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Even but low compression on 3.8 - timing slightly off?


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Before I run away screaming, would injbg being slightly off result in even but low compression on an engine rebuild? My initial take is no, since even if timing is off it both valves would still be closed at some point in a full rotation and this come up to expect expected 150 range?

Still a newbiebhere, feel free to educate me!


Deleted member 8987

Before I run away screaming, would injbg being slightly off result in even but low compression on an engine rebuild? My initial take is no, since even if timing is off it both valves would still be closed at some point in a full rotation and this come up to expect expected 150 range?

Still a newbiebhere, feel free to educate me!


Not sure what a injbg is to be off, but the subject line....since you are cranking with no spark to get compression, you can have the distributor fully removed and it would make zero difference.

So, a lot of questions, I guess.

Does it actually run, or are you doing a compression test on a non-running engine to determine if there are issues?

Do you know if your compression tester is even close to correct to begin with?

Even readings are good. Usually means nothing broken.

Now, one squirt of oil down each hole, crank it a couple of revs to spread the oil, re-do the readings (remember #1 is in the back if you are keeping records). This is especially important if it's a non-running engine.

What are your actual readings?

Cam timing can and does throw compression off and vacuum. BTDT. Cam timing is almost infinitely adjustable.

I would also get the necessary tools and manuals and check/adjust upper chain tension.


Country flag
Good morning - I thought you would find that title intriguing. Mostly from a "what the heck is he doing now, why do I have to keep bailing him out" sort of thing. Ha.

So here is the scoop even came across an otherwise driver quality car that has some potential. The car will run, but is obviously not well. The engine was rebuilt a couple years ago by another owner and the guy that owns it now is putting forth that he thinks the timing is just slightly off which is why the compression is so low.

The car is not local to me so I don't have the advantage of running over and doing everything in person.

Basically what I'm asking is if the timing is off by a couple teeth if that would actually result in a consistent 50 psi across all the cylinders and if correcting the timing would bring it up to a healthy indicated range.

Deleted member 8987

Ignition timing....distributor off a couple of teeth...no effect on compression at all. Running? Well, teeth make less difference than you'd think...IF they connected a timing light and actually set the timing.
Distributor is a tanged shaft drops into the top of the oil pump if I recall. "teeth off" would be how the timing chain and gears were set up for the auxilliary shaft. If it's off, and you cannot get the distributor cranked in, oh, lord, you don't want to go there.

My first question with those facts would be with this data, "The engine was rebuilt a couple years ago by another owner and the guy that owns it now is putting forth that he thinks the timing is just slightly off which is why the compression is so low.", did it EVER run correctly?

If you don't set things up exactly correctly initially, some stuff cannot be easily fixed later.

Give you an example.
On mine, there is a timing mark on the bottom front of the sump. AND one on the edge of the flywheel, visible through an opening behind the exhaust manifold.

Morons that set mine up, who know how many decades before I bought the car, did not locate TDC by the book, and just threw the flywheel on. Then they set the cams with the wrong TDC, and both cams were about 30 crank degrees retarded.
When you tear something down that has "issues" you try to do it forensically so you have some data for later.

Inlet valves were hitting the pistons, two valves bent slightly, they set the inlet valve lash to ZERO to minimize piston/valve interference....engine vacuum way low, compression off, and with it being a 2.4L, with all the BS added it had the power of a Briggs and Stratton lawnmower.

So, once you run a wet compression test (one squirt only Vasili) and compare to dry, you will know f the rings are toast. If the readings stay close, then you buy the car, drag it home, yank the cam covers, obtain the cam block plates, set the upper chain tension, ensure TDC is really TDC, set your cams, check your valve lash, and then report you readings.

Before you loosen up the cam gears to set cams, if the block plate does not fit, roll the engine by hand (plugs out makes it easier) until one drops in, check the timing marks, extrapolate the distance advanced to retarded, make a note, and report here.

Remember, at that point in time you OWN a Jag with an issue that the seller is trying to tell you is timing off.
Remember, that is pure unadulterated BS.
Nobody sells a car that "just needs a tune-up" or "compression is low because the timing is off" without fixing that and getting the higher dollar figure for it.

They either know from looking themselves or have been told what the issue it.

Deleted member 8987

Okay....since I ain't buying and I don't need to know where, what model Jag?
How much money?
What else is wrong with it?


Country flag
Right-ignition timing, who cares? Not related and stupid easy to fix.

Cam timing-totally different kettle of fish, and sorry if I was unclear about this being what I'm asking. it would seem to me that the cam timing could be off but the compression still near or at normal ranges, right?

Have not asked for a wet compression test, looking to make sure I understand how the cam timing could potential he/not be the culprit here.

Car is in Oregon, non-overdrive for speed, right hand drive.


Deleted member 8987

Yeah, Orey-gun is a bit far to pop in from SoCal to have a look-see.
Wet will tell you if the rebuild was garbage as far as rings and cylinders..maybe.

You're in SoCal.....I was born there...Remember (ethnic) rebuilds? Overhauls? "Oh, yeah, we pulled the valve cover and painted it, installed a new gasket, so it's rebuilt!"

Paperwork. Who did it and where are the receipts?

Did they do an in-frame?
Hone it and slap rings in it?
Was it bored with new pistons and rings?
Crank turned, new bearings?

It runs, but not well....does it get hot when it idles? What is cold oil pressure? Hot oil pressure?

If the cam timing is off, what else is off?

Remember #1 is at the rear...when I pulled my 2.4 apart, the morons center-punch marked the rods and mains from the FRONT.

They didn't know anything about it.

Cotter pins (12) through rod bolts and nuts...not one was there. But six were stuck in the sump and screen (over one inch of hard sludge in the bottom of the sump...probably not even cleaned....it now takes one more full quart and maybe a bit more oil to fill than it did before) and at least one pin went through the oil pump, splitting it. All that and it ran for eight years from when I got it.

No paperwork, no compression, if you buy it, gotta be cheap, and you have to be prepared to go through it, worst case scenario.

Valve timing off enough that the valves open before they are supposed to, you can lose compression.
Valves too tight.
If the cams are way off and they tried to correct loss of power with advance in the ignition, pinging can and will break rings. Resulting in loss of compression.

On mine, the power loss was so bad after they rebuilt it, they pulled off the original Solex carbs and replaced them with mismatched Mercedes Pontoon carbs. Hosed linkages, fuel lines, choke..all in an attempt to get more power.
Then they yanked the distributor, and instead of replacing the blown vacuum advance, brazed the breaker plate solid in three places.

Cams off, inlet lash to zero so the engine could turn over, welded advance plate, HUGE carbs, and it wouldn't get out of it's own way.

Proper carbs, fixed the distributor, rebuilt engine, set cams and lash properly, and it is at least double the power it had.

You won't know until you get it and dive into it.
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