Mercury 5 hp 4 stroke backfire through carb

[maineiac5586]

I have a 5 hp mercury 4 stroke and can't get it running. It fires for a second and on some trys it pops through the carb inlet. i rebuilt/cleaned the carb fully 2 times, checked for spark (it is fine), did a leakdown check to make sure the valves are fully closing and checkked compression. Any one have any ideas? This seems like such a simple motor I am stumped.I even sprayed starting fluid in the carb and there was no change. I ruled out the fuel pump by bypassing the pump and filling the bowl using the squeeze bulb on the fuel line, still no change. Please help if you can, thanks

So I was thinking maybe the keyway is sheared giving false timing. I removed the recoil but I don't have long enough bolts yet to use the puller on the flywheel. I took these 2 pictures of the keyway area. I can't tell if the keyway is there or not. Is it supposed to come flush with the flywheel surface? Anybody have any ideas?

 

[maineiac5586]

Re: Mercury 5 hp 4 stroke backfire through carb

anyone know?

 

[Texasmark]

Re: Mercury 5 hp 4 stroke backfire through carb

Firing back through the carb is fuel igniting while the intake valve is open. So why is fuel igniting at that time? First thought would be timing has changed. Did you do anything to change your timing? How have you been operating this engine? Do you possibly have some glowing carbon embers in the combustion chamber that could ignite the fuel at the wrong time?

Mark

 

[maineiac5586]

Re: Mercury 5 hp 4 stroke backfire through carb

I checked leakdown. It shows both valves are closing. I pulled the flywheel. Keyway is there as designed. The label on the motor says timing is not adjustable. I am at a loss here. I may take it to a dealer but I'm afraid they won't find the problem. Anything else I can check?

 

[raczekp1]

Re: Mercury 5 hp 4 stroke backfire through carb

its quite new motor,
carb is not blocked anywhere?

 

[1nebel0]

Re: Mercury 5 hp 4 stroke backfire through carb

The small four strokes are prone to float issues....take the carb apart and get the spec on float drop and float level..just rebuilt a 15hp carb and sure enough per my buddy at the marine shop the plastic float arms had been attacked by Ethanol. I rebuilt kit was expensive however as soon as I put the new float on and took measurement, the new float was per the spec's. They are non adjustable....I would have never guessed the float however he see's it all the time..just like he see's four strokes making water and customers thinking they have a cracked head, cracked block or head gasket..Turns out to be the thermostat sticking and the motor running too cold..This is on four strokes and especially the big ones that idle around a lot and don't build any heat to get rid of the inherent moisture issue...hope this helps.......