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Author Topic: Causes for low compression?  (Read 681 times)

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Offline Harper

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Causes for low compression?
« on: July 28, 2012, 03:49:46 PM »
750 K5
I have 65-80 psi across all four cylinders. A friend drove my bike and told me that after running at highway speed he stopped and noticed a spray of oil on the rear tire. He kept going, thinking the "funny sound" of the engine was a persistent mixture problem that I'd talked to him about. However, after he got back, I checked the oil and there was none (sweet!). The motorcycle will no longer start, fuel and spark are (to my eye and tests) fine. I cleaned everything thoroughly this morning and put in new oil to look for whatever huge leak pissed out all my oil, but didn't see anything immediately. I'm going back to check after I finish writing this.
So my questions as a novice are: what could cause that kind of blow out, and where would you begin to look? Is there anything that could cause my systemic loss of compression other than internal cylinder damage (it just seems strange to me that they would all reduce so evenly and so dramatically)? Finally, how could I check whether the lubrication system is getting oil to the cylinders, if there's a blockage somewhere?

Hoping this isn't ruined

Thanks!
Harper
1975 CB750

Offline phil71

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Re: Causes for low compression?
« Reply #1 on: July 28, 2012, 04:05:08 PM »
Whoa. Compression loss can happen for a lot of reasons.
1) normal wear and tear
2) warping from overheating
3) no oil (which also makes overheating happen)
4) poor valve seal
5) no oil

You say they all reduced evenly, but previously said they are between 65-80 psi, which is a range of 15 psi.
  I'd want to know why it shot it's oil all over the rear tire, but if he ran it at highways speeds with no oil, I'd say you don't need to do any more detective work. Open it up and brace yourself for some ugly.

Offline Psychonaut

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Re: Causes for low compression?
« Reply #2 on: July 29, 2012, 06:56:33 AM »
Did the bike previously sit for an extended time? Tight valves could give low compression but not the oil loss. Stuck rings could be the problem if it sat and was not stored properly.

Offline lucky

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Re: Causes for low compression?
« Reply #3 on: July 29, 2012, 07:53:02 AM »
No there is something else wrong.

"persistent mixture problem"?

Tell us more.
Like what kind of exhaust?
What kind of intake?

How long had your "freind" been riding the bike?

Had the bike been sitting around?

No compression. IS the air filter blocked with squirrel nuts?

For the compression to be that low across all cylinders EVENLY I suspect that the person taking the compression test did not have the throttle wide open? The choke was on? The battery was low? Three spark plugs were still in it???

Matter of fact there may be nothing wrong except for the oil leak.

The first thing ANY GOOD mechanic would ask is HOW MANY MILES!!!


Offline Harper

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Re: Causes for low compression?
« Reply #4 on: July 29, 2012, 08:59:39 AM »
Hello Lucky
thanks for your clarifying questions. And your insinuating quotation marks:) Here goes:
Mixture problem: I rebuilt carburetors, moved to high altitude, and damaged a condenser (just things that I suspect. I don't know the cause). the second and third cylinder ran significantly richer than one and four, repeatedly fouling the plugs over the course of a couple months. I, like a BAD MECHANIC, would just clean those spark plugs, everything would be good again because I was only using the bike for half mile rides on the dirt roads where I live, and I never dealt with it more thoroughly than that.
Exhaust: four into one from the PO. I'm not sure what brand.
Intake: stock everything. I'll check for any blockages.
The ride: a half hour shot out to the highway and back, with oil loss ten minutes from home?
The bike: had not been "sitting" though it rarely had long or hard use. Many daily short trips.
compression: maybe I did this test wrong, and I have an update. All that I did was pull all the plugs and sequentially screw in my compression tester first dry, then wet, kicking over the bike until the pressure stopped increasing. The throttle was closed and the choke was off. However, I re-tested yesterday and got the following results: dry 1 - 92, 2 - 75, 3 - 70, 4 - 70. wet 1 went to 110, the rest had negligible difference.
Oil Leak: still no big puddle, so I'm not sure how to pursue finding that.
Miles: 35,000

I have to move in a month to a place with no shop and no time, I'm working full time now, and so it might be time to try to cut my losses..
In any case, thanks for all your advice. Let me know if I've incompletely filled in any of this information.

Harper
1975 CB750

Offline lucky

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Re: Causes for low compression?
« Reply #5 on: July 29, 2012, 03:37:22 PM »
Thanks for all the info.

How would you damage a condenser? Forget that.
But if #2 and #3 were really rich there could be an electrical problem.
Just clip a timing light onto the plug wires and you can see if the light blinks when it fires. Look for consistancy.
If it (condenser) was not smashed you cannot test it with a regular volt ohm meter.

How high was the altitude???
2000 feet would not even affect it really.
You would have to be at 8,000 feet or more.


Oil leak must be resolved. Find out where it is coming from.

Compression must be checked with the throttle all the way open. Key off.
« Last Edit: July 29, 2012, 03:42:32 PM by lucky »

Offline Harper

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Re: Causes for low compression?
« Reply #6 on: July 29, 2012, 03:56:43 PM »
cool, I'll re-test at open throttle.
I'm at 6000 feet, so you're right, I doubt it's at the heart of the issue
If it runs again, I'll try your timing light trick. right now there seem to be larger issues
re the condenser:  I had a 15 mph "crash" trying to dodge escaped sheep and driving into a feed bucket. this mushed my timing cover and dented one of the condensers.

will report back w/r/t oil and compression.

thanks again
1975 CB750

Offline TwoTired

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Re: Causes for low compression?
« Reply #7 on: July 29, 2012, 04:18:24 PM »
The number one cause of "low Compression" in the SOHC4, is errors produced by the selected tool and misunderstanding how the tool is used or applied.
Tested with throttle closed and/or choke plates closed? (for example).

http://forums.sohc4.net/index.php?topic=369.msg476032#msg476032

Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC

Popeye: "Wrong is wrong, even when it helps ya."

Offline Bob Wessner

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Re: Causes for low compression?
« Reply #8 on: July 29, 2012, 04:27:58 PM »
cool, I'll re-test at open throttle.
I'm at 6000 feet, so you're right, I doubt it's at the heart of the issue
If it runs again, I'll try your timing light trick. right now there seem to be larger issues
re the condenser:  I had a 15 mph "crash" trying to dodge escaped sheep and driving into a feed bucket. this mushed my timing cover and dented one of the condensers.

will report back w/r/t oil and compression.

thanks again

New Paltz, NY... 6,000 ft??
We'll all be someone else's PO some day.

Offline Harper

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Re: Causes for low compression?
« Reply #9 on: July 31, 2012, 10:21:02 PM »
No, California. Near Bishop, the White Mountains. New Paltz is lower for sure.

the thing is, I took the cover off the alternator and all my oil fell out. really all of it.How could it all have gotten there? I finally noticed oil leaking around the three retaining screws in that left-hand cover.

I got worried about my test and read some of the compression stuff here, but when I retested following all the recommended procedures I got the same number +/- 5 psi. Maybe when you have half the compression you should technique becomes less important.
1975 CB750

 

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