7.5 Merc w/ Murphy's law

[BBBrady]

Hey guys, got a 1981 Merc 7.5 w/ a gremlin or just Murphy's law playing tricks on me. Bought the motor for a 14' jon and went to fire it up the other day, wouldnt hit a lick so i went through the usual culprits. Still nothing, Ive got good fire, good compression, fuel and exhaust are both clear of blockages. But still the most im get is a small puff of smoke from the exhaust from time to time. Any Ideas would be helpful, any any questions as to what Ive attempted already are welcomed. Thanks

 

[AMD Rules]

Re: 7.5 Merc w/ Murphy's law

Any chance you have the spark plug wires reversed?

Will it fire at all if you spray pre-mix into the carb?

Was it running okay before?

 

[JB]

Re: 7.5 Merc w/ Murphy's law

Howdy, BBBrady.

Welcme to iboats.

Did you go through the troubleshooting in "Outboard won't start" in the Engine FAQs?

Give that a try, and don't tell us such and such is good, tell us the evidence.

Let us know what you find, and good luck.

 

[BBBrady]

Re: 7.5 Merc w/ Murphy's law

Hey Guys, first off AMD Rules its possible but unlikely that the plug wires are reversed theyve been on there so long there essentially molded into position so if they were swapped it was long before me, Does not help spraying mix in the carb, as far as running the motor has been sitting for 5+ years thats the extent of my knowledge on its history. JB hadnt read your post "Outboard won't start" but have now and I have been through all the procedures you had outlined before posting here, Im a new to outboards but not new to two cycle engines this ones jes got me scrachting my head.

 

[JB]

Re: 7.5 Merc w/ Murphy's law

Have you checked the flywheel for a sheared key?

 

[BBBrady]

Re: 7.5 Merc w/ Murphy's law

yes that was the last thing i did last night before coming in from the shop.

 

[Robj]

Re: 7.5 Merc w/ Murphy's law

If you have spark, that problem is ruled out. Do you have fresh fuel? Is there good compression? What is the condition of the carb. If you carb is gummed up, it won't start.

Have a great day,

Rob.

 

[jbjennings]

Re: 7.5 Merc w/ Murphy's law

If you've really got good spark and you squirt some fuel mix in the carb, the darn thing ought to run for a few seconds at least...even with below normal compression.
JMO,
JBJ

 

[BBBrady]

Re: 7.5 Merc w/ Murphy's law

Alright guys I first have to tell you the issue has been resolved, I thought on the issue all night and day, thought that possibly I had just flooded it with my relentless efforts last night, so I left the plugs out last night to let it air and got home today to pull my brains out again, and i thought what i had posted to AMD about the plug wires being reversed "unlikely but possible" i figured what the hey worth a shot. swapped them and didnt get much more than a half a pull before the thing fired and purred like a kitten. Now just to be fair to myself, i had initially labeled all of my wires, they were swapped out before i got a hold of the motor but for some reason that doesnt make me feel any less stupid at the moment, but at the same time Im extremely overjoyed b/c thats resolved. Thanks for the help guys, and especially to AMD who got me thinking about that "unlikely but possible" thing.

 

[AMD Rules]

Re: 7.5 Merc w/ Murphy's law

Been there, done that.

Glad you got it working.

Cheers

 

[Motor Boater Bill]

Re: 7.5 Merc w/ Murphy's law

Dawggonit--that's good reading!

 

[jbjennings]

Re: 7.5 Merc w/ Murphy's law

AMD,
I'm just curious, how did you catch the reversed wires? Was it the poof of smoke every once in a while that he mentioned or just a thought. Either way, you are very observant---NICE ONE.
Glad to hear you got that 7.5 running BBBRADY.
JBJ

 

[AMD Rules]

Re: 7.5 Merc w/ Murphy's law

I fought with my 20hp last year. Same thing... sounded like it wanted to run but wouldn't. (On a pull start motor you can feel the flywheel kick-back, as it is trying to run in reverse due to the firing sequence.) All basics covered air/spark/fuel/compression. Ended up being the reversed wires in that case. I've seen it mentioned by other members a few times on this forum since then.

On a side note, something to watch for is a timing setting (if adjustable) so far out of whack it will only run with the wires reversed from spec. The fix isn't to reverse the wires from spec, but rather to correct the timing. I'd suggest to BBBrady to double-check the timing before assuming that this issue is put to rest. Hopefully he's out fishing.