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Chaps

A swift search seems to be yielding differing opinions on this.  So, in the hope of minimising my incompetence, can I 'umbly request confirmation this this is the failsafe methodology for pressure testing:

-  cold engine (all spark plugs out, natch)

-  test dry

-  hold throttle fully open

-  three or four rotations

-  look for variations in individual cylinders rather than a high average value

-  repeat wet if there are variations

I'm getting a lot of oil pumped out of the front three cylinders (based on eyeballing the stain variation on the garage floor and rear wall).  The head was replaced with a pukka rebuilt one about four years ago, so there cannot be an issue there, and I'm assuming that I've got cylinder wear or a broken ring somewhere.

Paul

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Hot test Paul, IMO.

You want to be as close as possible to running conditions as you can bear, where full engine is warmed through and everything that can expand slightly has done so.

The number of revolutions, basically spin until pressure stabilises. Because the starter doesn't spin the engine at any major speed it can sometimes take a while to reach max pressure. This is why you remove the other spark plugs, it makes it easier to spin the engine that bit faster.

Everything else is spot on. As you correctly say, the important info is the comparison of the values to each other, the actual value doesn't mean that much (though expect to be above say 120 on a solid engine). The wet test WILL increase your pressures compared to dry, again you are looking to see if they all increase a moderate amount compared to dry, or if one cylinder leaps higher (which suggests rings/bore issue). And then a low reading on one or more units that doesn't change much on a wet test is likely pointing to valve seat or gasket leak.

I hope this satisfies your 'umble request, 

Phil

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Does the engine actually use significant quantities of oil?

Assuming you are judging this by what comes out of the two tailpipes (one linked to front 3 one linked to back 3?), consider also that the same split exists on the induction side so a rich front carb/sticking choke might give the same effect....

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34 minutes ago, John I said:

Now showing my lack of knowledge, what is the difference between a dry and wet compression test? :craig:

Hi John,

A dry test is the engine as it. This will/may show air leaking passed the piston/rings etc

A wet test is when you squirt clean engine oil into the tops of the pistons. This will run down the piston walls and help seal this area. If you still have a pressure problem then it may isolate the valves(seats)

 

Roger

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Posted (edited)
1 hour ago, Nick Jones said:

Does the engine actually use significant quantities of oil?

Assuming you are judging this by what comes out of the two tailpipes (one linked to front 3 one linked to back 3?), consider also that the same split exists on the induction side so a rich front carb/sticking choke might give the same effect....

Thanks, Nick - tricky question to answer.  Yes, it consumes some oil, but I haven't been dilligent enough to measure how much over what mileage.  For a couple of years, the car has seen far less of the road than I would like.  It's a three-into-two exhaust: one pipe serves the front three cyls, the other the rear three.

The plugs don't give much clue - 1, 3, 4 & 6 are good, 2 & 5 are distinctly oily.  It's an HS6 conversion, so theoretically, both chokes are mechanically linked. And the pressure relief pipe feeds back into both carbs with equal length pipes, so no preferential burn of oil off the head (which I'm prepared to believe is as good as it can be).

No rattles to indicate worn bearings or broken parts.  Okay, let me rephrase that... no additional rattles to suggest that something is freshly amiss.

When the head was replaced, four years/c.6k miles ago, there was no scoring in any cylinder to be seen, nor any lipping at the top of the cylinder to suggest wear.  These pictures are pretty, but bu&&er all use for seeing anything in detail:

DSC_1384.thumb.JPG.b8939106e8da8dd70cb744e163a62149.JPG

DSC_1388.thumb.JPG.12951f869d8426050cec84136b05f800.JPG

Nor does the view of the old head give much away, other than to suggest that the front three cylinders spent a lot of their recent past running rich:

DSC_1390.thumb.JPG.942ce4ea523f5023f71047bbe38a5700.JPG

So I thought I'd endeavour to get a reliable reading from the pressure test to determine whether there is any good reason to whip the top and bottom off and tackle the rings.

Paul

Edited by PaulAA
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Posted (edited)

Hi Roger

I've not dismantled to check, but, given where (and how recently) the replacement head was machined and built, I'm prepared to vouch for them being in place...

... or is this a potentially rash assumption? :blink:

Paul

 

DSC_1391.thumb.JPG.d74ff7dae4602868106ea576819f7672.JPG

Edited by PaulAA
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They don’t have seals from the factory..... but various possibilities exist for retro fitting.

Although the chokes may be linked, the classic SU favourite trick is for one or both of the jets to not quite fully return to the adjustment nut when the choke is pushed home, causing richer than intended running. 

IIRC, 2 & 5 get slightly less flow with a standard manifold.

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Posted (edited)
16 hours ago, Nick Jones said:

They don’t have seals from the factory..... but various possibilities exist for retro fitting.

