Good compression on 8, High leakdown on 1 cyl
 

Looking at a boat with twin 502s. Approx 125 hours since last refreshed.

Compression numbers all in the 150-180 range.

One cyl on each motor is showing up at 65% Leakdown, which seems super high to me. The shop doing the testing says it could be due to being winterized, maybe a little rust on a valve preventing full seating, etc but I'm skeptical about it being one cylinder on each motor like that.

Walk away? Renegotiate? Other?

That can happen but yes seems odd one cylinder on each??? Not ideal way to ck under current conditions. How many hours total on engines and who did the REFRESH? What all falls under refresh? Any receipts? And who performed the work?

Does the boat and engine bay look meticulously maintained?

just some things I’d take in consideration.

They should be able to determine where the leakage is coming from. A true 65% would mean it needs to be torn down.
What % are the other cylinders??

His feeling was the leakage was top end because the compression was pretty good across the board. Shop has a good reputation (Boat Doctors of Gilbert SC).
Not sure what the other leakdown numbers were but I can find out.

Sidetrack: 180 psi ? How much static compression do these motors have ?

Sometimes leakdown will be bad on a few cylinders when engines haven’t been run. Did they listen to where air us escaping ?

Enough compression that the current owner runs 93 octane. Not sure of exact comp ratio.

I don't know if he heard where air was escaping but I did ask whether he thought it was top end or bottom end, and he felt pretty confident that it was top end, with the caveat that he can't be completely sure without a teardown.

An ear to the intake or the exhaust will tell you if its heads, same ear to oil fill cap will tell you if its bottom end

Other cylinders were between 5-15% leakdown.

When doing a leak down you always listen to where the air is leaking, WTH.

As a example that is a LOT more leakage then I have on my 680 hour HP500EFI's that were done cold. As a matter of fact when we did them they had not ran for months. Just to give you a comparison. It should be looked into a bit more.

I am also with SB 180 psi is up there. Don't forget around us on the water all you can get is 90, needing 93 will make a lot of our runs a PITA. Better be to the top for a Sebastian run and play.

WG, I know, I was thinking the same thing. I might take a pass on this one.

Tried to renegotiate down $7K in the price to account for top ends. Seller declined. So, I'm out. Thanks for the input all!

Good call. Might be as simple as a little corrosion (doubtful) however that’s a good number you negotiated for a piece of mind.

150-180 psi variance, up to 65% on a cylinder and not knowing where the leakage was coming from??? I'd have more confidence in the engines than the tester.

A stock 502 magnum will crank in the 155 psi neighborhood. That's an 8.8:1 motor with a [email protected] cam.

Upwards of 180 suggests oil in the cylinders artificially raising the psi.

I have to ask if the compression test and leak down were done on a winterized motor(s) or running motors after being warmed up and cooled down.
You need to run them up to operating temp, then Rev em several times, spin em up to 4000 or 4400 rpm. Then back to idle and let them cool down enough to pull plugs and run tests.
The reason is that fogging oil will screw with psi readings. The other reason is that carbon and fogging residue over a winter can Harden and prevent some valves from sealing as well as they should.
After you run it up to temp, Rev it some, then any partially sticky rings will be freed up, crunchy residue on valveseats will be hammered off, and any fluid/oil/whatever will be burned out of the combustion chambers.

Your wrench guys ought to know this. But sometimes you need to specifically ask...

I'm out of the deal at this point, and pretty comfortable with that decision, BUT, they're not stock 502s. They dynoed at 565 HP when originally built. So, it's likely the compression is a good bit higher than a stock 502 mag. I asked the seller if he knew the compression ratio and he did not.

Initially my impression of the boat was that this was a well loved and cared for "baby" but as things have unfolded I think it was an occasional toy that got hauled to the shop when something wasn't right, but didn't seem to get the love that I like to see in something I'm interested in buying.

Use a bore scope to take a look, first I'd take it for a good beating to dust off any rust and blow out any oil from winterizing.

I understand you have passed on the boat that you started this thread about. I just wanted to ask a question about it though.

Do you know which cyls were low?

Aren't cyls 7 & 8 susceptible to water intrusion from the exhaust? If so, isn't it possible the seats or valves could be damaged?

It was cyl 4 on one motor and 6 on the other. so, interesting that it's on the same side of each motor, but not sure what to infer from that.

I know you passed on this deal,and I get it, so this is just info for next one. I had the heads milled .020 on my 502mag with stock pistons and bore and it runs mostly 180 on compression check.

Good to know. These motors had Edelbrock aluminum heads, pretty big cams with over 0.600 lift, and had allegedly been refreshed twice since original builds. Too many unknowns and maybes.

sounds like reversion has been an issue.

Thanks for the reply.

I wonder if boats that haven't been drained of water in the engine have a problem with inclines? I know 1 PWC in particular that the water inside the exhaust for cooling rubber hoses, would run back and into the front cyl if you didn't blow the water out and backed the ski down a hill.

In my situation, I pull the drain plug from the bilge, but I have to back the trailer down a decent hill to where I park it, and normally drain the pipes when not being used for a few days. (fresh water)

Do I need to worry about water from the block filling the headers and running back into the exhaust? (CMI Fusion headers) My guess is no since the engine pulses aren't sucking the water backwards, but figured it's a good time to ask. Thanks guys..

In my case it was.

Engine builder removed the heads and a couple valve seats were pitted pretty good. (bought a used engine, ran for 1 season and already in for complete rebuild. :faint2:)

HP 500 carb. Engine had CMI headers on it in the video the seller sent me. Pits look to have been there for some time. I'm running CMI Fusion headers but was wondering if me backing the boat down a hill had something to do with the water intrusion. (se previous post ^^^^^)