What is the compresstion on an XJ6 motor?


(tony) #41

No, that is one of two other things I intend to do before a teardown, and have the tester

The perfectly equal compression on each cyl is “confusing”, but I guess it only proves out valve/seat

If a leakdown test fails, head will have to come off anyway

went back thru the build receipts I have, it mentions head rebuilt with new hardened ex-valve seats, stakedown kit, new valves & springs, but guides not mentioned


(Nigelplug) #42

Ok Jochen, thanks anyway. I’ll let you know what I discover
Best Regards
Nigel


(Frank Andersen) #43

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The standard indication is only described for worn valve guides, plural, Tony - and indeed vacuum vibrating excessively in idle indicates worn guides. Readings steadying as rpms increase…

Anyway, with worn guides the plugs should show heavy oil deposits - and with one guide loose; confined to that cylinder. And if we want to associate the rear carb whistle with a loose guide - it should be on one of the rear cylinders…?

While a guide problem may be related to the vacuum vibration - associating it with a carb whistle is a long bow drawn? Admittedly, varying air flow is how organ pipes work - but how this can occur at the carb is beyond my experience. However, if same whistle occurs also with a replacement carb - it’s difficult to discard…

In theory, the whistle may be caused by some internal leak in the carb - not revealed by an external leak test? I assume you included the butterfly spindle in the external leak test - I have had that whistle…:slight_smile:

The vacuum reaction to a worn guide is in itself odd; very little extra air can be drawn past warn guides - to me it sounds more probable that the valve(s) fail to seat ‘instantly’, but sort of ‘wriggling’ into place, slightly delaying closing?

The compression test will not necessarily show worn guides - the readings are performed over several valve cycles, recording maxima. Only a valve failing to close completely throughout the test cycle will show up…

The leakdown test may or may not, for the same reasons, not show a worn guide - but all tests possible should be performed before engine strip-down. Anomalies found after the strip-down may be conclusive - but any indications on what to look for is helpful…

Frank
xj6 85 Sov Europe (UK/NZ)
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(tony) #44

Thanks for all the good ideas

I observed the compression rise equally on each stroke, for each cyl,

apart from the leakdown test, the other one will be as per Hammill,
that is fit marked rods into the piston tops and see if they rise at the same rate, this one may give me an indication of unbalanced airflow

pulling plug wires on each cyl with engine running does not seem to show one weaker than any other

Clearly I dont want to pull the head, but will if I cannot rule out a problem
atm, I am completely refurbishing the headlining area, holding me up


(Mark Lee) #45

Based on my (domestic engine) rebuilds; the only way to check valve guide wear is by popping the head off and measuring. I suppose if it’s really bad, getting the cyl on a compression stroke, pop the cam and valve springs out, and see if the valve will wiggle at all. this will also gain you access to the stem seal. I’ve been lucky on occasion by just replacing the seals. Compressed air in the cylinder is always a good idea when dealing with a un-sprung valve. it will help to keep the valve from dropping in the cyl.

Borescope is another handy tool. run it down the intake port, and see if there is any oil build up on the intake valve. If so, either seal or worn guide or both. While I was writing this I had a Series I and early Series II in mind. This could be a bit of a problem for the FI engines. A bit tough, to find a port with the a plenum.

So chalk it up to this guy (me) doesn’t have a clue of what he’s talking about.


(Frank Andersen) #46

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In principle, Tony; if the compression is even, the pistons rise the same amount - and vice versa…

Anything to avoid a needless head removal. There are some tests that positively confirms the necessity - but few, if any, that positively confirms that head removal is a waste. One is of course if the cause of symptoms is positively found and fixed to be external to the head…

Worn guides would lead to abnormal oiling up of the plugs, but so might failing oil seals on the inlet valve. In principle; valve wobble, indicating worn valve guides, ‘might’ be detectable by removing valve springs and tappet, head in place - but honestly…removing the head is less hazardous!

Did you try the plug disconnections with the vacuum gauge attached? It will add vacuum anomalies, but cylinder difference may be detectable? However, vacuum flux with valve guide problems will disappear with increasing rpms - does it?

But the whistling carb should be pursued further - as a possible separate problem…

Frank
xj6 85 Sov Europe (UK/NZ)
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(tony) #47

regards vacuum readings, as the RPM increases, the needle movement flicker speed increases greatly, but is still about 2* range ( I think)

I tried 2 different vacuum gauges, and even found the air intake bore cross-section of the gauge affects readings

cant remember for sure if I checked the gauge while pulling plugs, but almost certainly would have

even though others have reported whistling carbs, its a very distinctive sound , an oscillation that increases with RPM…wish I could capture & post it!


(Frank Andersen) #48

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The standard description for worn valve guides are for flickering vacuum needle in idle to stabilize as rpms increase, Tony…

Mind you, it describes ‘worn guides’, assuming equal wear on all guides - what happens if one guide misbehaves may not apply…?

The vacuum reflects engine power, variation in vacuum generally means that one or more cylinders are not pulling their weight when their turn comes. If one carb is misbehaving, three cylinders will misbehave - and vacuum will certainly oscillate. More or less depending on the degree of misbehaviour - but likely oscillations will increase with rpms…

While I would hesitate to pull a head if compression readings are OK, valve clearances to specs and no coolant loss, doesn’t mean that it won’t be unnecessary. I’d just try to eliminate other factors to avoid it…:slight_smile:

Are the carbs properly synchronised; throttle gaps even - and using a carb sync tool to ensure synchronisation? Carb whistle may be unrelated to your vacuum issues - but it should not be there. Adjusting the carbs by ear is fair enough; sounding the same indicates rough sync - sounding different sync is at least not confirmed…

Frank
xj6 85 Sov Europe (UK/NZ)
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