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Unsustainable WOT

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Gregofracine

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1993 MCM 4.3LX with 600hrs. Accelerates well to WOT (4000 RPM), but only for 30 seconds. Engine rapidly decelerates, "pop-pop-popping" as it comes down off plane (overly lean?). Retarding throttle will catch idle RPMs. Engine will accelerate again, and decelerate again. Timing has been checked, ignition components have been changed, spark plugs have been replaced, a different fuel tank and line has been used, bypassing the fuel/water seperator, carburetor has been replaced, all with no improvement. Compression check reveals middle 2 cylinders at 80psi, remaining cylinders at 125 psi.
Engine has been removed. Heads show nothing extraordinary. Spark plugs look normal. Head gaskets appear intact. 2 Cylinders "show some rust near top with minor roughness". Exhaust risers have 1/2 of each water baffle missing. Past engine history includes 2 sparkplugs with electrodes melted away--gone. Timing was suspect as engine resisted starting rotation and might run on or diesel at shutdown. Timing issue has been corrected (I believe). Mechanic wants to install a Long Block, replace ignition components (again) and tear down to inspect and rebuild carburetor (3 seasons new). Thoughts?
 
Gregofracine said:
1993 MCM 4.3LX with 600hrs. Accelerates well to WOT (4000 RPM), but only for 30 seconds. Engine rapidly decelerates, "pop-pop-popping" as it comes down off plane (overly lean?). Retarding throttle will catch idle RPMs. Engine will accelerate again, and decelerate again. Timing has been checked, ignition components have been changed, spark plugs have been replaced, a different fuel tank and line has been used, bypassing the fuel/water seperator, carburetor has been replaced, all with no improvement. Compression check reveals middle 2 cylinders at 80psi, remaining cylinders at 125 psi.
Engine has been removed. Heads show nothing extraordinary. Spark plugs look normal. Head gaskets appear intact. 2 Cylinders "show some rust near top with minor roughness". Exhaust risers have 1/2 of each water baffle missing. Past engine history includes 2 sparkplugs with electrodes melted away--gone. Timing was suspect as engine resisted starting rotation and might run on or diesel at shutdown. Timing issue has been corrected (I believe). Mechanic wants to install a Long Block, replace ignition components (again) and tear down to inspect and rebuild carburetor (3 seasons new). Thoughts?
Click to expand...
Ayuh,..... Sounds like it was running lean quite awhile,.... Yer mechanic is probably right,..... A long block, 'n carb rebuild outa get it headed in the right direction,.....

Btw,.... Wot should be 'round 4800 rpms,..... 4000 is lugging the motor,.....
 
Based on what you have reported, it sounds like a good plan.

All cylinders should be ~ 165 psi. A remanufactured long block with heads should be a good choice.


Carb sounds like WOT issues, (lack of proper fuel level in bowl)
 
If two cylinders show low compression, I would have done a cylinder leakdown test but first, I would have used compressed air to find out if the valves were leaking.

When you posted the info, you mentioned 'middle cylinders- that has to mean 3&4- the firing order was verified?

If it cranked hard, that may have been caused by the adjacent cylinder's compression (this was with the spark plugs in, right?).

Did anyone see evidence of rust in the cylinders?
 
jimn said:
If two cylinders show low compression, I would have done a cylinder leakdown test but first, I would have used compressed air to find out if the valves were leaking.

When you posted the info, you mentioned 'middle cylinders- that has to mean 3&4- the firing order was verified?

If it cranked hard, that may have been caused by the adjacent cylinder's compression (this was with the spark plugs in, right?).

Did anyone see evidence of rust in the cylinders?
Click to expand...

Engine is out of boat
Heads and head gaskets removed.
some rust found.

To late for leak down but that may not have been needed anyway.
V6 blocks especially if over heated need machining, heads and deck. Add in the timing issues mentioned and possible a piston issue also....
 
kghost said:
Engine is out of boat
Heads and head gaskets removed.
some rust found.

To late for leak down but that may not have been needed anyway.
V6 blocks especially if over heated need machining, heads and deck. Add in the timing issues mentioned and possible a piston issue also....
Click to expand...
These had other problems a few years later, after the exhaust manifolds & cam were changed- water ingestion from rotating backward after run-on, bad raw water pump seals.....Mercruiser sent a service bulletin out that had about 9 things that could be tried. Their session at the Regal dealer meeting was just a bunch of damage control- we all asked why we should be doing their R&D for them.

The previous owner of a boat I bought redefined 'overheat' and warped the head, bent an exhaust valve and melted every piece of rubber or plastic after the exhaust- fortunately, the deck was flat. After reassembly, I couldn't have asked for it to run any better.
 
kghost said:
Based on what you have reported, it sounds like a good plan.

All cylinders should be ~ 165 psi. A remanufactured long block with heads should be a good choice.


Carb sounds like WOT issues, (lack of proper fuel level in bowl)
Click to expand...
Possibly low fuel level and sloshing once it was no longer accelerating, too. If the boat runs into a large wave or wake, the gas can leave the area of the pickup tube and suck some air- bad fuel sensors and letting the level go too low can be a big problem.
 
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