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1983 Honda vt750 Shadow
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Both of those test sites have good points. The engine should be warm. The throttle should be open. Then spin it for about 8 puffs. Use oil as a second test if you see a lower test the first time. Most of our bike are up in the 180-200 pound range.
It is possible to have a poor sealing oil control ring ( the bottom ring ) and still have good compression because the top ring is the compression seal for the most part.
The second ring is sometimes call the scraper ring, which helps control oil getting to the top also.

The second site says to remove all the plugs, but with these bikes with 2 plugs per cylinder you won't get pressure build up since it will be blowing out the other side. Only take out 1 plug per cylinder.

If you consistantly have low compression, a leak down test will pinpoint where the compression is leaking from.
 

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Discussion Starter #3
I have been trying to do a compression test with all of the plugs out and I haven't been able to get a reading, because the needle doesn't budge. Virtually all of the youtube videos I've seen suggest that all of the plugs should be removed, apparently they are not referring to bikes with 4 plugs. I thought something was wrong with my compression tester. Tomorrow with light I'll try it as you suggest.
 

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Discussion Starter #4
Does anything have to be disconnected i.e. coil, ignition etc.?
 

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1983 Honda vt750 Shadow
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That WILL be a problem. On a car you take all the plugs out since they usually always have only 1 plug per cylinder. These bikes need to leave one side in to build up pressure.
 

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You don't have to go to the bother of disconnecting the coils. My older bike if I shut off the kill switch it stops power to the coils but it will still crank the starter. If yours won't crank with the kill switch off, you can ground all the plug wire caps so you don't get sparks jumping around. Just put a piece of bare wire in each plug cap and lay it next to the head somewhere.
 

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Discussion Starter #7
Thank you! People such as myself (who don't know) often ask questions that appear to be readily available on you tube; but I have not seen one video explaining this as you did. Each and every one said the same thing i.e remove ALL of the spark plugs, and/or disconnect something. I appreciate you!!!
 

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I worked on Japanese cars for 40 years but I am still learning about my bike. If I can help a guy with my experience it is satisfying. I asked a lot of questions over the years also. Still learning !!!
 

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Discussion Starter #9
My compression is low! Does that automatically rule out any DIY fixes? Is the product "engine restore" effective in cases that are not too severe (hopefully mine isn't as it will effectively end my motorcycle career)? Is there any way to temporarily squeezeeeee a little more life from the engine?
 

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Just my 2¢ but low compression doesn't mean it's not a diy fix. Can be a few things causing it, bad piston rings, or poor valve seating for example. Both are pretty easy to handle if you have a manual and take your time. Any fix is a diy fix if your willing to get the info and do it.

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Discussion Starter #11
Your $2 are welcomed. I envisioned having to remove the engine, and buy all types of specialized tools (with money I don't have). Thank you!
 

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How low is low, and are the readings close to the same on both cylinders?
 

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I was going to ask the same thing. Maybe do another comp test with the battery jumped with a car battery and see if the pressures are better. That way you get the fastest crank speed and will get a more accurate reading. Use oil on the second try if it is still low. Then tell us the results. Remember open throttle. I did mine with closed throttle, and then open throttle, and picked up over 50 pounds.

I don't remember your bike but it may have adjustable valves which would be the first thing to consider.
 

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Your $2 are welcomed. I envisioned having to remove the engine, and buy all types of specialized tools (with money I don't have). Thank you!
The tools to rebuild the engine aren't that special usually. Just a handful of socket and ratchet for the most part. But you would have to take the engine out most likely if it needs a rebuild. That can all be done over the weekend from start to finish if you have the parts handy.

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Discussion Starter #15
I have never been more confused! The compression results were incomplete because after dry testing a cylinder (161 psi) I put a dab of oil in but then things went haywire because I could get no further reading i.e. the compression needle moved but continually jumped. The same thing happened with the other three plugs. This morning I went to the Honda dealership regarding the condition of the compressor and he refused to test it as I had purchased it several months ago and no longer had proof of purchase; but he did ask about the problems I am having. Upon explaining the circumstances and running condition of the bike, he said that my problem is more than likely fuel related and that the carb should be revisited and that I probably should pay close attention to the float level, valve and valve seat. The float cannot be adjusted, it would have to be replaced. He also said that the condition of the spark plugs could be the problem as far as the compression test is concerned. They are fouled (black) and need to be replaced to get accurate results. I need to do 2 or 3 hours of deep breathing.
 

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OK slow down and breathe.
If the needle was bouncing it probably got a piece of dirt or carbon in the schrader valve in the tip of the compression tester and was loosing the pressure. Just unscrew it with a tire valve tool, and work the valve and blow it off . Looks like a tire valve but very weak spring.
If there is a push release valve up near the head of the tester it could have gotten dirt also. You would hear the pressure escaping.
160 pounds is not bad for a dry test don't loose hope yet.

Leave 1 plug in each cylinder and only test each cylinder with the other side plug. 2 cylinders= 2 tests.

Get through this compression first and then we can talk about carbs.
 

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Discussion Starter #17
Yesterday I brought a service manual and for cylinder compression it says: (1)to measure the cylinder compression of each cylinder, remove only one plug at a time (2) warm up the engine (3) disconnect the spark plug caps and remove one spark plug at a time (4) shift into neutral (5) install a compression gauge into the spark plug hole (6) open the throttle all the way and crank the engine with the starter motor until the gauge reading stop rising.
So I am clear on the method of testing. I hesitated to tell you the results because I thought it was unbelievable the way the gauge was reacting. And that is why I went to the dealership to have the gauge tested, for my sake it is a good thing that you are not unfamiliar with reasons a tester can give erratic results. It seems that every step of the way presents some weird challenge.
 

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As I always say "Get a manual and study it. Because there will be a test".
There is always a test.
 

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How did you get from black sooty plugs to doing a compression test? Like the mechanic said, it's carb related. Have you checked your airfilter? A dirty airfilter can cause a rich condition. Low compression will not carbon up plugs.
 

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Discussion Starter #20
Big discrepancy! I changed the valve core on the compression tester and it is now showing 110 psi's on the front cyclinder(s) and 115 on the rear two.
 
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