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Overheated Merc 150V won't plane out

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BlackMax-ThunderToad

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Bad overheat at lake. Then new water pump parts, Tstats, poppit, plugs. Compression test good. Now good water pressure, good tell tale, starts fast, idles great, troll speed good. Will not plane. Bogs. Not O/B savvy but know auto mechanics. Seems like if it was a coil or two it would eventually plane? Like a car will run with one cylinder out. Seems like this means it's up the electrical chain? Is this true? What would fail at overheat to cause this? Was running great up until. Seems like if it starts good, idles smooth and trolls that the ignition system is good. What do you experts say? What is the next step that I can do? 1984 Merc 150 carb V6. Serno: 6552235
 
Warmed up on hose. Plugs out. WOT. Lanyard off.
Cylinders:
1. 110
2. 115 - "vapor" escaping for 4-5 seconds when plug removed after warmup
3. 121
4. 114
5. 120
6. 120
 
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Further; Coils, stator and trigger test show good numbers except stator inner sw box Blu to Red reads 8600 Ohms, book says 5400-6200. Everything else that I can test here is in spec...... Stupid boat motors!
 
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Well, I don't have DVA or leak down equipment. I don't know how to analyze it. So I will do what every red blooded American shade tree does. Buy parts by guesstimating what's wrong and info from all here and elsewhere. It's all we can do. We can't buy tech equipment that we will never use. At least the parts we buy can be used even if they don't need replaced. Then we can eliminate them as the problem and sell the used ones on Ebay. I know it must be hard for Gurus and Techs to help people like me, knowing we should be taking it to an expert, but I appreciate it. I hate taking my whole boat somewhere for weeks/months and then get a huge bill. Marine shops are now few and far between and the remainders are not cheap. Going to buy flywheel puller, OEM stator and trigger. I know auto and truck motors but not O/B, especially the electrics. I've read all about how it works and what all could be wrong and I have a headache. Maybe I will get lucky. I will report back if anybody cares. All cylinders fire on timing light but did not check under load, which is where my problem happens.

Another shade tree problem: lake too far. Poor me.
 
Many say, including posts here, that leak down not for 2 cycle. Compression test is conclusive. More applicable to 4 cycle. I thought about it and if you bring the piston to TDC, the skirt covers the ports/reeds so what are you checking besides the rings? It seems like the compression test shows motor has the compression it needs to fire? Which my motor does fire. I just can't test for weak spark/breakdown under load. I know it's not a fuel problem and I have compression so I'm betting on spark strength.... under a shade tree (or cactus in my case.)
 
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fastjeff said:
Since leak down is conducted at TDC the ports have nothing to do with it.

Jeff
Click to expand...

Agreed.

But, While doing the leak down test, you can let the piston down the bore just enough to open the port and check the reeds.

Pulling the heads is a little extreme at this point, I would find a good bore scope to borrow and inspect the cylinder walls for scuffing. If any scuffing is found you do not need to go any further. The engine needs a rebuild.
 
If a two stroke motor develops serious blow by, the intake charge is polluted, which leads to poor low speed power (and idle). Get it up into the power band, however, and the exhaust dilution is less of a factor--there's only a fraction of a second for the leak to occur.

I've seen motors that barely idled, and were HELL getting on plane, that ran 'normal' at high rpm. Perhaps this is the problem.

Jeff
 
Go ahead take one cylinder head off for a look.-----------See what cylinders look like.--------------No point jumping up and down and guessing.
 
Thanks for the help on this, thinking it thru with me.
I am not an O/B tech. I don't want to buy equipment. I don't want to buy parts and try them without some logic. I can't afford a boat shop. I can't do all the tests and don't want to start tearing down, yet. So I get on a forum and give motor history, symptoms and what I've tested and replaced in hopes of narrowing down my ultimate guess. So far I have purchased the parts mentioned, tested some, got a FW puller and am "scoping" out an inspection camera. Was going to buy stator and trigger but decided to think and ask a little more without jumping up and down. I appreciate anyone's further input and thoughts.
Motor starts quickly, idles smooth and will troll great. No plane power at all.
 
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Is it simply over propped? Can you try a borrowed prop with a lot less pitch, just to find out?

My buddy had too much prop on his in-line 6 Merc and it would not get on plane--even with me hanging off the bow. Two less inches of pitch and all was well.

Jeff
 
Thank you Jeff for responding. I want all ideas and advise. I've had the boat for almost 25 years. Nothing has changed except that it overheated and began this problem. (bog down under full throttle). I have done a water pump service and other per my above posts.
I still would like some answers to this "So scuffed cylinder walls will have good compression, start good, idle good? but no plane out power? "
 
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Over that 25 years the motor has gotten a bit tired--hell; you're older too, right!

It's entirely possible that less prop will cure the problem. I suggest you try it.

Jeff
 
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