Merc Gurus...I need some help....Final update....

[walleyehed]

1983 Merc 70hp..I've owned this engine for 14 years and last weekend it threw a fit.<br />When leaving one spot, I opened the throttle and it just bogged down....didn't quit, just won't run above 1500RPM.<br />Hit the choke, it done nothing until I let the key back, then it had a very short rise and then fell back.<br />I've decarbed every year, new plugs every year, and nothing but fresh gas and quicksilver oil. I just put a new fuel pump diaphram in it, but I've put about 30 hours on it since then.<br />I tore both carbs down monday, found no trash or varnish-this boat gets used year-round and averages 75-100 hours a year. I kitted carbs anyway, cleaned all jets and orifaces, checked float level, replaced all fuel lines under the hood, check all coils-all fire a 1/2" gap, compression is about 125 on all 3, just put new plugs in when I put the fuel pump diaphram in.<br />Mounted everything back up, checked all linkages, and lake tested...still the same...Idles great but when slowly advancing throttle it will bog about 1500-1700RPM and won't go any higher. At one point while playing with the throttle, I pulled it back to about half and hit the choke....it took off like a wild man and went on up to 5600RPM quickly...I ran about 2 miles and slowed down again and tried to plane back out..no deal, just bogs...<br />I pulled plugs and the top looked perfect, middle was a light to medium tan, and the bottom was a definite light tan-too light (Lean) and very clean on the face of the UL76V.<br />This one has cleaned my clock, and maybe I have missed something??? This has been a trouble-free engine for alot of years, and nothing has changed in my maintenance practices..<br />I pulled the carbs again today and can't find anything wrong....I'm very thorough with this sort of thing, and it's got me licked....<br />If anyone has a clue, I'm all ears..

 

[rodbolt]

Re: Merc Gurus...I need some help....Final update....

check the stator.<br /> if its an ADI ign system it has high and low speed coils. normally the low speed fails but occasionally the high speed will. if you suspect its fuel hook a vacum gauge inline just before the pump intake. it should have about 1" Hg. if more than 4"Hg its restricted, if less it may be pulling air.<br /> do you have a peak reading meter or the DVM with a DVA adapter ?

 

[dan jaycox]

Re: Merc Gurus...I need some help....Final update....

 

[dan jaycox]

Re: Merc Gurus...I need some help....Final update....

Had the same thing on a 78 70 hp merc quite a few years ago and I think it had something to do with the ignition. A coil I think.

 

[walleyehed]

Re: Merc Gurus...I need some help....Final update....

Got a peak and a DVM, rodbolt...<br />Vacume was about 1.5"

 

[rodbolt]

Re: Merc Gurus...I need some help....Final update....

vacum looks good.<br />if this is an ADI ign.<br /> usiing clips check the stator output on the red lead to the switch box. test for proper output at the speed its bad. you can also test for primary coil voltage at the 3 coils the same way.going by memory so double check my figgures you should have a resistance between the red stator lead and ground of about90-140 ohms with the blue and red leads disconnected. and a peak voltage of about 130-300V above1500 rpm or so with the stator wires attached to the switch box. your coil primary should be about 180-280V above 1K RPM's. you can test the trigger inputs with a peak reading as well. seems the spec is about 1-1.5 v min. but like I say its all from memory and I am getting old.

 

[walleyehed]

Re: Merc Gurus...I need some help....Final update....

Rodbolt, I know this is a bad thing, but I did a No-No with it in nuetral and was wondering if load will change it, but out of gear, I slowly went from Idle up through about 4000RPM and slowly back down....I preach not to do this, but wanted to see if I was limited somehow on RPM in general.<br />This may be a stupid question, as I'm not thinking clearly anyway on this deal, but will a load on the engine change the output of the stator and/or coil readings??? Or does this open another can of worms??

 

[walleyehed]

Re: Merc Gurus...I need some help....Final update....

^^

 

[rodbolt]

Re: Merc Gurus...I need some help....Final update....

kenny<br /> I doubt it. the electrical part is controled by the speed of the magnets.but I would try the test on the water. its time to isolate it for fuel or spark. usually if it will ocassionally run ok you can eliminate mechanical failues. my finding is once a mechanical subsystem fails it stays failed. however engine load can increase the electrical load on the plugs and the coil secondaries.

 

[walleyehed]

Re: Merc Gurus...I need some help....Final update....

Thanks for the help...<br />I have no system left as far as fuel system to go through, so unless I have missed something very obvious, and have looked so close I "Over-looked" it, it has to be an ignition issue....<br />I may be back...

 

[achris]

Re: Merc Gurus...I need some help....Final update....

Had a similar problem on a 1997 90 hp. Turned out to be the linkages that advance the spark were just catching enough to stop the spark advancing, thus the engine would bog and not make more than about 2000revs. But occasionally it would all work properly, usually when I was trying to find the problem.<br /><br />Inside the hub of the throttle arms is a spring loaded arm (which is connected to the trigger, up under the flywheel), when you advance the throttle the arm should follow. When you reach maximum spark advance the arm will stop moving but the carbs can be continued to be opened, ie WOT.<br /><br />Just a thought.<br /><br />Chris.............

