Mercury 500 Strange Ignition Problem?

[RickG79]

Need help diagnosing a motor that was given to me. I believe its a 1980 mercury 500 thunderbolt ignition. My buddy didn't have time to use it so gave it to me. All I know is wiring is new. Compression is good. Just did carbs and water pump. Motor starts easily and idles okay, but when you move warm up lever up and down its sporadic, like its dropping cylinders, then picks back up strong? It never "dies" though. It has max speed of 6-8 mph on small boat, im pretty certain its something in ignition because of this strange misfire. I pulled plugs when I got home and saw 1 and 3 look clean (especially 1). And 2 and 4 look like they burning good. This makes me believe 1 and 3 are dropping under load but I am not sure, any tips to go from here or what to test. Sorry for long write up but need advice

 

[racerone]

Does spark jump a gap of 3/8" or more on each lead ?

 

[RickG79]

Does spark jump a gap of 3/8" or more on each lead ?

Racerone, yes, I did spark gap test at 1/2 inch after pulling plugs. It was broad daylight, I should have done at night but saw a white flash on all four cylinders

 

[racerone]

Check to make sure all wires are going to the correct terminals on the switch box.-----Post compression test actual values.-----Open low speed mixture screws 1/8th turn at a time and test run.

 

[RickG79]

Check to make sure all wires are going to the correct terminals on the switch box.-----Post compression test actual values.-----Open low speed mixture screws 1/8th turn at a time and test run.

I checked and all wire colors match. I tried adjusting carb screws on water but did not help top end, and didn't get rid of bizarre low rpm misfire. I can try to screw out more though. I will get Guage to compression check. Thank you for advice on this motor

 

[racerone]

The carburetors have fixed jets for full throttle operation.----But low speed mixture must be correct for proper throttle pick up.

 

[The Force power]

Need help diagnosing a motor that was given to me. I believe its a 1980 mercury 500 thunderbolt ignition. My buddy didn't have time to use it so gave it to me. All I know is wiring is new. Compression is good. Just did carbs and water pump. Motor starts easily and idles okay, but when you move warm up lever up and down its sporadic, like its dropping cylinders, then picks back up strong? It never "dies" though. It has max speed of 6-8 mph on small boat, im pretty certain its something in ignition because of this strange misfire. I pulled plugs when I got home and saw 1 and 3 look clean (especially 1). And 2 and 4 look like they burning good. This makes me believe 1 and 3 are dropping under load but I am not sure, any tips to go from here or what to test. Sorry for long write up but need advice View attachment 330321

Intermitted miss firing when moving the warm-up lever sounds like the carbs. need to be set up properly also the timing & pick-up needs to be verified if set correctly
(Sorry I just saw that Racerone already said this) so I can only add
Inspect the wires coming off the trigger-arm, remove the wrap etc.
They tend to break/corrode incl. the terminals

 

[Faztbullet]

Inspect the wires coming off the trigger-arm, remove the wrap etc.
They tend to break/corrode incl. the terminals

This......

 

[RickG79]

Wow all this advice is great. To me I think there is electrical problem because motor seems to "die" down at times and fire back up if trigger is slightly moved up or down slightly, then it surges back up like it got a cylinder or two back. If I check voltages of trigger do I do this at idle or move throttle to different positions? Can trigger specifically make cyl 1 and 3 cut out because that would explain my spark plug pattern? The wires look very new but a part has to be bad.

 

[Faztbullet]

Trigger coils are paired...1 and 3 are same trigger coil and 2 and 4 is separate trigger coil. Movement of trigger can cause the wiring to loose connection as it rots up in the protective sleeve and is unseen.

 

[RickG79]

Okay I think engine might be blown. I rechecked compression (warm this time) after running in a barrel for fifteen minutes in neutral
1 - 128 psi
2 - 130 psi
3 - 128 psi
4 - 115 psi

Some may say its okay but here's the concerning part, after checking compression I noticed the pressure falling on #4 on guage, which I'm guessing is bad news bears, some kind of leak or hole somewhere in cylinder, or maybe cylinder is destroyed? It was really hard to tighten Guage on #4 but I'm sure it was on there.

