NEED HELP!!, 83 larson with 3.7 470 merc

sledhill

Cadet
Joined
Apr 14, 2013
Messages
10
hey everyone! new to this forum. first off, Im going to say thanks in advance for any and all help. I purchased this boat middle of last season, boat was running awesome for a month or so before I started having issues.

we were out on the water cruising, after a couple of hours the motor, almost bogged out to stall, I had to pull throttle back to keep it running, trying to limp it back to dock, couldn't get boat to plain as I had no rpms, after cruising like this for 4-5 minutes, all of a sudden the boat started to take off again and shot out of water and onto plain. eventually got back to the dock and loaded it on trailer.

a few days went by, I changed plugs, fuel filter, and gave it an oil change. when I ran it up, I forgot to turn water on for the muffs. I only ran it for maybe 30-40 seconds( obviously to long with no water)doh!

so... hoping the impellar was still good, dropped boat in water and away we went, before long temp was right up, and the exhaust bellows were smoking I believe the temp was up around 200-220( on the gauge). got towed back to launch and went home.

a few days went by, I dropped bottom end, and sure enough, just as I suspected. the impellar and housing were both melted. so I replace imepellar and housing, and run it up with water. seems to run good. so I headed out on water with family. just as I didn't expect, the boat overheated again.

so I took thermostat out on water, that didn't help. ended up getting towed back to the do. I eventually parked the boat for the winter.

So a few days ago I pulled the boat into the shop and started wrenching. I put a new cap and condencer on, rebuilt the carb, pulled front cap off of heat exchanger, found a few pieces of rubber, blew air through the sea water lines and everything seemed fine, and no obstructions.

I went to go fire the boat up, I finally got it to fire, but I thought the motor was gonna shake right out of the boat! I had it running with WOT(wide open throttle) and it was barely staying running until it died. after I shut the key off, I looked down the carb and raw fuel was still pouring in, for at least 5-8 seconds.

so I finally had enough. i am ready to take it to a mechanic.

the boat ran up until i changed cap and condencer and rebuilt carb this year, the only issue was over heating, so now i have a non running boat.

where do i start?

any help is greatly appreciated

Bryce
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
71,309
Re: NEED HELP!!, 83 larson with 3.7 470 merc

hey everyone! new to this forum. first off, Im going to say thanks in advance for any and all help. I purchased this boat middle of last season, boat was running awesome for a month or so before I started having issues.

we were out on the water cruising, after a couple of hours the motor, almost bogged out to stall, I had to pull throttle back to keep it running, trying to limp it back to dock, couldn't get boat to plain as I had no rpms, after cruising like this for 4-5 minutes, all of a sudden the boat started to take off again and shot out of water and onto plain. eventually got back to the dock and loaded it on trailer.

a few days went by, I changed plugs, fuel filter, and gave it an oil change. when I ran it up, I forgot to turn water on for the muffs. I only ran it for maybe 30-40 seconds( obviously to long with no water)doh!

so... hoping the impellar was still good, dropped boat in water and away we went, before long temp was right up, and the exhaust bellows were smoking I believe the temp was up around 200-220( on the gauge). got towed back to launch and went home.

a few days went by, I dropped bottom end, and sure enough, just as I suspected. the impellar and housing were both melted. so I replace imepellar and housing, and run it up with water. seems to run good. so I headed out on water with family. just as I didn't expect, the boat overheated again.

so I took thermostat out on water, that didn't help. ended up getting towed back to the do. I eventually parked the boat for the winter.

So a few days ago I pulled the boat into the shop and started wrenching. I put a new cap and condencer on, rebuilt the carb, pulled front cap off of heat exchanger, found a few pieces of rubber, blew air through the sea water lines and everything seemed fine, and no obstructions.

I went to go fire the boat up, I finally got it to fire, but I thought the motor was gonna shake right out of the boat! I had it running with WOT(wide open throttle) and it was barely staying running until it died. after I shut the key off, I looked down the carb and raw fuel was still pouring in, for at least 5-8 seconds.

so I finally had enough. i am ready to take it to a mechanic.

the boat ran up until i changed cap and condencer and rebuilt carb this year, the only issue was over heating, so now i have a non running boat.

where do i start?

any help is greatly appreciated

Bryce

Ayuh,.... Welcome Aboard,... Did ya replace the Points,..?? set the Dwell, 'n Timin',..??

