a spring change may be necessary, but not for what you are doing. Rough rule of thumb: 9:1 compression look for 25 deg total advance at 2000RPM and 30 deg after 2500 RPM, 10:1 compression no more than 20 deg total advance at 2000RPM reaching 30 deg after 3500 RPM.BMW corteco oil seals.
This is covered in more detail in Distributor advance rate adjustments – compensating for high mileage wear and for modified engines: Setting up ignition timing for modified engines.
If compression higher than standard, try one grade lower in spark plugs.
That’s the end of the simple checks, after that it’s a case of remove the head, check the squish area/chamber size/head type/cooling holes/hot spots etc.BMW corteco oil seals.
The later MGB heads had the same chamber capacity as the early MGB/MGA1622, but had a shallower chamber with less squish area; most had the same valve sizes, but some had bigger inlets so one of these would be the one to get if you could. Refer my web page B-series Cylinder head markings for notes on the various MGB heads.
Lowering the compression ratio is an option for addressing some of these problems. BMW corteco oil seals. However, I’d be reluctant to give away compression ratio as a cure to detonation – compression increases both power and economy. The B series engine functions perfectly happily on 9:1 to 9.5:1. 10:1 is very viable with a little care and setting the advance curve correctly and even 11:1 is reasonably comfortable on 96 octane if everything else is optimised.BMW corteco oil seals.
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