How much varience allowed between cylinders's compression?

[Cannondale]

Just sold a 1990 50hp Force Saturday. Buyer checked compression (I stated comp. was between 130-135# on the two cylinders) and got 135# on top cylinder, 131# on lower cylinder, whereupon he immediately says "You know what that means, don't you?"

My response was......yep, what you'd expect to see in a 20 year old motor, <5# difference between the cylinders.

His response....."No, it means the lower cylinder is about to fail. You trying to sell me junk?"

So, I asked him to show me any sort of documentation proving his statement and offered the use of any of my computers and any length of time to find it.


He refused, paid what I was asking and drove off......grumbling just low enough that I couldn't hear it, but the wife made an assessment about him I really cannot repeat.


So, was I wrong? I'd always been told that up to a 10% variance between cylinders was acceptable, and this was <5# difference, almost within instrument tolerance (+/-2%).

 

[lncoop]

Re: How much varience allowed between cylinders's compression?

I believe agreement is pretty well unanimous on no more than ten percent variance. I would think if the guy had been that confident you were selling him junk he wouldn't have bought it, much less at your price. Then again, takes all kinds. Especially with regard to those of us crazy enough to play with boats. Notice I didn't make any snide Force comments? Someone will I'm sure.

 

[Lostfisher]

Re: How much varience allowed between cylinders's compression?

I'd say that the motor has Excellent compression. He was probably just trying to talk you down and didn't really know anything about motors in general. I have also been told that there is a 10% tolerance between the cylinders.

 

[robert graham]

Re: How much varience allowed between cylinders's compression?

He was just jerking you around...some folks enjoy doing that just to intimidate people. If you'd had time to think about it, you could have acted real surprised and told him you wouldn't feel right selling him a faulty motor like that, then see what he said! If he thought that cylinder was failing then he wouldn't have bought the motor.

 

[Robbabob]

Re: How much varience allowed between cylinders's compression?

Yup, he was hoping you didn't know anything much about it.. then he'd get you to drop your price. You pretty much heard him grumbling how he didn't stand a chance getting a better price.

He obviously got a good enough deal; since he paid you, but some people just want things their way.... as if Burger King was waiting on them all the time!

Good on ya!

 

[TOHATSU GURU]

Re: How much varience allowed between cylinders's compression?

It's more a relative issue to the given hours on the engine and the loss of compression. IE a 20 year old engine with 5% difference isn't bad, a 10% difference isn't great, but okay and a 15% difference should be examined. A 5 year old engine with few hours would have an issue with a 10% difference. It's all about how much slop anyone is willing to allow on any given day on any given engine...As an engine re-builder I'm less tolerant, because I'm more of a paranoid, than say....JB

 

[Cannondale]

Re: How much varience allowed between cylinders's compression?

Thanks for the replies! Good to hear that the 10% variance wasn't a figment of my imagination.

The main problem was I was suffering from a case of the flu the day the motor sold and I really was "slow" that day......and this was last Saturday. Today, Thursday, and the flu symptoms are finally clearing enough to be able to stand being out of bed for more than an hour.

But thanks much for reinforcing what I thought I knew.