 

[RickG79]

Okay I think engine might be blown. I rechecked compression (warm this time) after running in a barrel for fifteen minutes in neutral
1 - 128 psi
2 - 130 psi
3 - 128 psi
4 - 115 psi

Some may say its okay but here's the concerning part, after checking compression I noticed the pressure falling on #4 on guage, which I'm guessing is bad news bears, some kind of leak or hole somewhere in cylinder, or maybe cylinder is destroyed? It was really hard to tighten Guage on #4 but I'm sure it was on there.

Nevermind, Guage should hold peak pressure, the valve must have got bumped. Do you think compression is good enough at 115?

 

[The Force power]

Nevermind, Guage should hold peak pressure, the valve must have got bumped. Do you think compression is good enough at 115?

Okay I think engine might be blown. I rechecked compression (warm this time) after running in a barrel for fifteen minutes in neutral
1 - 128 psi
2 - 130 psi
3 - 128 psi
4 - 115 psi

Some may say its okay but here's the concerning part, after checking compression I noticed the pressure falling on #4 on guage, which I'm guessing is bad news bears, some kind of leak or hole somewhere in cylinder, or maybe cylinder is destroyed? It was really hard to tighten Guage on #4 but I'm sure it was on there.

Is it "hard" to screw in the spark-plug on #4? (tread damaged?)

 

[RickG79]

Is it "hard" to screw in the spark-plug on #4? (tread damaged?)

No damage, its just deep below the cowling so hard to get access to. I can't use a socket to tighten on the plug I have to use an open wrench.

 

[RickG79]

Update. I bought the manual and started testing EVERYTHING on the motor from stator, trigger timer, coils everything. Stator and trigger are correct ohms. But All four coils tested out of spec, 1270 ohms from plug wire to ground. This didn't make sense to me so I pulled off plug wires and noted they are aftermarket. I put back on the original plug wires which are yellow and ohms came to 940ish on all 4 which is correct per the manual. Do you think this could have been issue of low power, and occasional miss??

 

[racerone]

You say you have spark that jumps a gap of 1/2" on all coils.----That would make me think coils are all OK.----Remove bypass covers to look at pistons / rings on this motor.-----Or take boat for a spin with timing light hooked up to one lead at a time.---Observe the flashing light and look for possible spark issues.

 

[RickG79]

You say you have spark that jumps a gap of 1/2" on all coils.----That would make me think coils are all OK.----Remove bypass covers to look at pistons / rings on this motor.-----Or take boat for a spin with timing light hooked up to one lead at a time.---Observe the flashing light and look for possible spark issues.

Racerone, I think I may have screwed up the test. I did test in daylight and just saw white flash on all 4 and called it good at the time. I am redoing this test in dark to verify, I don't want to be too confident but think this may have been issue. Have you seen bad plug wires cause problems before? All ohms now are 940 compared to 1270ish when reading to ground (mounted to block). Will retest spark gap and then do a test run. If it doesn't work I will take up your recommendation and look inside cover.

 

[RickG79]

Engine sounded a little better after changing wires but kind of hard to tell, the idle is definitely higher now and needs to be reset. Going to go through carbs again to make sure I didn't miss anything before next test run. Does anyone know how to remove the red gasket sealant stuff - it is completely covering the "idle tube" and welch plug? Im going to soak these for a whole day instead of just spraying them with the cleaner this time.

 

[RickG79]

You say you have spark that jumps a gap of 1/2" on all coils.----That would make me think coils are all OK.----Remove bypass covers to look at pistons / rings on this motor.-----Or take boat for a spin with timing light hooked up to one lead at a time.---Observe the flashing light and look for possible spark issues.

Racerone, before retesting ignition, I wanted to look inside engine, I felt like I was going to snap a bolt on side cover so I looked through plug holes with camera and what I found looks like bad news. 2 horizontal scratches on right side (port) on cylinder #1, cylinder #4 had 2 horizontal scratches, one on each side of engine. Didn't find anything on #2 or #3. know very little about engines, time to throw in the towel or worth pursuing the ignition problem? For ignition, im pretty positive its trigger/power pack because near the idle stop it sometimes turns off like a switch if you just barely move the throttle, not a sputter or slow down or anything. Thank you

 

[racerone]

Removing the INTAKE bypass covers should be easy.-----The EXHAUST cover is often harder to remove because cooling water get into bolt threads.-----Remove intake bypass covers.----But those pictures of cylinders are not encouraging.----Free motors can be expensive.