1971 Mercury 7.5hp

[Coldwaterangler]

I was hoping that someone could help me with my "new" 1971 mercury 7.5hp. I recently purchased this motor from a gentleman that said it had not been run in over 10 years. I hooked it up to the gas tank the other day and after 100 pulls, nothing. I rebuilt the carburetor and fuel pump, still nothing. I have a spark plug tester and it appears that I have spark to both plugs. I also inspected the insulator blocks at the points and they were intact. Even after spraying gasoline directly into the spark plug holes, it still would not fire. The only thing it will do every once in a while is backfire if I turn the throttle down from the start position to the shift position. I have no idea where to go from here. Any help would be greatly appreciated! Thanks!!

 

[impatico]

Re: 1971 Mercury 7.5hp

test the plugs themselves they may be bad, even if brand new, pull them out plug them into the wires and rest them on the head then pull if a bit make sure it has strong spark, other than that maybe the trigger wires got mixed up or something

 

[Faztbullet]

Re: 1971 Mercury 7.5hp

What color is the spark? It should be a bright blue and jump a 3/8 air gap,if not it plugs will not fire under cylinder compression.

 

[Coldwaterangler]

Re: 1971 Mercury 7.5hp

Thanks for the info. I will definitely check the spark plugs directly. I have one of those "in-line" testers that lights up when I pull the starter. Do these not indicate true spark at the plug? Also, I am not sure how to test for a 3/8" spark on the surface gap plugs. I know how to check spark on the other types of plugs but not these.

 

[Coldwaterangler]

Re: 1971 Mercury 7.5hp

Ok. So the plugs have a nice strong bright blue spark. I tried to restart the engine after confirming and still nothing. I even tried spraying gasoline directly into the spark plug holes again and it will not start in neutral. However, I can get the engine to start if I put the motor in forward gear and crank the throttle up. It will run for a few seconds and then kill each time. Does this sound more like a carburetor issue? Thanks for the input!

 

[CharlieB]

Re: 1971 Mercury 7.5hp

The motor needs three things to run.

Compression, ADEQUATE spark, and fuel.

Notice the adequate was in caps. An ignition can make a weak spark and NOT run, or may start and run at low speed but die when throttled or load applied once in gear. A hot blue arc of at least 1/4 inch or more is required. The points ignitions used a 'plastic' insulator where the wire comes out of the points housing, sometimes this insulator cracks, or gets a carbon trail burnt into it from arcing to ground. Often the motor can show a small spark when tested, yet fail to spark under compression and the motor either will not start or runs poorly from a weak spark. Close inspection of the the insulator is necessary. Since the parts is No Longer Available thru Merc, CDI Electronics has them available.

If the motor starts, runs a short time and dies, try squeezing the primer bulb to 'assist' the fuel pump. You may have a valve incorrectly installed or air leaking into the hose anywhere before the pump and the motor may be running out of fuel. Disconnect the fuel line to the carb, crank the motor and observe the fuel flow, temporarily install a clear hose and watch for bubbles once you start the motor which may prove air sucking into the system.

 

[Laddies]

Re: 1971 Mercury 7.5hp

You have fire just not at the right time to fire the fuel, that leaves the ign. insulator blocks arcing causing random spark.

 

[Coldwaterangler]

Re: 1971 Mercury 7.5hp

I did inspect the insulator blocks at the points. They do not look cracked but one of them does have some carbon build up on the top surface. Do you think these still need to be changed even though they are not cracked? Also, the motor will start and run for a few seconds if I crank the throttle in gear and try to start it that way. Can the insulator blocks still be a problem, even though I can start it in gear?
Thanks

 

[Laddies]

Re: 1971 Mercury 7.5hp

The points control the timing, the only other thing it could be is cracked wiring to them. The system is high voltage to the points and fires when ever the wire from the coil can find ground so even the stud on the points when not coated with neoprene can cause random spark. As the rpm increaces so does the voltage and the liklyhood of a arc, so you have a problem that moves around and drive you nuts

 

[Coldwaterangler]

Re: 1971 Mercury 7.5hp

Also, could someone please give me the part number for the insulator blocks?
Thanks again!

 

[Laddies]

Re: 1971 Mercury 7.5hp

CDI part number 994-3981

 

[Coldwaterangler]

Re: 1971 Mercury 7.5hp

Excellent! Thank you for the help. I'll try to give you a report after I install the new insulator blocks.

 

[Coldwaterangler]

Re: 1971 Mercury 7.5hp

Ok. So I installed the new insulator blocks. There has been a slight improvement with the engine. It now sputters everytime I try to start it in neutral but will not stay running. Once again, the motor will start in forward gear but now will stay running if I keep the throttle turned up. As soon as I try to idle down, it kills. Any ideas? Thank you.

 

[Laddies]

Re: 1971 Mercury 7.5hp

You need to go back to the carb, clean the low speed pickup tube and check the float level

 

[rdmotors]

Re: 1971 Mercury 7.5hp

Have you checked the compression I have found that if it is low what you are describing is the way it acts.

 

[db21]

Re: 1971 Mercury 7.5hp

Did you ever get your engine running? If so did you confirm the problem? I have the same one. Runs at WOT in gear, but at no other time!

 

[Windnsails]

Re: 1971 Mercury 7.5hp

Don't know if my 1971 Merc 75 (Serial #between 25xxxx and 27xxxx) is the same as your 1971 7.5, but I found that the fuel pump cover was cracked where the threaded brass nipple screwed in. This meant air was getting into the pump on the wrong side of the pump diaphragm so there was no pressure pulsation to operate the pump.

I replaced the cover last weekend and now it runs.

My continuing issues however are a 1. low speed miss and 2. gear oil leaking out of the propeller seals. 3. Water pump seems weak too though some cooling water is getting through.

For item 1., some other threads on 2 cycle engines said to back out the idle/mixture screw 1/4-1/2 turn. So I'm going to try that next.

I dread trying to find parts and a repair manual for this old motor.