Low compression

[Chuck73]

Hello all I have just got a 1997 Royal Star that has 40,000 on it.......it has a popping at a steady RPM so I decided to do a compression test so I found the right front is 51 PSI the right rear is 75 PSI and the left side are both 99 psi I have no records of maintenance so I am wondering if it's because of carbon buildup that is common with these bikes or is it rings or possibly the valves need adjustment thank you in advance for any help troubleshooting my issues.......Chuck73

[MiCarl]

Those numbers are just not believable on a 40k mile bike unless it's been severely abused.

 

Compression test should be done with:

 

1) All spark plugs removed.

2) Throttles wide open.

3) Fully charged battery (I prefer to also connect a boost box).

 

If all three were true I'd get my hands on another gauge and try it again.

[Karaboo]

Check valve clearance.

[djh3]

A couple things. All of the above. Also check the book to see what is window for difference. Usually around 10-12% high to low. After the first compression test all around, do a "wet" compression test. A little oil maybe a 1/2 teaspoon per cyl and roll it over again. If they come up to allowable range it would indicate rings. As mentioned, if the valves are out of adjustment they would "hang" open some and lower compression on those cyl. Dont panic yet, low milage, any reason to think something major is not a worry yet.

[frankd]

Measure your valve clearance, and if it's real low on your weak cylinders, install the proper shims. However if the valve clearance is excessive and you have low compression and popping there are other possible causes. Let us know how much clearance you find.

 

Frank D.