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1991 Mercruiser 350 Magnumno power

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puzzolente

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"HELP!
I've got a 19

"HELP!
I've got a 1991 Rinker 236 Captiva Cuddy with a Merc. 5.7 350 magnum. The boat has been laid up for 6 years and although winterised and serviced...she's not been 'wet' run. After an intensive 3 month restoration the boat is now in the water and looking fantastic, however....it's backfiring and has absolutely no power! It will 'bog-down' at approximately 2750 rpm and backfire like hell (through the air filter). We've replaced the whole ignition system, stripped and cleaned the carb, replaced the fuel filter, adjusted timing etc. etc. etc. Our last assumption has been a sticky valve so we've poured brake cleaner through the carb and it's made no difference........help!!!! Out of gear, it will rev freely and sounds clean, the moment it's under load, it will start backfiring. Any advice?"
 
"Before we get into speculatio

"Before we get into speculation too deep, did you own the boat and use it before the lay-up? Do you know for sure how it was running when pulled from service or why it was pulled from service.

Was the engine thoroughly prelubed before starting up the first time after storage?

I agree that extensive backfiring is likely valve(s) problem, but I have also seen several times when a distributor shaft bearing was worn and the shaft ran in egg shape pattern causing much missfiring.

A little more history please."
 
If you think you have sticking

If you think you have sticking valves pour a can of SEA FOAM into the oil and let it idle for 30 minutes to dissolve any gum and varnish in the oil. Change the oil and filter and substitute a quart of ATF for 1 quart of oil. Run the engine under 3500 RPM for 10 hours and change the oil and filter again. Engine should be clean inside.
 
"Thanks for the response folks

"Thanks for the response folks, the boat was owned by a close friend of mine for 13 years, it was only laid up due to personal circumstances. He knows the boat inside out and is as puzzled and desperate to see her running properly as we are!!!! Time is acting against us a bit as the English summer is notoriously short, shall I just 'bite the bullet' and get the heads off????? once again, thanks for your help. By the Guy, did you mean 10 hours or 10 minutes? It would be cheaper to go and buy a brand new engine than run that beast at 3500rpm with UK fuel prices!!!!!!!!!!!!!!!"
 
"Ayuh,...

It experincing Fu

"Ayuh,...

It experincing Fuel Starvation,.....

The Problem can be Anything between the Gas Tank,+ the Carb, or either of Them as well....

I'd start with the Fuel Filters,+ move both ways from There....."
 
"Just run it, once started and

"Just run it, once started and seaworthy, for about 10 hours under normal use and change the oil & filter. That is sufficient time for the ATF to clean all the sludge out. Watch the oil pressure during that time; if it begins to climb above what it has been running, change it then."
 
"Guy...thanks for that, when r

"Guy...thanks for that, when running with the ATF should I labour it through the bog-down/backfire stage or just cut back to a point where it runs smooth? Bondo....we've changed the filter, stripped and cleaned the carb etc. but it really does feel like fuel starvation/running to lean"
 
"Ayuh,...

What about the Fu

"Ayuh,...

What about the Fuel Tank, It's plumbing fittings, the hoses, the fuel pump,..??..??

What did the Carb look like,..??
How much Crap was in the Fuel Filter,..??

Put a Fuel Pressure gauge tester on it.....

For that matter,...
A Compression Test will tell you if there's a hung up valve,+ Which 1....
And if it was a Valve,... It Wouldn't Ever run Smooth.....
"just cut back to a point where it runs smooth?""
 
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