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1973 7.5hp Mercury - Starting issues

S

Sockman96

New member
Hey all,

I have a 1973 merc 7.5, I bought it and brought it home, after testing compression (100 psi in each cylinder) and checking spark and that I had fuel to the carb I tried starting it. Second pull it went!

Fast forward to the lake and the dang thing won’t run.

I’ve redone my ignition wires (as the insulation was bad on them all) re-gapped my points to 0.020”. And put a new fuel pump in the carb (it’s an integral fuel pump)

Re checked compression and I have the same values, spark is great as ever, but after pulling pulling and pulling the motor refuses to run!

Any help would be appreciated!
 
Does spark jump a gap of 5/16" , yes or no ?-----I suspect there may be an issue with crankcase compression.----Did you install a new impeller in water pump?
 
racerone said:
Does spark jump a gap of 5/16" , yes or no ?-----I suspect there may be an issue with crankcase compression.----Did you install a new impeller in water pump?
Click to expand...
I will double check my spark today. But I believe it jumps that amount. They’re the flat face permagap style plugs.

I did not mess with the impeller in the water pump as the very first time I had it running it was spitting really strong.

If it is crankcase compression how do you check that?
 
Not that easy for a novice.-----Could be the upper and lower seals.-----Labarynth seal between the 2 crankcases.-----Chipped reed valves as well.
 
racerone said:
Not that easy for a novice.-----Could be the upper and lower seals.-----Labarynth seal between the 2 crankcases.-----Chipped reed valves as well.
Click to expand...
I’ll keep this thread updated after I check my spark jumping capabilities.

I have a hunch it’s fuel related. After pulling and pulling and pulling I pulled the plugs to smell for fuel and there isn’t a strong smell of fuel. I took the carb apart last night and cleaned everything and made sure my passages through jets all flow which they do.

Which with what you’re saying racerone if would be likely my crank seals are bad?
 
A carburetor is a very simple device.----It responds to the motor pulling air through it.----If it is not pulling the right amount of air it will not work.---Dees choke flapper close and stay closed after you pull the recoil ?------Do this ultra simple test.----Remove both sparkplugs.-----Put 5 or 6 drops of fuel in the cylinders.----Install plugs.---Pull cord a few times.-----Does motor bark and roar like a cornered lion , yes or no ?
 
racerone said:
A carburetor is a very simple device.----It responds to the motor pulling air through it.----If it is not pulling the right amount of air it will not work.---Dees choke flapper close and stay closed after you pull the recoil ?------Do this ultra simple test.----Remove both sparkplugs.-----Put 5 or 6 drops of fuel in the cylinders.----Install plugs.---Pull cord a few times.-----Does motor bark and roar like a cornered lion , yes or no ?
Click to expand...
The choke works as intended and I can’t get the motor to start with it closed or open. I did as you recommended and put a little bit of mix fuel in the cylinder. 1 ml in each cylinder. I gave a good 10 or so pulls and there was not a bark or roar. In fact not even a whimper, sadly.

I checked my spark, and it does not jump 1/4inch, but that’s me holding it to a ground. I need to test this in an in-line tester to achieve a bit better results I think. It was sparking, but not arcing the 1/4inch. It arc’d maybe 1/8th
 
The motor is not starting properly because you have weak / no spark in the cylinder.-----A little bit of spark on a plug laying on the block just is not good enough.
 
Coil packs were tested, but I’m not sure if I did it correctly.

But I need to test the condenser and windings in the stator still! On the to do list for the coming days I’ll keep you updated.

Spark is definitely weak and will only jump about 1/16 to an 1/8th.
 
The condenser I tested was not to spec, as well as the high-speed winding. I've ordered new high and low-speed windings as well as a condenser. Hoping this cures my spark issues.

I think the coil packs are good.
 
Small update:

Installed new high speed and low speed winding, along with a condenser, re-tested components and re-measured spark. Spark is still weak and will not fire. Re tested ignition coil packs, which seem to be out of spec. Ordered new ones and we will see if that fixes spark issues. Ive now entirely replaced the ignition system so hopefully the coil packs will test well.

updates to come, these old thunderbolt mercs are not exactly cheap to work on.
 
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