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Need to know compression?

Rob S.

Rob S.

Forum Guru
Past Site Supporter
I'm gathering parts (making a list and checking it twice) for Suzi's winter work, and a compression test occurred to me. I'm fairly certain my mechanic, who's always looking to save my money and his time, will say, "You don't need to know."

She runs darn near perfect (starting, idling, pulling to redline). I've convinced myself not to request a compression test. The winter work will include valve check/adjust, electrical maintenance, tune up, etc. but no other engine work.

Am I correct that, in this instance, ignorance is bliss?
 
I'm one to think that knowing is understanding. I'm betting that you don't have an issue but knowing what the current compression is will inform you in the future if things are changing. Do a compression check and get a baseline.
 
Would be good to have a baseline. Especially before and after the valve check.
 
If you know your mechanic is experienced with your particular model or the general type (older UJMs), and did a "seat of the pants" check, and said it was fine, then you ought to go with that. If it starts and runs fine, just let it be. Unless you are building a race bike, or have money to waste, finding out what percentage your compression pressure is relative to the published spec. isn't likely to mean much. Save your money for when blue smoke starts coming out the tail pipe.
 
JamesWhut? said:
If it starts and runs fine, just let it be. Unless you are building a race bike...finding out what percentage your compression pressure is relative to the published spec isn't likely to mean much. Save your money for when blue smoke starts coming out the tail pipe.
Click to expand...

That's the way I'm leaning.

I mean, if I found out they were all high and equal, I'd ride around with a puffed out chest for a while.

But even if the numbers were pretty bad, I'm not going to rip her open when she runs so great.
 
I would put retorquing valve stem caps on the list , maybe just below the compression test.
 
Pick up a compression tester and do it yourself. They're not that expensive. It's one of the tools that you'll use over and over. Every bike I purchase I do a compression test right there in front of the current owner. If it's good then great. If not then it's a negotiating tool. I do the same with the charging system using my DVM.
 
You can rent a compression tester. It's more than likely ok, but I would do it if I'm going that far into the engine. FYI. When I bought my bike, cylinder 4 was way down. Added some oil and it came up... When I got it running again I beat on it and ran it around town. Guess what? Runs just fine now. Never rechecked the compression, but I suspect the rings reseated.
 
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