1978 mercruiser 470 will not start

[blasep]

this boat is new to me so not sure about everything, the first time I had it out it ran good for a while. just as we were coming in the rpm guage started fluctuating... but the engine sounded fine... for about a minute then it died and would not come back.. it turns over but only starts for a second then dies.. I removed the fuel line and changed filters, it does have fuel to the carb. I read somewhere that a shorted tachometer could prevent the engine from starting. I havent checked for a spark yet but it doesnt seem like ignition problem since it does start but only for a split second.. the tach is still going crazy and I dont even know how to troubleshoot it any help would be greatly appreciated.. I also tried disconnecting the gray wire from the tach and starting but same problem.

 

[Limited-Time]

Re: 1978 mercruiser 470 will not start

Does it die as soon as you release the key from the start position?

 

[blasep]

Re: 1978 mercruiser 470 will not start

yeah pretty much, it only fires for that one second, and yes when i release the key it dies..

 

[blasep]

Re: 1978 mercruiser 470 will not start

ok, ruled out a fuel problem.. I have no spark, I connected a jumper from the battery to the coil and she fired right up.. I disconnected the jumper and she fired up again.. ran for a couple minutes then died as it did before.. so I got no juice to the coil I guess? would a bad voltage regulater cause something like this?? I am not sure where to go from here? thanks..

 

[Don S]

Re: 1978 mercruiser 470 will not start

No, the regulator is for charging, not for ignition.
You have a problem in the power circuit to the ignition. Start from the ignition switch and check for power (with the key in the run position) on the purple wire and follow it back to the coil. Somewhere in there the circuit is open. Could be a bad safety kill switch, bad ign. switch, corroede connection, cut wire, faulty main plug on the engine, burned out resistor to the coil. Lots of places.

 

[blasep]

Re: 1978 mercruiser 470 will not start

ok ill check the ignition switch wires....should I have a voltage at the coil with the key in the on position?? because i have nothing now with the key on.. I had a hard time following the ignition wires since it seems they are in a harness in the back and off of there the wires seem to be hidden. but i will definately check again. thanks for you help..

 

[Don S]

Re: 1978 mercruiser 470 will not start

Those of us that do this for a living, start at the coil, if there is no power, I go to the ign. switch. If I have power there, I check both sides of the kill switch. If power there, I check the main engine harness plug. If I have power there, it's trace wires, cut wire tie bundles and follow the wire to find some other connector or chafed wire.
It's not always easy, but it is how it's done.

 

[jclemmojoe]

Re: 1978 mercruiser 470 will not start

You should have two wires attached to the plus terminal on the coil. Probably a solid purple and a purple/yellow wire. The solid purple wire is either a resistance wire or is wired to an external resistor. The purple wire should only have voltage when the ignition switch is in the run position and the voltage will be reduced to 7-9 volts. The purple/yellow wire will have B+ voltage only when the ignition switch is on the momentary start position.
If you have 0 voltage at the + terminal with switch in the run position you have an open resistor wire or open external resistor.
You can test this by temporarily bypassing the resistor/wire by running a jumper wire directly from a B+ source to the coil. If the motor runs now you've confirmed the trouble. Don't run the engine too long as the resistor is there to reduce current flow through the points/condenser. There are other components wired in series between the ignition switch and the resistor/wire that could be shorted and cause the lack of voltage to the resistor/wire, the tach is just one of these components. Most likely it's the resistor/wire.

Good luck!

Joe

 

[blasep]

Re: 1978 mercruiser 470 will not start

ok, thanks for the quick responses.. I will start testing and see what i can come up with..
thanks
Nick