Compression Numbers

S

s_montgomery

New member
Finally got around to changing my plugs and testing the compression in my '80 Fairmont w/ 200 six.

1 130 (143 after adding oil)
2 150
3 130 (152 after oil)
4 155
5 145
6 150

Also plugs 3-6 were really black from running rich lately (i assume) from adding a 2bbl and getting it running right. The car sat for about 4 years but I've been driving it occasionally for a few months. New plugs helped with some midrange drivability issues and I just sorted out a malfunctioning choke problem.
I was a little dissappointed to see some low numbers. I plan on adding some oil to my gas from now on to help the rings out some.

Should I be concerned about these numbers? I feel like theyre almost close enough to not be a huge ploblem? Would like to put a t5 in it and a header or a least delete the cat.
 
Howdy:

The numbers are a bit on the low side, but not critical. Black on the plugs could be from oil or rich carb. Is the black wet and shiny looking or dry and fuzzy looking? Most likely the valve guide seals are history on this engine. If so you will notice a black puff of exhaust after a hot shutoff and restart within a half an hour. IF the choke was rich and/or sticking get that fixed right away. Another crutch is to switch to a spark plug heat range one step hotter then what you have now.

Since the numbers came up after adding oil, it's likely that the rings are weak and not sealing like they should. Have you put a vacuum gauge on it to see if the valves are leaking?

adios, David
 
The only oil I use in my gas is Marvel Oil… :nod:
I don’t leave home with out it..!! :nod:

You want to fallow David’s suggestion with the vacuum gauge.
That little guy is worth its weight in gold..


HERE
is a site that will help you with the vacuum gauge.
At the bottom of page are illustrations..
Good luck.
 
CZLN6":1r7z71ff said:
Howdy:

The numbers are a bit on the low side, but not critical. Black on the plugs could be from oil or rich carb. Is the black wet and shiny looking or dry and fuzzy looking? Most likely the valve guide seals are history on this engine. If so you will notice a black puff of exhaust after a hot shutoff and restart within a half an hour. IF the choke was rich and/or sticking get that fixed right away. Another crutch is to switch to a spark plug heat range one step hotter then what you have now.

Since the numbers came up after adding oil, it's likely that the rings are weak and not sealing like they should. Have you put a vacuum gauge on it to see if the valves are leaking?

adios, David
Click to expand...

Very black, dry/fuzzy. After talking with my dad, he told me pouring gas in the carb to start isn't great for the plugs either. I think my carb and choke issues are sorted out. I'm using 46's now, maybe I'll have a look at them again soon after they see a few miles. I'll have to pick up a vacuum gauge it looks like they have indash ones at advance for just over 20 bucks. I may be picking up a wideband o2 sensor soon for my daily driver; maybe I should at least get an idle reading with it in the tailpipe on this engine. Thanks for the input!

echo1955":1r7z71ff said:
The only oil I use in my gas is Marvel Oil… :nod:
I don’t leave home with out it..!! :nod:

You want to fallow David’s suggestion with the vacuum gauge.
That little guy is worth its weight in gold..

HERE
is a site that will help you with the vacuum gauge.
At the bottom of page are illustrations..
Good luck.
Click to expand...

Which is worth it's weight in gold? David, the vacuum gauge, both? Just kidding, thanks for the link. I was thinking of getting one just for setting the ignition timing; I would have never guessed you can tell so much from vac readings... The link makes it look very simple, but I bet once you have more than one problem at once it must get tricky! Oh and I use the oil additives for small engines, boats, etc the main thing I've gathered is that its designed to pass through filters. (But, yeah I'm cheap)

mugsy":1r7z71ff said:
Did you get all of the old gas out of the tank and system and replace it with fresh?
Click to expand...

I just mixed new with the old and have gone through a few tanks by now without a problem.
 
I've had a vacuum gauge in the car for a week or so now. I had only 8" vac until I found one leak. There was a hose running off the firewall through the fender I think to the Fuel EVAP box which is just an open system now. So I now have 18-20" at idle.

At times at idle, usually when cold and/or I think just idling too low, The vacuum sort of ticks back and forth between 16-18". I guess its doing well for a old car...I think it has either 115k miles or 215k miles. Thanks for the advice to get the gauge I would have never known about that leak! Havent noticed more power but seems to have helped a lot for gas mileage (along with the new plugs).
 
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