Try changing the valve seals, can be done with just removing the valve cover. Try some 20-50 vr1. Probably your piston rings. Look for excessive blowby coming out of the valve cover breathers.

 

Re: valve seals. I'm somewhat mechanically inclined...replaced the manifolds/risers this past spring. Do own brakes on cars/trucks, oil changes, etc. Have pulled the rocker boxes off of old Harleys to redo gaskets. So...doing valve seals....difficult levels? Special tools required?

Will try some 20-50 oil..or 25-50 if I find it. Guessing straight 60 weight like I used to use in my old Harley Shovelhead is too thick. ;/

No "breathers" on the valve covers...only devices are 1) Oil filler cap, 2) on same bank...the breather hose going to flame arrestor/intake. Might be just an elbow, or could be a PCV valve....I'll examine it closely next week. But where it goes come to the flame arrestor/intake..that area is just "slightly" black...does not look like oil is splattered like I'd imagine if there was a lot of positive pressure in the valve cover/case.

 

Compression seems real good. I'm leaning towards valve stem seals myself. How do the spark plugs look?

 

Quite decent....not oily or fouled.

 

Was that a misprint on the compression numbers, saying that's your port engine ?
If it was the stbd numbers, they are high for a non-vortec engine and would be an indication that there is an excess of oil boosting the reading, which would point to rings.

 

Was that a misprint on the compression numbers, saying that's your port engine ?

Dangit....that's what I get for posting before more coffee. Good catch. Sorry...yeah, that was for port engine which is running fine. I could have sworn when we purchased it work was done on the starboard engine including a compression check...but I was apparently incorrect...it was the port engine which is running fine now. But during sea trials port engine was backfiring badly and I had the yard do a compression check plus change rotor/cap/wires.

So...OK...I have no compression check numbers to go on for the starboard engine. Will get some done.

 

Yes that elbow. If it's puffing oil like a chimney when disconnected its rings.
So the valve seals...get the piston at top dead center, chuck up some air into the spark plug hole, with your compression tester adapter, and remove the 2 rocker arms. they make a pry bar like tool for like $10 you attach to the rocker stud and you put pressure on it to remove your valve springs. Then R/R the seal. You can probably find a youtube vid on it, simple stuff.

 

I'm thinking since it's relatively clean around that elbow..and the end of the hose going to the air intake...we're good on the rings. So yeah I'll dig up some "how to" videos on you bube. Thanks!

 

k-d no. 912 is the cheap prybar for the springs. ebay $16

 

Order up a set of retainers (or keepers). There are 32 of them on the motor, Murphy’s law will dictate that you will drop one and lose it if you don’t have any spares.

Order some extras. (That should guarantee that you don’t need them!)

 

Whooo! Slow down for a minute. First of all there are no PVC valves on a marine engine, only breather tubes that direct engine blow by from the valve cover to the top of the engine intake. If the amount of oil accumulation on the two flame arrestors are the same then rings are not the problem. Yes it could be valves seal but I would be willing to bet there is a pinhole leak in the starboard engine oil cooler. That would account for both the excess oil consumption and the oil around the exhaust outlet. If it were rings or valves seals the engine would be burning off most of the excess and you would not see oil accumulation around the exhaust port.

 

Well they most certainly do put pcv's on marine engines, and the sbc engines usually don't have oil coolers.

So...

 

i have them(PCV) on my 350 EFI motors and no oil coolers.

 

I'll take pics of the exhaust ports..comparing port 'n starboard. The exhaust port isn't "wet oil" dirty...nor slippery. It's just barely tinged/sooted enough to notice. Not black or horrible looking by any means, it's not all sooty like an old coal locomotive. Just trying to make as many observations as I can to give more details.

Here are 2 pics....first one showing that breather tube..source and destination.
Second one showing closer up of that "valve"..either PCB...or just elbow....

 

The 2nd pic looks like a pcv. Go to Mercruiserparts .com, punch in your engine # and look at the diagrams yourself to see what you have.

Instead of observing from the outside. You warm the engine up fully and pull that pcv out of the valve cover and see how much oil vapor is coming out when its hot and running. The pcv could easily vacuum the blowby out without leaving a trace on the outside. The valve will also be plumbed to the manifold and not the flame arrestor

 

I thank everyone for their input and time/knowledge.
Should be at the boat this Thursday night, will take "up close" pics of what that PCV/elbow, also where it fixes to the intake. And I'll hit Mercruiserparts.com and see what it tells me.

 

So a few weeks later I finally get around to uploading pics. I changed them out not long after posting this, pulled them the day I took these pics...got the part number, went down to NAPA...got 2 of them for $8.84.

These pics show the old ones...pre-change. The starboard engine...note the hole the PCV goes in is a bit rusty...and the PCV itself is bent. So I figure "Ahh..since it's bent, probably broken...so my oil consumption will be gone!" However in looking at the PCVs..and shaking them, they rattle. So if they rattle, guess that means they are functional...and I cannot blame them for oil loss.

Regardless, cheap enough, so I swapped 'em. The port engine had a newer looking PCV..and the hole in the valve cover was clean, smooth paint. But both are swapped with new now.

I did 2x runs with the boat, 1x short run 2x weekends ago up to Lord Cove by East Haddam, and last weekend...a 4x (2x hours each way) hour run from Deep River to Noank...Fords Lobster, and out around RAM island. On the way back several sprints at steady 4k rpms. So that was more hours, and more RPMs...than the run to Greenport LI. This weekend I'll pull my rear lounge chairs and deck plates and pull the dipstick on the starboard engine and I'm curious what the oil reading will be.

