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MG Midget and Sprite Technical - Assessing how tired an engine is
|I am thinking of doing a bit of refreshing of my engine. But the question is how much? So to start with it is down to some diagnosis of the amount of wear.|
The engine is a 1330 which has done 70,000 since its last major rebuild. It is running well, revving freely and starts "on the button"
However it is burning a bit of oil - blows some blue smoke when hot, and not just on the over-run or after idling.
First stage assessment is spark plugs. - 1 and 4 are fine, but 2 and 3 are a bit sooty. Dry black soot, not too wet and oily.
The dry readings are pretty well exactly the same as on the last 2 test, it hasn't got any worse. (I routinely test and record compression every 6 months)
The wet readings are higher than they should be - I think I was over enthusiastic with the amount of oil I added! They are normally 10 to 20 psi lower than that.
Will probably take the head off tomorrow to check gasket condition between 2 and 3 and take a look at the bores. I think if it was blowing between 2 and 3 it would have got a lot worse in the last 6,000, but test as at April 2011 is almost identical.
|Here are the plugs as removed. They are NGK BP5ES.|
#1 is on the right, so the blackest one corresponds to the lowest compression. Normally with adjacent cylinders reading low I would suspect the HG, especially with the middle pair. But as it hasn't got any worse I am thinking maybe worn rings.
|#1 looks like it's still glowing with heat :)|
|LOL, yes it does a bit. It was cool enough to take out though! - Don't worry Nigel, it will effectively get a service treat of a set of new plugs!|
|as Lawrence would tell you if he were here, you're being wasteful just clean them up and they'll be good for another 70,000 :)|
|Nigel, you think you are joking, but those HAVE done 70K ! Time for a change, but being me, they will be kept as spares!|
Actually I cracked the insulation on one of the original set and one of these is an odd one out. Can you spot it?
|tut, tut, tut, you go to the trouble of having a 1330 engine and then you stifle it performance by having plugs past their prime - they can look ok but be past their best, I'll see if I can find a veeery long article on this :)|
I was looking for the plug that looked the least decrepid and thought of three then noticed the seal looked crudded so looked at the set again . . . and then noticed three was a different type
I'm lucky to see the monitor screen let alone any details, just remember if you ever see me driving on the road :)
I'd see if you can just get away with a hone and new set of rings.
check the bearings and replace if needed
|Yes nr 3, but why?|
Onno, I was thinking it may need new rings, or possibly a set of those Cords rings, depending on what the bores look like.
Yes I will probably put new shells in. The crank was ground, line bored and balanced, together with a lightened flywheel. Its had regular 3000 mile oil changes and hopefully any wear will be on the shells rather than the journals.
|because the others are pre-1980 design :)|
|No 3 is a different shape and less rusty.|
Aren't BP5ES a bit on the hot side? I thought that a 6 was a standard replacement and a 7 for a 'hotter' tune?! I used to run 8 on the racer.
|fit new plugs (full set, correct spec) and take a fresh look in a couple of weeks...|
|nr 3 is a BP 6ES the others are 5s.|
I fitted grade 5 when it first went on a rolling road. They don't explain the varietion in compression!
This thread was discussed between 29/01/2012 and 30/01/2012
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