Backfire Through One Cylinder During Compression Test
Thread Starter
Cruising
Joined: Apr 2011
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Backfire Through One Cylinder-UPDATE 7/18 With Visual of Problem Help Needed!
hello,
I just finished doing a compression test on my 1968 327/300 Vette to see if a backfiring problem I have been having was due to a problem inside the engine. Solely based on the PSI results, it would seem as if this wasn't the case, since all of the cylinders had a strong output of between 160 and 190 PSI (factory specifications for my engine is 160). However, when I was testing one cylinder (so this one particular cylinder is plugged with the compression kit while the rest are open), I heard a popping sound which I identified to be backfiring through the carb - seems too coincidental to be insignificant? This backfiring did not occur when any of the other 7 cylinders were being tested. Does anybody know what this signifies? Does that mean I have a sticky valve or some other internal problem even though the compression in that cylinder was strong?
Thanks for the help
I just finished doing a compression test on my 1968 327/300 Vette to see if a backfiring problem I have been having was due to a problem inside the engine. Solely based on the PSI results, it would seem as if this wasn't the case, since all of the cylinders had a strong output of between 160 and 190 PSI (factory specifications for my engine is 160). However, when I was testing one cylinder (so this one particular cylinder is plugged with the compression kit while the rest are open), I heard a popping sound which I identified to be backfiring through the carb - seems too coincidental to be insignificant? This backfiring did not occur when any of the other 7 cylinders were being tested. Does anybody know what this signifies? Does that mean I have a sticky valve or some other internal problem even though the compression in that cylinder was strong?
Thanks for the help
Last edited by SFT; Jul 18, 2011 at 06:52 PM.
Instructor
Joined: Jun 2011
Posts: 183
Likes: 1
From: Litchfield CT
carb pop
hello,
I just finished doing a compression test on my 1968 327/300 Vette to see if a backfiring problem I have been having was due to a problem inside the engine. Solely based on the PSI results, it would seem as if this wasn't the case, since all of the cylinders had a strong output of between 160 and 190 PSI (factory specifications for my engine is 160). However, when I was testing one cylinder (so this one particular cylinder is plugged with the compression kit while the rest are open), I heard a popping sound which I identified to be backfiring through the carb - seems too coincidental to be insignificant? This backfiring did not occur when any of the other 7 cylinders were being tested. Does anybody know what this signifies? Does that mean I have a sticky valve or some other internal problem even though the compression in that cylinder was strong?
Thanks for the help
I just finished doing a compression test on my 1968 327/300 Vette to see if a backfiring problem I have been having was due to a problem inside the engine. Solely based on the PSI results, it would seem as if this wasn't the case, since all of the cylinders had a strong output of between 160 and 190 PSI (factory specifications for my engine is 160). However, when I was testing one cylinder (so this one particular cylinder is plugged with the compression kit while the rest are open), I heard a popping sound which I identified to be backfiring through the carb - seems too coincidental to be insignificant? This backfiring did not occur when any of the other 7 cylinders were being tested. Does anybody know what this signifies? Does that mean I have a sticky valve or some other internal problem even though the compression in that cylinder was strong?
Thanks for the help
If you are, the backfire is coming from one of those 7, and not form the one with the compression gauge attached.
if all of your other plugs are removed, and you have only the one cylinder sealed (with the comoression tester), you may have a bad intake valve, sticking lifter, bent pushrod gettign wedged in the head, or a problematic rocker.
I would loosen the rockers on the suspect cylinder to eliminate everything except the valve when making your compression test.
Team Owner
Joined: Jan 2006
Posts: 37,637
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From: Crossville TN
Very difficult to do a compression test with the other plugs installed. Engine turns much easier with plugs out. You should also pull the center coil wire so that the engine cannot fire at all. I'm not sure what you are describing as a "backfire" when doing a compression test. With no spark, it cannot 'backfire'.
Thread Starter
Cruising
Joined: Apr 2011
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I also removed the coil wire when I performed the test.
Maybe "backfire" was the wrong term. It was a consistent popping sound of air coming through the carburetor with each turn of the engine....and it was only on this one cylinder all of the rest were normal.
Maybe "backfire" was the wrong term. It was a consistent popping sound of air coming through the carburetor with each turn of the engine....and it was only on this one cylinder all of the rest were normal.