 

Whelp..been a while. Took a bit to get her hauled and staged for winterization/wrappage. Requested the yard to a compression test on the starboard engine. Head mechanic just called me..said they fired up the engines on a very still day and he saw a tiny bit of rainbow on the water surface. He said at that point he hoped I had oil coolers...pulled the deck ...and...yup no oil coolers.

He ran a compression check, said most cyls were around 160 within some variance, but one back was a little lower..and one of those cylinders in that bank was ~120.

As he was pulling the plugs to do the comp check..he noted most were a little oily.
I asked for his advice..he said he could run a leakdown test, it can help point if more rings or top end.

I asked him for what he thinks the best course of action is, he said a reman "long block"...run around 10k.

YIKES!
I think next summer I'll shift to straight weight oil...40 or maybe 50...and just get through the summer...saving for a reman the following year.

Open to suggestions of temp snake oil miracle cure all fixes to slow down that oil burn. Heh.

 

https://rislone.com/

https://www.amazon.com/Rislone-4416-6PK-Ring-Smoke-Repair/dp/B008YFHL78

 

I would check the transmission coolers also.
When they fail you will see an oil sheen in the water.

 

Wow! $10K will buy a lot a oil!
My first two cars burned and leaked oil but ran for many years. As long as it cranks, runs reliably, and your not polluting a sheen behind you, run it!!!
That’s just me though.

 

There is a fuel/oil treatment by Schaeffers called Neutra Fuel Stabilizer 131. You can run this in your fuel but also in your oil to help break down carbon deposits. IF if it's a stuck ring or valve, this could help free it up. Read online. The stuff works.

 

You didn't say what the actual oil usage was after you replaced the PCV valves. Even though one cylinder is lower than the others, it is still within spec (within 70% of the others). You need to do another compression test with a leakdown test to determine where that cylinder is loosing compression. Reworking the heads is a lot cheaper than a long block. So it's using some oil - the engines are 23yrs old - nothing at all wrong with running it while you figure this out. And personally I would think twice before I put any significant $$ in a 23yr old boat - if you like the boat and intend to keep it then do it, but don't think you will ever get the $$ out of. For the $10k you mechanic is talking, you could buy a complete reman.

 

You didn't say what the actual oil usage was after you replaced the PCV valves.

Same....replacement of PCVs didn't impact the oil consumption.

Realistic about not getting the money back on sale of the boat. It's like spending 85 grand putting in an all new kitchen or roof on your house before selling it...doesn't mean the price of the house went up 85k. But it can help sell the house if the whole price is right.

While we dream of upgrading to a 37-40....that's not for a couple of years..so we'll have this boat at least another 2-3.

 

So my next step would be to find out what is really wrong - could be you just need the heads reworked, if so that is not too bad. If you are getting into the bottom of the motor, I would really look at a complete reman like a Michigan Motorz. By the way, the older 300's with v-drives are nice boats, worth investing the $$ in to keep in my opinion.

 

5.7's are plentifull and cheap. Buy a long block for around 2k and bolt all your stuff to it.

 

Lucas makes some really good additives. I have seen them work miracles in many car engines, transmission and steering. Lucas used to be for the big truck industry years ago.

 

Just had to re-read this thread I made a couple of years ago....
Made a run to Mystic today from Deep River, was a 2.5 hour cruise...majority of it at 3,200 rpm/19-20 knots.
Departed with both engines at top of the dipstick line
Arrived...port engine down about 1/2 qt, starboard engine down 1x qt.

Have made several short runs this season, this was the first longer one. Will be repeating in 2 weeks and another run to Greenport LI.

She doesn't visibly smoke...looking behind me when on plane, no smoke. Traveled with a couple of other boats this weekend, one of which was behind me most of the way...asked him about smoke..he didn't notice any.

Even starting up, at idle, no puffs of smoke.

Suppose it's really time to do a proper investigation, that "leakdown test".
Approx 720 hours on engines.

They fire up quickly..and run well. Sometimes I do swear I hear a "whistle" sound when above 3krpm....someone mentioned that piston blowby will cause the PVC to whistle into the intake.

 

I thought 1 QT in jet boat with a 350 at 3,500 RPM as a lot after 30 hours of use. Look at it this way you spent $10 on oil and $600 on gas. I use strait 30 weight oil. Oil type and manufacture can make a difference in oil consumption. What does your manual say to use.

 

Not knocking what is working for you but have you tried 15/40? We used to run the straight 30 in our 5.7 Kodiak with a 3 stage Hamilton in our 20' Ali-craft. It seemed to be a common choice with jet boaters. The result was similar to yours. Switching to 15/40 greatly reduced it's consumption and extended the service interval.

 

I'm inclined to go with the "oil is cheap" group, but I understand you're wanting to know why the port engine is better than the starboard and it'd be nice to find out what the cause is. Someone had mentioned earlier about the valve guide seals; definitely good advice. Most OE seals are rubber "umbrella"-style seals that merely divert the oil out and around the guides. Engines that old are prone to having their rubber parts go bad. I've found more than one seal split, allowing oil to pour directly down onto the guide where it then gets drawn into the engine.
As for working insitu; rather than relying on the leakdown tester to maintain pressure on the valves to hold them closed, you can use an old snowmobiler's trick to remove flywheel and clutch bolts:

1) Grab a length of limp nylon rope that can easily slide through the sparkplug hole in the head (3/8"?)
2) TIE A BIG KNOW ON ONE END!!! Yeah, you don't want the rope disappearing down the hole...
3) For the cylinder you're working on, bring it up close to TDC of the compression stroke (~90* BTDC)
4) Snake as much of the rope down the sparkplug hole as will easily go in
5) Continue rotating the engine towards TDC. You'll know you've been successful when you feel the crank lock up and can't turn it any further

At this point, you've essentially hydraulically locked the piston and valves in place against the wad of rope, allowing you to take the valve springs off with no worries of them falling into the cylinder.