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High Speed Problems with 1972 Mercury 7.5 Thunderbolt Outboard

Can anyone give me any hints on troubleshooting my 1972 Mercury 7.5 HP Outboard.

This little gem runs fine at low speed but struggles with high speed.

I have changed spark plugs, coil wires, fuel pump, and cleaned carb.

I suspect ignition coil problems since I believe there is a low speed and high speed section in each coil.

Any suggestions.

Thanks,
Simfish3618
 
Yes, it runs when I put it in gear - when I throttle up it lags and sputters then runs very erratic - sometimes it just dies.

I have attached pictures of the coils --- can you give me a site where I can purchase this type of coil?
 
Serial Number on Motor is 3281871 ----- 1972 7.5 HP Mercury Thunderbolt.

I will pull coils and clean and resistance test -- still looking for replacements that are compatible.

1972 7.5 HP Mercury Thunderbolt coils small.JPG
 
Test first before spending $$$$------------Does the spark jump a gap of 3/8" yes or no !!!----What does a compression test show on this motor ?-----------Has the waterpump impeller been changed?
 
Spark plugs test fine --- compression test 80 lbs on each side -- water pump has been changed.

Gear Box pressure tested -- will not hold pressure - ugh, guess seals need changing.

Attached a picture of my home made lower unit pressure tester.

Ignition coils pulled and cleaned --- part number is 339-5748-4877 ...

How do I test coils with a volt meter? Tried resistance test but ohms reading fluctuates.

Can I tell if my coils are good before spending $$$$ to replace?

Looks like I can find coils part #336-4528 for around $100 bucks each.
7.5 mercury coils.JPGlower unit pressure tester.JPG
 
OK - so just did some testing with an inline spark tester and make some progress.

Both cylinders fire just fine on low speed.
On high speed one coil is not firing steady -the other is - I swapped coils and the trouble stayed with the input to the coil.

So, the trouble is coming from the source to the coil - I will have to look under the flywheel now for the culprit.
 
OK - so just did some testing with an inline spark tester and make some progress.

Both cylinders fire just fine on low speed.
On high speed one coil is not firing steady -the other is - I swapped coils and the trouble stayed with the input to the coil.

So, the trouble is coming from the source to the coil - I will have to look under the flywheel now for the culprit.
 
.."Both cylinders fire just fine on low speed.
On high speed one coil is not firing steady -the other is - I swapped coils and the trouble stayed with the input to the coil."

As suspected. Buy one NEW coil and make it go--avoid used ones,, and here's why: Seen a bunch of those old coils do that. The miles of wire inside (coated to prevent short circuits) breaks down over decades of use.

Jeff
 
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