MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGA - MGB head

Hello, I have fitted a new MGB head on a 1600 engine that did not run hot befor. What can I do to get the temp down? Could there be a blockage in the new head or is this normal? Does not make sense to me. The engine also backfires now upon starting and never did befor. Thanks, Neal Turner
Neal Turner

timing?
John Greenlee

Which MGB head did you use? I am not sure, but I believe some of them had shallower combustion chambers which would raise the compression ratio and require higher octane fuel or retarded timing. I think that some MGB heads also require valve relief notches to be cut into the engine block.
http://www.mgbmga.com/tech/mga3.htm

Jeff Schultz

Backfires on startup sounds too lean. verify head as above.
R J Brown

Hi, This head is 67 vintage with compression at 145 psi. The timing has not changed but the carbs have been fooled around with by some one who is supposed to know a whole lot more than me. I am suspect of the carb settings (too lean) and or the possibality of more flow more richness required. The old head ran perfectly and I never had a problem with heat even on the hottest days when I was stuck in traffic behind a tar truck at high noon. I also ran Amoco high test 93 and the engine ran and cut off perfectly. Now it deisels with the same gas and requires air craft fuel to prevent deiseling. What a pain for a few more hp. Thanks for your patience, Neal Turner
Neal Turner

Compression here is not measured by psi but by a ratio like 8:1,8.5:1 or real high 10:1 etc. It is dependent on the size and shape of the pistons and the volume of the combustion chamber. Higher compression can make more power but can cause problems like over heating and pinging.
Do you know what the bore size is? (standard, .010, .020, .030, what pistons are fitted? was the head milled? with that info someone on this site could give you an idea of what you comression ratio is.
R J Brown

Hi, The engine bore is standard and pistons are flat. I do not know the depth of the head but will try to find out from the machinest Monday. I haven't taken a comp check lately but the last check indicated 145 psi across the board. The engine runs and performs well otherwise. Thanks, Neal Turner
Neal Turner

This page has specs for MGA heads, http://www.mgbmga.com/tech/mga6.htm and this page has specs for MGB heads. http://www.mgbmga.com/tech/mgb25.htm
The standard 1600 head has a combustion chamber volume of 38.7cc and the 67 MGB is 43cc, so unless the MGB head was shaved, the compression ratio should go down. Was anything else changed at the same time?

Like RJ said a compression test will not tell you the compression ratio. If you took a compression reading before and after changing the head it might give you some indication of the relative compression ratio, but there are so many other variables that can alter compression test readings that it is not a reliable indicator of compression ratio.

Perhaps it is just a matter of getting the jetting correct on the carbs. Like has already been mentioned, a lean mixture can cause the same types of problems. What carbs and needles and jets are you running? It is also possible that you have an air leak somewhere which is leaning out the mixture.

Jeff Schultz

This thread was discussed between 17/09/2005 and 18/09/2005

MG MGA index

This thread is from the archive. The Live MG MGA BBS is active now.