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Poor Compression

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#1 surfblue


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Posted 07 June 2019 - 02:50 PM

So my Mini had been heard to start recently over the winter when I would start it every month or so to keep things moving. Went to get it out at the start of May and it wouldnt start at all, Discovered the head gasket had gone with the cooling system pressurised and mayo gunk in the rocker cover.

Took the head off, ground in all the valves and re built it with a new head gasket.
Still wouldnt start,
Checked its getting sparks and drained and replaced the fuel, its getting petrol but still not starting. Timing hasnt been altered.
So bought a compression tester and popped it on today.
Results are I assume poor:

No 1, 100 psi
No 2, 65 psi
No 3, 100 psi
No 4, 95 psi

Gave each a squirt of engine oil to ss if that improved things then checked again:

No 1, 115 psi
No 2, 75 psi
No 3, 110 psi
No 4, 100 psi

So obviously No 2 is significantly worse that the other 3 but is it so bad that the engine shouldnt be starting, even on 3 cylinders running roughly? Is it possible that the replacement head gasket hasnt sealed correctly around No 2?

Can anyone advise what a standard 850 engine in good health should be producing by way of cylinder compression?

Although getting sparks at each plug they dont seem to be the biggest , brightest sparks Ive ever seen so Im wondering if a different coil or points and condensor might improve things?

I realise that this engine isnt performing at its best and the plan will be to do a rebuild in the fullness of time however I dont have much space in my garage for an engine out job at the minute. We're moving house next year when I'll have a large garage / workshop with as much sapace as I'd need and that would be the sensible time to have it done. Id just like to get it running for a little use this summer,

Any ideas or suggestions anyone or am I wasting my time with those readings?

Edited by surfblue, 07 June 2019 - 02:54 PM.

#2 nicklouse


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Posted 07 June 2019 - 02:59 PM

first off what was the head gasket like?


mayo and pressure are not a sign of head gasket issues on their own.


milky oil and blowing the water out of the system would be. mayo is just a sign of it never getting up to temp through short drives or on and off starting in cold weather.


ok so the compression test.


are these your averaged figures or just the only set you took?


for accurate reading you should charge the battery. remove the plugs and hold the throttle open.


take readings from, say, 1 to 4 and the repeat in reverse. this takes out any change in battery power, rpm etc.


but saying that it does look like you have a small issue on 2  but I would say your problem is somewhere else though.


what dizzy and coil?


what voltage is the coil getting?


point set right?

#3 unburntfuelinthemorning


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Posted 07 June 2019 - 03:00 PM

They are low.  I'd expect to see 150 psi. 


Did you test with the throttle open? 


Did you let it turn over a few times on each test?


Are the valves adjusted correctly?

#4 surfblue


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Posted 07 June 2019 - 03:17 PM

There was no sign that the gasket had failed anywhere obviously when I checked but it certainly was giving indications of it.

The throttle was held open while tested  with all 4 plugs out. Battery fully charged.

Valve clearances are correct. Points correclly adjusted. (I had changed these from a newly fitted set back to the originals which know to be working

My multimeter has a broken lead at the minute so cant actually check voltage at the coil which is a recent Lucas sports coil, I do have the one I replaced it with so might chuck that back on just to rule that out, (cant remember why it was changed now but know the old one still works)

#5 surfblue


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Posted 07 June 2019 - 03:50 PM

So just made a temporary lead for the multimeter.

Coil -ve 12.5v

Coil +ve 0.14v

Changed over the coil also, both coils very similar readings.

(Its an early car still positive earth, if that makes a difference?)

Standard 25D Lucas distributor, points and condensor.

Edited by surfblue, 07 June 2019 - 03:50 PM.

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