Low Compression

[SeriesVtime]

Hello all,

I have a Series V that I've been working on for a while years now. She's getting close to being back on the road. Our current issue, and I'm hoping our last, is that it's running rough.

I swapped out the Strombergs with a pair of DCOE 40 Webers, which I rebuilt, and seem to be running fine, except cylinder 1. For grins, I swapped the carbs to rule them out. Still cylinder 1 fires intermittently. When I pull the plug it's smelly and covered with gas. I popped a screw driver into the plug wire and it's got plenty of spark.

So with gas & spark present I decided to check the compression. Cylinders 2-4 have around 150 lbs of compression (is that good, high, low?). But, cylinder 1 has around 90 pounds - argh.

So, the things that come to mind are: rings, valves, cylinder warn or damaged, valve setting and possibly head gasket/wrapped head etc..

Am I missing anything?

Before I start ripping the head off or other major steps, I wanted to check with the group to see if I'm overlooking something stupid.

Some relevant background: shortly after getting it running, it got hot and I lost the head gasket. I tore it down and had the head checked - it required resurfacing, which I did. I have not re-torqued the head since re-installing it.

The weather is great in California right now. I need to be driving my Alpine and not working on it! Any and all help would be deeply appreciated.

Thanks!

Jeff

 

[Bill Blue]

Jeff, if you pull the valve cover and see nothing amiss (improperly adjusted valves, bent push rod, etc.) I think you will have to pull the head. Might try squirting oil into #1 and checking compression. If this brings the compression up, the problem is rings. If not, valves. But you still will have to pull the head.

Bill

 

[Green67Alpine]

Put some oil in the low comp. cylinder(squirt some in the plug hole) if the comp goes up bad rings, if it stays about the same valve job............
Went through the same thing last year, great weather Alpine on jack stands
Tom j

Gee, Bill almost the same reply at the same time !!

 

[SeriesVtime]

Great feedback!

Bill, Tom,

This is great feedback! Thank you both.

I didn't know that a little squirt of oil would help isolate the problem. I suspect that I'll need to turn it over a few times to allow the oil to get down to the rings, all the way around, but I suspect I'll know pretty quickly.

I'd much prefer (duh) a valve related problem, but being able to know is a great step forward.

As a parting thought - is there any chance that it could be a head gasket? That's changed recently might be a problem?

Thanks again!

Jeff

 

[SeriesVtime]

What is normal

One additional thought/question: what is normal on a recently rebuilt motor? Does the ~150lbs sound right?
Thanks!

Jeff

 

[RootesRacer]

One additional thought/question: what is normal on a recently rebuilt motor? Does the ~150lbs sound right?
Thanks!

Jeff

Depends on your cam grind and static compression ratio.

Aggressive cams will register lower readings due to overlap.

 

[Alpine 1789]

In the interests of ruling out easy things first, how about the valve lash? Couldn't poorly adjusted valves in #1 give the same reading?

 

[Nickodell]

One additional thought/question: what is normal on a recently rebuilt motor? Does the ~150lbs sound right?
Thanks!

Jeff

With a stock engine, that's borderline. 170-180 is good. Just as important, as a rule of thumb there shouldn't be more than 15% difference in any cylinder from the highest, or 10% from the average of the other 3.

I would suggest that you do the "oil test" with more than a "squirt." Put about an ounce (30 cc) of engine oil in (SAE 50 if you have it) so as to coat the entire piston and rings, and don't wait for it to soak in or it'll simply drain down into the sump. - whip the comp tester back in and turn it over a.s.a.p. I assume you know to wedge the throttle wide open with the coil HT lead disconnected!! so as to get the proper reading, and le the engine turn over four of five times.

 

[SeriesVtime]

checking this afternoon

Hi Nick, Thanks for the details. I wasn't sure about the viscosity, duration and timing (how fast I need to take the measurement).

Hey Jim, if you mean the valve setting, I used the book value of .014 for the exhaust and .012 for the intake. What's not ideal is that I set them while before the motor got up to full temperature - just wanted a quick adjust to get it running. All of the cylinders were set this way, so they're consistently set incorrectly if that's any value. They should all be close.

I'll post my findings after I add the oil and retake the compression readings. I plan on taking them all with and without oil and will report my findings this afternoon.

Thank you all for your help! I really want to be driving this machine as soon as possible.

Jeff

 

[SeriesVtime]

Maybe next summer

Ok, I rechecked each cylinder and they all had 150 except the first which had around 95 pounds. So I put a small amount of oil in cylinder 1, disconnected the coil wire, wired the throttle wide open and when I cycled the motor, and took the reading, the gauge went to its max - over 300lbs.

I did the exact same thing to cylinder 1, just to compare, and it went to 180. I became nervous about what the oil and additional compression may have done to cylinder 1 so I put the plugs back in and started it up.

Needless to say there was a fair amount of smoke. The bad thing is that the smoke didn't go away.

After running it now for well over an hour the smoke isn't bad at idle, but any acceleration, or actually deceleration, the girl starts smoking. A whitest acrid smoke.

So my working guess is that there was something wrong with one or more rings on cylinder 1 and the additional compression took it out.

I think it's time to retire my efforts for now. I've lost my stomach for trying to get this car running at this time. Thanks to all for your suggestions.

