350 MAG Backfire Through Intake
JimGuy341
Member Posts: 104 ✭✭
Hoping to tap into a little forum knowledge here...
Last season around this time we started getting a backfire through the intake at around 4000 rpm on the port engine (stbd engine is running fine). A full tune up was done two years ago. My local mechanic plugged into the engine - no codes, fuel pressure good, timing good, fuel filter replaced, no water in the fuel.. He wanted to do some research. So while I was waiting to hear back from him, I replaced the cap and rotor just for fun. This solved the problem. The replaced parts didn't have any obvious failures. But I was happy to have the problem resolved so we didn't continue the investigation.
Boat ran good this season until a week ago when the backfire @ 4000 rpm returned again on the port engine. I replaced the cap and rotor and the backfire went away.
The engine runs good under 4000 and above 4100 up to WOT. It's just around 4000 that the symptom occurs.
Any ideas where I should look next?
Last season around this time we started getting a backfire through the intake at around 4000 rpm on the port engine (stbd engine is running fine). A full tune up was done two years ago. My local mechanic plugged into the engine - no codes, fuel pressure good, timing good, fuel filter replaced, no water in the fuel.. He wanted to do some research. So while I was waiting to hear back from him, I replaced the cap and rotor just for fun. This solved the problem. The replaced parts didn't have any obvious failures. But I was happy to have the problem resolved so we didn't continue the investigation.
Boat ran good this season until a week ago when the backfire @ 4000 rpm returned again on the port engine. I replaced the cap and rotor and the backfire went away.
The engine runs good under 4000 and above 4100 up to WOT. It's just around 4000 that the symptom occurs.
Any ideas where I should look next?
2008 350 EC
Comments
Get a leak down on each cylinder. An exhaust lifter could be weak and losing pressure at that rpm on an individual cylinder, and perhaps have tiny debris in its port, which can make it sporadic.
Given the relationship with the cap and rotor, it would more likely have to do with the timing chain.. those dang things have nylon between links to keep them quieter, but which wear as you can imagine.. that allows tiny amounts of slop that create more slop over time and which is apparent at higher rpms. It's a six pack job- no more than a couple hours. You can test for slop by pulling cap off and watching the rotor button while hand turning the engine and observing how quickly the rotor responds when you turn engine- should be instant. That's just a shade tree test, though.
Look at the caps inside to see if there is indication of spark spray- if the cap has any age on it there should be pretty clear little lines across each point and uniformly worn- you'll be able to see, if you're lucky, constant fire from one point to the next indicating where it drags spark between the points, in affect firing them both at same time or pretty dang close to same time, and which indicates back to slop in a specific link or series of links in the timing chain.
If you have to replace the chain, get a good one. Double roller without nylon inserts/liners...
One more thing- get a vacuum gauge on that thing. Having two matching engines you should have equal load on each engine at same load and rpm.. if one is less vacuum you have a leak that rears it's head when drawing the most (not loaded down and steady rpm), and that causes air:fuel issues with ignition still happening when the intake valve opens (being rich) and ignites the intake runner- hence backfire through intake.
Alright, one more one more thing: Your timing could simply be off, too. A degree advanced or retarded could cause spark spray.
A backfire through the intake is NOT to be taken lightly on a boat. Things can go boom quickly.