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1986 AQ 151 lost power overnight VERY CHALLENGING not for the faint of heart

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dazk14

Contributing Member
"1st- I am the mechanic on the below stern drive and I am STUMPED! I work on Outboards, but this is family...

1986 Sunrunner sb220 with Volvo AQ 151A 290 outdrive.
Overheated to 240deg on meter(per owner), shut down, towed, lost 700rpm WOT on startup next day -idle appeared to remain unchanged.

Please note, the top area of the coolant box leaked, lowering the coolant level. The raw water system worked perfectly - cools manifold, oil cooler, etc..

Compression is 170+ all cylinders.

Timing belt has (sadly) NOT jumped.

Has Pertronix(love them) and TAT is functioning properly.

All plugs look identical, very smooth black finish on outer rim and electrode - insulator is white and looks nearly new!! Seems like a clue? SEE PIC BELOW.

Pulled exhaust manifold, after 4 months sitting in dry dock from winterizing, tiny water globules at cyl 4 and slightly toward cylinder 3. At layup, I drained the lower drain at the oil cooler, but did not drain the exhaust manifold (stuck), so this may be a false lead.

The above still wouldn't explain why 1&2 plugs look identical(to 3&4) after post overheat 25hrs of run time.

FYI- When I couldn't solve the initial problem, I dropped the prop pitch from 15" to 11" to make it usable (kept max rpm ~ 5300). Boat barely planes.

Engine runs smooth at all RPMs, installed outboard primer bulb before fuel pump, stayed hard while running, pumping did NOT affect engine speed in any way. Acc. pump functions properly.

One thought...I'm wondering if the exhaust in the lower end is being greatly restricted...

Or...Something after the crankshaft could be totally bound up...but

NO unusual noises from the outdrive.

Of course maybe the head gasket blew... but it didn't appear to be using coolant(can't verify 100%) and the plugs all looking the same...


This one is torture... so any insight - especially from the "Top Dogs" would be greatly appreciated...THANKS!!!!

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"Hi, here are a few things you

"Hi, here are a few things you can try:

1) Remove the exhaust bellows and see if the flapper (if it has one) has become loose and got stuck in the exhaust inside the outdrive.

2) Conduct a leak-down test and see if compression is being lost somewhere.

3) Make sure all 4 cylinders are firing.

4) Make sure the PDS does not have a seized bearing.

5) Check the throttle cable is correctly attached to the carb.

There are obviously more possibilities, but that should be a good start."
 
clean the flame arrester.. tha

clean the flame arrester.. that leaking coolant might have gummed it up
change fuel filters
 
"Thanks guys,

Is there an e

"Thanks guys,

Is there an easy way to check the PDS (primary drive shaft I presume)? I don't feel any binding when I rotate the engine via the crank pully.
For kicks I'll try rotating engine by hand with it in gear.

I can't get compressor to boat(for leak down), so I'm going to pull the head and send it out. Engine/outdrive have 600hrs, so it can't hurt.

All 4 cylinders were firing and throttle is properly adjusted.

Saltwater boat and outdrive covered with crud, so I can't pull the outdrive yet. Too cold to pressure wash.
Maybe I can get enough crud off to get the exh. bellows off.

The head has the cyl 4 steel ring insert, so maybe a previous mechanic thought he'd add the flapper style bellows for additional protection.

If you have any other thoughts, I'm all ears.

Thanks a ton!"
 
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