1974 Mercury 500 (50 HP) Won't fire, has compression, stumped.

djanzen186

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Apr 9, 2016
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Have a 1974 Mercury 500 (50 HP) hasn't ran in a year and a half. Last owner said last time it was ran it started smoking white smoke out of cover was shut off and not turned back over. Was never properly winterized. Below is a list of what I have tested and done so far.

Compression Test: Cyl. 1. 120 - 123, Cyl 2. 119 - 121, Cyl 3. 105 - 112, Cyl 4. 110 - 118 (Note: Test was done cold engine, electric start, 5 Cranks Per test, 3 tests per cylinder)

Repaired wiring harness, cleaned all grounds, coated all with dielectric grease.

New Impeller Kit (note: old impeller only had half of a vein left was completely shredded)

Carbs were pulled and were clean, parts washed and changed all gaskets, Idle set screws are 1.5 turns CCW

New clean fuel tank and line with new 87 Octane mixed 50:1 with cap of Seafoam stabilizer.

Fully Charged Battery

New Fuel pump diaphragm and Gaskets reinstalled.

New Spark Plugs (note: when checked after starting have fuel on them, mixed with some black soot)

Not engine related but swapped out the gear oil, was milky, so fresh oil installed bottom up.

When attempting to start noticeable exhaust exiting port, no popping.

When priming fuel bulb can see fuel come out overflow on the carbs. Fuel bulb stays firm but not hard slowly decreases in pressure over time.

Type III Ignition system, no spark test as I don't own a spark tester, but 1/4" jump with noticeable arc. Now when engine grounding arc also shocking through the key and cable.

In closing I guess I am at a loss as what to test next or where to go. I am always starting it with a flush kit attached at high water pressure and with a battery charger hooked up.
 

merc850

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Jul 7, 2010
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2,046
Don't worry about comp it sounds good I think you have an ignition problem.
That system uses a magneto style ignition with no points so the battery only turns motor over; have you checked the ignition driver on the port side of motor?
It has a cap, rotor and plug wire posts and should be clean with no corrosion it's held on by 4 bolts so it comes off easily.
My quick fuel test is to tilt engine on to stops and pump fuel ball until fuel runs out of carb, lower motor and try to start it.
1975-500small.jpg
 

djanzen186

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Apr 9, 2016
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Will check the ignition driver tomorrow, in the meantime I just wanted to check the timing markings and with the cover on the distributor and with the #1 Cylinder at the top of the stroke the marking appears to be 45 degrees or around 8 teeth off from the mark is this correct in assuming that the timing may be off. Will check driver first before messing with timing.
 

merc850

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Jul 7, 2010
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2,046
Take the cover off the dist. pulley (5/16") bolt and see if it is the same as this manual page, are you using L77V plugs they are surface gap no side electrode.
Merc-500-lightning.jpg
 

djanzen186

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Apr 9, 2016
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Cleaned the ignition driver, and was corroded, the timing was off moved it and she fired right up.
 

djanzen186

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Apr 9, 2016
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So just had some time to work on the boat in the evening here. First time I fired it up I was short on time and had to go. So here is the new issue and symptoms:

Electric start with manual choke pulled at 3/4 of Idle throttle.
Will Idle at 1/2 of Idle throttle stalls out under that.

Went to adjust idle set screws out another 1/4 turn ccw

Still won't idle and noticed arcing from screw driver touching front cover and jumping to idle set screw. The lead running from the magneto to the ignition driver does touch the carb. could that be inducing and electric charge.

Checked all grounds and wiring no points found also ran continuity checks.

Stumped again.
 
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