 

[walleyehed]

Re: Merc Gurus...I need some help....Final update....

thanks Chris, I've checked for wear on all timing/throt linkages and greased where needed. timing advance is smooth and properly adjusted for my altitude.<br />This one has me scratching my head.....I've worked on many of these, and 99.9% of the time, I find the issue, but not so in this case.<br />I'm looking into ignition by advice of rodbolt, and if I don't find something there, rodbolt will be receiving a merc in the mail.....

 

[walleyehed]

Re: Merc Gurus...I need some help....Final update....

Rodbolt, I broke the manual out and followed the book...all testing has confirmed normal conditions exist.<br />If I can't figure this one out, I have no business working on anything....<br />I believe it's beat me???<br />I guess I'll haul it to a Merc Dealer and pay the fine...

 

[Clams Canino]

Re: Merc Gurus...I need some help....Final update....

You got me stumped too - unless you overlooked something. <br /><br />That little choke trick seems to indicate fuel system. BUT everything else also fingers the high-speed winding on the stator.<br /><br />-W

 

[rodbolt]

Re: Merc Gurus...I need some help....Final update....

walleyed<br /> its time to forget you ever worked on it and restart with the basics. test compression, thest spaark insure an adequate supply of fresh fuel. I usually bypass the quick connector and go directly to the fuel pump intake. now we have top go to the water. its imperative to be aable to load the engine and still turn it at 4500 or so. someplace there is a merc tech bulliten on cutting down props to use as test wheels. now I would use my CD-77 and my snap on DIS spark tester. this tester measure secondary ign voltage. test for proper voltage and steady voltage to each coil primary. you may wish to index the flywheel and add timing marks for cyl 2 and 3. this will allow you to insure that each cyl is firing at the proper time every time. you can also use this time to spray pre-mix down the carb throats.<br /> occasionaly I have to use the start all over with the basics method of trouble shooting. but a test wheel or a dyno will be mandatory, if the servicing dealer does not have it odds are he will find the same problems as you.<br /> I think the twst wheel I had for my old 3cyl 65 was a cut down 10 1/4 by 15 cut to a 8x12. call a good prop shop and tell them what your trying to do but I know the tech bulliten exists cause we got a copy of it in a suzuki school in 1993. if fuel is ok you can have a bad high speed stator winding, bad trigger or wiring between the trigger and switchbox, bad switchbox, faulty wiring between the box and the coils or a bad coil. but you have to be able to load the engine for any tests.

 

[Rabbitdawghunter]

Re: Merc Gurus...I need some help....Final update....

I use a clear piece of hose between a engine fitting and a tank fitting that I can place in line quickly. This will let you see any air bubbles in the vacuum side if present. Some don't show up until just enough vacuum is placed on the line to start the air sucking in. Then it will go away when the throttle is backed down. Usually the bubbles show up then the engine will bog a few seconds later at or near WOT.

 

[BillP]

Re: Merc Gurus...I need some help....Final update....

I had that symptom happen on a new motor and it was a clog in the fuel line at the bulb. Gasket material debris from a built in tank was the culprit. It would let a little fuel in but not enough for high speed. I had a clear filter in line but it it didn't get that far. My first impression was the line was clear but pumping the bulb helped me ID it. <br /><br />A couple weeks ago one of the ss links came unsnapped from a carb. It bogged when I hit the throttle but ran ok at idle. That one took me by surprise but I found it after a few cuss words.

 

[walleyehed]

Re: Merc Gurus...I need some help....Final update....

I have a 15P prop that had bent the blades half-way in...I actually did cut the blades half-off and made a template to get all 3 the same, and my ignition tests were done on the lake.<br />I won't give up yet, but where the problems have led me to believe fuel is the problem, I am 100% confident the fuel is not the issue, tank, lines, pump, carbs or other-wise...It's got to be ignition related, I just can't figure out way the values are all where they should be....I don't even have a component that is border-line???? Guess I'll get it one way or another...<br />My Dad was flown out flight for life a few days ago (all is coming around good for him) and it has got me away from the merc for awhile so maybe I can start fresh and come at it as I've never seen it before....<br />Thanks Guys.

 

[agrazela]

Re: Merc Gurus...I need some help....Final update....

Walleyehed,<br /><br />Longshot, but I had similar symptoms to what you describe on my '78 70 hp Merc. All was fine in the driveway on the bell, but bogged intermittently upon acceleration on the water or in a test tank with a cut-down aluminum test prop.<br /><br />I did the spark tests, and all tests on the stator, coils, etc. from the Seloc manual..all OK but still had the problem. Was getting spark to all cylinders and advance checked out OK, so assumed (or hoped!) the powerpack was OK (no real way for the home mech to test it, as far as I know).<br /><br />I replaced all the fuel lines, filter, diaphraghm, kitted the carbs, still had the problem.<br /><br />Finally traced it with an ohmmeter to an intermittent short in my internal harness between the mercury switch lead and ground. Temporarily disconnecting the orange lead at the powerpack made the problem disappear (though I had to choke the engine to shut it off, as this disconnects the key shut-off).<br /><br />I figure that either vibrations or humidity (while on the water) would cause intermittent shorting and cause cylinders to not fire intermittently.<br /><br />Of course, the ground was not in the wiring, but inside the plug itself so no way to repair...had to replace the harness.

 

[dsrchamp04]

Re: Merc Gurus...I need some help....Final update....

wheres laddies?