So I finally got a compression tester, and I can't say I'm really excited about the results:

#1 90PSI
#2 90PSI
#3 20PSI
#4 80PSI
#5 90PSI
#6 80PSI

The car has always had pretty rough idles, but has never had oil burning issues, blue smoke, coolant leaking into oil, etc. My mileage has always reeked of fecal matter, on the other hand. Like 14-16 highway. Now I'm beginning to wonder if this was the issue all along.

Ignition is totally new, as I just replaced the old one with HEI. Obviously, that has nothing to do with compression, but I'm curious what the experts here would do. Any other tests I should look at? Will I need to get a new or rebuilt head?

This is sounding expensive already.

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63 Plymouth Valiant V200 Convertible.
225, HEI, Super Six with 38/38 Weber, Electric Fans, Scarebird discs, FirmFeel front swaybar.

If you have the original 63 engine, (or any SL6 up to 1980) pull the valve cover and check the valve lash adjustment, then re-check.
DD

Don't get discouraged yet. As Doc says, the valve adjustment has to be right or the results will be wrong. valve adjustment procedure

Also, the compression test has to be done correctly. Engine at operating temperature, all spark plugs removed, battery fully charged up, choke and throttle fully open.

Try again and see what you get.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Valve lash, definately, need to look at that first, before assuming the worst.

Any other history? Was this ever a rebuild? I'd think the camshaft timing would have to be way off, like a couple of teeth, to do something like this. But if the mpg's have always been bad... How has the power been?

When you did this, you had the air cleaner off, choke open, throttle open, correct? But, even if you didn't do that, it wouldn't explain that dead cylinder. That hopefully is from too tight valve lash, although it could be a bent valve or dead cam lobe. But I'd think you'd have a steady or at least intermittant miss at 20psi. So... I'm hoping it's that you didn't measure quite right, that it's just one cylinder with bad lash, and that's something else causing low mpg's.

With clean looking oil, that at least says the bottom end is ok.

I'll re look at the valve lash. I just did it about a week ago, but Dan's post makes me think I could probably be a little more precise here. I'll check it out tomorrow and get back.

Also, I did the compression test with only ONE sparkplug removed (like one at a time). Also, air cleaner was not off. Would this make a difference?

This is not the original 63, as far as I know, but I'll see what code is written on the engine. It was rebuilt in the early 2000s.

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63 Plymouth Valiant V200 Convertible.
225, HEI, Super Six with 38/38 Weber, Electric Fans, Scarebird discs, FirmFeel front swaybar.

Yup, with the other spark plugs left in, the starter will spin the engine much slower, lowering the readings. And with the throttle closed, readings will be lower. Maybe the engine was cold, too?

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I would not think that the air cleaner should matter--if in good shape it should have very low restriction--but it can't help matters. [But I'll defer to any properly written procedure.] Choke and/or throttle closed, though, will. If the engine, while it is spinning, can pull any amount of vacuum, then that much less air will go into the cylinders--and give a compression reading that is that much lower.

Every procedure for reading compression that I've read has stated to pull the plugs. You wouldn't think it would matter--but the engine should spin faster without those plugs (no compression stroke on those other cylinders means less work for the starter). At slower speeds pressure can bleed off, and give lower readings, so I'm guessing it does matter. I'd pull all 6 to test.

But as Dan points out, engine should be warm (forgot that tidbit). Maybe take it out for a good drive, then double check the lash, and then try rechecking compression?

Ok, sounds good. I did it with throttle open, BTW. Engine was hot and choke was open, too.

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63 Plymouth Valiant V200 Convertible.
225, HEI, Super Six with 38/38 Weber, Electric Fans, Scarebird discs, FirmFeel front swaybar.

Advice so far is excellent... but doesn't explain the disparity in cylinder #3.

If on the retest the other cylinders rise above 105 or so, then you might get away with another couple of seasons of driving without a rebuild.

However, you'll still have to deal with that #3 cylinder. It could be a bad valve, bad head gasket, or broken rings. If it's the rings then it's definitely time for a full rebuild.

- Mac

Actually, it might. If a valve on cylinder 3 is adjusted tighter than the valves on other cylinders, the #3 valve will be held open farther than on the other cylinders and could build less pressure because of it.

I wouldn't take those compression readings as conclusive of anything (other than an indicator that SOMETHING isn't right) until the valves are readjusted and the compression test has ben performed properly (all plugs out, throttle blocked open).

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Casually looking for a Clifford hyperpak intake for cheap.

Reed, you're right.

I thought about that possibility... it seems unlikely that he overtightened just that one valve, but anything is possible.

Another test (after valves are adjusted, and assuming #3 is still very low) would be to push compressed air into the cylinder using a threaded spark plug hole adapter.

- If air comes rushing out the intake or exhaust (with engine rotated so both valves are closed), you have a bad valve or seat.


- Mac

Here's what I got now with the compression test done right:

#1 80
#2 110
#3 30
#4 120
#5 110
#6 105

Did the valve lash again, and everything looks good there. Still. It's ugly anyway you look at it. Where do I go from here?

Can Cylinders 1 and 3 possibly be brought up to snuff without a complete rebuild?

Thanks!

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63 Plymouth Valiant V200 Convertible.
225, HEI, Super Six with 38/38 Weber, Electric Fans, Scarebird discs, FirmFeel front swaybar.

Why? I've done it plenty of times!


63valconvert- Your compression readings are in the low-acceptable range, except for 1 & 3. Given that the rest of your compression readings are OK, I think that you should pull the head and check the condition of your valves. You might have a couple cracked valves. But before you do that, try squirting a bit of oil in cylinders #1 and #3 and see if your compression does up.

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Casually looking for a Clifford hyperpak intake for cheap.

I agree with Reed here.

3 is dead, 1 is only slightly less dead, and 6 is ill. Whether it winds up being rings or valves, you are looking at major engine work or an engine swap. Be careful -- if you discover it's lunched valves leaking your compression, and the rings aren't newish, don't just swap the head; you'll quickly finish off the bottom end.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.