Jeff

 

[Nickodell]

Jeff, a compression reading of 300 psi, or even 200, is impossible in a stock engine cylinder that had been reading less than 100 earlier! If you got this on the dial, your gauge is duff (maybe the original low reading was bogus too).

If you're getting white smoke* you're cycling coolant through one or more cylinders. Since you mentioned blowing a gasket before, my guess is that your head wasn't milled completely flat and you aggravated the situation by not re-torquing hot. My money is on a gasket blow between the coolant jacket and #1 cylinder. Take the head off and have it carefully checked for plane by someone who knows what he's doing. While it's off, examine the cylinder bores. Can you feel a ridge on the cylinder wall at the place the top of the piston reaches? Can you actually move the piston laterally? If so, the piston, rings or cylinder are badly worn.

Exhaust smoke:
Black .... raw gas being burned
Blue ..... oil " " " " " "
White .... coolant being cycled through cylinder(s)
Green ..... normal color of the money being poured into a 40-year-old British car.

 

[SeriesVtime]

Hi Nick, thanks for your assessment. I'm sure that once I get a bit of distance from today's experience I'll be up for removing the head. I'll update this thread when that happens. Thanks again - Jeff

 

[Green67Alpine]

Green ..... normal color of the money being poured into a 40-year-old British car.


Ain't that the truth, but that's why they're so much fun !!

Tom j

 

[RootesRacer]

How much oil did you put in there?

Only a small squirt will do, put in 20ccs or so and you compression will double.

 

[wframe]

Sorry, but that sure sounds like you have bad rings in #1. The oil you added to cylinder #1 temporarily sealed the ring leaks and bringing up the compression reading. The continued smoking is because the rings are not sweeping the oil off the cylinder wall, thus it is left to be burned during the ignition.

Good luck!

 

[Nickodell]

How much oil did you put in there?

Only a small squirt will do, put in 20ccs or so and you compression will double.

Not by my calculation:

1725 divided by 4 = 431 cc per cylinder
At an 8:1 c.r., combustion chamber volume at t.d.c. = 54 cc.
Add 20 cc of oil, and new volume = 34 cc.
New effective c.r.: 8/34 x 54 = 12.7 (less any oil that has drained past piston). The effective c.r. will also be reduced by the - admittedly small - compressable volume of the compression gauge hose and bourdon tube.

If the original pressure was 90 psi, new pressure should be 90/8 x 12.7 = 143.

I would never, of course, suggest putting more than maybe 10 cc into a cylinder for compression check where it was already reading 120 or more p.s.i.

You're much more of an engine expert than I am (that's not meant sarcastically), so please tell me where I'm wrong.

 

[RootesRacer]

Not by my calculation:

1725 divided by 4 = 431 cc per cylinder
At an 8:1 c.r., combustion chamber volume at t.d.c. = 54 cc.
Add 20 cc of oil, and new volume = 34 cc.
New effective c.r.: 8/34 x 54 = 12.7 (less any oil that has drained past piston). The effective c.r. will also be reduced by the - admittedly small - compressable volume of the compression gauge hose and bourdon tube.

If the original pressure was 90 psi, new pressure should be 90/8 x 12.7 = 143.

I would never, of course, suggest putting more than maybe 10 cc into a cylinder for compression check where it was already reading 120 or more p.s.i.

You're much more of an engine expert than I am (that's not meant sarcastically), so please tell me where I'm wrong.

OK the combustion chamber is about 43CC and the compression ratio on the alpine is 9.2/1 not 8/1.

If you half the CCs in the combustion chamber by putting in 20cc of oil, you will approximately double the net pressure.

After doing the math the net compressioin ratio is 18.74/1 (based on 43 cc chamber and 20 cc oil), or a net non adiabatic corrected pressure of 275.5 PSIG. Throw in some adiabatic heating and you will be well over 300PSIG till the fluid cools.
BTW equation is 14.7 * 18.74 where 14.7 is 1 atmosphere PSIA and 18.74 is the net compression ratio ignoring VE.

Lastly, when he put the oil in, that kinda temporarily sealed the rings, so what was reading 90psi, would suddenly read 150psi like the rest of the other cyls (ignoring the extra pressure you would read due to the chamber volume loss of the extra oil).

 

[husky drvr]

OK the combustion chamber is about 43CC and the compression ratio on the alpine is 9.2/1 not 8/1.

If you half the CCs in the combustion chamber by putting in 20cc of oil, you will approximately double the net pressure.

After doing the math the net compressioin ratio is 18.74/1 (based on 43 cc chamber and 20 cc oil), or a net non adiabatic corrected pressure of 275.5 PSIG. Throw in some adiabatic heating and you will be well over 300PSIG till the fluid cools.
BTW equation is 14.7 * 18.74 where 14.7 is 1 atmosphere PSIA and 18.74 is the net compression ratio ignoring VE.

Lastly, when he put the oil in, that kinda temporarily sealed the rings, so what was reading 90psi, would suddenly read 150psi like the rest of the other cyls (ignoring the extra pressure you would read due to the chamber volume loss of the extra oil).

The 9.2 ratio would also presume that the head has not been milled and/or the block has not been rebored in the past. In either case the initial CR would be higher than 9.2.