compression test results 454

Leak down test.

The weak cylinders may be slightly rusted and not sealing well.
You mentioned installing new gaskets? Head gaskets?
The rings may be stuck in the ring lands of the pistons from sitting so long. Sitting an engine will have some valves open and some closed.
Closed valves usually sit well with no damage. Open valves will take moisture from the air while sitting and eventually there will be some rust on the valve, head and cylinder wall. Squirt a good amount of ATF fluid in the low cylinders, it is high in lubricants and cleans well.
Some mix the ATF with Acetone 50/50 and claim that works even better. It may free up a stuck valve or piston ring and get you ready to go. If rusted severely a cylinder will not rebound and seal up. If the rust is moderate it may seal up just fine. If not you are looking at a valve job or maybe a bore and hone on the block. Maybe you have a bore scope that you can shine into the cylinders and look prior to pulling off the heads. Harbor freight has a cheap bore scope for $150 or less.

So those numbers seem kind of high for a leak down test but considering it hasn't been run in so long chances are you have multiple rings which are either not seating or stuck. also you said the bores were rusty? thats your first clue of possibly needing to pull the motor. rust doesn't just accumulate, it takes the place of apparent material. in removing the rust you may have opened up your piston to wall clearances. And if you had one cylinder that was at the top of the tolerance to begin with and had to resurface and hone it you could have easily taken it out of tolerance. But with results like that and the history of the engine it sounds like rings are going to give you an issue if you just assemble it.

not sure your capabilities but if it were mine, i would do the quick rebuild check if i were looking to get use out of it this season. Pull it, hone the walls clean and check the piston to wall clearances, if ok, new rings, oil pump, seals, gaskets. check all the bearings, check the thrust while you have the crank out. Then re-assemble. It sounds like you already have the top end freshened up why not go through the lower end? there has to be a reason the heads were off when you bought it.


-Tony

The rust on in the bores came off with just rubbing an oily cloth on them, so no metal loss to speak of there. I think the stuck rings theory is most likely.
Oh, the PO had the heads pulled because he thought the engine was frozen, but it was just a bad ignition switch connection as far as I can tell. He also removed the intake manifold with the carb still on, and the hard gas line still attached! Weird.
I have a 90 SN in great shape to use this summer, so this is just a project/hobby to work on. Pulling the engine parked in my sloped driveway could be a real pain. I have a garage and shop on the family ranch I could use if it came to that. It's just nice to have it here so I can play with it in the cool mornings for a couple of hours while everyone is still asleep. Boys and their toys.

I have to agree as much as it goes against all logic lol. You've already got the heads on. Bolt up the intake and give it a go. On the small chance the rings free up (im never that lucky)