Compression test

I borrowed a compression tester from O'Reilly Auto Parts to check my compression on my 82 XJ650. I didn't do this when I bought it a couple of years ago but decided to do it now before I do things like cleaning and rebuilding the carbs, rebuilding the clutch, and rebuilding the starter. The compression tester looked like crap...the glass on the face of the gauge was cracked all the way across, the o-ring on the 14mm screw in portion looked pretty rough. I decided to go ahead with the test anyway. Here's what I got:

#1: 120
#2: 115
#3: 105
#4: 120

These numbers were with the engine cold. My starter sounds like it's tired and struggling even with a brand new battery and all the spark plugs out. I know those numbers are really low but I don't feel like I can trust them with the junky gauge. I tried doing #2 and #3 over again but couldn't get the gauge to seal very good. I tightened it as much as I could but had to hold it a certain way before it would build pressure. I think the gauge is just crap. My bike has just under 20,000 miles on it and runs pretty decent. It feels a little flat in the higher rpms where it should scream but I'm thinking that's more carb related...I think but who knows. I get about 35 to 38 mpg. My question is how much does the starter being tired affect those numbers? Also, what's a good gauge to get so I can be confident in the results?

Thanks,
Stacy

 

I think your numbers look ok. Did you have the carbs wide open and did you try a little oil in each cylinder to see if the numbers came up? That would be my only suggestion other than making sure your valve clearances are in spec.

 

The throttle was wide open but the air filter was not removed, and the valves should be fine as I did them about 1000 miles ago. The minimum for the XJ650 is 128psi, normal is 156psi, and high is 171psi. I didn't account for barometric pressure or sea level. Plus, I'm pretty sure the gauge wasn't sealing as good as it should have, and my starter wasn't turning as quick as it should....or at least it seemed like that to me. I didn't try any oil in the cylinders cause the gauge was acting up and not sealing as good the more I used it.

 

auto parts store should have more than on tester or go to another location.
follow the test procedure in the info overload. it aalso has reasons as to why your compression is low read it
The Information Overload Hour
then do a third test when warm. remove the filter
there is a little debate on stone cold vs warmed engine

650, 750 air-cooled engines:
Minimum: 128 psi
Standard: 156 psi
Maximum: 171 psi
Max. variance between lowest and highest: 14 psi

your altitude is 1345
- 1000' above seal level, multiply your readings by 1.029 to get a "true" compression reading.
- 2000' above seal level, multiply your readings by 1.060 to get a "true" compression reading.

just out sside the 14 variance spec
adjusted for alt
123.48
118.335
108.045
123.48

starter rebuild kit is around 30$ you will need it sooner or later when do you want to rebuild it now or mid summer when your 100 miles from home
get another tester at another store
do the test right
keep the charger on the battery charge between cylinders
your bike even with those numbers is far from unridable or dead

 

Thanks for the info. That's the only parts store in my immediate area that loans tools and they only had one compression tester. I think I'm gonna buy one... Just not sure what a good one would be as far as brand or design. I'll probably go ahead and do the starter. I think with that done, a good gauge, and filter removed, the numbers will be better. I'll also add oil and try it warmed up. I just didn't do it this time cause the gauge didn't seem to be sealing correctly when I went back to do the lower reading cylinders again. I didn't see the point in wasting time with this junky gauge. Any recommendations on a compression test gauge kit?

 

they sell a BOSH setup that has the different adapters for 50$ at O'Reilly Auto Parts. a cheaperBosh one that does not have the 12mm adaptor

 

OK thanks. I'll check it out.

 

I bought the Bosch FIX 7828 compression tester from O'Reilly for $44.99 plus tax. It's a very nice tester and I would highly recommend it. I couldn't wait to use it on my bike so I did my tests again just as the instructions say in the Information Overload. Here's the results corrected for distance above sea level:

Engine cold
#1: 140.97
#2: 138.92
#3: 133.77 (Since this one was a little lower, I added a teaspoon of oil and tested again: 149.21)
#4: 151.26

Engine warm
#1: 146.12
#2: 140.97
#3: 142.00
#4: 154.35

The new tester performed great and I trust these numbers. I think they look pretty good. Cylinder #3 seems to be a little worn but it's still not too bad. When warm, they are within the 14 psi variance. Also, the new AGM battery spun the starter pretty fast when starting the bike and worked great. I may still rebuild the starter cause I'm sure after 34 years it needs it. This makes me feel much better about my bike and makes me feel comfortable putting more money into it. It runs ok now but I'm sure the carbs need to be gone through. That scares me a little bit. I have a bike that runs ok now and I'm afraid that if I start trying to do the carbs, I'll end up with a bike that won't run at all. I'll probably tackle it when I get my nerve up. Anyway, I feel good about the compression numbers. Thanks for the help guys.

 

The idea of adding oil to the cylinders is to see how much the compression comes up. If the numbers stay close to the same your rings and pistons are good. On low cylinders that wont come up with oil in the cylinders mean the valves are tight or they are burnt.
On the same note if your bike has been sitting for some time the rings can stick. A good trick is to put sea foam or ATF in the cylinders and leave sit for several days. This can loosen up the rings so they can seal again. Also the sea foam is good for cleaning carbon out of the ring groves.