1989 DT65 compression number variations?

[tinkerr]

I bought the motor with the lower unit in a box, but checked compression first. The compression #s (via 25yr old tester)were 120T/115M/120B after installing lower unit, ran the motor, it idled, but poorly and smoked. I took it to a Suzuki dealer for carb rebuild and once over. He checked the compression and got 140T/140M/130B, he scoped cylinders and looked at the reeds found some scratches but nothing significant, cylinders still had the cross hatching marks. He said it could run forever or break next week, but couldn't find anything to cause the variation in compression. He cleaned the carbs and says motor idles great, no strange noises, everything looks good. But I'm concerned, I've got $1300 dollars invested in this thing and haven't mounted it on the boat yet. A lot of motors have couple psi variation from cylinder to cylinder, but not 10psi. Should I be concerned about the variation, is it a indication of bad things to come or just a fluke.

 

[robert graham]

Re: 1989 DT65 compression number variations?

Accepted rule of thumb is 10% difference between cylinders....sounds like yours are O.K. And within tolerance....besides differences in test methods, procedures and equipment are common....

 

[tinkerr]

Re: 1989 DT65 compression number variations?

I'm going to borrow a friends compression tester and recheck the #'s. I bought this motor to replace my 1984 mercury 35hp that runs excellent, but doesn't move the boat that well. It's a lot of trouble to swap motors and run all the wiring and controls including the 2" monitoring gauge just to have a problem after I sell old reliable. thanks for the reply.

 

[robert graham]

Re: 1989 DT65 compression number variations?

I doubt you have a compression problem on that 25 year old motor....but if it runs good maybe consider doing a de-carb to loosen any carbon-stuck piston rings to improve overall performance....Yamaha sells an additive called Ringfree that can help prevent carbon and actually reduce carbon at the "de-carb" dosage....

 

[James R]

Re: 1989 DT65 compression number variations?

The compressions are good with either gauge reading.Gauges do vary considerably, even so called brand names. I have two gauges, one is my daily gauge and the other is my new back up. Both give the same readings. If you are still worried get a leak down test done. Frankly I agree with robert graham. Suzuki's are great motors and you will be very pleased the the super power from it.

 

[tinkerr]

Re: 1989 DT65 compression number variations?

I had time to retest the compression today. I used my old Craftsmen first, the numbers were 115psi on all cylinders. I then tested with my buddies Mac tools tester and got 135psi on all cylinders. Although the compression numbers are different from gauge to gauge the values are equal on all three cylinders, this gives me piece of mind there is nothing wrong with the motor. I'll install in in the next couple of weeks and add the Ringfree to a couple tank of gas to hopefully get the inside as clean as the outside. Thanks for all the responses


As a side note I use the hunt and peck method when typing. Why do I get logged out before?
I finish the reply?