Instructor
Joined: Jun 2011
Posts: 183
Likes: 1
From: Litchfield CT
cool
If it pops back through the carb now, your intake valve is probably burned. It wil lbe heard to hear with the engine cranking, but listen to hear if it huffs out the exhaust.
If you have an air compressor, some compression testers have a Schrader valve on them that will enable you to pressurize the cylinder. If you can pressurize the cylinder, you should be able to hear the air rushing out into the intake manifold or exhaust manifold, if either of the valves are bad, burned, or stuck.
You can also use a broken spark plug, break out the ceramic part then bore a hold through the electrode section. Clean this uo, the nscrew it into the spark plug hole ( if it is accessible), then use the little blow-off attachment from your comoressor (mine has a rubber cone on the end) bu stuffing it into the headless spark plug, then oull the trigger to pressurize the cylinder.
Thread Starter
Cruising
Joined: Apr 2011
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hello again,
I just removed the valve cover and believe I found the source of my problem. One of the push rods is coming out at the wrong angle and so its rocker arm is not connected in the spot where it should be...I tried to simply move the rocker arm back in place but it won't let me. I believe it is because the push rod is coming out at the wrong angle (maybe it is bent? I'm not sure).
Can anyone tell me how serious of a problem this is and how I can go about fixing it? Is it simply adjusting the rod or pulling it and replacing it or does this signify some other problem deeper inside the engine?
The engine ran perfectly for about a week until I was pulling onto the freeway one day and heard a popping sound which I identified to be backfiring through the carb. This must be the cause of my problem.
I tried to attach pics but I couldn't figure out how to so instead I took a youtube video of the problem so you all can see it firsthand....as they say a picture is worth a thousand words.
http://www.youtube.com/watch?v=QE-oShxiPsA
Thanks for the help.
I just removed the valve cover and believe I found the source of my problem. One of the push rods is coming out at the wrong angle and so its rocker arm is not connected in the spot where it should be...I tried to simply move the rocker arm back in place but it won't let me. I believe it is because the push rod is coming out at the wrong angle (maybe it is bent? I'm not sure).
Can anyone tell me how serious of a problem this is and how I can go about fixing it? Is it simply adjusting the rod or pulling it and replacing it or does this signify some other problem deeper inside the engine?
The engine ran perfectly for about a week until I was pulling onto the freeway one day and heard a popping sound which I identified to be backfiring through the carb. This must be the cause of my problem.
I tried to attach pics but I couldn't figure out how to so instead I took a youtube video of the problem so you all can see it firsthand....as they say a picture is worth a thousand words.
http://www.youtube.com/watch?v=QE-oShxiPsA
Thanks for the help.
Instructor
Joined: Sep 2007
Posts: 198
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From: Colorado Springs Colorado
I can't tell much by the video. I would loosen the rocker arm, and remove the pushrod, making note of which end was up (so it can be installed in the same way it came out). Place the pushrod on a pane of glass and roll the pushrod. If it is bent, it will be very evident. If bent, replace, and re-adjust the rocker arm.
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I'm guessing it's an exhaust valve not opening. You need to pull the rocker and pushrod to see what shape it's in. Hopefully just a busted rocker and bent pushrod. Since compression seemed good I'm guessing you don't have a bent valve. Shouldn't be too hard of a fix. Look closely at rocker for bluing from no oiling etc.
OK...just looked at video...yep an exhaust valve. Either it wasn't in place well when adjusted or it got over revved maybe and lifter came apart or something. Pull rocker and pushrod and look down in there to make sure c-clip is still in top of lifter.
JIM
OK...just looked at video...yep an exhaust valve. Either it wasn't in place well when adjusted or it got over revved maybe and lifter came apart or something. Pull rocker and pushrod and look down in there to make sure c-clip is still in top of lifter.
JIM
Thread Starter
Cruising
Joined: Apr 2011
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Update: I just removed the push rod. It was both cracked and bent! Definitely the source of my problem. I know that this is an old engine and sometimes parts just fail, but is as far as causation (other than the part simply failing) goes, is there anything else I should look into to try and figure out if there is a deeper problem?
Also, on another note, once I purchase a new push rod, is there any trick to installing it or should it just slide right into the lifter? I don't want to drop it into the engine...that would be a disaster.
Thanks
Also, on another note, once I purchase a new push rod, is there any trick to installing it or should it just slide right into the lifter? I don't want to drop it into the engine...that would be a disaster.
Thanks
