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Compression woes

MY66AH

Senior Member
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Well the good news is I have spark and fuel but only compression on the front 2 cylinders and next to nothing on the back 2. Either the rings went bad or burned a hole in the pistons or maybe head gasket?

 

nomad

Yoda
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Most likely head gasket. Find a way to blow air into the spark plug hole and listen to where it is leaking out.

Kurt.
 

LarryK

Obi Wan
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HF has a cheap leakdown tool, make it easy. Might just need valves adjusted.
 
Last edited:

dklawson

Yoda
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The HF leak down tester doesn't have a great reputation. For about the same money you can make your own following plans on the net. If you are fortunate enough to have some surplus air fittings around the garage it costs next to nothing to make your own. Of course your time is valuable also.

Regardless, the leak down test will tell you more than the compression test alone and the direct connection of air to the cylinders as Kurt suggested can often tell you all you need to know. Just don't forget to take off the radiator cap when hooking air up either directly with a hose or with the leak down tester.
 

LarryK

Obi Wan
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I only mention HF as I use spark plugs adapted to air lines. HF is cheap, just trying to get him a jump without scary prices.
 

nomad

Yoda
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Man I love that color- especially as it ages. (Don't know how to spell "patiniates")

I've always liked primrose as well...only the aging thing is what I DID'NT like. Seemed to fade big time.

Kurt.
 

SaxMan

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I have the HF leakdown tester. It works fine. The only trick is establishing the baseline pressure is like steering a big ship. If you're low and you increase the flow too quickly, you overshoot your mark, and vice versa. You can also let it run "wide open", which can allow you to listen/observe the air leak if it is in the valves.

Head gasket is a likely culprit, however, I know in my case and other folks on BCF that the head gasket failure tends to be between cylinders 2 and 3, not 3 and 4 (or is it 1 and 2? I always forget which way to start counting).
 

dklawson

Yoda
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Start at the water pump and count back. Between 2-3 seems to be the most common HG failure location.

I chose to build my own leak down tester. I opted for the 2 gauge configuration. With it, all you do is supply a bit more than 100 PSI to the tester's regulator, dial the regulator output to 100 PSI, and read the downstream gauge after the inline .040" orifice. There are no other adjustments.
 

nomad

Yoda
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Start at the water pump and count back. Between 2-3 seems to be the most common HG failure location.

I chose to build my own leak down tester. I opted for the 2 gauge configuration. With it, all you do is supply a bit more than 100 PSI to the tester's regulator, dial the regulator output to 100 PSI, and read the downstream gauge after the inline .040" orifice. There are no other adjustments.

I started to build one by knocking the porcelain out of an old plug and brazing in a air fitting. When I hooked up the air and used a length of hose to listen through the burned valves on my MGA became very obvious so that was as far as I got. If I was trying to determine wear I would have to finish the project.

Kurt.
 
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