That, 'n yer carb sounds like it needs rebuildin', might help...

Unfortunately, the worst of yer problems are, ya got a 470, 'n ya Overheated it....
No doubt, it's blown the head gasket somewhere...
I'm guessin' that's causin' the continued Overheatin'...

A Coolin' system pressure test will confirm it...
 

sledhill

Cadet
Joined
Apr 14, 2013
Messages
10
Re: NEED HELP!!, 83 larson with 3.7 470 merc

i did not replace the points, set the dwell, or adjust the timing

what after market ignition system would be a good replacement? i am not a big fan off points.

i will be doing a pressure check on coolant system as soon as i can

bryce
 

Bondo

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Staff member
Joined
Apr 17, 2002
Messages
71,309
Re: NEED HELP!!, 83 larson with 3.7 470 merc

i did not replace the points, set the dwell, or adjust the timing

what after market ignition system would be a good replacement? i am not a big fan off points.

i will be doing a pressure check on coolant system as soon as i can

bryce

Ayuh,.... I sure Pertronics does an ignition conversion for the 470s,...
But as I said, I think ya got Bigger problems right now....
 

sledhill

Cadet
Joined
Apr 14, 2013
Messages
10
Re: NEED HELP!!, 83 larson with 3.7 470 merc

I will do the pressure test to start off, what pressure should I test at? And for how long? If it drops, I will be changing the headgasket, anything specific I should do while its apart?
 

Bondo

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Staff member
Joined
Apr 17, 2002
Messages
71,309
Re: NEED HELP!!, 83 larson with 3.7 470 merc

Ayuh,..... 10/ 15 psi is Plenty.... it should hold it indefinitely...
 

Don S

Honorary Moderator Emeritus
Joined
Aug 31, 2004
Messages
62,321
Re: NEED HELP!!, 83 larson with 3.7 470 merc

Drain the coolant first.
Another test you can do with the coolant in the block to see if the head gasket blew is to fill the coolant tank and leave the cap off.
Bring each cylinder up on TDC of the compression stroke and pressurize the cyliner through the spark plug hole (Like a leak down test but the gauges aren't needed) then see if you get bubbles in the cooling system. If you do, you know the overheat moved the cylinder and damaged the head gasket.
The open deck design of the block is why the cylinders move during an overheat.
 

stonyloam

Vice Admiral
Joined
Mar 13, 2009
Messages
5,827
Re: NEED HELP!!, 83 larson with 3.7 470 merc

Ugh, yeah you got problems! Go to Autozone and borrow the compression tester and the cooling pressure system testing kit (you shouldn't have to drain it to do the test) and do both tests. Chances are you have a blown head gasket. You will be looking for low compression on at least one cylinder and a pressure build up in the cooling system (while running) to diagnose. let us know the results. Oh yeah you probably trashed your exhaust shutters too. Good luck.
 

sledhill

Cadet
Joined
Apr 14, 2013
Messages
10
Re: NEED HELP!!, 83 larson with 3.7 470 merc

I wont be able to work on the boat until next Friday, but thanks for all the help. if and when I replace head gasket, is there anything else I should be doing while I have the head off? also is the exhaust shutter located in upper or lower pipe off the back off reservoir?

I have no problem changing a head gasket, I was also thinking of changing ignition, motor has always seemed hard to start for me, which would be good to get away from points? pertronix?
 

stonyloam

Vice Admiral
Joined
Mar 13, 2009
Messages
5,827
Re: NEED HELP!!, 83 larson with 3.7 470 merc

You want to use a genuine OEM Mercruiser head gasket, and while you have the head off you might consider a valve job, maybe new seals, and when I did mine I had hardened exhaust seats put in. For the ignition: Petromix 91146a Ignitor II, and a 45011 (oil) or 45111 (epoxy) Flamethrower II coil. Make sure you supply a full 12V to the Petronix by replacing the resistor wire that goes from the choke to the coil + with a stranded copper wire. Good luck.
 
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