1974 Mercury 50 HP - dies when decreasing throttle

rcknrbn

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Hey guys, great information on these forums. Last year, my father in law gave me his 1974 King Fisher with a 50hp Mercury (I told my wife this proves how much her dad hates me but I'll save that for another thread).

Long story short, did lots of minor work on it last year (water pump, fuel pump diaphragm, etc). Been sitting up since last fall. Took it out a week ago and was shocked at how well it ran. Flawlessly! Well, I chirped. The following day, as I was decreasing the throttle from wide open, the motor abruptly died. I assumed it may be because the 6 gallon tank was low. Swapped tanks. Engine fires right up with the idle lever all the way up to it's highest idle. Not missing, sounds good. When bringing the idle lever down, it dies. Abruptly. Literally like just turning off the key. No sputtering, etc. Almost impossible to get it into gear because the motor dies so quickly when bringing the lever down to put it in to gear. When I was able to, it ran fine. Until I backed off the throttle and quickly died again.

I've changed gas tanks, hoses, checked for leaks. Tore into the carbs a couple of days ago to eyeball them. The bowls had quite a bit of crud in them but it didn't appear the floats were sticking. Needle valves looked good. Regardless, used a good carb cleaner and followed the SELOC manual I bought last year. Carbs were so clean you could eat out of them when I finished. Meticulously put everything back (removing these carbs was a pain in the rear). End result....nothing. Same as before. Runs fine at high idle. When backing off the idle lever, it dies.

Any ideas? I'm leaning towards trying to find carb kits to change out gaskets, bowls, needle valves. Parts are hard to find on this old motor. Maybe even changing plugs and plug wiring. Possible electrical problems with the stator, rectifier? My knowledge is limited on this stuff and I can't find ANY marine shop in my area (Jackson MS) that is willing to look at something this old.

Thanks for letting me ramble. I look forward to your input!

Mark
 

merc850

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Have you set the carb idle screws? Turn them in lightly until they seat, then back them out 1 1/4 turns; run the boat in water in F at about 1000 rpms then turn each screw in or out until motor runs smoothly then give a 1/8 turn out more. Test settings by accelerating from idle to full to check for bogging.
 

rcknrbn

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Hey Merc, thanks for the input. Yeah, I had adjusted the idle screws close to what you suggested. Couldn't get close to idle again before it died, just like before I tore into it.

What is interesting is that this all started while running at WOT as I decreased throttle, it just dies. I'm kind of at a loss as to what is going on. I've ordered a carb kit (gaskets, needle valves, floats, filter strainer) that should be in later this week. I'll install it all over the weekend but am somehow leaning toward an electrical issue. Can't say why I speculate that. Just, everything I've ever owned that had carb problems (chain saws, lawn mowers, generators), all seemed to sputter and die with carb problems. The old Merc just flat dies when deceasing the throttle. Stator, rectifier problems maybe? I just don't know.

Mark
 

merc850

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This could be a trigger problem but we have to know what type of ignition you have, take and post a photo of the distributor area.
 

rcknrbn

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Merc, just a quick update on my progress over the weekend. I pulled both carbs again and soaked them in Chem-Dip. Pulled and cleaned everything (idle tube, plug screws, main fuel jet, main nozzle, idle mixture screw and spring). They are spotless and have no obstructions. I replaced the floats, needle valves, seats, filter screens and all gaskets. I replaced the 4 plugs (odd looking plugs on the Mercury with no adjustable gap). No leaks.

I primed the bulb and it fired right up with the warm up lever all the way up. I adjusted the idle adjustment screws a bit and it sounds really good. No missing or sputtering, ran really well. As soon as I drop the warm up lever down, the motor shuts off again (just like before!).

Side note: when slightly adjusting the upper carb idle mixture screw, you could tell by ear when you hit the "sweet" spot. When adjusting the lower carb idle mixture screw, there was no significant difference in the way the motor ran. I tightened it all the way in and it seemed to run the same. If I did that with the upper carb, it would die on me. Does this possibly point to an issue with the lower carb?

A couple of overview pics are attached. Thanks in advance for the input!



B3DDC0B2-6470-498D-B5BC-AECABA7A6417.jpg E3340CF2-6969-4F87-903B-CBB78851B994_1.jpg
 

merc850

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Did you do a compression test? Should be 100+ on each cyl.
Did you check the wiring for corrosion especially between the distributor and switchbox?
Here's the trigger check
litning-driver.jpglightning-energizerS.jpg



I would buy a Mercury manual if you want to keep the motor and repair it yourself. You can buy parts here http://store.oldmercs.com/category_s/887.htm
 
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