Which 0X66 is the good 0X66?

[seasick]

Yamaha advises not to crank with plug wires disconnected.
Normally a mechanic would connect a clip on switch at the starter relay to enable cranking. In that instance the ignition would be off. If you do turn on the ignition, pull the safety lanyard. That should kill all spark.
The compression numbers vary the engine model and horse power. In general for 2 strokes, compression could be between about 58 psi and 150 psi
A good guess an at typical number would be 100 psi. Most importantly cylinders should be within about 10% of each other keeping in mind that the two lower cylinders 5 and 6 have a slightly higher compression ratio and therefore potentially a slightly higher compression number.

The oil question is interesting. You can take a dry compression reading and then inject a few squirts of oil and take another reading. A large difference 'might' indicate stuck or binding rings. It could also mean a weak or broken ring. It's not a fool proof test. Remember that if you fogged your cylinders for winter storage, they will still have some oil in them

 

[Halfhitch]

Thanks seasick, I haven't seen the boat in person. It is a 1992 Grady 246G with a 250 0X66 on it. I kinda wonder if that's enough power but the specs say the hull is only 3500 lbs and the max power was listed at 300hp so maybe it's OK. It's about 90 miles from me so I think I'll run over and have a look. They are asking $12.5K with bow pulpit and Lewmar 1000 windlass, hardtop with Radar, set of clears for the hardtop. I will take your advice and jump the solenoid if I wind up cranking it. The owner may have the recent results anyway.

https://gradywhite-fdcdn-blob-dqabgae8e7g5fygv.a02.azurefd.net/media/21331/1992.pdf

Yamaha advises not to crank with plug wires disconnected.
Normally a mechanic would connect a clip on switch at the starter relay to enable cranking. In that instance the ignition would be off. If you do turn on the ignition, pull the safety lanyard. That should kill all spark.
The compression numbers vary the engine model and horse power. In general for 2 strokes, compression could be between about 58 psi and 150 psi
A good guess an at typical number would be 100 psi. Most importantly cylinders should be within about 10% of each other keeping in mind that the two lower cylinders 5 and 6 have a slightly higher compression ratio and therefore potentially a slightly higher compression number.

The oil question is interesting. You can take a dry compression reading and then inject a few squirts of oil and take another reading. A large difference 'might' indicate stuck or binding rings. It could also mean a weak or broken ring. It's not a fool proof test. Remember that if you fogged your cylinders for winter storage, they will still have some oil in them

 

[seasick]

Bring a soft faced hammer, plastic is OK and tap all over the hull from the keel up to the rub rail. Then repeat on the transom. Wet/rotten transoms are common. I can't comment on the asking price. There are many factors to consider.

 

[family affair]

A 250 ox66 is plenty of power for that hull. Ours was a 248 with a 225 ox66. Power was never an issue.

 

[DennisG01]

John, I don't know what the exact spec differences are between my Offshore w/bracket and what you're looking at, but I suspect they're probably pretty darn close. I have MORE than enough power - the thing rips out of the hole and I never have an issue with power in the snotty stuff.