Although the chokes may be linked, the classic SU favourite trick is for one or both of the jets to not quite fully return to the adjustment nut when the choke is pushed home, causing richer than intended running. 

IIRC, 2 & 5 get slightly less flow with a standard manifold.

Just checked the original invoice for the head build and there is indeed a line item for 'viton stem seals'.

I too have read about the constricted flow to 2 & 5 in the carb manifold.  Some feedback elsewhere suggested that the late CF cars had an improved casting and that the problem was much reduced, but I tried contacting a certain Triumph head expert about getting the manifold shipped to him for some internal shaving and flow improvement, but I failed awaken his interest.

In the meantime, a buddy with too much money and a beautiful post-Jensen Volvo P1800 has just acquired (as you do...) a rare Judson supercharger and accompanying manifold, thus releasing the pair of Mikuni HSRs that he had bolted to B20 lump.

Tempt me not, satan!

Paul

Edited by PaulAA
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39 minutes ago, PaulAA said:

Tempt me not, satan!

Why ever not....? :huh:

40 minutes ago, PaulAA said:

CF cars had an improved casting and that the problem was much reduced

The long branch manifolds are supposedly better, dunno how much better.

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1 hour ago, Nick Jones said:

Why ever not....? :huh:

 

The terms 'buddy' and 'too much money' do not preclude a healthy desire to recoup his investment with Messrs Performance Parts of New York. A little too rich for my wallet at the moment...

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Fair enough :biggrin:  You might not notice much difference anyway!

Suggest just checking that the choke mechanism allows the jets to fully return (every time!) as a first step and, unless oil/fuel consumption are horrific.... go out and enjoy the thing!

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41 minutes ago, Nick Jones said:

Fair enough :biggrin:  You might not notice much difference anyway!

Suggest just checking that the choke mechanism allows the jets to fully return (every time!) as a first step and, unless oil/fuel consumption are horrific.... go out and enjoy the thing!

Sage words  :thumbsup:

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Paul,

Please tell us when you have some compression figures, a la Phil.   it's been a largely theoretical thread so far.   

Would a cheap mobile phone borescope let you inspect them, in case there has been a broken ring and wall damage?

Your pic of the old head with sooty front chambers might indicate that here was an oiling probelem even then.

John

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To me, any clear difference between front three and back three on a 6 with twin carbs, is telling you that there is a difference between front and rear carb settings. 

On a car with a 6-2 exhaust, divided front three/rear three, as here, I suppose it could also indicate a difference/ problem there, but seems improbable......

My previous advice stands.... :smile:

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Posted (edited)
30 minutes ago, JohnD said:

Paul,

Please tell us when you have some compression figures, a la Phil.   it's been a largely theoretical thread so far.   

 

This is true.

I'm hoping to be granted a leave of absence from the domestic cage this evening and will report back with figures.  Unfortunately, it is currently snowing, so I will only get to do the cold test.

Paul

Edited by PaulAA
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I've not had the opportunity to take the hot figures - the snow has stopped, but it's not very inviting out there.  In the meantime, the cold and dry figures are:

1:   165
2:   165
3:   160
4:   172
5:   160
6:   168

So, range is 12 psi, 7.5% more than the lowest.  Looking pretty good, no?

But, but, but... there is quite a lot of oil around (all of) the plug openings, so I think I'm going to have to pull the rocker cover off at least and renew the seal. Postage from the UK and an indefinite wait for a new gasket. Oh, joy...

Paul

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On 4/8/2021 at 2:28 PM, JohnD said:

Paul,

Would a cheap mobile phone borescope let you inspect them, in case there has been a broken ring and wall damage?

John

John

I have one of those and it is somewhere north of useless.  It seems that the world of borescopes falls into two catergories - cheap, small & useless or brilliant, substantial & hideously expensive.

My brother-in-law offered me some of his old dental mirrors, which seems like a better option...

Paul

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If you can get one of those through the spark plug hole, and a torch, and are able to see anything, then you are a rectal surgeon.

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2 hours ago, PaulAA said:

So, range is 12 psi, 7.5% more than the lowest.  Looking pretty good, no?

Look pretty excellent to me.

Tip for fitting the rocker cover gasket. Clean both mating surfaces and thoroughly degrease the rocker cover. 
 

Put a thin smear of grease on the cylinder head surface.

Now stick the cork gasket to the rocker cover with a good quality silicone gasket goo (victor reinz is my current fave) and smear a layer on the head facing surface too - in fact I typically aim to fully encapsulate the cork in in a thin layer of silicone.

Then fit the cover as usual. Do not over-tighten.

The grease on the head surface means the rocker cover and gasket come away clean removal and you can re-use indefinitely. The one on the Vitesse has been in service at least a decade on two engines and survived multiple removal/refit cycles.

 

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