1990 Mercury 100 hp Cough Sputter Backfire

[nwboater45]

I have a 1990 Mercury 100hp 2+2 outboard that I have got to idle and purr like a kitten but as when I jump to say to 1500 to 2000 rpms it run fine for about a minute then it backfires through the exhaust about every 15 seconds or so. I have gone through the service manual. TDC line up exactly on the mark. I have gone through the link and sync and made all the adjustments. Gone through all the fuel lines and have tightened everything. I just got done rebuilding the fuel pump. And nothing changes. I am not sure now if this is a fuel issue or ignition. Anyone had this issue and can lead me to the fix. Next step is to rebuild the top two carbs. If that doesn't work I may have to start replacing ignition parts or maybe buck up and take it to the shop which have 6 week back logs. Any guidance or help is appreciated.

Thanks.

Serial number: OB339?84

Link to Youtube vid: https://youtu.be/ILAmpPVeAO8

 

[gbrowne9]

check the plug wires for shorts,leaking spark first.then ckeck the accelerator pump to see if it is giving the bottom 2 cylinders a shot of fuel when the throttle is opened fully.what are the mixture screws of the top carbs set at?.in the vid sounds like a lean miss.should be 2.5 to 3 turns out from lightly seated.just a thought.hope it helps.

 

[nwboater45]

2.5 to 3 turns out? Service manual states 1.25 to start with. I put all new fuel lines in, got the timing between 3 and 4 BTDC, running directly from fresh fuel. I am stumped. Ready to take it to the shop. Anyone have any suggestions?

 

[jimmbo]

Why are you concerned about the TIming at Idle? You set the Max timing, set the Pickup timing and then you retard the timing to whatever it needs to be for the desired idle speed

 

[flyingscott]

+1 for the accelerator pump.

 

[Watermann]

Take a look at my thread in the link below nwboater45 I have your fix... very common issue on these 2+2 motors.

I know you said you did the link & sync but you have to do everything, no skipping anything they say to do, exactly how they say to do it in the instructions. As you can see and hear in my videos, the issue is identical and now it's solved on my 115.

https://forums.iboats.com/forum/engi...ems-need-ideas