1975 850 merc just cuts off at WOT

[roger3645]

I’m running a 1975 850 85 hp mercury serial #4300733. Yes and I’m very glad that it still runs as well as it does. The only problem I’m having is it will just cut off. This occurs at WOT. It will run for a while at WOT then just shut off. Just like flipping a light switch. It will start right back up. I’m wondering if I’m exceeding maximum rpm’s. Don’t have a tach. Running 2 bladed 19p prop on a 16ft tri hull bass boat. The boat will top out at 33 mph. Did try a 21p prop which over propped the boat (max of 21mph) Do these motors have an rpm inhibitor to shut down the motor if max rpm is exceeded?

 

[parkert50]

Re: 1975 850 merc just cuts off at WOT

Eventhough this motor is older than I am. I have checked with the master tech in my shop and he has told me that this motor will not shut down if over reved. With the price of ignition parts for this motor I don't suggest that you dump a whole lot of money into finding the problem.

 

[Laddies]

Re: 1975 850 merc just cuts off at WOT

Either hook a voltmeter to the white switch box terminal to see if you are loseing power from the switch or if you don't have a meter use a jumper wire between the red and white terminals to bypass the wiring harness. If that don't work we can help you with the ign. testing, but it sounds like your losing power to the system

 

[roger3645]

Re: 1975 850 merc just cuts off at WOT

Thanks guys. I'll try running a jumper from red to white to see that'll help.

 

[roger3645]

Re: 1975 850 merc just cuts off at WOT

Just remembered something that’s probably important. With the motor running there is a hum (more of a crackling noise) coming from the control box in the area of the key switch. Could this be the culprit?

 

[Clams Canino]

Re: 1975 850 merc just cuts off at WOT

Could be.... the red-white jumper test will help in that area. Red is always +12, white goes +12 when key is "on".

 

[Troy_from _Oz]

Re: 1975 850 merc just cuts off at WOT

I had a similar problem with my merc 650. It turned out to be the choke switch playing up, which was shorting out and putting choke on at WOT. Instantly would kill the engine but would allow me to start it again immediately afterwards. ended up rewiring it with external toggle switch due to difficulty in finding parts - no problems since<br /><br />to test this you could disconnect the electric choke at motor and see if the problem goes away.<br /><br />just a thought

 

[roger3645]

Re: 1975 850 merc just cuts off at WOT

Thanks! Troy I'll give that a try too. Hope to get out on the lake over the Xmas weekend.

 

[Texasmark]

Re: 1975 850 merc just cuts off at WOT

Crackling is a burning inside the switch caused by a deterioriated switch. I am curious about 2 things: 1. That it only happens at WOT and 2. That it repairs it'self so fast after being bad enough to stop the engine.<br /><br />Regardless, isolate what is crackling and replace it. Then proceed.<br /><br />Might be that the power (amperage) pulled through the switch is highest when the spark plugs are firing the most often which would happen at WOT. The higher current produces the most heat and that is enough to cause your problem. Still don't understand why it would correct so fast.<br /><br />Might open up the switch when you replace it and see what it looks like inside. If burned and carboned up you may have your problem.<br /><br />Merry Christmas.<br /><br />Mark

 

[Elmer Fudge]

Re: 1975 850 merc just cuts off at WOT

This could well be a fuel related issue, check that the fuel vent is not clogged and the fuel lines are air tight, you may also try replacing the fuel pump diaphram.<br />If the problem persists, then move on to the electrical system.

 

[roger3645]

Re: 1975 850 merc just cuts off at WOT

Thanks!<br /> Had the boat out on Dec 27th. Problem worsening with the motor shutting off more often and not just at WOT, and bit harder to restart now. I was going to try the jumping the red to the white wire but forgot the jumper wire. I have another key switch from another control box that I’ll switch out before my next outing. Thanks for all the advice.<br /><br />PS: Don’t think it’s fuel related as it has new lines and fuel pump.

 

[burnboyy]

Re: 1975 850 merc just cuts off at WOT

Wow, i have a 75 merc 850 and had the identical problem. For sometime, after it stalled at WOT I would fire it right up and it would be fine. Oringally i thought that the fuel line was some how vacuming but that wasn't the problem, very recently the motor did the same thing, but never restarted again, the motor turns over fine but when i took the boat out of the water i noticed i had no spark, might be worth your while to do a spark test.

 

[Texasmark]

Re: 1975 850 merc just cuts off at WOT

Just an afterthought.<br /><br />After you get your problem solved you might investigate performance. 33 mph on a 16' boat with an 85 hp is low. You ought to get right at 40 with that rig. Trim or prop is wrong. <br /><br />You were talking about overloading the engine with too much pitch. Sounds to me like it's just the opposite. I'd be expecting you to be over revving with only 33mph on that boat/engine combination. If you went to a 21" pitch (from a 19)and dropped to 21 mph you may have a trim problem; bow too heavy, or other problem; overweight, barnacles on the hull, aluminum prop or something else.