1983 V-150 rpm variations, leaking bottom crankshaft seals?

[jdaghir]

1983 V-150 S/N 6313170 on a 28' foot Lowe Regency pontoon.

After a fair amount of work last season, motor has ran well for most of this season, but recently it has become somewhat difficult to start and I also noticed that the rpms tend to vary or float around a little bit above 2000 rpm - for example, the motor will be running smoothly at say 3500 rpm, but then will drop down to 3300 rpm for 5 seconds or so, then go back to 3500 rpm and stay there for a minute or so, and then repeat.

Since I hadn't put new spark plugs in at the start of the season, I hoped that possibly it was just fouled plugs, so I put new plugs in (NGK BU8H). While changing the plugs I noticed that the bottom plugs on both the starboard and port sides were very clean and looked almost brand new, while the other 4 plugs all looked the typical black and sooty around the center electrode.

That didn't seem right, so I used a cheap USB boroscope and took a look inside the starboard cylinders. The tops of the top two pistons were completely covered in carbon, but the top of the bottom cylinder was mostly covered in carbon with a few clean and shinny spots. I think the clean spots more or less lined up with the intake & exhaust ports. I clumsily managed to drop the boroscope and break it before I had a chance to take a peek at the port cylinders.

I took the boat out and ran the motor for an hour or so and the new plugs seems to have resolved the difficult starting, but the rpm variations under load are still there. I pulled the plugs again, but at this point in time they all still look brand new & clean.

After doing some research, I suspect the cause of the clean & shiny bottom plugs/piston top is water getting into the cylinders and causing one or both cylinders to occasionally miss or not fire which could be the source of the rpm variations. The water is most likely coming from leaky bottom crankshaft seals or less likely a leaky exhaust manifold gasket or cylinder head gasket.

Questions:
Does this sound correct or should I be looking for some other problems?

Should I start with replacing the crankshaft seals first since they are the most likely suspects, or should I replace the exhaust and cylinder head gaskets first since they are easier to get to?

Assuming it is in fact bad lower crankshaft seals and I pull the powerhead to replace them, do I need anything other than 2 new seals, a new o-ring for the end cap and a new exhaust plate gasket?

Is there anything else I should consider doing while I have the powerhead removed? Specifically should I replace the packing & carrier assembly that is on the end of the crankshaft while it's exposed?


Thanks in advance for sharing your knowledge and experience!

 

[Dukedog]

check exhaust divider plates for tiny crack/cracks and gaskets before ya pull tha power head.... if they have any "clean spots" and/or block exhaust runners have any "clean spots" replace both plates.

 

[jdaghir]

Thanks Dukedog! That's exactly the kind of info I was hoping for. I've got two new divider gaskets and a seal on order. Once they arrive I'll pull the exhaust cover off and see how things look.

I did take a closer look at the exhaust cover and there does appear to be an accumulation of black, sooty oil on the bottom edge of the exhaust cover and around the heads of the bottom two bolts on the exhaust cover. Possibly a sign of a gasket failure, or maybe it just collects there since it's a low spot when the engine is tilted down. I noticed that all of the exhaust cover bolts have paint chipped off of them, so I suspect someone has had it off at one point in time. Hopefully that means the bolts won't give me too much grief when I go to remove them.

I ordered gaskets from Sierra rather than Mercury, which from other threads I've read means I will need to use gasket sealer on the the gaskets, and red loctite on the bolts. Is that correct?

 

[jdaghir]

Oh, and when you say "replace both plates" did you mean replace the divider plate and both gaskets? There is only one divider plate, right?

 

[Dukedog]

two parts to it,, reason for two gaskets.. you will see when ya get it off.. did you get tha "spaghetti" gasket also?.. looks like a piece of clear tubing...

 

[jdaghir]

Thanks for the clarification - I appreciate it. And yes, I ordered the tube-shaped seal along with the gaskets.

Again, much thanks!

 

[jdaghir]

Finally got time to get back to this. The exhaust cover bolts, the cover and the divider plate all came out easy. Too easy in fact - makes me suspicious the bolts were not properly torqued. The cover has definitely been off before and the gaskets seemed to be in descent shape.

Most of the bolts looked like I expected but a couple had a fair amount of corrosion on them and the bottom three bolts were black and oily. The bottom edged of the inner gasket was also black and oily. The divider plate itself was clean and free from carbon build up on the bottom quarter of the plate.

I didn't seen any obvious holes or cracks in the divider plate, but I will take a closer look after I have cleaned it up. I'll also try and see it warped much, although I don't have any machinist-grade straight edges or flat-surfaces available to me - I'll have to make due with a plain old steel ruler.

I've attached some photos - can someone take a look and give me their opinion on them? Was water leaking past the divider plate and into the exhaust cavity and from there into the cylinders?

Thanks!

 

[Faztbullet]

There is something wrong with #5 as plate and exhaust port is washed,,it a fuel wash. Either jetted to big,carb is flooding or electrical problem(incomplete combustion).Is this a 20 or 25 inch shaft??

 

[jdaghir]

Great eye faztbullet - The bottom carb was spitting fuel out the front of the carbs. I had rebuilt it earlier this summer but apparently set the float level too high. I used Sierra floats and was a little uncertain as to what exactly was the top of the float. A few weeks ago I adjusted it down further and tested it using the primer bulb to confirm that the bowl filled to the top but then stopped filling. I've only had it out briefly once since then.

Also the wire from the switch box to the coil for #5 was kinda loose in the terminal. The factory harness was in bad shape, so last year I made my own and used a terminal that was a little too large for the wire gauge. I probably accidentally pulled the wire loose while monkeying with it at some point in time. I'll replace it with the correct size terminal when I button it back up.

 

[jdaghir]

Oh, and it's a 20" shaft. Just for my own education, how does that affect anything?

I bought Sierra gaskets which look to be plain paper, not silicone impregnated, which I believe means I will need to use some kind of gasket sealant.

I have both Permatex High Tack Gasket Sealant 98H & Permatex Aviation Form-A-Gasket No 3 on hand. Would either one those work?

Thanks!

 

[Faztbullet]

Use the AV sealer....On some 25" model the main jets as a tad smaller

 

[jdaghir]

Got the divider plate cleaned off today.

For the record, the cooked oil and carbon deposits on the divider plate laughed at a 2 hour soak in SeaFoam Deep Creep and told it to be quiet while the adults talked. They responded with a lot more respect to old-fashioned, brain-cell-killing paint remover and a stripping scrubbie pad.

Good news - I didn't find any cracks or pin-holes in the divider plate.

Bad news - the divider plate is warped. Best I could tell using a steel ruler and feeler gauges is that the center of the divider plate is approximately 0.012" out of flat. In other words, if you were to hold the divider plate in position on the power head, the top and the bottom of the plate would be touching, but the center of the plate would be bowed out away from the block by 0.012".


Options:

1. Buy a new divider plate at $220.

2. Buy a used one on Ebay for around $100, and hope I win the lottery and get one that isn't warped, cracked or have holes in it.

3. Double up on gaskets on both sides of the plate and use overly generous amounts of gasket sealer and hope for the best.

4. Install it normally with one gasket per side, gasket sealer, proper torque, tightening sequence and thread locker, and not worry about it. The tightening sequence does start in the middle of the plate and then progresses up and down from there, so it should have a tendency to pull it back into flat.

5. Take it to a machine shop and have it machined flat.

6. Something else?


I'm leaning toward #4 but I'm open to suggestions.

 

[Faztbullet]

#4........