1970's 85 Merc dies out

[corvair60]

Used to do this only occasionally, but a lot more frequent now. Gas seems good (either of two gas tanks have same result).Starts up fine, lots of batt power, idles okay, get 'er out and up to speed, is okay for awhile and wham - dies. Cuts right out to nothing. Starts right up right away usually and same thing after a bit - cuts right out. Doesn't bog or choke or anything first. Doesn't seem to be speed related, cuz can cut at low speed too. Plugs look okay, but should likely change em (2 years old), but I'm thinking electrical in my limited experience with this. Any ideas how I'd go about troubleshooting?
Thanks

 

[russelleotero]

Re: 1970's 85 Merc dies out

do you have proper ventalation on your feul tanks, sounds like it is starving for feul.

 

[corvair60]

Re: 1970's 85 Merc dies out

Yes, should have mentioned I opened the twiddly bits on each tank and even tried it with the caps off - same result. Thing is, when it cuts out, it CUTS OUT. Fast and immediate and complete. No choking or anything like a reduced gas flow might indicate - but that's just my gut feeling.

 

[Chris1956]

Re: 1970's 85 Merc dies out

Sounds like a loose connection in the ignition system. If that motor has a distributor, check to make sure the ignition wire is clean and tight. The ignition wire is the white wire on the switchbox, just below the red one. Both wires need to have +12V on them to have any spark.

 

[emckelvy]

Re: 1970's 85 Merc dies out

I had one once with a busted wire in the plug-in harness and everytime the motor would turn a certain way, the wire would part and you'd lose ign.

Try wiggling the external harness and the internal wiring too. If you want to experiment, you can run a jumper wire directly from the battery and power up both the red (constant +12V supply) terminal on the switchbox, and the white (switched +12V) terminal on the same side.

Needless to say, you won't be able to turn the engine off with this jumper in place! You could always rig up a switch inline with the jumper, and this would allow you to control the ign in an emergency, without having to frantically rip wires off the battery!

See if it quits with this jumper installed, if not then you know the problem is in the 12V supply somewhere. Could even be in the ign switch or problem with wiring in the control box.



HTH........edc

 

[corvair60]

Re: 1970's 85 Merc dies out

Thanks gentlemen. I live in Ontario and am in Newfoundland on vac'tn until the end of the month...not with my boat tho! Both suggestions sound very viable and I look forward to giving them a shot when I get home.

 

[Motor Boater Bill]

Re: 1970's 85 Merc dies out

If you hotwire the ignition, you should still be able to kill the engine with the choke. Look over your internal wires carefully--mid-1970's Mercs nearly all have bad internal wiring, something wrong with the wire they used at that time. It's a well known fault. Might also try disconnecting the Mercury switch, which is a kill switch.

 

[corvair60]

Re: 1970's 85 Merc dies out

Thought I'd post the end result - may be of use to someone. Did several tests as indicated in the manual and by the respondents, but no concrete results and problem continued.

Had a tech over. With the boat on it's trailer we started it with water muffs on. Then he proceeded to yank back and forth on the the main plug going into the side of engine. Guess what - she balked a couple times, then cut out. He repated this test a few times with the same result on each, so it appeared he was on to something. I had previously checked for corrosion in the terminals, and spread the pins a tiny bit, but hadn't been able to narrow down the problem like the plug yanking did.

So the problem is either the big plug itself, or some of the interior wiring in the engine. He said these plugs "go" all the time, and can cost a lot of money for a new plug/cable.

So we bypassed a couple of the plug wires (ignition ground and hot?) and went directly from the battery to the engine using a very robust cable pairing I didn't otherwise need that was laying in another boat I had recently picked up. All controls and so on work the same. So w gave the old Merc a "double bypass surgery" that seemed to suit her quite nicely! Have had her out on the water since and she works like a charm.

 

[Chris1956]

Re: 1970's 85 Merc dies out

Probably 20 years ago the ignition wire on my '77 merc 1500 broke at the motor,inside the external connector. I ended up doing some surgery to splice in a new wire and solder it to the connector insode the female plug. A bit of silicone sealed it up well.

 

[Sailor77.7]

Re: 1970's 85 Merc dies out

if you do the surgery on the exterior cable, you can always buy some heat shrink to put over the cut away section. This and the silicone will protect all the internal wiring. Heat shrink is great for wire repair.

Vr,

Sailor77.7