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Old 01-17-2016, 02:04 PM
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Unbalanced piston compression

so my 74 chevy pickup has been bogging down when it gets up to around 40 mph and sometimes it wont even go past 10 mph. did a compression test,(v8 engine) and on the dry tests, the pistons produced around 120-130 psi. Pistons 3 and 5 produced 105 and 110. From what i read, it could be a blown gasket or a crack on the engine. Any suggestions or wisdom to offer is much appreciated.

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Old 01-17-2016, 02:23 PM
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Quote:
Originally Posted by hcompton View Post
Did you try it hot with a tea spoon of oil in the cylinders. Sometimes a small difference is not that bad. You may want to try a leak down that may help find a blown head gasket better.

What else have you checked compression is not good but should still let the engine run unless its blowing bad when hot.

Is there any oil in the water or water in the oil.
yes, i had an oil can and squirted oil for a wet test after each dry test. the engine still runs but at high rpm, it'll bog down like its out of gas. in neutral, it wont act that way but when driving around, it'll happen at 40mph then get worse during the drive, especially if i stop at a light and accelerate again. Not sure how to look for oil in the water or vise versa.
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Old 01-17-2016, 02:35 PM
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Sounds like a plugged exhaust to me.
Loosen the exhaust from the manifolds , lower the pipes down enough to create a gap, eliminating any back pressure from the exhaust.Then drive,see if it is better.




Could also be starving for fuel.Maybe be a plugged strainer in the fuel tank. Putting a "T" fitting in the suction line and running it to a vacuum gauge will answer that question.a few inches of vacuum is OK. If it is pulling more than just a couple inches of vacuum, then the strainer is plugged.
Also check fuel pressure/delivery. Might do that first as the exhaust could be rusty and a bit more challenging.
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Old 01-17-2016, 02:55 PM
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Originally Posted by LATECH View Post
Sounds like a plugged exhaust to me.
Loosen the exhaust from the manifolds , lower the pipes down enough to create a gap, eliminating any back pressure from the exhaust.Then drive,see if it is better.




Could also be starving for fuel.Maybe be a plugged strainer in the fuel tank. Putting a "T" fitting in the suction line and running it to a vacuum gauge will answer that question.a few inches of vacuum is OK. If it is pulling more than just a couple inches of vacuum, then the strainer is plugged.
Also check fuel pressure/delivery. Might do that first as the exhaust could be rusty and a bit more challenging.
I'll definitely check that out. I don't think it's the fuel though, if anything it might be getting too much fuel. I have an inline filter between the pump and the carb, when idling, the filter is full of gas and when I took the top off of the carb, there was plenty of fuel in the bowls.
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Old 01-18-2016, 02:44 PM
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What did plugs look like? Were they fouled, normal brown or bone white? Are they in decent shape with correct gap?

Oil looks like a milkshake if its got any significant water. You're not gonna have a blown head gasket or cracked head unless you got it really hot or let it freeze. My bet is this is ignition or fuel related. If the truck was running fine and then started to run like crap over a few days, I'd suspect carb condition. Sometimes the float sticks and it starves for gas when driving. You take it apart and it may look OK, but the float could be sticking or the needle/seat could be sticking. If it's sticking open (flooding) you should see smoke and fouled plugs. If its sticking shut it may backfire from the carb and show lean plugs. What carb is it? Holley or Q-jet or carter/AFB clone (edelbrock)?

Could also be a loose vacuum hose, stuck advance system, etc.

Can you give some history, is this a new project truck that you dont know history, or something that was running fine but took sick one day?

Last edited by Chiphead; 01-18-2016 at 02:52 PM.
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Old 01-18-2016, 03:18 PM
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Quote:
Originally Posted by Chiphead View Post
What did plugs look like? Were they fouled, normal brown or bone white? Are they in decent shape with correct gap?

Oil looks like a milkshake if its got any significant water. You're not gonna have a blown head gasket or cracked head unless you got it really hot or let it freeze. My bet is this is ignition or fuel related. If the truck was running fine and then started to run like crap over a few days, I'd suspect carb condition. Sometimes the float sticks and it starves for gas when driving. You take it apart and it may look OK, but the float could be sticking or the needle/seat could be sticking. If it's sticking open (flooding) you should see smoke and fouled plugs. If its sticking shut it may backfire from the carb and show lean plugs. What carb is it? Holley or Q-jet or carter/AFB clone (edelbrock)?

Could also be a loose vacuum hose, stuck advance system, etc.

Can you give some history, is this a new project truck that you dont know history, or something that was running fine but took sick one day?
The plugs looked fine. That was the first thing I checked. Gapped them and cleaned any soot off the plugs. If anything, they might have had a little browning on the ceramic stem but that's it. We checked the carb too, they did look fine, I guessed it might've been sticking too but I thought I would be able see an issue. The carb is a rebuilt edlebrok. Took a rebuild kit to it a couple months ago. We checked the vacuum advance like as well and when we plugged the hose, there was zero change in the idle. I got the truck about 6
Months ago. It had quite a bit of issues, fixed most of them but this one has me stumped
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Old 01-24-2016, 01:27 AM
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Your cylinders should be with in 10% of each other. The 105 is low.
Put fresh plugs in let it get warm and run it as hard as you can...Then check the plugs again - especially the low compression cylinder. If it looks dirty - and
if the exhaust is proven to be clear, and the carb is getting fuel - I would suspect a worn/broken ring.
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