My 1979 MGB had a engine rebuild some years ago and has a persistent pinking even if I retard the ignition. The cylinder head was gas flowed and the compression was raised. I have been told the pistons would need to be drilled to bring down the compression. Since this engine is often tuned I cannot understand that the compression is so high it causes the pinking. Any thoughts ?
The other more recent problem is the overdrive. When hot overdrive fumps in on overrun and light acceleration.Reverse becomes inoperable. The overdrive is engaged in all gears and it kicks in wihtout moving the switch. The gearbox oil has been changed.
The car only covers less than 2000 miles a year.
Any advice greatly appreciated.
Cheers
Peter
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The pinking suggests that the comp ratio is too high for fuels available now. Use a mix of one of the higher octane pump fuels (Optimax etc) with a dose of an octane improver such as Millers or Castrol.
Both our old racecars, Lola Chevrolet and Ford Anglia need high octane fuel and this is the route we go down if AvGas (105 octane) is not available.
The overdrive problem is probably electrical, I had a similar prob on my 3litre Marcos when O/D used to drop in and out when it fancied due to the inhibitor switch for 3rd and 4th jammed.
Andrew
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Simplicate and add lightness!!
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Place a wire between the OD solonoid and a light in the drivers cell. Go for a run and see if it is electrically cutting or just the hyrualic valve is faulty. On some models there is a 3rd and 4th gear enable switch which could be a fault but I doubt it, Is this a steering wheel stalk engange switch, dash or button on to of gear stick. There is aan overdrive relay in some versions. Come back with more info after establishing whether it is electrical or not. Regards Peter
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The standard MGB was designed to use 5 star petrol (100 octane)
so a raised compression ratio will result in pinking.
As Andrew says, you need an octane enhancement.
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Peter C, My brother's chrome bumper MGB 1.8 had a dashboard o/d switch plus a vacuum switch to make you close the throttle to engage the o/d, to stop you power shifting.
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Yes, I would start with using the higher octane fuels as it's the simplest solution, also, I wonder if a colder spark plug would help?
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Check the running temp and the idle speed. Bs are prone to running hot so normal chec's stat, radiator flow etc. DON'T remove the stat, no 4 will overheat and may crack the head. Check the tickover speed if much above 800RPM that will promote pinking and running on. Good dose of Redex treatment may help reduce carbon deposits.Also check that the mixture isn't weak as this alos givs problem.
Agree with others diagnoses of O/D problems. Don't reverse with Overdrive engaged you'll destroy it...
Jim
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If I remember correctly, standard MGB CR was 9.5:1 so if the heads had the usual 40-60 thou skimmed off it could be 10-10:5:1 which is a bit high running standard unleaded petrol. Agree with others and think it would be best to use super unleaded or better still Optimax. The extra mpg would recover the extra cost so not a wise move to have the CR lowered IMHO .
Also check for broken springs in distrbutor. If fitted with performance air filters these can weaken the mixture and make pinking worse. Try plugging the vacuum advance on the distributor.
In my experience running high CR enines, tappets, plug gaps, carb balance, points dwell etc. have to be absolutely spot so check carefully.
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I recall a conversation with a couple of guys at Dunfermline Car Club about de-rating gaskets. A solid metal gasket used in conjunction to a standard gasket to lower the compression ratio. Surely the MG owners club would know all about this problem. As far as drilling the piston “ I wouldn’t be doing that then”. Super Unleaded is not that common now and I am sure you have tried it to discover it still gives you a problem on light acceleration and cruise. There were a couple of alternative heads available with swirl pots and all sorts of combustion chamber mods to control the burn process. You may recall when unleaded first came out there were complaints about older series engines burning out exhaust valves mainly due to latency in the burn process causing higher temperature exhaust gases, even still burning gases leaving the pot. I am sure the MG owners club will be able help. Regards Peter
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