I have a 1969 with a 1098 engine.
What should the compression be?
I have read lots of threads and the answers vary from 125 to 150.
I have 2 black plugs and 2 grey plugs, and the MOT tester said I had high Hydrocarbon readings
I have 3 cylinders at 132 and one at 140, do I have a problem?
Compression
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philthehill
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Re: Compression
The compressions are not good.
I would be looking for all four cylinders to be at least 150.
There is only a problem if you cannot live with the readings and engine condition you have.
I personally would be looking to overhaul, at a minimum new rings and a de-coke in the near future.
Unfortunately you cannot determine the condition of the bores which will allow you to decide whether you can get away with just new rings or if the bores are bad undertake a re-bore without removing the head which is the down side.
It may be best to find a S/H engine and fit that to your car and then overhaul your original engine in slow time.
I would be looking for all four cylinders to be at least 150.
There is only a problem if you cannot live with the readings and engine condition you have.
I personally would be looking to overhaul, at a minimum new rings and a de-coke in the near future.
Unfortunately you cannot determine the condition of the bores which will allow you to decide whether you can get away with just new rings or if the bores are bad undertake a re-bore without removing the head which is the down side.
It may be best to find a S/H engine and fit that to your car and then overhaul your original engine in slow time.
Last edited by philthehill on Wed Mar 22, 2017 4:38 pm, edited 1 time in total.
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dalebrignall
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Re: Compression
they are not far off i would do a wet test to see if the values go up a lot then its rings but i daught thats the case . a moggy does not need an emissions test just a visual one , to be honest i would not worry too much .
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oliver90owner
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Re: Compression
Are the two pairs of black and grey plugs adjacent? Is the plug deposit oily or sooty? Does the engine burn oil? Does the engine smoke when pulling away after a period of idling?
Compression test guages are not necessarily that accurate. Even pressures are more important and even starter motor condition can affect the readings. Even check that the plugs are the same grade, if a 'new-to-you' engine.
I would not necessarily be jumping for the boring bar, or even re-ringing, before checking out the top end. New pistons, or even just rings, can stress a weak bottom end, so consideration of the whole reconditioning process needs more thought before a decision is made - so more info required, I'm afraid.
Even a small air leak into the inlet manifold might be causing a high emission reading, although the comment that the high readings were of hydrocarbons, rather than carbon monoxide might be relevant.
I would not be overly worried if it passed the MoT. Time to save up for any repairs, at least, if necessary.
RAB
Compression test guages are not necessarily that accurate. Even pressures are more important and even starter motor condition can affect the readings. Even check that the plugs are the same grade, if a 'new-to-you' engine.
I would not necessarily be jumping for the boring bar, or even re-ringing, before checking out the top end. New pistons, or even just rings, can stress a weak bottom end, so consideration of the whole reconditioning process needs more thought before a decision is made - so more info required, I'm afraid.
Even a small air leak into the inlet manifold might be causing a high emission reading, although the comment that the high readings were of hydrocarbons, rather than carbon monoxide might be relevant.
I would not be overly worried if it passed the MoT. Time to save up for any repairs, at least, if necessary.
RAB
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Budgie
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Re: Compression
Just for info. My 1098 engine has 120 pressure on all four cylinders and I recently had the whole lot out of the car to carry out a gearbox refurb. So while I was at it I checked the engine and all seemed ok with regards to bore, wear etc.. She runs as sweet as a nut and uses no oil, pulls very well and starts nice and sharp. The plugs are the colour they should be and she sounds smooth to the ear. Long may it continue 
Re: Compression
Many Thanks all....................I think I'll live with it, and worry about if it gives up the ghost or fails an MOT !