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 Post subject: Lost Compression?
PostPosted: Wed Mar 27, 2019 6:50 pm 
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Location: North Yorkshire
I've a problem with my Sprint engine and hope the club gurus can help me.

Over winter, I decided to correct the valve clearances which have caused clattering since I’ve owned the car.
Before disassembly I decided to carry out a compression test, which showed I had just under 130psi on all cylinders. While this was not earth shatteringly good, I didn’t think it too bad for a 100k plus, 44-year-old engine.

Stripping the rocker gear and cam from the head, I discovered a few mismatched bolts and dodgy thread repairs, so decided to remove the head and send to my local machine shop for repairs.
While there, I asked them to ‘face’ the head, removing the minimum amount of material to clean the gasket face and removing any corrosion.
Once returned, I shimmed the valve clearances on the bench and refitted the head, carbs exhaust etc.
The head was torqued down in three stages 20, 40, 58 lb/ft backing off an 1/8 of a turn each time, following the sequence in the factory manual, onto a Payan head gasket supplied by Rimmers.
I attempted to start the engine and could immediately tell the difference in the speed which it turned over on the starter
With difficulty, the engine started but did not run cleanly and would not idle.
Repeating the compression test i found the following:
Cylinder No.1 – 85psi
Cylinder No.2 – 95psi
Cylinder No.3 - 95psi
Cylinder No.4 – 95psi

So now I’m stuck for ideas as to why.
After a few mugs of tea and some head scratching I have come up with these possible problems.
1. I have adjusted the valve clearances too tight (or they have changed during assembly) to the point where the valves aren’t fully closing.
2. I have somehow bent a valve – I was very careful to avoid this, turning the away from TDC during assembly
3. The machine shop have not faced the head correctly resulting in the head not seating correctly
4. The cam timing has moved very slightly leaving the valves open during the compression stroke (is this possible)?
Can anyone help?

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 Post subject: Re: Lost Compression?
PostPosted: Wed Mar 27, 2019 10:57 pm 
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My money is on the timing being a bit out, since it affects all cylinders. Not sure I could see it being a shimming issue, some on here say you can take the tolerances higher than it says in the book and the engine is cold when you are doing the test.

If they have skimmed the head I would have thought you would have got higher compression, instead of lower. Did they lap in the valves and/or put the valves back in the right order? I suppose if they mismatched them it could cause problems but I can't imagine professionals would do that.


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 Post subject: Re: Lost Compression?
PostPosted: Thu Mar 28, 2019 5:16 pm 
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Joined: Mon Nov 23, 2015 7:43 pm
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Thanks for the reply.
I struggle to picture in my mind how the cam timing can move when the top pulley is supported on the bracket and the camshaft only has two fixing bolts.
Assuming the bottom end hasn't become detached, the camshaft can only be either correct or 180 degrees out and the engine wouldn't run?
I don't think they removed the valves when skimming the head, so they won't have been re-ground.
I guess I'll only know when I strip the head off again. :roll:

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 Post subject: Re: Lost Compression?
PostPosted: Thu Mar 28, 2019 5:49 pm 
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Pour some oil in each bore and retest compression. If it is the same you have a leaky valve situation.
If it rises you have a ring / bore problem.
Rings expand with heat and close up slightly so check when the engine is hot.
Are you sure the tension was kept on the chain when the head was off?

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 Post subject: Re: Lost Compression?
PostPosted: Sun Mar 31, 2019 8:10 am 
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Check valve clearances while head is still on. Massive clearance = bent valves (they cant make it home to the seat). Tight clearances = well, just too tight. On the exhaust side just waggle the rockers to see if theres clearance for quick check. With crank on the timing mark TDC, you can look down the oil filler cap and just see the cam timing marks. If you cant turn the crank a full turn and look again.

Tony

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 Post subject: Re: Lost Compression?
PostPosted: Sun Mar 31, 2019 1:14 pm 
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I have had a few heads skimmed in the past and the valves always came out. otherwise all sorts of crap will end up underneath them. Plus when you have come that far, grinding them is a must.


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 Post subject: Re: Lost Compression?
PostPosted: Mon Apr 01, 2019 6:14 pm 
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Thanks for you advice everyone.
I stripped the ancillaries of the head at the weekend ready to remove it and measured the valve clearances.
I set them all at 8 thou" (0.2mm) but i'm not sure where I got this figure from as both Haynes and the factory manual states 18 thou" (0.42mm), so this is likely to be my problem.
I've also check the timing marks (the crank is difficult to see) but they look correct.
The valves must have been removed for facing the head, as I checked for swarf in the ports and could see none.

My plan now is to remove the head, lap the valves in, and re-shim the valve clearances to the factory manual.
I'll make sure I compression test it before refitting the carbs and report back.........

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 Post subject: Re: Lost Compression?
PostPosted: Mon Apr 01, 2019 8:01 pm 
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Before taking all apart i would check the timing. The valveplay you have is ok. The factory figures are way too large. When checking the valve play make sure the head it torqued down. A "bench" play check or adjustment is not totally accurate because of the valvetrain is normally under the headstuds torqued tight to the head. The few little bolts holding the caps on the "bench" don't tighten the caps 100%.

Lapping valves is not just grinding some valves in some seats. The contact area of the seat and valve should be around 2mm. More gives less pressure per square mm2 so more easy for valves to leak or to go leaky. Too much contact area will also have less hot valves making it easier for contamination build up.

Succes.

Jeroen

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 Post subject: Re: Lost Compression?
PostPosted: Tue Apr 02, 2019 10:15 pm 
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If you do a compression test with the engine cold, would the valve clearances matter? Surely the valves would only be held open when all the components have expanded.


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