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1978 50hp thunderbolt ignition

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dde428

New member
"The motor has been running great until a few weeks ago. Took it out on a Tuesday and did fine but wouldn't start on Sat. It turned over but wouldn't fire. I jiggled the plug wires and it runs for about 5 seconds then dies. Will not fire until I move wires again. New plugs, fuel, and clean carbs. Whats killing the motor?

1978 500 Thunderbolt 4 cyl"
 
"Remove one plug wire at a tim

"Remove one plug wire at a time from the spark plugs. Do a continuity test from the plug end to the battery connection at the rectifier, (a convient spot) his will check the plug wires and the coils at the same time. Your meter should read battery voltage equally on all wires. More than likely you have some bad insulation shorting out your ign. system. Also check the tilt kill switch wires, the little cylinder just above the starter solonoid, and, try removing the kill wire on the switch box. (The middle wire on the right side row.) Actually do this last step first."
 
"I'm getting good continui

"I'm getting good continuity on all four plug wires, and the tilt kill swith was already disconnected. I pulled the plugs and noticed the #4 cylinder wasn't firing. The plug had moisture on it, no oil/gas. Shot some starting fluid directly into cylinder and ran for 10 seconds."
 
"clarify, You shot starting fl

"clarify, You shot starting fluid into the #4 cyl only, The engine ran on all cylinders briefly then would not restart??

Did you ever try removing the kill wire from the switch box. The box is the part with all the wires connected to it on the Starboard side of the engine. The kill wire is the Middle wire in the row of 7 wires on the rt side."
 
"correct, the motor ran smooth

"correct, the motor ran smooth for 2 seconds then began to misfire and died out. The switch box only has five wires on the right side but I disconnected the middle wire which goes to the kill switch and still had the same problem. Do I have a valve sticking? I haven't checked the compression either but I'll do that tomorrow. Any ideas would be helpful. Thanks"
 
"I have a 77 500/50 that I hav

"I have a 77 500/50 that I have been working on daily for the last month trying to get the bugs out, I finnally succeded a few days ago. Thats why your motor has special meaning to me!!!
My problem turned out to be a wireing harness full of frayed wires. But in your case it sounds like a fuel problem. I would check your fuel supply from the tank to the carbs. Be sure fuel lines are clear and not leaking. Check the fuel screens in the carb pumps for dirt. Look for an air leak, vent on tank closed, recheck the carbs. Possibly the fuel pump diaphram or gaskets have been installed wrong. You might try putting fuel mix into an oil squart can and shooting a small amount of fuel directly into the carb throats. That would confirm your problem, fuel supply or spark."
 
"another thought. Fuel qualit

"another thought. Fuel quality. Fresh fuel?? 50:1 mix? No water in the tank?? Something else to confirn are the fuel lines from the motor connector to the carbs. and the T fitting between the carbs. The T fitting holes are pretty small and can get clogged easily. I may be over thinking your problem, but iam am just remembering all I did to get mine running. Take that compression test. #4 is a bitch to get to for sure,"
 
Re: 78 50hp thunderbolt ignition

replaced all the fuel lines and primer bulb, cleaned carbs again and found a small amount dirt in the fuel screens, flushed the lines ,put back together and ran for a few seconds then died. I noticed the #1 spark plug wire popped off. I put it back on and it ran for a few seconds then died. Everytime I wiggle the plug wires or pump the fuel bulb it runs and dies. I don't know whats going on fuel or spark? Gimme a break Mercury!!
 
Re: 78 50hp thunderbolt ignition

dde428 said:
replaced all the fuel lines and primer bulb, cleaned carbs again and found a small amount dirt in the fuel screens, flushed the lines ,put back together and ran for a few seconds then died. I noticed the #1 spark plug wire popped off. I put it back on and it ran for a few seconds then died. Everytime I wiggle the plug wires or pump the fuel bulb it runs and dies. I don't know whats going on fuel or spark? Gimme a break Mercury!!
Click to expand...

Do you have access to a inductive type timing light????

You could check each plug wire for spark witout removing anything. Works like a charm. You can SEE the missfire or lack of spark on each cylinder. Just clip it on one plug wire at a time and see what you got. Should help point you in the right dorection./
 
Re: 78 50hp thunderbolt ignition

just cant help thinking this is fuel. Losing spark would cause it to stop suddenly. How about some ideas guys. dont let me sink alone on this one!!!!
 
Re: 78 50hp thunderbolt ignition

You said,I jiggled the plug wires and it runs for about 5 seconds then dies. Will not fire until I move wires again. New plugs, fuel, and clean carbs. Whats killing the motor?
Stator? Switchbox? Your plug leads may be showing continuity when cold but if there is a crack in one ,when it heats up the crack opens and continuity lost,same with stator.
 
Re: 78 50hp thunderbolt ignition

haventaclue said:
You said,I jiggled the plug wires and it runs for about 5 seconds then dies. Will not fire until I move wires again. New plugs, fuel, and clean carbs. Whats killing the motor?
Stator? Switchbox? Your plug leads may be showing continuity when cold but if there is a crack in one ,when it heats up the crack opens and continuity lost,same with stator.
Click to expand...

Remember also, each cylinder has its own ig system. Not likely all 4 would be bad at the same time. Im now thinkin if fuel is not the issue I would put my 2 cents on the trigger...

The loss of spark for your reasons given would show up with the timing light test.

Still want to elimanate a fuel problem. How about closeing choke as motor slows and stalls?? Any response to that? Or use the manual choke knob, put choke at half on. Did you ever try squerting fuel in the carb throats??
 
Re: 78 50hp thunderbolt ignition

I bypassed my fuel connector on the engine and and put the fuel line directly to the fuel pump and solved the problem.Just food for thought.also my floats were old and were sticking (replaced)
 
Last edited:
Re: 78 50hp thunderbolt ignition

I bought a new brass fitting for the fuel line into the tank, and plug wires, runs fine. The bright side is I learned how to take everything apart! Thanks for all of the help
 
Re: 78 50hp thunderbolt ignition

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