Compression test

[Walt]

Hi everyone... ok, so now that the new starter is in I was finally able to do my first compression test on my car - 64, 110hp, PG. So I pull all of the sparkplugs and everything else to get ready, test cylinder one and get a 100 pounds... crap. Ran the test a couple times got the same result each time. So I moved on to cylinder 3 and got a 158, cylinder 5 165, cylinder 2 145, cylinder 4 155 and cylinder 6 150. I try cylinder one again and still just get 100. So I pop a little oil into cylinder one and do a wet compression test and I get a 150! Am I correct in understanding that that means it is most likely not a piston and ring issue but something going on in the valves? And if so is my next best step really to pull the head and take a look at things or is there some other test I can run? When I drive this car for a 1/2 hour or so pinging and detonation happens a whole bunch and the last time I drove it was really bad with a semi new knocking sound... I was hoping since I just put a new tank of gas in maybe I got some bad gas... but I was planning on doing this tune up anyway, And I really do not like the knocking sound... The spark plugs all look good they are all champion L86Cs... And while most of them had the proper gap, a couple were little large so I have adjusted them for when I do finally put them back in. With one cylinder being low like this is it something I really shouldn't be driving around on and should either pull the head or look at any other test any of you might suggest? Thank you in advance for all the wisdom that I know is coming my way! :)

[61SuperMonza]

I would perform a valve adjustment before I did anything else. Then recheck your compression. It may bring the low cylinder up.

[Walt]

ok, very good, thanks!

[terribleted]

Has the engine been run recently? If it has been sitting I would do nothing except get it operating and through a couple hot and cold cycles and then re-test compression. Somewhat dry cylinder or something a little gummed up that may clear with just a little operation is likely. If at this point #1 still reads 100 then adjust the valves on #1 running and see if it makes a difference. (of course inspect the rockers and pushrods on #1 to see if there is an obvious issue). If compression is still low after adjustment there is a deeper issue.

[joelsplace]

Ted - I think he has run it enough to overheat it several times.
Walt - the oil test is rings. The thick oil helps bad rings seal. One thing that will mess up the test is too much oil because it increases the compression ratio.
Check the valve adjustment like Ted mentioned and no matter if that fixes it or not I would figure out the overheating problem before I worried about the low compression.

[terribleted]

Ted - I think he has run it enough to overheat it several times.
Walt - the oil test is rings. The thick oil helps bad rings seal. One thing that will mess up the test is too much oil because it increases the compression ratio.
Check the valve adjustment like Ted mentioned and no matter if that fixes it or not I would figure out the overheating problem before I worried about the low compression.

This post says nothing about overheating, but, some of the symptoms he describes could be overheating and yes that is a primary concern for sure!!

[joelsplace]

"When I drive this car for a 1/2 hour or so pinging and detonation happens a whole bunch and the last time I drove it was really bad with a semi new knocking sound..."
Ted, that is just how I translated this and I think it was detailed in another post but I may be confusing this with someone else.

[Walt]

Hi Everyone! Thank you for chiming in... I got this car in June of this year and have put 1000 miles on it since... so yes, I've been driving it quite a bit so it has not been sitting. And I was looking forward to doing all of the tune-up tests Ive learned about in the manual and videos and posts from here so I know how this car is set up AND I need to figure out the pinging/detonation issue under load after I've been driving for a half hour or so... but then I had to replace the starter and just finally got to the compression test this past weekend. So actually I was exactly wrong about the adding oil thing? I guess that makes sense... low compression, but add some oil and that fills the gaps of the piston rings that perhaps have worn down or worse... I will pull the valve cover, visually inspect and adjust and test #1 again. Then I was going to check/set the timing and then I was going to do the whole carb-sync process to make sure they are set up properly and make sure the pinging isn't from too lean of a fuel position... I will update here when I have some results. Thanks again!

[Walt]

Just thought of a quick question and couldn't find the answer anywhere... so tomorrow I am putting the spark plugs back in, firing it up, checking/adjusting the timing, checking the idle and balance of the carbs... so I was planning on doing the valve adjustment after... every tutorial I see for doing the compression check states to take ALL of the spark plugs out and then do the testing... if I am just needing to recheck cylinder #1, can I leave all of the other spark plugs in while I test the compression or do I have to pull them all again?

[azdave]

if I am just needing to recheck cylinder #1, can I leave all of the other spark plugs in while I test the compression or do I have to pull them all again?

Pulling the plugs helps your battery make it through testing all six cylinders with the least amount of sag and also achieves the highest cranking speed for the test. If you are only curious to see if a major compression change has happened with #1 you can do the test without pulling the other plugs however, you will have to keep in mind that the numbers will be lower than when you did the initial tests with the other plugs removed. For the best comparison to your earlier test numbers you should pull the other plugs as before. You could also test #1 compared to one of the others to see how the numbers are more balanced. You would only pull two plugs that way.

[Walt]

ok thank you very much for your reply

[azdave]

ok thank you very much for your reply

I should add to check all the easy stuff like you are now but then decide if it warrants a teardown and repair if it goes that far. Back when I had little money to play with and starting a family, these Corvair were all we owned and were our daily drivers. I drove engines for years where one cylinder was quite low compared to the others (but not zero of course). They were not the smoothest engines at idle but once above 2000 RPM you could not feel any difference and no other issues came up from driving them that way.

[Walt]

ok, good input, thanks!

[Walt]

Ok so a minor update... I put the spark plugs back in -this is the first time doing this with this car - and found that the threads on cylinder 1 -which had the low 100lb compression reading - must be at least partially stripped as I could not put the spark plug in tighter than hand-tight. So new side question: if that was the only issue would that be sustainable/reliable or do I need to get that hole tapped?

Back to the bigger issue... I literally just created a youtube channel to post a video that shows my new knocking sound... with new white smoke coming out of the tailpipe... this is warmed up right after putting plugs back in:

https://www.youtube.com/watch?v=LK3B3OGX7uw

Sorry it's so short, I was thinking I could upload it to the forum but found out that is not the case...



I would appreciate it if all of you in the know could take a quick look/listen and give me your opinion...

Thanks!

Walt

[bbodie52]

You can embed a YouTube video on the Corvair Forum. I edited you last post to show you how. The URL that is needed is obtained on YouTube by clicking the SHARE button below your YouTube video. The copy the displayed URL (formatted somewhat differently than the URL address at the top of the screen) and thenpaste that SHARE URL between the share_youtube markers on the Corvair Forum. When you save or Preview the edited post, your YouTube video will be displayed.

[Walt]

thank you, i will try that next time... any comments on the knocking noise in the video?