7.5 merc wont start just backfires ser#

[tony11905]

i have a 7.5 hp merc with electric start with issues wont start and backfires ,could it be timing,? what? i beleave it is a 1981 with blue cover,the serial nimber for it is 611958 thats all i see on the plate, please help with what is a possible problem thanks

 

[JB]

Re: 7.5 merc wont start just backfires ser#

If your backfire is a loud bang from the exhaust it is timing.

You could have sheared the flywheel key or you could have the spark plug wires reversed. Either is a simple fix.

 

[tony11905]

Re: 7.5 merc wont start just backfires ser#

it is not a loud back fire ,its almost like its muffled fromthe exhaust.

 

[JB]

Re: 7.5 merc wont start just backfires ser#

If the foot is in water it could still be one of those things. Check them out.

 

[tony11905]

Re: 7.5 merc wont start just backfires ser#

i have cheched them out and everything semms fine , the only thing i found is that the coils are not the right coils they are from a diffrent motor a much bigger hp motor would that do it?

 

[buddhapi]

Re: 7.5 merc wont start just backfires ser#

I'm putting an impeller in a 7.5 right now (7168002)Canada, and if you're standing behind the motor, facing the prop. the left coil wire goes on the bottom plug, and the right coil wire goes to the top plug. that is if other wires haven't been spliced and switched around.

 

[tony11905]

Re: 7.5 merc wont start just backfires ser#

that how they are set up , but like i said before i have 2 diffrent coils 1 is the original coil to that motor and the other coil goes to a 150 hp merc, would that make this problem?

 

[buddhapi]

Re: 7.5 merc wont start just backfires ser#

I would start by getting the correct coil for your motor. that may be your whole issue. what year 150hp did the coil come off?

 

[mrteeman]

Re: 7.5 merc wont start just backfires ser#

did you check for good spark on both plugs?
carb not too rich?
no backfires through carb?

 

[Chinewalker]

Re: 7.5 merc wont start just backfires ser#

My bet is the primary wires from the pack got switched around on the coils... I believe the solid green goes to the top cylinder (No. 1) and green/white goes to the bottom cylinder (No. 2).

 

[mrteeman]

Re: 7.5 merc wont start just backfires ser#

My bet is the primary wires from the pack got switched around on the coils... I believe the solid green goes to the top cylinder (No. 1) and green/white goes to the bottom cylinder (No. 2).

Why? Not as a critique but for learning purposes.

My theory for your point is that spark will come on exhaust stroke when exhaust ports are opened making the mixture explode in cylinder and pressure is relieved through exhaust port?

Regards Teackele

 

[Chinewalker]

Re: 7.5 merc wont start just backfires ser#

If the spark occurs at the BOTTOM of the stroke (180 degrees off where it should be), then the exhaust port is open and the flame goes right out the exhaust and into the crankcase, not being confined to the combustion chamber as it should be. This is a backfire...

 

[tony11905]

Re: 7.5 merc wont start just backfires ser#

no backfire through carb? how do i adjust the carb?

 

[tony11905]

Re: 7.5 merc wont start just backfires ser#

as far as wires from coils i was told before that solid green goes to pos post and green w/white stripe was neg this was for bottom cyl, and blue w/white stripe was neg and solid blue was for the top if this is wrong i need to know thank you

,

no backfire through carb? how do i adjust the carb?