8hp mercury missfire

seasalt

Recruit
Joined
Aug 31, 2007
Messages
2
engine was very bad to start from cold.traced fault to diagphram @ replaced same. engine now starts like a dream. but, after starting the engine will run but has a missfire/pre ignition when the throttle is opened ! new spark plugs were fitted at the same time. the motor has been in my possession for 4 years now been no problems. could any1 throw any light on the subject !
 

Windykid

Lieutenant Junior Grade
Joined
Apr 17, 2007
Messages
1,177
Re: 8hp mercury missfire

What year is it?

Plug fouled?

Water in fuel?
 

Laddies

Banned
Joined
Sep 10, 2004
Messages
12,218
Re: 8hp mercury missfire

Does the engine have a ign miss or simply backfireing when throttled up
 

seasalt

Recruit
Joined
Aug 31, 2007
Messages
2
Re: 8hp mercury missfire

engine has missfire when at tickover but this is intermittent & still missfires/backfires when the throttle is opened. it is a 1997 motor . no fuel probs it is new fuel. the motor is off the boat in the vice on the bench in a tub of water for testing purposes only. i have no workshop manual for this motor so any set up information would be greatly appreciated.
 

Laddies

Banned
Joined
Sep 10, 2004
Messages
12,218
Re: 8hp mercury missfire

Heres the test for your ign from CDI. it's tough to find that kind of problem without a induction tach or dva, if you have a induction timing lite it can find the bad cyl at idle and just above
 

Laddies

Banned
Joined
Sep 10, 2004
Messages
12,218
Re: 8hp mercury missfire

Sorry this didn't post the first try

scan0007.jpg
 
Top