1979 Mercury 7.5 Speed Problems - Please Help

[imported_D]

Hi,

Please, I need advice.

Just purchased a 12ft JON w/ a 7.5 Merc off of ebay. I ran it and overheated it b/c the impeller was not working. So, I replaced the impeller, ran seafoam through, checked compression (90 on both cylinders). Now it starts up quickly everytime and stays cool, however, I can't get any speed out of it....maybe 3-4 mph. So, I marked the prop to see if it is slipping. After running it full blast the prop appears to not be spun....lines still matched up. I think this boat should do around 20mph (using the speed formula Sqrt (HP/WEIGHT) / 180.
Anyone have any suggestions? Any help would be appreciated.

 

[imported_D]

Re: 1979 Mercury 7.5 Speed Problems - Please Help

Two other things I forgot to mention, It;s a flat bottom JON boat and I also changed out the spark plugs and gaped them @ .40.

 

[imported_D]

Re: 1979 Mercury 7.5 Speed Problems - Please Help

bump

 

[Chinewalker]

Re: 1979 Mercury 7.5 Speed Problems - Please Help

Are you running on both cylinders? That's usually where I start on an obvious power loss issue. Warm up the motor. Pull one plug wire and see if there is a difference. If the motor dies completely, then you pulled the one it was running on. If it stays running with no change, then you pulled the dud. Two-cylinder motors can run quite nicely and sound good on one cylinder, but they'll be gutless...

- Scott

 

[imported_D]

Re: 1979 Mercury 7.5 Speed Problems - Please Help

Are you running on both cylinders? That's usually where I start on an obvious power loss issue. Warm up the motor. Pull one plug wire and see if there is a difference. If the motor dies completely, then you pulled the one it was running on. If it stays running with no change, then you pulled the dud. Two-cylinder motors can run quite nicely and sound good on one cylinder, but they'll be gutless...

- Scott

Scott, thank you for the advice!!! I pulled the top plug and the motor died. So, it is only runing on one cylinder! This is expensive isn't it :% Any suggestions on what to do next?

 

[ukejser]

Re: 1979 Mercury 7.5 Speed Problems - Please Help

I noticed "So, I replaced the impeller". Can you make a picture of the bolts that need to be unscrewed to get to the impeller.

 

[Harker]

Re: 1979 Mercury 7.5 Speed Problems - Please Help

Are you running on both cylinders? That's usually where I start on an obvious power loss issue. Warm up the motor. Pull one plug wire and see if there is a difference. If the motor dies completely, then you pulled the one it was running on. If it stays running with no change, then you pulled the dud. Two-cylinder motors can run quite nicely and sound good on one cylinder, but they'll be gutless...

- Scott

Scott, thank you for the advice!!! I pulled the top plug and the motor died. So, it is only runing on one cylinder! This is expensive isn't it :% Any suggestions on what to do next?

What year is the motor?

 

[Harker]

Re: 1979 Mercury 7.5 Speed Problems - Please Help

Whoops...79..didn't pay attention. Can you change your coils around to see if the fire migrates? If it does then get a new coil.

 

[perkdp]

Re: 1979 Mercury 7.5 Speed Problems - Please Help

Wasn't long ago I bought a DOA plug.
So, you may want to swap the plugs and see if the dead one moves with the plug or stays on the same cylinder.
If it moves with the plug,,, buy a new plug.
If it stays on the same cylinder, then swap the coils.

And so on... do the simplest & cheapest stuff first