Crappy compression options - Steve Saunders Goldwing Forums

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post #1 of 31 (permalink) Old 01-19-2017, 07:28 AM Thread Starter
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Crappy compression options

77 GW. I've had some minor running problems in the past; carbs are really crappy with oxidized/rotted away main jet posts. I have a little $ right now and was going to buy a set of carbs and actually good rebuild kits (the chinese ones I bought a year ago already had a needle break in half, haha). Before I dropped cash on this and a new back tire, I decided to check compression (overnight cold soak).

Sitting on the bike facing forward and looking down, I got:

Front left: 100psi
Rear left: 100psi
Front right: 100psi
Rear right: 75psi

Ouch. On a car, this would be a garbage cylinder. I am assuming (perhaps incorrectly) that this holds true on a GW as well.

What are my options here? I have no problem doing anything, I just want to know what's cost effective.

I am going to try adjusting the valves and rechecking all cylinders first.

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post #2 of 31 (permalink) Old 01-19-2017, 07:39 AM
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Adjust your tappets (cold engine) and retest. Hopefully Right Rear has a tight tappet keeping a valve open too long.

When was the last time the engine was run? A lot of ills are cured with exercise.
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post #3 of 31 (permalink) Old 01-19-2017, 08:00 AM
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Originally Posted by billythegrape View Post
77 GW. I've had some minor running problems in the past; carbs are really crappy with oxidized/rotted away main jet posts. I have a little $ right now and was going to buy a set of carbs and actually good rebuild kits (the chinese ones I bought a year ago already had a needle break in half, haha). Before I dropped cash on this and a new back tire, I decided to check compression (overnight cold soak).

Sitting on the bike facing forward and looking down, I got:

Front left: 100psi
Rear left: 100psi
Front right: 100psi
Rear right: 75psi

Ouch. On a car, this would be a garbage cylinder. I am assuming (perhaps incorrectly) that this holds true on a GW as well.

What are my options here? I have no problem doing anything, I just want to know what's cost effective.

I am going to try adjusting the valves and rechecking all cylinders first.
A cold compression test does not mean much. I would warm the engine and recheck with throttle blocked wide open, plugs removed so the starter can spin real fast. A single cylinder that is slightly lower than the rest (10% variance is acceptable) is not usually an issue right now. You might just notice it does not run quite as smooth as you might like. The cylinder balance is off as that one cylinder does not make quite as much power as the rest. I wouldn't over react at this point. Compression tests are over rated for engine diagnosis on older engines.

Mike

Worked on the "big rigs" for 45 years now just riding my Wing whenever I can. Gets cold in Wisconsin.
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post #4 of 31 (permalink) Old 01-19-2017, 11:04 AM Thread Starter
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I've never met an engine that was to be tested warm for compression, but again I am a car mechanic. The bike is driven 20 miles every weekday, mostly interstate. I will adjust valves and get back.
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post #5 of 31 (permalink) Old 01-19-2017, 11:25 AM
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Originally Posted by billythegrape View Post
I've never met an engine that was to be tested warm for compression, but again I am a car mechanic. The bike is driven 20 miles every weekday, mostly interstate. I will adjust valves and get back.
I am not familiar with your bike, but here is the procedure in GL1500 Service Manual showing it should (not should, must!) be warm.
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post #6 of 31 (permalink) Old 01-19-2017, 12:18 PM
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Quote:
Originally Posted by billythegrape View Post
I've never met an engine that was to be tested warm for compression, but again I am a car mechanic. The bike is driven 20 miles every weekday, mostly interstate. I will adjust valves and get back.
Always test compression on a warm engine. I was an auto mechanic all my working life and never heard of any other way.

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post #7 of 31 (permalink) Old 01-19-2017, 01:31 PM
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post #8 of 31 (permalink) Old 01-19-2017, 02:03 PM
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Quote:
Originally Posted by billythegrape View Post
I've never met an engine that was to be tested warm for compression, but again I am a car mechanic. The bike is driven 20 miles every weekday, mostly interstate. I will adjust valves and get back.


As mentioned by others , a compression test should be done on warm engine , - on all engines !

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post #9 of 31 (permalink) Old 01-19-2017, 02:54 PM Thread Starter
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I must be in the twilight zone. Never have I heard of, performed, or seen anyone do a compression test on a warm engine, ever. Probably because it's the wrong way to do it, but I guess this IS the internet, so...

Anyway, my bike is ridden every weekday. Don't know where the weekend came from, as the original post says weekday.

I will adjust the valves and perform another cold compression test, as performing one warm would be useless.

Here's why:

When an engine is warmed up, unless it has just one cylinder, the compressions will vary in cylinders due to uneven heating, physical material distribution, etc. Especially in a carbureted engine. Especially in a 40 year old carbureted engine.

When cold, all the metal shrinks down allowing for a more accurate test of what's really going on with compression. This is why many (not all) engines require the engine to be cold to adjust the valves. This is also why we do leakdown tests on a cold engine.

I have never, EVER heard of anyone in the car business doing compression tests on warm engines because it just isn't done that way. Sorry.

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post #10 of 31 (permalink) Old 01-19-2017, 03:04 PM
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I'm agnostic when it comes to warm vs cold. But when running a wet compression test (5 ml oil added to cylinder immediately before taking the reading) that the oil MUST BE DELO 400!

Popcorn, anyone?
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Current Bikes:
---------------------
74 CB750 K4
77 CB550 K3
82 CM250C
82 GL1100 Interstate w/ Auburn Gemini (doublewide) Sidecar

Past Bikes:
------------------
What?? SELL a bike??

------------------
Can I have a motorcycle when I get old enough?
If you take care of it.
What do you have to do?
Lots of things. You've been watching me.
Will you show me all of them?
Sure.
Is it hard?
Not if you have the right attitudes. Its having the right attitudes that's hard.


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