low compression, --inevitable strip down??

so-- I finally got round to checking my engine, during my rebuild.
the engine is on the bench, and has been sat a couple of years, with carbs removed and rags stuffed in the intakes and exhaust.
I got the valves in speck a couple of years ago, and ran the engine, and all seemed fine, no bad noises, or overheating.
so I did a compression test today, and the numbers are---

#1----0
#2----45
#3----100
#4----85

squirted a few presses of the oil can in each cylinder, and now

#1----110
#2----155
#3----160
#4----100

so #1 and #4 still low, so this would indicate stuck rings right?.
before I tear into it, what "magic potions" could possibly free these up.
as you can see from my profile I'm from uk, and the chemicals you get off the shelf in u.s. , are just not available easily here.
cheers,
stu

oh, p.s.
engine has 47k miles

 

A engine that has been sitting for a few years will most likely have a few sticky valves. Your compression numbers show that. Pull all the plugs, put some penetrating oil in each cylinder. Spin it over to mix the oil in and let it set a week. Do the compression test again and see if the numbers improve.

 

I don't trust those numbers.

You should get some compression even from totally shot rings. To truly get zero you need a major valve problem or a holed piston. I suppose a valve sticking partly open could do it too. A shot of oil won't bring those up.

 

use a 50/50 mix of atf and acetone. let it soak a few days and try the comp test again.

CN

 

Bensalf - did you check the valve clearance??

I got a "100" reading on a good motor with a valve way out of spec.

 

Compression readings which improve by adding oil to the hole generally indicate the hole has some Ring Related issue.

If you had a Valve or Gasket-related problem, ... the "Wet-test" would have shown little or no improvement.

 

rechecked valve clearance on #1 today, and in spec at, int= .127mm , ex = .2mm
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Compression readings which improve by adding oil to the hole generally indicate the hole has some Ring Related issue.

If you had a Valve or Gasket-related problem, ... the "Wet-test" would have shown little or no improvement.
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yep, that's why I did it.

so-- redid the compression numbers again today with still a trace of oil in there, and-----

#1----25
#2----75
#3----105
#4----90
so it appears the original numbers were correct

I managed to get some acetone today, (from of all places , a pharmacist)
mixed it with a.t.f. , and put a couple of tablespoons in each cylinder,
as cyclenoob suggested.
so i'll let it sit for a few days, and see how we go.
thanks for your help so far
cheers
stu.

 

so I let the acetone/a.t.f. sit in the cylinders for above a week, and redid the compression test.
#1 was back down to zero, other 3 improved slightly up from original test.
so ,as I said in my original heading "inevitable strip down"
so did just that.


squirted some penetrating oil in the upturned head, and it leaked straight out of #1 exhaust valve

other 3 held on a little longer

#4 held longest at about 1hour

pistons measured (with a digital caliper) in spec.
ring end gap is in spec when placed back in cylinder.
nothing to measure the internal bores with but I can see some crosshatching in there from honing.
are the bores honed from manufacture ,or would this have been done recently?
so definitely some valve issues,
but hopeing bores are ok.
stu

 

oh, and, rings were all free, motor has 47k miles
stu

 

The bores would have been honed at manufacturing. If the crosshatch is even on all cylinders then you can put all of your effort into getting the valves in order. I would go ahead and clean up the pistons (paying attention to the ring lands), and the bores with the petrolium-based solvent of your choice, just to make sure any remaining detritus dosen't contribute to premature wear. Ideally you'd remove the rings to clean the lands, but given how your rings are in good shape, and how easy it can be to break them during removal, I'd suggest leaving them in place.

 

bensalf, excellent thread and photos!
Thanks for sharing this.

 

doesn't 47K seem like a lot for still having original cross hatch?
is there a lip at the top of the cylinders?

 

So what's wrong with the exhaust valves? Can you see anything wrong?

I spent January fixing this. Not even close to an XJ, but thought you might like inspiration:

 

Both good questions. I have had, in front of my very eyes, engines with more miles that still have the original crosshatching. It's not common, but it is very possible. The main point being that there is no sign of excessive blowby at the piston shoulders, and the ring end-gaps are in spec. The cylinders could be egg shaped, but if they were I'd expect to see clear signs of blowby on the pistons. What I'm seeing appears to be leakage from the valve guide seals.

Even so, it certanly is a good idea to measure the bores while the engine is apart to confirm my assumption.

 

Rice.......I'm not seeing anything wrong with the EXHAUST valves, but that INTAKE valve looks like it might let a little too much fuel in..........unless you're figuring that with THAT condition, it's become an exhaust valve, too. LOL

The smaller ones are the intake, and the larger ones are the exhaust, iirc.......

dave f

 

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The smaller ones are the intake, and the larger ones are the exhaust, iirc.......
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er, no
t'other way round.
look at my pic of the head, smaller ones exhaust.
stu

 

dave

I just checked in my manual for the Seca900 and they say the head diameter of the valves is 36.1 mm for the intake and 30.0 mm for the exhaust. I would have thought the opposite.

 

take a closer look. the results of not checking and adjusting the valves at the proper intervals.


valve by bmonkey36, on Flickr

CN

 

Nice picture! No wonder the compression was very low on that cylinder!

 

Oops Well I never intended to muddy the waters with that pic, but seems I did...

First off, that pic I posted is not of an XJ. In fact, it's not even a motorcycle. It's from one of the DD's in the family. I posted the pic simply as inspiration and as an example of valve damage that can be seen with the naked eye. I had no compression on that cylinder, and after digging into it, found the reason why. I was just wondering that since bensalf's compression numbers were as gruesome as mine were, that maybe he could see something clearly wrong with his valves as well.

Hogfiddles, No, it's an exhaust valve. Burned up real good, huh? The general configuration is for exhausts to be smaller than intakes.

cyclenoob, it was a joke... Hogfiddles saw the problem. He was making a funny. Also, it's academic, but my valve problem was not caused by improper clearances and lack of maintenance. It was caused by carbon buildup on the seat that prevented proper closure. The hot gasses being pushed through the small opening ate away at the valve.

Sorry for any confusion guys... Back to your regularly scheduled investigation!