Compression test results
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Post Fiend
Joined: Mar 2002
Posts: 19,402
Likes: 38
From: Bonita Springs FL
Compression test results
Ok, I did a compression test on my '86 300 today. Got some interesting results.
1,2,3,4 all came in around 130 lbs, give or take
5 - 154 lbs
6 - 164 lbs
What's up with 5 & 6? It could be excessive carbon buildup, but only those two cylinders? Seems strange to me, so I thought I'd ask the experts.
1,2,3,4 all came in around 130 lbs, give or take
5 - 154 lbs
6 - 164 lbs
What's up with 5 & 6? It could be excessive carbon buildup, but only those two cylinders? Seems strange to me, so I thought I'd ask the experts.
Postmaster
Joined: May 2002
Posts: 3,241
Likes: 3
From: Broken Arrow, OK
Compression test results
Are those numbers wet or dry?
5 and 6 might be high because the valve seals aren't doing their job and oil is leaking into the cylinders. The engine sits slightly nose high, then if you park on a slightly sloped surface it is even more nose up. That would make oil accumulate at the rear of the head and not the front, so none would be in the front cylinders.
5 and 6 might be high because the valve seals aren't doing their job and oil is leaking into the cylinders. The engine sits slightly nose high, then if you park on a slightly sloped surface it is even more nose up. That would make oil accumulate at the rear of the head and not the front, so none would be in the front cylinders.
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Post Fiend
Joined: Mar 2002
Posts: 19,402
Likes: 38
From: Bonita Springs FL
Compression test results
Dry. Because the first four cylinders were all pretty close, I didn't wet them with oil. However, the engine was up to temp before I ran the test - if the valve seals were leaking, wouldn't they already be wet? I did notice some oil on the plugs when I pulled them.
New User
Joined: May 2003
Posts: 12
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From: Jersey Shore
Compression test results
Hmm,
Why not try the test again, this time wet. (You take out each plug and squirt some oil in there and then screw in your compression gauge). IF your compression readings go up when wet, it confirms bad rings in those cyls that went up.
Maybe what you're seeing is 4 cyls with bad rings?
-Dan
Why not try the test again, this time wet. (You take out each plug and squirt some oil in there and then screw in your compression gauge). IF your compression readings go up when wet, it confirms bad rings in those cyls that went up.
Maybe what you're seeing is 4 cyls with bad rings?
-Dan
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Post Fiend
Joined: Mar 2002
Posts: 19,402
Likes: 38
From: Bonita Springs FL
Compression test results
No, I just bought this thing a week ago.
However, I did successfully rebuild the carburetor. It idles like a champ now. But the carb needs to be adjusted to compensate for the large screw I fished out of the venturi. I'm going to take it to the shop tomorrow to have that done, have the timing checked, fix the driver's side motor mount, and have it smogged.
*Then* I'll do the comp test again and let y'all know how it looks.
However, I did successfully rebuild the carburetor. It idles like a champ now. But the carb needs to be adjusted to compensate for the large screw I fished out of the venturi. I'm going to take it to the shop tomorrow to have that done, have the timing checked, fix the driver's side motor mount, and have it smogged.
*Then* I'll do the comp test again and let y'all know how it looks.
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Freshman User
Joined: Apr 2003
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Compression test results
What did you mean by:
"But the carb needs to be adjusted to compensate for the large screw I fished out of the venturi. "
Jerry
"But the carb needs to be adjusted to compensate for the large screw I fished out of the venturi. "
Jerry
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Post Fiend
Joined: Mar 2002
Posts: 19,402
Likes: 38
From: Bonita Springs FL
Compression test results
Sorry, I should have been more clear. 
I rebuilt the carb yesterday, and the last mechanic (I use the term loosely here) dropped a screw into the carb. It got stuck in the venturi. Then he apparently adjusted the carb to compensate for the screw being there, as it runs like a dog now that I took the screw out. Before, it had a rough idle but ran great down the road. Now, it idles perfectly, but backfires and has no power while driving it.
Between this and the steel nugget I fished out of the rear diff, I'm amazed the thing ran at all.
I rebuilt the carb yesterday, and the last mechanic (I use the term loosely here) dropped a screw into the carb. It got stuck in the venturi. Then he apparently adjusted the carb to compensate for the screw being there, as it runs like a dog now that I took the screw out. Before, it had a rough idle but ran great down the road. Now, it idles perfectly, but backfires and has no power while driving it.
Between this and the steel nugget I fished out of the rear diff, I'm amazed the thing ran at all.
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