[bonitoman]

I rebuilt my 130 yamaha and it has 2 hours idling on the flusher. I am ready to go for a ride on the river. I also have one of those flush mufflers that create back pressure simulating it in the water. Plus some of my neighbors don't like the sound of outboards. So after 15 years of running them I bought a flush muffler. My question is that I set the pilot screws at 7/8 ths and then 1 1/8 trying to richen the mixture thinking it is backfiring because it may be lean? Still no change. I have 135 compression and spark on all 4. All are firing. When i pull the plug wire on #3 there is no drop in rpm like the others. That does have spark. I am running double oil for the break in. I picked up this motor blown. The engine runs cool. Originally #4 was blown when I got the engine. Now thats great. I rebuilt the carbs and put a new diaphram kit in the fuel pump rebuilding that. Any suggestions apprecited. Thanks John

[bonitoman]

Pick up timing at idle is set at 5 degress atdc and wot is set at 20 btdc. I have a new plastic omc fuel filter on instead of the yamaha since I have a dozen omc fuel filters laying around. All the fuel line around brand new connected with the black fuel tie wraps.

[rodbolt]

hello double check the idle circuits on the top carb. the tiny jets in the fuel bowl can become clogged with anything. is this backfire from the exhaust and sounds like a bang or is this back fire like a deep cough and makes the motor jerk? you may have a restriction in the idle circuit. a lean backfire in a 4 stroke causes flame to pop out of the intake/carb shutters. a lean backfire in a 2 stroke with reed valves gets trapped in the crankcase and tries to move the piston against the compression. good luck and keep posting