7.5 Ignition Issue

[JAR55]

I have a 1976 Mercury 75 (7.5 hp) ser# 4399xxx that has spark, compression and fuel but will not start. Compression is 111 on top cylinder and 112 on bottom. I see good spark on both plugs(new), I get 195 VAC on the stator windings and 170 VAC from switch box to both coils. Resistance test on both coils is good and both look great. The trigger has 2.5 volts AC. I rebuilt the Carb but also tried spraying a little fuel directly into the cylinders. It won’t even cough. When I pull the cord it give 2 rotations but the spark seems to last the entire time. My first guess if the switch box but since one is almost more than the motor is worth, Hate to buy one on a hunch.

 

[racerone]

How long have you owned it ?-----Have you checked the flywheel key ?------Perhaps you need to switch plug wires.

 

[JAR55]

Motor hasn’t ran for years. Was collecting dust in neighbors garage so he gave it to me. DId not run when he put it in the garage. I did find one of the wires from the Switch box deteriorated and as well as the kill wire. Fixed both wires. Flywheel key is good, visually checked the stator and trigger coils and they look good. Spark plug wires are stranded wire with 0 resistance.

 

[racerone]

You say there is spark on the plugs.-----I say there is no spark on the plug in the cylinder !---Or spark is going to the wrong cylinder.

 

[merc850]

Go to the neighbor and ask him what happened to make him put it away, maybe he messed something up as is usually the case.

 

[JAR55]

He said it wouldn’t start so he took the cover off, saw the deteriorated wires and oxidation where the wires(kill switch) sparked against the motor and figured he burned it up.

Could a defective switch box just pass the stator voltage to the plugs. When I pull the cord it seem like I have a constant spark not just the two I would have expected from rotating the engine twice with the pull cord.

 

[JAR55]

Took video of spark plugs firing when I pulled cord. It appears that both are firing at same time when I slow-mo the video. Also took reading of stator winding connected and disconnected. Voltage is more than 3X when disconnected than it is connected. I ordered a switch box.

 

[JAR55]

Installed the switch box. Same issue. I have worked on a lot of 2 cycles and this one has me stumped, Good compression, spark and fuel but wont even fire.

 

[merc850]

Have you followed this wiring diagram maybe there's a crossed wire somewhere

 

[JAR55]

Thanks for the response. Yes I have this diagram and looked several time to make sure wires were exactly as shown. Also tried to swap brown wires. No change. I think the replacement switch box I had was also be bad - returned it today. The blue wire and ground were both stripped and crossed at one spot. All resistance and voltages with Switch box disconnect are correct. May have to find another switch box Unit.

Am I right to assume that the switch box is a CDI unit?

 

[racerone]

Switch box is the CDI unit.----Maybe there is an issue with reed valves ?-----But I can not see or hear your motor.

 

[merc850]

Take out both plugs and ground them, remove recoil assembly, put a pencil in the top cylinder and turn the flywheel - watch the pencil move when it reaches near TDC replace the recoil and look at the timing mark on it; it should line up with the line on the flywheel. A timing lite will show where it's firing or you can rock it back and forth to and past TDC and see where it sparks.

 

[JAR55]

I replaced the CDI unit and no change. I was able to put a timing light on the unit and it is 30 degrees below TDC without retarding the spark in the light. Did the pencil in Cylinder and the mark is exactly at TDC. Also it appears the this CDI fire both cylinders at each cycle. Going to try and adjust the switch coil.

 

[JAR55]

No success... I’m beginning to believe the Ignition coils are bad. Spinning the motor with a drill. I can definitely hear a difference when I advance/retard the timing light Iam not getting compete ignition. I believe the reed switched are working because I am getting unignited oil/gas coming out of the exhaust.

 

[JAR55]

Changed the ignition coils - motor starts and runs like new. Individually replaced all changed parts with original and engine will not run with any of the original ignition parts. I guess they were all bad...