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Discussion Starter · #1 ·
I don’t think this is the correct numbers for a compression test. 120 front and 165 rear. How can I get it to rise in the front to get the same as the rear. I heard there is a way to add oil thru the spark plug but what’s the process. And will that work? Do I even need to do anything. I have a 1991 vn750.
 

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I don’t think this is the correct numbers for a compression test. 120 front and 165 rear. How can I get it to rise in the front to get the same as the rear. I heard there is a way to add oil thru the spark plug but what’s the process. And will that work? Do I even need to do anything. I have a 1991 vn750.

Oil can be added for testing, but it's not a fix.

When you tested the front, did you have a plug out of the rear cylinder? And did you open the throttle while taking the reading?
 

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Discussion Starter · #3 ·
Oil can be added for testing, but it's not a fix.

When you tested the front, did you have a plug out of the rear cylinder? And did you open the throttle while taking the reading?
i had all plugs in exceptfor the e one I would test. So three spark plugs total each time I tested. No I did not open the throttle I just had the compression gauge In spot for the spark plug I would test and I would just hold the start button until the need would stop increasing and stay steady.
 

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Discussion Starter · #4 ·
i had all plugs in exceptfor the e one I would test. So three spark plugs total each time I tested. No I did not open the throttle I just had the compression gauge In spot for the spark plug I would test and I would just hold the start button until the need would stop increasing and stay steady.
I also had the carbs off for cleaning during this as well.
 

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Ok, having the carbs off would take the place of using full throttle.

The manual wants you to have plugs removed from the opposite cylinder of the one being tested. It's to increase cranking speed.

However, if you tested both cylinders the same way, the numbers should have been closer together.
 

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Discussion Starter · #6 ·
Should I retest and take all the spark plugs out? Also do I need to take a fuse out or do something so no current is running through the ignition coils. I’m not sure exactly how to do the test. Just from what I’ve seen on YouTube but they do really say and it’s not for the vn750.
 

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Should I retest and take all the spark plugs out? Also do I need to take a fuse out or do something so no current is running through the ignition coils. I’m not sure exactly how to do the test. Just from what I’ve seen on YouTube but they do really say and it’s not for the vn750.
Could try it again, the increased cranking speed might get you numbers that are closer together.

Of course, one plug will have to remain in the cylinder being tested.

If you add oil to a cylinder and get a much higher reading, that's an indication of worn rings.
 

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Discussion Starter · #8 ·
Could try it again, the increased cranking speed might get you numbers that are closer together.

Of course, one plug will have to remain in the cylinder being tested.

If you add oil to a cylinder and get a much higher reading, that's an indication of worn rings.
Tried it warm today and got the same readings 120 from 170 rear. And I let it idle and it was fine then it just died out of nowhere again. Battery still was good and it lets me start it up again but struggles and then dies after less than a min. I'm lost
 

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Discussion Starter · #10 ·
It was the ignition coils. I replaced them with eBay ones a while back and just now put the original ones back on and it didn’t die after riding.
Mao now it’s back to normal.
However should I do anything about the difference in compression? Or let it ride till it’s below a real low amount.
 

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Compression should be tested with one plug removed from each cylinder, and the throttle open. If the carbs are off, that's the same as the throttle being open. Your compression is WAY low in the front, slightly low in the rear. I have 177 front and 182 rear. Still a little to much difference for me, but the bike has 31K miles on it and runs fine. About the oil. By putting oil in the cylinder through the spark plug hole before testing the compression in that cylinder, you can determine where you are losing compression. If the compression goes up noticeably, you are losing compression past the rings. If there is no change, you are either losing compression past the valves, have a leaking head gasket, or a crack somewhere. An easy way to determine where you are losing compression is by pressurizing the cylinder with air, then see where the air is leaking from. Remove the oil filler cap. connect a hose from an air compressor to the spark plug hole. If air comes out the intake (where the carburetors go) it is leaking past the intake valve. If air comes out the exhaust, then it is leaking past the exhaust valve. If air comes out the oil filler, it is leaking past the rings. Take a bottle of soapy water and spray it around the head gasket. It will make bubbles if the head gasket is leaking. It is possible to lose compression from more than one place, especially in a worn out engine.

But whatever it is, a difference of 45 psig between the front and rear cylinders shows a serious internal engine problem, and the engine will have to be torn down to fix it. It may run that way, but it will not run well. And you will probably notice the spark plugs from the cylinder with the lowest compression tend to get fouled easy, as that cylinder will not have good combustion. Is there any smoke coming out the exhaust? I don't know what oil you are using, but if you are losing compression due to loss of ring seal, heavier oil will help the rings seal better. I have never used anything but 20w50 in a motorcycle. You could even try a straight 50 weight. With compression numbers like that, you don't really have much to lose. That engine has serious problems.
 

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I don’t think this is the correct numbers for a compression test. 120 front and 165 rear. How can I get it to rise in the front to get the same as the rear. I heard there is a way to add oil thru the spark plug but what’s the process. And will that work? Do I even need to do anything. I have a 1991 vn750.
Change your motor oil, use 20-50 synthetic m/c oil and Rislone Restore. Add 3oz per quart of oil. All my bikes are vintage / classic that can use some help with compression. The tech at Rislone confirmed using this additive with wet clutches is ok. I've never had a problem. Good luck, ride on!
 

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FREEBIRDS MC CENTRAL NY
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On an old xs400 I put marvel mystery oil down the plug hole and let it sit for a couple of weeks and it freed up the rings on one cylinder

Sent from my A501DL using Tapatalk
 
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Yes the rings in the low cylinder could be stuck. That is usually caused by long term sitting. Marvel Mystery Oil is actually pretty good stuff. If I suspect stuck rings I usually spray a bunch of PB Blaster, Seafoam, or Chevron Techron into the cylinder. Let it soak for a long time, then put some engine oil in the cylinder, turn the engine over a few times to blow out the excess, put the plug back and try to start the engine. Stuck rings almost always result in a lot of oil smoke coming from the cylinder with the problem. I'm talking about rings stuck in the piston grooves. If the iron rings were rusted to the cylinder walls, there is likely to be permanent damage to both the cylinder and the rings. I've had a lot of experience with that, when various departments of the city I used to work for left equipment sitting for 6 months outside without touching it. Engines were often rusted solid, and would not turn over. Sometimes they could be saved, but usually required extensive repairs or replacement.
 
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