Lower compression in one cylinder boat won’t start… ideas before I tear down 😒

[IselaLuv]

It’s a 2002 sea ray mercruiser with about 200 hrs it was initially overheating, impeller was done and the hose from impeller housing had a hole, ran it to test was good until about 30mph it would overheat so I shut off and took it back, changed thermostat but it was good, checked the elbow riser, that was clogged n rusted. I changed it, did spark plugs and as I was changing the oil saw water in it… though it be because of the elbow riser clogged, but darn it to heck it now this thing won’t start.
We tried it with starting fluid and little popping sounds on carb… then fire with starting fluid but didn’t start. He said maybe a valve is stuck open…
I did compression test. 150/120/150/~100
I am thinking on redoing it, maybe I didn’t do it correctly even thou I did do it a couple of times and same cylinder #4 was lower. Technically it should start thou right? I’m new to boating, I am mechanically inclined…
I just want to see if anyone can chirp in any possibility of being something simpler, something I am missing? Or is it just luck I’m missing, THANKS in advance.

 

[alldodge]

Yes it should start even with the low compression on 4
What is the motor serial number?
Have you checked for spark, and if so how?

 

[Rick Stephens]

I get the impression this is a new to you used boat? And probably a 3.0L merc in it?

If so, along with figuring out the motor issues, you need to start going through all the regular maintenance items, front to back. Lots of clues here that the boat is poorly maintained. It over heats - needs an impeller replaced, water in the oil, needs the engine cooling system pressure tested (very simple to do on a 3.0L). Low compression, probably the easiest way to test that is a leak down test, which will indicate if you have a valve problem, ring problem or other..

Lot of work ahead. The list of maintenance items is much longer than I listed. But you must assume previous owner let everything go to hell.

 

[IselaLuv]

Yes it should start even with the low compression on 4
What is the motor serial number?
Have you checked for spark, and if so how?

Serial is OM413330
My friend checked with the little light tester that tells you it has spark.

 

[IselaLuv]

I get the impression this is a new to you used boat? And probably a 3.0L merc in it?

If so, along with figuring out the motor issues, you need to start going through all the regular maintenance items, front to back. Lots of clues here that the boat is poorly maintained. It over heats - needs an impeller replaced, water in the oil, needs the engine cooling system pressure tested (very simple to do on a 3.0L). Low compression, probably the easiest way to test that is a leak down test, which will indicate if you have a valve problem, ring problem or other..

Lot of work ahead. The list of maintenance items is much longer than I listed. But you must assume previous owner let everything go to hell.

Yup! Learned that the hard way… now I know low hours means crap if you let it sit for a couple of years and the thing rots

 

[alldodge]

Serial is OM413330
My friend checked with the little light tester that tells you it has spark.

That's the drive serial number (alpha Gen 2)
Does the light go in line with the plug wire? If so then spark gap tester maybe

 

[alldodge]

Does it have EST distributor and a TKS carb?

 

[Bondo]

We tried it with starting fluid and little popping sounds on carb… then fire with starting fluid but didn’t start. He said maybe a valve is stuck open…

Ayuh,.... Welcome Aboard,.... Sounds like it ain't gettin' any gas,....

 

[achris]

Yup! Learned that the hard way… now I know low hours means crap if you let it sit for a couple of years and the thing rots

Indeed!

It's not using a boat that kills it, it's not using a boat that kills it...

If rather a 20 year old boat with 2,000 hours than 200 hours.

 

[IselaLuv]

Does it have EST distributor and a TKS carb?

I hate to say this but I don’t know. Here is a couple of pictures.

 

[IselaLuv]

Indeed!

It's not using a boat that kills it, it's not using a boat that kills it...

If rather a 20 year old boat with 2,000 hours than 200 hours.

Yup I now know that to be so damn true

 

[IselaLuv]

Ayuh,.... Welcome Aboard,.... Sounds like it ain't gettin' any gas,....

Ugh I’m starting to understand why the two happiest days are the day you buy the damn thing and the day you sell it

 

[San_Diego_SeaRay]

We tried it with starting fluid and little popping sounds on carb… then fire with starting fluid but didn’t start. He said maybe a valve is stuck open…

In my opinion a valve that is stuck open is the best case scenario for low compression. You asked for suggestions before tearing it apart. My suggestion would be to remove the valve cover and look at the valve assembly for the questionable cylinder while you turn over the engine w/o starting it. It's a long shot, but something you should probably do prior to tearing it down.

 

[achris]

In my opinion a valve that is stuck open is the best case scenario for low compression. You asked for suggestions before tearing it apart. My suggestion would be to remove the valve cover and look at the valve assembly for the questionable cylinder while you turn over the engine w/o starting it. It's a long shot, but something you should probably do prior to tearing it down.

Or just back the rocker stud nuts off half a turn and recheck the compression.

 

[Rick Stephens]

Ugh I’m starting to understand why the two happiest days are the day you buy the damn thing and the day you sell it

Well, you ain't there yet. Lots of help here. This forum is for do it yourselfers. If you are in that mindset, you can get all the information you ever wanted or needed to get things up and running.

At this point you need to slow down and get really systematic. Start on the compression issue, don't even bother trying to start it yet. Work your way through the basic stuff and fix things as you find em.

And to do that you gotta get the gen-you-ein mercruiser service manuals for your motor and your drive. Those are essential guides to doing it yourself, or doing it in a professional shop. Absolute must have.

Then get dirty and start in on it. If you find it fun you will enjoy every minute of play while you aren't at work.

 

[IselaLuv]

Or just back the rocker stud nuts off half a turn and recheck the compression.

Thank you I’m gonna give that a try. Then off w the valve cover plan if no difference. Thank you

In my opinion a valve that is stuck open is the best case scenario for low compression. You asked for suggestions before tearing it apart. My suggestion would be to remove the valve cover and look at the valve assembly for the questionable cylinder while you turn over the engine w/o starting it. It's a long shot, but something you should probably do prior to tearing it down

Well, you ain't there yet. Lots of help here. This forum is for do it yourselfers. If you are in that mindset, you can get all the information you ever wanted or needed to get things up and running.

At this point you need to slow down and get really systematic. Start on the compression issue, don't even bother trying to start it yet. Work your way through the basic stuff and fix things as you find em.

And to do that you gotta get the gen-you-ein mercruiser service manuals for your motor and your drive. Those are essential guides to doing it yourself, or doing it in a professional shop. Absolute must have.

Then get dirty and start in on it. If you find it fun you will enjoy every minute of play while you aren't at work.

Thank you! I really needed this little pep talk. I am getting me that mercruiser service manual asap.

 

[alldodge]

That's a Merc carb

 

[IselaLuv]

Well, you ain't there yet. Lots of help here. This forum is for do it yourselfers. If you are in that mindset, you can get all the information you ever wanted or needed to get things up and running.

At this point you need to slow down and get really systematic. Start on the compression issue, don't even bother trying to start it yet. Work your way through the basic stuff and fix things as you find em.

And to do that you gotta get the gen-you-ein mercruiser service manuals for your motor and your drive. Those are essential guides to doing it yourself, or doing it in a professional shop. Absolute must have.

Then get dirty and start in on it. If you find it fun you will enjoy every minute of play while you aren't at work.

Man I just got my copy of the service manual and I have to say I love you!!! Thank you thank you thank you